eden food waste technology trial - top eco visitor attraction

2005-2007

Eden Project Food Waste Technology Trial

Investigating the viability of in-vessel

composting for site-based food waste disposal

for the commercial sector

Prepared for The Eden Project by Tim Stokes, environmental consultant, 07809 762545 The author has taken due care in the preparation of this report to ensure that all facts and analysis presented are as accurate as possible within the scope of the project. However no guarantee is provided in respect of the information presented, and the author is not responsible for decisions or actions taken on the basis of the content of this report.

Acknowledgements

The in-vessel composting trial has been run through the Eden Project’s Waste Neutral

Programme (WNFP).It has consequently indirectly benefited from the funding and

support supplied to that programme through the Landfill Tax Credits Scheme, the South

West Regional Development Agency (SWRDA), the Combined Universities of Cornwall

(CUC) R & D programme (Objective One European Structural Funds) and the BOC

Foundation.

Prior to the start of the technology trial, WNFP’s main funder, Viridor Credits, had

supported the purchase of the Neter and the earlier development of the recycling

compound within which it operates. This support was matched by funding for the

broader activities of the Eden Project received from SWRDA and the Objective One

Programme. Additional landfill tax funding allocated to the composter, recycling

compound and adjoining visitor centre, was supplied through the English Environment

Fund.

In 2004, the BOC Foundation approved a research grant towards the monitoring and

evaluation of the potential benefits of on-site disposal of food waste specifically in terms

of its impact on greenhouse gases.

Towards the end of 2004, a bid for funding was submitted through Defra’s New

Technology Demonstrator Programme. Although the bid succeeded in getting through

to the second stage of the funding process, it was ultimately unsuccessful.

The most significant funding for the trial was supplied through the CUC’s R & D

programme, using European Structural Funds supplied through Cornwall’s Objective One

office.

The WNFP has also received funding through the SITA Trust for awareness and public

learning activity relating to composting and food waste issues in general. This included a

compost exhibit partly funded by the Environment Agency. The trial has both benefited

from and added value to this activity.

CONTENTS

Title

Page no.

Executive summary...................................................................................................... 1 1. Introduction.................................................................................................................... 3 2. Background................................................................................................................... 4 2.1 The issue of waste........................................................................................................ 4 2.2 Eden Project’s Waste Neutral Programme................................................................... 6 2.3 Food waste at the Eden Project.................................................................................... 7 2.4 Aim of the food waste technology trial.......................................................................... 7 2.5 The choice of technology.............................................................................................. 7 3 Overview of the composting process used in the food waste technology trial.............. 8 3.1 Collecting, sorting, transportation and storage of feed-stocks...................................... 8 3.1.1 Food waste.................................................................................................................... 8 3.1.2 Green waste.................................................................................................................. 9 3.1.3 Carbon supplements..................................................................................................... 10 3.2 Inputting feed-stock into the Neter 30 composter......................................................... 10 3.3 Description of the Neter 30 composter.......................................................................... 11 3.4 Energy input.................................................................................................................. 13 3.5 Staff input...................................................................................................................... 13 3.6 Outputs of process........................................................................................................ 14 4 Findings of the trial........................................................................................................ 15 4.1 Background................................................................................................................... 15 4.2 Effectiveness of composting process............................................................................ 17 4.2.1 Temperature.................................................................................................................. 17 4.2.2 Acidity............................................................................................................................ 18 4.2.3 Moisture levels.............................................................................................................. 18 4.2.4 Rate of progress of composting mass........................................................................... 19 4.3 Effectiveness of compost as a horticultural product...................................................... 20 4.3.1 Growth medium trials.................................................................................................... 21 4.3.2 Mulching medium trials.................................................................................................. 21 4.3.3 BSI PAS 100 tests......................................................................................................... 22 4.4 Bioaerosol tests............................................................................................................. 24 4.5 Gas emissions....................................................................................................... 25 4.6 Design, operational and other issues arising from the trial........................................... 26 4.6.1 Design issues................................................................................................................ 26 4.6.2 Operational and other issues........................................................................................ 27 5. Financial appraisal........................................................................................................ 29 5.1 Introduction................................................................................................................... 29 5.2 Set-up costs.................................................................................................................. 29 5.3 Management, administration and research and development costs............................. 29 5.4 Operational and maintenance costs.............................................................................. 30 5.5 Cost benefit analysis..................................................................................................... 32 5.5.1 Assumptions.................................................................................................................. 32 5.5.2 Annual running costs and savings................................................................................ 33 5.5.2 Environmental and social impacts................................................................................. 34 5.5.3 Summary of costs and benefits projected for 2007/8.................................................... 35 5.5.4 Potential savings using Neter and other in-vessel technology...................................... 37 5.5.5 Conclusions of financial analysis.................................................................................. 39 6. Conclusions................................................................................................................... 40 7. Recommendations........................................................................................................ 42

Food Waste Technology Trial Report Page 1

EXECUTIVE SUMMARY

As a society, the UK is consuming resources at an unsustainable rate. At the heart of our unsustainable consumption of resources is the issue of waste. Around 100 million tonnes of waste are generated by commerce, industry and households every year. Food waste makes up a considerable proportion of the total waste stream that is sent to landfill where it rots down under anaerobic conditions to produce methane, a powerful greenhouse gas that is contributing to global climate change.

The Eden Project is pioneering an approach to waste called “Waste Neutral” aimed at stimulating the market for recyclates and encouraging people to think of waste as a resource. As part of this approach, it installed a Neter 30 in-vessel composter in April 2005 and set up a trial to test the efficacy of this technology for the processing of food waste.

This report is an open account of the Eden Project’s experience of setting up and operating the composter. The aim of the report is to inform others considering investing in this technology of the benefits and potential pitfalls of such an approach.

The Neter 30 operates on the principle of aerobic digestion - a process through which micro-organisms consume organic matter in the presence of air and convert it into a stabilised compost that can be applied to land.

Food waste is collected from the central food preparation facility and seven catering outlets on the site and transported to the Waste Neutral Recycling Compound (WNRC). Here it is fed into the Neter 30 alongside shredded green waste and wood pellets which help to improve the composting process.

After initial teething problems, the Neter 30 has been producing compost since September 2005. The compost produced has been submitted to various tests to ascertain its quality and in the main has been shown to be suitable as a soil improver for use on the site. In particular, samples submitted for testing under the British Standards Institute Publicly Available Specification 100 (BSI PAS 100) scheme have passed in two out of three instances.

The quantity of food waste being processed by the composter is lower than originally expected and well below the capacity of the machine. This is mainly due to problems that have been experienced in establishing the optimum feed-stock for the Neter. Also, the level of food waste available was overestimated at the design stage of the project partly due to the success of sustainability initiatives across the site which have subsequently reduced food waste.

The most reliable feed-stock mix identified to date consists of approximately two parts food waste to one part green waste, with a 15% addition of wood pellets.

Analysis of the financial costs and savings arising from the use of the composter indicates that the process operating at the Eden Project cannot currently be justified from an economic perspective. The operational costs incurred outweigh the savings made from diverting food waste from landfill and producing compost for use on the site.


However, the Eden Project is committed to the principles of sustainability and there are environmental and social benefits that arise from diverting food waste from landfill. In particular, treating food waste on site follows the ‘proximity principle’ of managing waste close to where it is produced. Composting food waste also results in a reduction in emissions of methane, a powerful greenhouse gas.

Furthermore, the findings indicate that the economic viability of the technology tends to increase with the throughput of food waste and as knowledge of appropriate feed-stock mixes increases. The rising costs of sending food waste to landfill by virtue of increasing environmental taxation will also substantially increase cost savings over time from diverting food waste from landfill.

Arising from the Eden Project’s experience of in-vessel composting using the Neter 30, the following recommendations are made:

1. Organisations considering investing in an in-vessel composter should:

• seek to understand their food waste issue and take appropriate steps to minimise the amount of food waste being generated;

• make an accurate assessment of the amount of food waste generated as it could have a major bearing on the financial payback;

• seek to ensure that the capacity of the machine purchased closely matches the volume of food waste and other feed-stocks composted;

• ensure that they have given full consideration to the implications of complying with the relevant legislation including amongst others ABPR 2005 and the EPA 1990; and

• be committed to the principles of sustainability and thereby have a full appreciation of the benefits of the process, over and above the current financial returns.

2. At lower levels of throughput, in addition to the aforementioned precautions, the following conditions should ideally apply:

• A suitable free or low cost source of carbon should be available as a supplement to the feed-stock.

• A steady source of green waste should also be available free or at low cost

• The compost produced should be either of use to the operator as a soil improver or processed into a saleable product.

• Any additional operational capacity employed in the processing of food waste needs to be closely matched to the nature and level of work.

3. It is important to input an appropriate mix of feed-stock into the composter. Failure to do so could result in a situation where the machine will be operating inefficiently or not at all.


1. INTRODUCTION

The Eden Project was conceived to celebrate the wonder and variety of the plant world and to raise awareness of the interdependency of people and plants. Since its opening in 2001 it has consistently welcomed over one million visitors every year to view its iconic biomes.

The Eden Project strives to ensure that its visitors enjoy the whole experience of their visit. For this reason, there is a range of catering outlets on the site where a variety of meals and refreshments are served. Inevitably in such an operation, there will be left-over food. But what happens to this food waste? In most commercial catering establishments, it will be put in a bin, for collection and transportation to a landfill site. There, it will rot down along with other waste emitting, in particular, methane – a powerful greenhouse gas which is contributing towards global climate change. With no other option available at the time, the Eden Project used to send all its food waste to landfill. This approach was very much at odds with the ethos of sustainability underpinning its activities. It also contravened the ‘proximity principle’ which recognises both the need to avoid passing on the environmental cost of waste disposal to communities that are not responsible for its generation and the need to reduce the financial and environmental costs of transporting waste. However, the Eden Project’s food waste stream constituted 'catering waste' and as such was governed by the UK Animal By-product Regulations (ABPR 2005) and required special treatment. This necessitated using new, in-vessel composting technology capable of safely processing food waste containing animal by-products. To ‘close the loop’, the compost produced would be used on site as a growing medium, soil improver or mulch, thus returning nutrients to the soil, improving its structure, and aiding plant growth.

After considerable research, the chosen solution came in the form of the Neter 30 in-vessel composting system manufactured by a company called Susteco. Though a number of smaller composting facilities using similar technology are now operating in the UK, this was the first time that an installation of this scale and capacity had been installed. It was also only the third Neter composter to be installed in the world.

Recognising its potential for improving resource efficiency amongst small to medium sized enterprises (SMEs), the Eden Project decided to set up a trial to examine the operational, environmental and financial implications of investing in this technology.

This report is an open account of the Eden Project’s journey down that route. It is hoped that this experience in attempting to deal with its food waste more sustainably will help others seeking to go down the same path, raising awareness of the benefits and potential pitfalls of such an approach.


2. BACKGROUND 2.1 The issue of waste

As a society, the UK is consuming resources at an unsustainable rate, estimated to be three times the carrying capacity of the earth.

At the heart of our unsustainable consumption of resources is the issue of waste. Waste is one of the biggest environmental issues currently facing the UK. Around 100 million tonnes of waste are generated by commerce, industry and households every year1.

Existing landfill sites are fast approaching capacity and the space and desire to open fresh landfill sites is rapidly diminishing. Indeed, in Cornwall, full capacity will be reached in a matter of a few years. Moreover, it is now widely acknowledged that disposal of waste in such a manner is an unsustainable practice. Food waste represents a significant proportion of the total waste stream that is sent to landfill. The decomposition of organic wastes in landfill produces methane, a powerful greenhouse gas which is contributing towards climate change. It is estimated that approximately 40% of the UK’s methane emissions arise from uncontrolled emissions from landfill sites. Methane emissions account for around 8% of the UK’s total greenhouse gas emissions2. Food waste in itself has a considerable impact on the environment even if it is not sent to landfill, simply from the embodied energy used in its production, transportation and preparation. There are a number of reasons why food waste arises. Some food waste is unavoidable as inedible or unpalatable components of meat, fish, vegetables and fruit have to be discarded in the preparation of food. In a commercial catering setting there are a number of other causes of food waste including:

• Poor planning which can result in food going off before it is used • Providing portions that are too large • Providing menus that are too broad in a setting where food is pre-prepared, thereby

consigning less popular dishes to the bin • Providing poor quality food • Buying more food than is required – particularly as a consequence of “buy one, get one

free” offers Recently EU and UK legislation, and fiscal measures have been introduced to discourage the disposal of food waste (and other wastes) in landfill sites.

1 Waste Strategy for England, Defra 2007

2 Waste Strategy for England, Defra 2007


The Landfill Directive (99/31/EC) has set binding targets for the UK to reduce quantities of biodegradable municipal waste (BMW) sent to landfill. The targets are as follows: • By 2010 to reduce the amount of BMW landfilled to 75% of that produced in 1995. • By 2013 to reduce the amount of BMW landfilled to 50% of that produced in 1995. • By 2020 to reduce the amount of BMW landfilled to 35% of that produced in 1995.

The Waste and Emissions Trading Act (WET) 2003 has been introduced as the main mechanism for meeting the Landfill Directive targets in the UK. The WET Act provides the framework for the Landfill Allowance Trading Scheme (LATS) which through a system of tradable allowances and permits will help waste disposal authorities to reduce the amount of BMW that is sent to landfill.

The Landfill Tax Escalator is set to increase landfill tax from £24/tonne in 2007/8 by £8 per tonne per annum until at least 2010/11, by which time the landfill tax will have doubled to £48 per tonne. The banning of non-hazardous liquid waste and untreated waste from landfill under the Landfill Directive from October 2007 may also place upward pressure on the cost of disposing of catering waste. In practice, these initiatives will result in increased costs for commercial organisations wishing to send food waste to landfill, and so, they will be incentivised to seek out alternative methods of disposing of food waste. Options for the disposal of food waste include:

• Mechanical and Biological Treatment (MBT) combines mechanical and biological

processes to treat municipal, non-hazardous and commercial wastes. Essentially, a range of mechanical processes are used to separate out dry recyclables, and biological processes such as biological drying are used to stabilise the organic fraction of the incoming waste. The resultant outputs can be used to produce an energy rich refuse derived fuel, an organic fraction that is suitable for composting or anaerobic digestion and a biologically stable residue that is suitable for composting. It is, in essence, a method of pre-treating waste.

• Mass burn incineration treats unsorted municipal waste by combustion in air under controlled conditions, reducing refuse to dry ash. Most incineration plants now generate electricity and heat from the burning of refuse.

• Pyrolysis and gasification turn waste into energy rich fuels. Pyrolysis is the thermal degradation of waste in the absence of oxygen to produce char, pyrolysis oil and syngas. Gasification breaks down hydrocarbons into a syngas by carefully controlling the amount of oxygen present. The by-products of these processes can be used as fuel and as a feedstock for petrochemical and other applications.

• There are now a number of commercial companies operating in the UK employing aerobic digestion technology to produce compost or anaerobic digestion technology to


generate electricity and heat and produce a digestate that can be used as a soil improver.

• Companies can, under licence from the Environment Agency (exemptions can apply), compost their own waste on site employing aerobic or anaerobic digestion technology.

2.2 Eden Project’s Waste Neutral Programme

The Eden Project can play an important role in raising awareness of alternative ways of thinking about waste – in particular the recognition that potentially there is no such thing as waste, only unrecognised resources.

In 2004, the Eden Project set up its Waste Neutral Programme. The aim of the programme is to make best use of the resources that the Eden Project uses, minimising waste, and ultimately leading to a situation where the volume of waste and recyclates leaving the site after reduction and re-use, is equal to or less than the volume of products made from recyclates that are bought in. This approach encourages both the careful use of resources and the purchase of goods made from recyclates thereby stimulating the market for ‘waste’, recognising that unless recyclates become part of the resource chain, they will simply become ‘value-added’ landfill.

The mantra of the Waste Neutral Programme is Reduce, Re-use, Recycle and Reinvest. It is the reinvestment in products made from recycled materials that stimulates the market for waste.

The Waste Neutral Programme is being implemented as follows:

• Environmental and waste criteria have been fully integrated into the Eden Project’s purchasing policy, and accounting and auditing systems.

• The Eden Project is working proactively with its suppliers and service providers to

minimise waste and maximise the use of recyclates, redesigning products and services if necessary in order to achieve this aim.

• The Eden Project is promoting the programme to individuals, communities and

organisations through on-site and off-site educational programmes to encourage them to adopt a similar approach.

• Recycling stations have been set up throughout the Eden Project site to enable visitors

to participate in the Waste Neutral programme and to allow for the separation of recyclables.

• The Waste Neutral Recycling Compound (WNRC) has been set up to sort, store and

process waste arising from the Eden Project’s activities.


• In April 2005 the Neter 30 in-vessel composter was installed in the WNRC in order to process food waste generated on the site

2.3 Food waste at the Eden Project

Food waste is a significant concern for Eden. Prior to the inception of the Food Waste Technology Trial and installation of an in-vessel composting system, an estimated 230kg per day of catering waste was being sent to landfill. The cost of sending this waste to landfill in financial terms was at least £5,000 per annum, a figure that was likely to increase rapidly with the rising costs of disposing of waste in landfill. There was also a cost to the environment in using this form of disposal. For example:

• The material had to be picked up and transported to landfill consuming vehicle fuel and causing exhaust emissions to the environment

• Once landfilled, the material would most likely break down anaerobically to produce methane which would have a negative impact on the environment unless recovered and used as a fuel to generate electricity.

Finally, there was an opportunity cost of sending a resource to landfill that could otherwise be put to use on the Eden Project site.

2.4 Aim of the food waste technology trial

The aim of the food waste technology trial was to investigate the viability of new technology for complete, in-vessel treatment of food waste as a solution for comparatively small-scale on-site producers of food waste such as SMEs, schools, prisons, hospitals and community-based composting partnerships for the disposal of their food waste. The trial was set up to assess:

• The performance of the technology in treating food waste and producing usable outputs (e.g. compost, digestate, biogas);

• the ability of the technology to meet UK animal by product regulations (ABPR) in relation to the treatment of food waste; and

• the operational, financial and environmental implications of utilising in-vessel technology.

2.5 The choice of technology

Though other solutions for the treatment of food waste are available, the Eden Project narrowed its focus to a choice between two in-vessel treatment processes that it felt would have greatest applicability to the scale and nature of its catering operations, namely aerobic and anaerobic digestion. The final decision to utilise aerobic digestion for the trial was made for the following reasons:


• The energy input required for aerobic digestion is comparatively low because it is

essentially a natural process.

• Anaerobic digestion technology needed to be scaled-down to match the Eden Project’s food waste output. Even the smallest scale anaerobic digester would normally process higher quantities of food waste than produced by the Eden Project. The capital cost of the anaerobic digester option was also comparatively high making it less economic to operate at lower levels of throughput.

• Aerobic technology is generally easier to control and operate than anaerobic

technology, having fewer outputs and less complex maintenance requirements. The financial savings likely to arise from diverting small quantities of food waste from landfill would be too low to justify the base-level operational and maintenance costs of an anaerobic digester.

• Aerobic digestion is more complementary to other composting processes taking place

on site (e.g. of green waste) and to the home composting process. It was therefore felt that the installation of this technology and accompanying interpretation would reinforce the Eden Project’s support for home composting.

• It was felt that the simpler design of an aerobic system would be easier to replicate in

SMEs.

3. OVERVIEW OF COMPOSTING PROCESS USED IN THE FOOD WASTE TECHNOLOGY TRIAL

This section sets out the process through which food waste and other inputs are converted to compost, tracing the route of the inputs to the composting vessel and describing the composting process and its outputs.

3.1 Collection, sorting, transportation and storage of feed-stocks

The achievement of optimum conditions for the production of compost in an in-vessel system is dependent on the mixture of the feedstock applied. In particular, it is important to attain the correct carbon to nitrogen ratio, moisture levels and levels of acidity. The three key material inputs of the composting process at the Eden Project are food waste, green waste and carbon supplements.

3.1.1 Food waste

All food is cooked at a central preparation facility located next to the Foundation Building. Many of the vegetables received by this facility are pre-prepared off-site by the Eden Project’s catering suppliers. The proportion of food waste that constitutes raw vegetable material is as


a consequence quite low. The vegetable waste generated by the Eden Project’s catering suppliers is used as a feedstock for pigs and goats on a farm. Food waste generated at the central preparation facility is deposited into 90 litre wheelie bins.

The food is distributed from the central preparation facility to the seven catering facilities that operate on the Eden Project site, five of which are for visitors and two of which are for staff. At these facilities some final preparation takes place including the presentation of food on plates for customers. The total capacity of these facilities is 1180 covers.

At the restaurants and cafes, left-over food, packaging and utensils are separated by restaurant staff to minimise contamination of the food waste by items which are unsuitable for composting. Certain non-food items are permitted including serviettes and kitchen towel, some egg boxes, coffee filters, wooden cutlery, tea bags and occasional small bits of paper. The food waste is again deposited in 90 litre wheelie bins.

The food waste arising from the catering operations at the Eden Project consists of cooked and raw meat, carcases and bones (e.g. from chickens), bread, cereals, some vegetable waste and some compostable packaging and eating utensils.

The wheelie bins are collected at regular intervals (along with other waste and recycling) by staff from the WNRC in a diesel tipper van with a tail-lift and transported to the Waste Neutral Recycling Compound.

At the WNRC food waste is either input into the Neter, or picked up by the Eden Project’s waste carrier and taken to landfill, dependent on the operational status of the composter and balance of feedstock required to ensure the optimal functioning of the composting process.

In recent times, the breadth of options available on menus has been reduced in order to minimise wastage. The decision to reduce choice was based on financial grounds as much as it was on the level of waste that was arising from lack of take-up of certain dishes on the menus.

The overall volume and weight of food waste produced is not routinely monitored. However, the weight of food waste was monitored during August 2006 and November 2006 providing an indication of the scale of the catering operation at the Eden Project. During August 2006, the total weight of food waste produced was 9532kg (173,037 visitors). During November 2006, the total weight of food waste was 4571kg (35,095 visitors). Generally August is the busiest month of the year and November is one of the quietest. So the two weights probably represent the opposite ends of the scale for food waste.

3.1.2 Green waste

Through its extensive horticultural activities, the Eden Project produces large amounts of green waste. Most of this is shredded and then composted in open bays on site. Some of this green waste is shredded and transported to the WNRC to be used as a feed-stock in the in-


vessel composting process to help increase pH and to provide suitable microbial populations potentially lacking in the food waste that would aid the composting process.

3.1.3 Carbon supplements

The optimum carbon to nitrogen ratio for composting is considered to be between 30 – 35:13. The manufacturer of the composter, Susteco, recommended the use of wood pellets made up of compressed sawdust to help increase the carbon ratio and reduce the moisture content. The Eden Project has trialled other carbon sources such as wood shavings, shredded office paper, shredded paper towels, shredded corrugated board, waste dust collected from the corrugated board shredding process, and chopped straw. Currently, the wood pellets are favoured due to their efficacy and the stability of supply. However, wood pellets are externally sourced and represent a cost to the Eden Project and to the environment (e.g. transportation and embodied energy). The pellets are supplied at a cost of approximately 31p per kg.

3.2 Inputting feedstock into the Neter 30

The food waste is fed into the Neter along a conveyor belt which transports the feed-stock to a shredder. Although some segregation of food waste and green waste is inevitable, efforts are made to ensure that the two feed-stocks are fed into the Neter together as this has been found to improve the efficiency of the composting process. The wheelie bins containing food waste are automatically raised and emptied onto the conveyor belt providing an opportunity for an operative to remove any unsuitable items from the food waste stream.

The conveyor belt transports the food to a spiral conveyor that terminates with a knife system. From here it passes to a shredder which mashes it into fragments of approximately 2cm in diameter. Wood pellets and green waste, if necessary, can be fed into the in-auger via a hopper to allow early incorporation of these feed stocks. The volume of green waste and wood pellets added is adjusted to take account of the nature and moisture content of the food waste. The mixture of food waste, green waste and wood pellets is transported by the in-auger into the composting cylinder.

The machine is designed to process up to 1800kg of material per day, operating on a range of inputs including vegetable and meat waste, and garden waste. The mix of inputs determines the quality and utility of the compost produced.

The cylinder rotates periodically, slowly moving the food waste from the front to the back end of the machine. The rotation increases access of the contents to atmospheric oxygen, enabling the material to be aerobically digested. After a period of time (usually 40-60 days under optimum operational conditions), the material is fully broken down through a process of aerobic digestion into compost and discharged from the rear of the machine via an out-auger.

3 McGaughey and Gotaas (1985)


3.3 Description of Neter 30 composter

The composter consists of a horizontal, stainless steel cylinder which is approximately 8.5 metres long and 2 metres in diameter with a central spindle. The cylinder is rotated by externally mounted hydraulic rams against fixed end walls. (see fig 1 below).

Figure 1

The composter is housed in a partially insulated, stainless steel outer-casing which is 9.5m long, 2.7m wide and 2.6m high. The outer casing acts as a safety barrier against the hazards of the machine’s moving parts and provides a degree of buffering from external temperature fluctuations.

The rotation of the cylinder tumbles the material being composted, mixing and aerating it whilst moving it along the vessel. The periods of rotation (‘run’) and standing (‘wait’) can be altered according to the requirements of the composting process at any one time.

Five circular steel plates are affixed to the central spindle at 1.4m intervals. Each plate holds four temperature sensors and one water inlet nozzle for adding moisture to compost as required (see figure 1, picture B inset).


Figure 2

Air is drawn by a fan across the top of the composting mass during the ‘wait’ periods and through the mass during the ‘run’ periods. This removes unwanted gases, water vapour and excess heat and replenishes oxygen levels (see figure 2). The fan speed can be adjusted to suit internal conditions. The exhaust air is passed through a biofilter to be cleansed prior to being discharged into the atmosphere. The biofilter uses natural processes to eliminate odours and contains bark which is periodically treated with an enzyme solution

There are four inspection hatches through which the process can be observed and any maintenance requirements can be administered.

The compost is produced through an aerobic process, through which micro-organisms consume organic matter in the presence of air and convert it into a stabilised compost.

Theoretically, there are three temperature phases through which the organic matter must normally pass in order to break down into compost. Under optimum operating conditions, the matter quickly enters the thermophilic phase which is characterised by high moisture content of the composting material and temperatures of between 55 - 65=C. This phase should last for 12 to 18 days. The next stage is the mesophilic stage during which the volume of the material diminishes and the temperature of the compost falls to between 25 and 40=C. This stage normally lasts for between 18 to 24 days. The final phase is called the maturation phase. This lasts for approximately 10 – 15 days during which there are further reductions in volume and in the temperature of the composting matter.

The reduction in bulk of the original material is between 70 to 90% of its original input. Subject to meeting the ABPR 2005 the compost material derived through this process can be used in a range of applications.


3.4 Energy input The Neter composting process involves an exothermic reaction, thereby providing its own

heat, though a small electric heater has been incorporated into the machine in case of extreme (cold) weather conditions. The machine uses electricity to power the:

• conveyor belts; • spindle for revolving the inner cylinder; • shredder; • aeration fan; • temperature sensors; and • electronic information systems

The average power consumed per day is around 48kwh/day broken down as follows: Component Average kwh/day

Feeding machinery 14.45 Cylinder and fan 29 Out-conveyor 0.55 Sensors 4.2 Total 48.22

. 3.5 Staff input

Five permanent and one seasonal members of staff are engaged in operational roles relating to the Neter. Two staff collect wheelie bins containing food waste from the catering facilities on the site and deposit them at the WNRC. Their role also involves the collection of other waste and recyclables deposited in the recycling and waste bins that are located throughout the site. All staff are involved in operating the Neter. Activities include:

• feeding the Neter, • checking the volume of compost in the Neter, • checking backend compost • emptying compost into the compost bay • general maintenance of Neter and bio-filter

• checking and logging temperatures • checking equipment is operating

correctly • cleaning the Neter • cleaning bins

In addition to high visibility jackets worn by all staff working in the WNRC, operatives feeding the Neter are required to wear the following additional personal protective equipment:

• Heavy duty gloves • Waterproof trousers and jackets • Safety wellington boots

• Particulate respirator face masks • Ear protectors


A number of staff from other sections also have roles in relation to the Neter, principally in relation to monitoring the biological functioning of the Neter, its emissions and the quality of the compost. The level of input from such staff has necessarily been higher than would be the case under normal operating circumstances as they have been involved in developing the knowledge base for the trial. Nonetheless, it is anticipated that other users of this technology would require access to a degree of scientific knowledge of the composting process and compost outputs.

WNRC operatives and Eden’s science team liaise closely in order to determine feed-stocks for the Neter. WNRC staff will use their experience to report any concerns regarding the status of the biodegrading feedstock to the science team. The science team will on occasion specify new feed-stock regimes for the purpose of the trial.

3.6 Outputs of the process (i.e. compost, emissions , waste)

The aim of the process is to produce a compost that can be used (ideally without further treatment) for a range of applications. Emissions to air and water are minimal through this process. However, it is important that a suitable use can be found for the compost, otherwise it effectively becomes waste. The Animal By Products Regulations (ABPR 2005) require composting material with a maximum particle size of 40cm to reach at least 60=C for two days, twice, with a barrier between the 2 treatments to prevent cross contamination - or, alternatively two periods at a temperature of greater than 70=C for at least one hour, with a maximum particle size of 6cm4. The legislation also requires that measures be put in place to ensure that pre-treated and composted materials cannot be cross-contaminated. In practice, this means that there should be a ‘dirty’ area where food waste is delivered to the site, an area where vehicles and equipment can be cleaned, and a ‘clean’ area where the finished compost is stored. Ideally a physical barrier should be installed. Failing this, procedures should be in place to ensure that operatives loading food waste into the composter do not transgress into the clean area without taking precautions to ensure that they are clean. Procedures to achieve this have been set out for composters using Hazard Analysis and Critical Control Point (HACCP)5.

Compliance of the composting process with Animal By-Products Regulations need not be achieved for compost produced from catering waste that is generated, composted and used on a single premises – provided no ruminant animals or pigs are kept on the site and the composting waste is not accessible to poultry.

4 “Guidance on the treatment in approved composting or biogas plants of animal by-products and

catering waste”, Defra 2004.

5 ‘Hazard analysis and critical contraol point (HACCP) for composters’ Evans 2003.


4. FINDINGS OF TRIAL

4.1 Background

The Neter 30 composter was installed on 20th April 2005. The trial period for the purpose of this report lasted until July 2007. However, there are a number of different permutations of food waste that require composting and so in practice the Eden Project will continue to trial different combinations of additives and operating conditions in order to improve its knowledge of the composting process. It is hoped that others considering or already using similar technologies can learn from the Eden Project’s experience.

During the trial period in question, a total of 33 different treatments (see table 1) were tested involving different mixtures of feed-stocks and running parameters. The number of treatments reflects not only a desire to gain a wide knowledge of the different permutations of feed-stocks, but also the reality of an operational environment in which volume and types of food waste available as a feed-stock are variable. A number of different combinations of ‘run’ and ‘wait’ times and fan speeds were also trialled to determine optimum operating conditions for the aeration and mixing of inputs.


Table 1: Effects of feed stock treatments T1 – T33 on temperatures, pH and moisture contents measured at sampling point A (figure 1) at which thermophillic conditions should be well established Turn cycle Temperature Category No Treatment ‘Wait’ (h) ‘Run’

(min) Fan speed

(m s-1) n Range Mean (SD) pH (SD) %

moisture (SD)

Progress A/D/U/S (Ascending,

Descending, Uneven or Stable)

T1 Food waste + carbon 4 12 7.8 76 20-34 28.5 (2.89) - - U T3 Food waste + carbon 4 12 7.8 21 28-40 36.2 (2.95) - - U T2 No feeds 4 12 7.8 19 21-27 23.6 (1.80) - - S T23 Food waste + green waste + carbon 2 6 3.2 10 36-39 37.0 (1.33) 4.3 (0.14) - S T33 Food waste + green waste 8 8 3.2 22 37-43 39.4 (1.56) 4.4 (0.05) 59.6 (0.74) S T17 Food waste + carbon 2 6 3.2 5 37-42 38.6 (2.07) - - A

1

T24 No feeds - - - 38 30-49 38.7 (5.38) 4.6 (1.05) 27.7 (-) A T5 Food waste (<50kg week-1) 4 2 7.8 19 54-33 47.2 (5.56) - - D T12 No feeds 0.45 0.25 3.1 10 56-47 50.0 (2.91) - - D T16 Food waste + green waste + carbon 2 2 3.2 35 54-39 45.9 (2.90) - - D T20 No feeds 2 6 3.2 14 51-46 48.1 (1.82) - - D T22 No feeds 2 6 3.2 15 47-38 42.5 (2.90) 4.7 (0.13) - D T31 Food waste + green waste + carbon 8 8 3.2 14 47-40 43.3 (2.43) 4.6 (0.14) 56.6 (-) D T18 Food waste + carbon + lime 2 6 3.2 45 32-48 41.4 (3.48) 5.0 (0.15) - U T21 Food waste + carbon + lime 2 6 3.2 24 43-52 49.1 (2.27) 5.2 (0.35) - U T7 Food waste + carbon 4 4 7.8 13 46-54 49.0 (2.48) 4.8 (-) - S T8 No feeds 4 4 7.8 16 42-51 48.9 (2.13) - - S T15 Food waste + green waste 0.45 0.25 3.2 15 43-47 44.7 (1.16) - - S T4 No feeds 4 12 7.8 38 34-48 40.3 (3.58) - - A T14 Green waste (<50kg week-1) 0.45 0.25 3.2 16 39-46 41.6 (1.89) - - A T19 Food waste + green waste + carbon 2 6 3.2 21 42-55 48.5 (3.67) 5.2 (0.12) - A

2

T32 Food waste + green waste + carbon 8 8 3.2 11 39-45 43.4 (2.06) 4.7 (0.57) 55.8 (1.39) A T11 Food waste + carbon 0.45 0.25 3.1 26 64-55 59.9 (2.48) - - D T28 Food waste + green waste + carbon 8 8 3.2 3 54-49 52.0 (2.65) 4.3 (0.35) 61.3 (0.71) D T30 Food waste + green waste + carbon 8 8 3.2 23 61-45 50.8 (4.73) 4.5 (0.23) 58.6 (2.02) D T13 Food waste + green waste + carbon 0.45 0.25 1.4-3.1 61 40-60 53.1 (4.45) - - U T29 Carbon only (<50kg week-1) 8 8 3.2 19 47-65 51.9 (4.09) 4.4 (0.19) 59.6 (0.88) S T6 Food waste + carbon 4 2 7.8 16 42-63 56.7 (5.14) - - A T9 Food waste + green waste 0.45 0.25 3.1 21 50-66 57.7 (4.96) - - A

3

T25 Green waste only 8 4 3.2 8 51-57 53.5 (2.07) 7.5 (0.52) 45.9 (0.93) A T27 Food waste + green waste + carbon 8 8 3.2 12 70-51 61.9 (5.85) 5.9 (1.92) 59.0 (6.51) D T10 No feeds 0.45 0.25 3.1 11 59-68 62.0 (3.29) - - S

4

T26 Food waste + green waste + carbon 8 4 3.2 25 56-72 62.9 (4.66) 7.0 (0.82) 48.4 (8.28) A


4.2 Effectiveness of composting process 4.2.1 Temperature

The temperature of the composting mass was measured through the built-in temperature sensors, the use of temperature probes and through data-loggers that were passed through the machine between 28/4/2006 and 10/7/2007.

Thermophilic temperatures have been achieved at some point within the composter since August 2005 and usable compost is being produced. However, frequent, and at times considerable, teething problems were experienced at the beginning of the trial.

Initially, problems were centred on the excessive moisture content of the food waste. Indeed during the first four months of the trial, temperatures within the vessel were insufficiently high to produce compost. A period during which only green waste was fed into the machine helped to stabilise the process before food waste could be re-introduced.

The temperature of material within the composter was monitored at sampling points throughout the vessel. However, it was the impact of feed-stocks on the temperature between sampling points A and B (see fig 1, page 11) which was of greatest interest since themophilic temperatures (55 - 65=C) would be required in this section of the vessel in order to ensure that the process was operating optimally.

Temperature data provided by the internal sensors indicated that the majority of feed-stock treatments (22 out of 33) did not result in thermophilic temperatures being attained between sampling points A and B, thereby representing non-functional states at that point (see table 1). The remaining eleven treatments did result in temperatures at or above the minimum required for thermophilic processes to take place

Whilst achieving thermophilic conditions near the front of the vessel is desirable for the effective and efficient operation of the Neter 30 composter, compost can still be produced as long as thermophilic temperatures have been achieved at some point within the vessel. And this was the case during much of the trial period with the exception of the period between April and August 2005 (see figure 3). Additional temperature data provided by the data-loggers placed in the composting vessel between 28/4/06 and 10/7/07 indicated that:

• during the period in question, temperatures increased in the machine with progress through the vessel; and

• when temperatures in the vessel were higher, the rate of composting was faster, and the transit time of material through the vessel was shorter.


Figure 3

Interestingly, for 60% of the periods during which thermophilic temperatures were achieved at points A and B, green waste made up a significant proportion ranging between 100 - 33% of the feedstock. The biggest temperature spikes achieved during treatments 9, 10, 26 and 27 (see table 1) were also achieved when green waste made up a significant proportion of the feed-stock.

4.2.2 Acidity

A strong relationship was also observed between acidity levels of the feed-stock and the rate of change from mesophilic to thermophilic conditions. The most effective treatments produced pH values of 5.9 and 7. Eden Project’s food waste was generally measured at pH5, prior to treatment, but dropped quite rapidly following the shredding process on entry into the composting vessel. The lower pH values that resulted from the food waste were generally associated with a slower transfer from mesophilic to thermophilic conditions between sampling points A and B.

4.2.3 Moisture levels

Moisture levels in the materials of between 55% and 65% and pH values of between 4 and 5 were associated with a cluster of lower temperature readings. These conditions occurred when macerated food waste was added with the wrong balance of accompanying feed-stocks. They were also associated with periods of electrical/mechanical breakdown resulting in materials at the front end of the composter not being mixed or moved for a period of time. This situation allowed acidogenic fermentation to take place in the macerated food component.


Once acidic conditions became established at the front end of the compost, they tended to persist. Better conditions could be re-established through the addition of large quantities of finished compost from the out auger through the foremost inspection hatch, or in later treatments through the hopper system. Suspending the addition of feed-stocks for a period of several weeks also tended to increase pH, though this could not be guaranteed. The relationship between moisture, pH and temperature is set out in figure 4 below.

Figure 4

4.2.4 Rate of progress of composting mass A range of markers of varying sizes and shapes were placed in the composter in various locations to determine the rate of progress of the composting mass through the machine. These included rubber balls of varying sizes and densities and plant labels cut to different lengths (see fig 5), the aim being to determine whether objects of different sizes and weights would pass through the composter at different rates of progress. The progress of the larger balls was also used to assess the risks of damage or loss to data-loggers of a similar shape and size that would be used to monitor the temperature of the compost. In addition to the markers, some feed-stocks were used to mark the point of change between two treatments. For example, large pieces of shredded plant material present in green waste were used to track the progress of compost mixes containing this feed-stock.


Figure 5 The marker tags took between 150 and 255 days to pass through the vessel. The size of the tags did not seem to have any significant bearing on their rate of progress through the vessel. Once it had been established that the risk of loss of, or damage to the dataloggers was low, nine ‘egg’ dataloggers were placed into the composting vessel at a point close to the foremost inspection hatch. These provided temperature data and transit times for discrete batches of compost material as they passed through the composter. These took on average, 95 days to pass through the vessel. The main influences on the rate of progress of materials through the vessel were the volumes of feedstock loaded into the vessel, and the rate at which finished materials were removed at the out-auger.

4.3 Effectiveness of compost as a horticultural pro duct

Though there was always an intention to use the compost produced by the Neter on the Eden Project site, the principal driver for the trial was the need to reduce waste from catering operations. Though the composting initiative has succeeded in reducing the weight of catering waste sent to landfill by approximately 29% (April ‘05 to July ‘07), unless the compost produced was suitable for use on the Eden Project site, a waste problem albeit reduced, would persist. Moreover, if good compost could be produced by the Neter, this would reduce the need for the Eden Project to purchase compost from external sources, thereby saving money. It was considered that the compost might be suitable both as a growing and a mulching medium. Three trials were therefore conducted to assess the suitability of the Neter compost for use as a growing medium and a further three trials were conducted to assess its suitability


as a mulching medium. The detection of any potentially phyto-toxic effects was of great importance since the presence of such would severely restrict the utility of the compost.

4.3.1 Growth medium trials

The trials were conducted on two commonly grown vegetables:

• Lactuca sativa - ‘Little Gem’ lettuce (two trials) • Lycopersicon esculentum - ‘Gardener’s Delight’ tomatoes (one trial)

The progress of vegetables grown in 100% Neter compost was compared with that of those grown in a commercially available, peat free preparation used for the majority of propagation and container growing at the Eden Project. Mixtures containing different ratios of Neter compost to the peat free compost were also prepared.

Importantly, no signs of phyto-toxicity were found in any of the plant trials conducted. Measurements of plant growth in the Neter compost produced variable results. Growth in the lettuce was substantially increased by the presence of Neter compost in the mix. In fact the greater the concentration of the Neter compost, the stronger the growth in the shoots. Conversely, the growth of the tomatoes was weaker, the greater the concentration of Neter compost in the mix. It is not yet understood why this was the case though the trials took place at different times of the year and variations in feedstock and accordingly the end-product may have influenced these results.


Figure 6

4.3.2 Mulching medium trials

The mulching medium trials were conducted on:

• Lactuca sativa – ‘Little Gem’ lettuce • Capsicum annuum - ‘De Cayenne’ peppers • A newly planted mixed hedge

The lettuce and pepper seeds were propagated in the normal peat free preparation used by the Eden Project up to a point where small seedling plants had developed. Freshly produced Neter compost was compared with proprietary ‘green waste’ compost (West Country Compost, Ecosci,) as a mulching medium. Plants were also grown in untreated compost as a control for the test. In the pot experiments with the lettuce and pepper plants, no significant differences were seen in shoot growth between the Neter compost, the West Country Compost and the control. Plant heights were only recorded in the pepper plant trial. These were significantly higher for plants treated with the Neter compost and West Country Compost than they were for those left untreated.


In the field trial, the mean heights of the plants used in the hedge were not found to be significantly different amongst the different treatments applied. However, because all plants assessed in the hedge are hardy and long-lived, further assessment of height will be made over time. Mineral analysis of the soil under the hedge indicated that neither the Neter nor the West Country Compost mulches have had any significant impact on the mineral composition of the underlying soil to date.

Again, no signs of phyto-toxicity were observed during the mulching medium trials.

4.3.3 BSI PAS 100 test

The British Standards Institution Publicly Available Specification for Compost Materials or BSI PAS 100 test specifies minimum standard requirements for the processing of source segregated biowastes. The aim of the scheme is to give end users and specifiers the confidence that compost meeting the standard is quality assured, traceable, safe and reliable. It covers all the key elements of the composting process including:

• Process control • Input materials • Composting activity (sanitisation and

stabilisation) • Compost quality requirements • Product preparation

• Compost maturation • Compost sampling and analysis • Classification of compost • Labelling and marking • Monitoring and traceability

The Eden Project submitted three samples of Neter compost for testing. Two samples have passed the test, but one failed on stability, plant top growth, and on an unacceptably high level of contaminant particles (which in this case were predominantly shreds of office paper which had failed to biodegrade). A summary of the findings can be found in table 2 below. Table 2

Sample number PAS 100 upper limit 1 2 3

Sample date - 11/01/07 26/02/07 16/05/07 Passed or failed PAS 100 - Passed Failed Passed Probable treatment number & category (see Table 1)

- T21/T22 (2)

T23/T24 (1)

T26/T27 (4)

Human pathogens

Escherichia coli (Colony forming units (cfu) g-1

1000 <10 <10 10

Salmonella (Present/absent) Absent Absent Absent Absent Potentially Toxic Elements

Cadmium (mg kg-1) 1.50 0.11 0.11 0.15 Chromium (mg kg-1) 100.00 6.55 9.30 6.76


Copper (mg kg-1) 200.00 10.3 22.3 16.4 Lead (mg kg-1) 200.00 3.44 6.27 4.97 Mercury (mg kg-1) 1.00 <0.05 <0.05 <0.05 Nickel (mg kg-1) 50.00 3.96 5.41 2.67 Zinc (mg kg-1) 400.00 44.5 90.4 60.6 Stability (mg CO2 g

-1 OM day-1)

16.0

13.1

21.2

14.9

Weed seedlings (no. germinated)

0 0 0 0

Plastics, glass, metal, sharps or other contaminant particles >2mm (%of air dry sample)

0.50

0

1.33*

0.21

Plastic >2mm (% of air dry sample) 0.25 0 0.14 0.01 Stones in ‘mulch’ >4mm (% of air dry sample)

16.00 1.24 0.86 0.07

Stones in other than ‘mulch’ >4mm (% of air dry sample)

8.00 1.24 0.86 0.07

Plant growth tests

Plants germinated (no. as % of controls)

80.0 93.3 93.3 103.0

Plant top growth (mean g plant-1 as % of controls)

80.0 93.0 74.7 89.8

*Particles were predominantly paper

4.4 Bioaerosol tests

Bioaerosol tests were undertaken by a representative from the University of Hertfordshire to assess the level of potentially harmful micro-organisms arising from the composting process. Air was sampled at eight stations situated in and around the WNRC to assess counts for total bacteria, total filamentous fungi and Aspergillus fumigatus.

The total number of bacterial colony forming units (cfu) m-3 present in the samples were less than the nominal reference value of 1000 cfu m-3 limit proposed by the Environment Agency (2001) in all except one site. The limit was breached at the sampling station next to the finished compost bin where a reading of 1780 cfu m-3 was taken. The reading was taken with the bin cover off. With the bin cover replaced, the reading dropped to 167 cfu m-3.

The count of total filamentous fungi exceeded the maximum of 1000 cfu m-3 at three sampling stations. However, the concentration of spores was considerably less than the level at which sensitisation might occur under conditions of continuous exposure. The report from University of Hertfordshire recommended that workers wear face masks as a precaution when working in the vicinity of the compost collection bin, when the cover is off at the feeding station and when accessing the biofilter.


Aspergillus fumigatus was found at all stations but in all cases at significantly lower levels than the 1000 cfu m-3 limit and were largely within the normal background range. A summary of these findings is set out in table 3 overleaf.


Table 3

Counts of colony forming units (cfu m -3) Sample Sample location Total

bacteria Total

filamentous fungi

Aspergillus fumigatus

1 Background On roadside footpath upwind of

the Waste Neutral compound 127 600 14

2 Biofilter

(uncovered) On top of the biofilter unit (figure 2) with cover open

200 1130 36

3 Filter tube End of the filter tube with the filter

removed 136 400 109

4 Compost bin

(cover on) Above the ‘finished’ compost bin with the plastic cover sheet in position

167 430 42

5 Compost bin

(cover off) Above the ‘finished’ compost bin with the plastic cover sheet removed

1780 1160 67

6 Food waste input Directly above the conveyor belt

during food waste loading operation

504 1736 18

7 Hatch 1 Open Hatch 1 (figure 1) during

loading of food waste* 736 722 41

8 Hatch 3 Open Hatch 3 (figure 1) during

loading of food waste* 263 345 41

*Sample collected 30 minutes after rotation was switched off. 4.5 Gas emissions

A schedule has been devised to test for concentrations of oxygen, carbon dioxide, methane, hydrogen sulphide and carbon monoxide using a gas analyser. The concentrations of oxygen and carbon dioxide were found to be well within expected and safe levels confirming that the composting processes within the Neter vessel are most likely operating aerobically as designed. The results of gas emissions tests undertaken to date are set out in table 4.


Table 4

Gases in exhaust air

Sampling date

Sample location & conditions

CH4 (%)

CO2

(%)

O2

(%)

CO

(ppm

)

H2S

(ppm

)

Atmospheric pressure (mm Hg)

7/02/07 Valve 1: Vessel

stationary

0 0.5 20.4 0 0 987


stationary

0 0.5 20.0 0 0 992

Valve 1: Vessel

rotating

0 2.9 18.4 99.5 0

Inside biofilter 0 3.3 17.9 174 0


stationary

0 0 22.8 4 0 996

Valve 1: Vessel

rotating

0 0.9 21.7 11 2

4.6 Design, operational and other issues arising fr om the trial 4.6.1 Design issues

Some design problems have been experienced with the Neter particularly in relation to the in-feed area of the composter which has subsequently been modified as a consequence. For example:

• The lack of containment at the food waste entry area resulted in cleansing problems and

created difficult working conditions for staff.

• Initially wood pellets were fed into the shredder along with the food waste through the same entry point. This blocked the shredder system. To remedy this problem, the shredder face was changed to increase the size of particles that could pass through the system.

• Continuing difficulties experienced in feeding wood pellets and green waste through the

shredder necessitated the addition of an in-feed hopper that would enable the wood


pellets and green waste to be mixed with the food after it had been shredded. This was fitted to the in-feed auger connecting at a point on the vessel side of the shredder.

4.6.2 Operational and other issues The average daily amount of food waste composted at Eden between January 2006 and

September 2007 was 66kg. In the last six months (April 2007 to September 2007), this rate has risen to an average of 88kg per day. The principal reason for the relatively low throughput of waste is the difficulty which has been experienced in maintaining high front-end temperatures for the composting mass. As knowledge of the optimum feed-stock balance has increased, so the volume of food waste composted has increased. Down time resulting from mechanical breakdown and from falling temperatures within the vessel has also been a contributory factor to the relatively low throughput of waste.

Figure 6

Wood pellets are currently being used as a carbon supplement for the composting process. Though they are effective in promoting a good composting process, their use incurs a financial cost to the Eden Project. They are also inert, and do not contribute any microbial activity to the composting mass. Their biggest disadvantage is that they have to be transported to the site and are not part of the Eden Project’s waste stream. The Eden Project recognises this issue and is exploring alternatives. A brief summary of materials trialled is set out below:


• Initial trials of wood shavings have indicated that the compost mixture produced has a good structure. Supply of this material is limited.

• Dust from corrugated card has a high pH value (which helps counteract the low pH of the other feed stocks) but supply is limited.

• The Eden Project is considering using ‘woody’ green waste as it is felt that this may help

both increase the carbon to nitrogen ratio and improve the structure of the compost • Shredded paper (towels and office) and corrugated card did not completely break down

- the shredded paper was partially responsible for the failure of one of the PAS 100 trials.

• Straw has been rejected as a feedstock because it tends to create a ‘wattle and daub’

effect within the vessel disrupting the mixing and flow of composting materials. The recent addition of a hopper may provide scope for using a mix of wood shavings, dust (from corrugated card) and woody green waste according to supply and the nature of the food waste being input.


5. FINANCIAL APPRAISAL

5.1 Introduction

It is recognised that there are a number of factors that can influence the cost of running an in-vessel composter such as:

• The nature and volume of feed-stocks available • Staff costs and availability • Staff knowledge of the composting process • Potential for use or sale of the compost produced • Disposal costs for food waste

Summary of key findings:

• Thermophilic temperatures have been achieved at some point in the vessel for most of the duration of the trial. Adding substantial quantities of green waste to the feedstock appears to raise temperatures and pH levels and encourage good composting conditions.

• Excessively moist food waste tends to reduce pH. It is important to input the right proportion of green waste and carbon supplements when waste of this nature is fed into the composter.

• The compost has passed two out of three BSI PAS 100 tests. The failure was due to the lack of compost maturity, poor plant top growth and to the presence of shredded paper which failed to break down in the composter.

• Trials of the compost as a growing medium have produced mixed results though there is evidence that it is nutrient rich and in most circumstances promotes plant growth. All trials of the compost as a mulch have produced positive results.

• There are no concerns in relation to bioaerosol or gaseous emissions.

• Finding the right balance of feed-stocks has been difficult. At times the rate of the composting process has consequently been slow, resulting in relatively low levels of food waste being processed on a daily basis.

• Issues relating to the design of the front end of the composter have also slowed the rate at which food waste is being composted.

• The Neter 30 has been relatively easy to operate and understand and the process has been positively embraced by staff.


• Economies of scale

Each individual organisation seeking to operate an in-vessel composter will have its own unique set of circumstances. However, there is much that can be learnt from the Eden Project’s experience in this regard.

5.2 Set-up costs The set up costs incurred at the start of this trial were as follows:

Item Cost Neter 30 purchase and installation £179,000

5.3 Management, administration and research and dev elopment costs Because Eden has been trialling this technology, there have been considerable management,

and research and development costs associated with the project. These have tended to increase as the project has progressed (see figure 7), reflecting in particular, the time implications of undertaking scientific research, responding to operational issues and improving the design of the composter. The two graphs overleaf set out the costs incurred on a monthly basis for the last financial year (2006/7).


Figure 7

5.4 Operational and maintenance costs

Operational and maintenance costs have tended to reduce over time (see figure 8), though there was a significant jump in costs in March 2007. This related to additional maintenance costs, incurred due to the connection of a hopper to the front end of the composter for feeding in wood pellets and green waste.


Figure 8 If operational costs are separated from maintenance costs, a clearer indication of declining day-to-day costs can be observed. This may reflect the development of a better understanding over time of the factors that affect the operation of the Neter and a consequent reduction in operational input.

Figure 9

The Eden Project site may be atypical of most catering establishments with several dispersed outlets operating on site. Travel time between catering outlets and the WNRC is therefore probably higher than would be experienced in a typical catering scenario.


5.5 Cost benefit analysis An analysis of costs and benefits has been undertaken in order to ascertain the value of

adopting such an approach to the disposal of food waste. The following assumptions have been made:

5.5.1 Assumptions Projected data for the period April 2007 to March 2008 have been used to reflect the most current operational cost data available. Projections are based on actual results for the first half of the 2007/2008 financial year. An assumption has been made that similar conditions will persist from October 2007 to March 2008 as no significant seasonal factors are currently influencing the amount of food waste composted. To illustrate this point – though the overall level of food waste is likely to fall during the quieter winter months, it is still likely to exceed the amount of food waste being composted given the current rate of throughput. Data from 2006/7 is also set out to provide a comparison. Only costs relating to the operation of the Neter have been incorporated into this cost benefit analysis. The research and development, and management costs incurred by the Eden Project relating to the process of trialling this technology are disregarded since they are not relevant to the ongoing, operational costs of running a Neter composter. However, it is likely that some scientific input will be required by early adopters of this technology, if the degree of trial and error involved in establishing feed-stock combinations that work is to be minimised.

The Eden Project is fortunate to have scientific staff at hand who have been able to contribute from the start of the project to the process of producing compost using the Neter. Their input has been invaluable in the quest to establish a range of feed-stock mixes and operating conditions that will produce a safe and satisfactory compost, and a reasonably efficient process. Although a degree of trial and error has been inevitable, the scientific staff have been able to minimise the potential for error. Their monitoring has also meant that lessons have been learnt from ‘unsuccessful’ feed-stocks as well as from those that have resulted in good composting conditions.

Technical support is available from Susteco, and no doubt from other providers of similar composting technologies. However, the availability of in-house scientific expertise has been invaluable to the trial. Other organisations considering installing in-vessel technology to deal with their food and green waste may not be so fortunate as to have access to such scientific expertise and may therefore need to consider the procurement of such advice in their cost projections. The estimate of savings from food waste being diverted from landfill is based on a cost of £67 per tonne during 2006/7 and £70 per tonne during 2007/8 for the collection and disposal of such waste. These costs are estimates only. Charges will vary amongst different waste carriers and waste disposal authorities. The £3 increase in charge between 2006/7 and


2007/8 represents the increase in Landfill Tax over that period. From April 2007, Landfill Tax is set to rise by £8 per annum until 2010/11. The value of the compost produced is based on the cost of composted green waste which is supplied to the Eden Project at a rate of £11.58 per tonne. Smaller scale operations are likely to be purchasing compost at a much higher rate and therefore greater savings are possible. To some extent, the composted green waste can be displaced by the Neter compost. However, the high nutrient content of the Neter compost means that it will usually need to be mixed with a lower nutrient compost such as composted green waste in order to provide a satisafactory growing medium for plants on the site. Energy consumption is assumed to be constant at a rate of 48kwh per day. Electricity is assumed to be purchased at a rate of 7.8p per kwh, though a small amount of energy consumption will occur overnight when this rate is reduced to 4.8p per kwh. Wood pellets are purchased by the Eden Project for 31.25p per kg.

5.5.2 Annual running costs and savings

Running costs of the Neter 30 composter for the per iod April 2006 to March 2007 Item Quantity Cost Wood pellets 3,069.8kg £959.31 Operational staff time 1,678.25 hours £12,413.20 Maintenance and repair costs 43.5 hours £1,157.06 Energy consumption 17,264.5kwh £1,346.63 Total cost £15,876.20

Savings from Neter 30 composter for the period Apri l 2006 to March 2007

Item Quantity Saving Food waste diverted from landfill 22,906kg £1,534.70 Compost displaced 6,872 kg £79.58 Total cost £1,614.28

Projected running costs of the Neter 30 composter f or the period April 2007 to March 2008

Item Quantity Cost Wood pellets 4,896 kg £1,530 Operational staff time 1,139 hours £8,679.18 Maintenance and repair costs 28 hours £455 Energy consumption 17,264.5kwh £1,346.63 Total cost £12,010.81

Projected savings from Neter 30 composter for the p eriod April 2007 to March 2008

Item Quantity Saving Food waste diverted from landfill 32,500 kg £2,275 Compost displaced 9,750kg £112.91 Total cost £2,387.91


The results and projections indicate that the composting operation is currently operating at a substantial loss. The main reasons for this situation are as follows:

• The amount of food waste being composted on a daily basis is relatively low both by comparison with the amount of food waste generated and with the capacity of the Neter 30.

• Operational costs are comparatively high for the scale of the operation. Should a

substantial increase in food waste be composted, economies of scale are likely to apply.

5.5.3 Environmental and social impacts

There are a range of environmental benefits that arise from Eden’s use of this technology, against the alternative of disposing of food waste in landfill. These are as follows:

• Composting food waste helps to avoid the negative environmental impacts that might

normally arise from disposing of such waste in landfill. For example:

� Food waste that is disposed of in landfill without further treatment will rot down. Often, it will be covered by other waste which markedly reduces the amount of oxygen available. These anaerobic conditions lead to the production of methane, a greenhouse gas that is estimated to be 21 times more powerful than carbon dioxide6. In some instances, the negative impacts of this will be mitigated where the landfill has facilities for generating heat and power from landfill gases. These facilities do not exist at the two principal landfill sites in Cornwall

� Food waste has to be transported to a landfill site and processed. The main environmental impact of this activity is the carbon dioxide emissions that arise from fuel consumption

� Putrescible waste at landfill sites is the main cause of unpleasant odours experienced in and around landfill sites.

• The Eden Project is practicing a more sustainable approach to the disposal of its food

waste and actively promoting this to its visitors and to a wider audience. Through this approach it can influence the way in which households, businesses and other organisations view and dispose of their food waste.

• Provided the compost produced is of appropriate quality and contains adequate nutrients, it can displace some of the compost purchased from external sources, thereby avoiding the use of fuel (and accompanying emissions) that would arise from the production and transportation of the compost.

6 “Climate change and waste management: The link”, Defra


• By dealing with its food waste on-site, Eden is following the ‘proximity principle’ which recognises the need to avoid passing on the environmental cost of waste disposal to communities that are not responsible for its generation.

The main negative environmental impact of using this technology is the embodied energy of the composter. For example, the machine is largely made of stainless steel. Energy will have been used in the reprocessing of steel and manufacture of the composting vessel. Energy will also have been used in the transportation of the machine onto the site. It should be noted, that though the machine uses approximately 48kwh per day, the electricity is currently supplied through a green tariff from renewable energy sources and therefore is rated as having no impact on carbon dioxide emissions.

5.5.4 Summary of costs and benefits

A summary of the costs and benefits projected for 2007/8 is set out below in table 5:


Table 5 Financial:

Costs Benefits Item Value Item Value Wood pellets £1,530 Food waste diverted from landfill £2,275 Operational staff time £8,679.18 Compost displaced £115.77 Maintenance and repair costs £455 Energy consumption £1,346.63 Total £12,010.81 Total £2,390.77

Environmental & social:

Costs Benefits

Item Value Item Value Resources used in manufacture of Neter assuming reprocessed steel used (energy)

Increased CO2 emissions arising from energy use

Reduced methane emissions from 32.5 tonnes of food waste diverted from landfill

Reduction in methane emissions estimated to be in the region of 13,000m3

Reduced vehicle emissions arising from transportation of food waste and compost

Reduced CO2 and particulate emissions

Promotion of zero waste message to households

Potential impact on climate change and more sustainable resource usage

Managing food waste on-site (proximity principle)

Social and environmental impacts of managing waste off-site in a community that did not generate the waste are theoretically avoided


5.5.5 Potential savings using Neter and other in-ve ssel technology

Though the Eden Project’s composting operation is currently running at a financial loss, the following factors should have a positive impact on the cost-effectiveness of the composting operation:

• Improved knowledge of the process should lead to an increased throughput of food waste

• Greater use of alternative (to wood pellets) carbon sources from the Eden Project’s own waste stream should reduce costs

• The cost of sending food waste to landfill is also set to rise substantially As landfill costs rise, the market for alternative processing methods will become more attractive to investors. Indeed, this is one of the intended consequences of the landfill tax escalator. This may lead to an increase in larger scale biogas plants or other food waste handling facilities whose operating costs will be immune to increases in the rate of landfill tax. Disposal of food waste through such a route is likely to be less expensive than sending waste to landfill. However, it is likely to take time for food waste handling capacity to be developed through the installation of such facilities, not least due to the scale of investment required and the time that it generally takes to obtain planning permission for such installations. It may be safe to assume for the next few years at least, that for most commercial organisations the only viable option for the disposal of food waste will be landfill along with its associated costs Table 7 overleaf sets out the potential savings that could arise at different levels of throughput and taking account of the impact of the landfill tax escalator which will increase the cost of disposal by at least £24 per tonne by the year 2010/11. The Government has not yet set out its plans for the level of landfill tax beyond 2010/11 and accordingly it is difficult to project savings from composting food waste beyond that financial year.


Table 6 Kg/day food waste composted

Tonnes/annum food waste composted

Annual saving @ £70/tonne (2007/8)




Tonnes/annum compost * x 4 years

Value of compost @ £12/tonne �

Total saving (2007/8 to 2010/11)

100 36.3 £2,541 £2,831.4 £3,121.8 £3,412.2 43.56 £522.72 £12,429.12

200 72.6 £5,802 £5,662.8 £6,243.6 £6,824.4 87.12 £1,045.44 £25,578.24

400 145.2 £10,164 £11,325.6 £12,487.2 £13,648.8 174.24 £2,090.88 £49,716.48

600 217.8 £15,246 £16,988.4 £18,730.8 £20,473.2 261.36 £3,136.32 £74,574.72

800 290.4 £20,328 £22,651.2 £24,974.4 £27,297.6 348.48 £4,181.76 £99,432.96

1000 363 £25,410 £28,314 £31,218 £34,122 435.6 £5,227.2 £124,291.2

*assumes weight of compost produced is 30% of the weight of food waste composted, and that the proportion of feed-stock additives (green waste and carbon) to food waste remains constant.

� Savings based on Eden Project’s costs and are likely to be higher for organisations purchasing lower quantities of compost.


5.5.5 Conclusions of financial analysis

Historically and at present, the Eden Project is diverting only relatively small amounts of food waste from landfill resulting in an estimated saving of just £1570.49 (food waste and compost) for the year April 2006 and March 2007. The amount of food waste composted is increasing, but even if Eden were to compost all of its estimated 84 tonnes of food waste per annum, a saving of only £5,880 would be achieved (at 2007 waste collection and disposal rates). This is less than current operating cost for the Neter. As landfill tax rises, and the composting operation at Eden becomes more efficient, it is likely that the breakeven point will be reached. However, it is unlikely that the capital cost of the investment in the Neter will be recouped over its lifetime unless other sources of food waste can be composted on site. The requirement of a licence to dispose of waste from off-site origins has to date deterred the Eden Project from pursuing this course of action. To minimise the costs of producing compost the Eden Project is aware of the need to identify a suitable carbon source (preferably) from one of its waste streams as an alternative to the wood pellets. The Neter 30 is over-sized for the scale of the operation at the Eden Project. The Neter 10 which has a capacity of 600kg per day, would with the benefit of hindsight have represented a better investment. For organisations processing less than approximately 400 kg per day, this technology is likely to represent an additional operational cost. Savings derived from food waste diverted from landfill may be insufficient to cover the additional operational cost of operating this technology. The Eden Project’s experience indicates that operational costs are likely to rise with an increase in the weight of food waste composted but will do so at a slower rate than the rate of increase in savings from food waste diverted from landfill and compost produced. There are therefore economies of scale in dealing with larger throughputs of food waste, which contribute towards making this technology a sound financial investment in such circumstances.


6. CONCLUSIONS

The Food Waste Technology Trial has represented a useful learning experience for the Eden Project team from which it is hoped others considering installing this or similar technology will benefit.

After some initial teething problems, the Neter 30 has been regularly producing compost which is of good enough quality to be used in a range of applications and to meet the requirements of the BSI PAS 100 test. However, this is not always the case and the final compost product will need to be routinely monitored to ensure that standards are maintained. In the Eden Project’s experience, material that has not attained this standard can still be safely used on the site of production as a good quality soil improver.

Achieving the right feed-stock balance for optimal composting conditions has been a difficult process requiring a degree of ‘trial and error’. However, the Eden Project now has a much better understanding of the feed-stock process and as a consequence, good conditions for composting are more regularly attained.

The front end of the machine has been a source of many of the problems experienced in the trial. In particular, there have been problems with the entry points for food waste, green waste and wood pellets. Modifications to the front end have been required as a consequence including the addition of a hopper for wood pellets and green waste.

Operationally the machine has caused few problems though more down-time has been experienced than is ideal due to competing priorities of the site’s maintenance operative. The technology is relatively straightforward and operatives have found it easy to understand how it works. Staff have also positively embraced the process, possibly because they are able to clearly see that their work is helping to produce a useful resource for the Eden Project.

The level of food waste being processed through the composter is substantially lower than the capacity of the machine. There are two reasons for this. Firstly, the level of food waste anticipated was overestimated during the procurement process. The capacity of the machine is therefore much larger than is required for the level of food waste being generated at the site. Secondly, when larger proportions of food waste to green waste are treated, there is a tendency for lower temperatures to develop at the front end of the machine, inhibiting the efficiency of the composting process.

The technology is financially viable at higher levels of throughput and provided an appropriate mix of feed-stocks can be established that is conducive to a good composting process. At lower levels of throughput the financial viability of the project may depend largely on the cost of providing operatives. The financial viability of this technology should improve substantially over the next few years as the landfill tax escalator increases the cost of disposing of biodegradable waste in landfill. However, the increase in landfill charges is likely to stimulate the commissioning of new large-scale composting or anaerobic digestion plants to handle food waste. This should eventually begin to stabilise


the costs of food waste disposal and may impact on the future cost-effectiveness of small-scale in-vessel composting operations.

There are a number of environmental benefits from using this technology, not least the diversion of waste from landfill and the accompanying reduction in methane emissions that can result from such treatment.

Generally, the technology seems to work well and there are a range of options to suit various budgets and levels of food waste including the bespoke solution such as the Neter and the cheaper ‘off the shelf’ solutions such as the ‘Big Hanna’. A number of other companies also provide this technology.


7. RECOMMENDATIONS

Organisations considering investing in an in-vessel composter should:

• seek to understand their food waste issue and take appropriate steps to minimise the amount of food waste being generated;

• make an accurate assessment of the amount of food waste generated as it could have a major bearing on the financial payback;

• seek to ensure that the capacity of the machine purchased closely matches the volume of food waste and other feed-stocks composted;

• ensure that they have given full consideration to the implications of complying with the relevant legislation including amongst others ABPR and the EPA 1990; and

• be committed to the principles of sustainability and thereby have a full appreciation of the benefits of the process, over and above the current financial returns.

At lower levels of throughput, in addition to the aforementioned precautions, the following conditions should ideally apply:

• a suitable free or low cost source of carbon should be available as a supplement to the feed-stock.

• A steady source of green waste should also be available free or at low cost

• The compost produced should be either of use to the operator as a soil improver or processed into a saleable product.

• Any additional operational capacity employed in the processing of food waste needs to be closely matched to the nature and level of work

It is important to input an appropriate mix of feed-stock into the composter. Failure to do so is likely to result in a situation where the machine will be operating inefficiently or not at all. A rate of 2:1 food waste to green waste with 15% carbon in the form of wood pellets has worked reasonably well for the Eden Project. However, the exact proportions of food waste, green waste and carbon supplements are likely to vary according to the nature of the wastes available at a site.

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Investigating single stage, in-vessel, site-based, treatment of food waste for the commercial sector:

Scientific evaluation of the efficacy of the Neter 30 composter for treatment of Eden Project’s food waste under commercial operating

conditions

M Newton, D Bullock, N Knight, M DeVille, T Keay, T Pettitt

Eden Project, Bodelva, Cornwall, PL24 2SG

SUMMARY

The efficacy of the Neter 30 in-vessel composter, the largest machine of its type to date, was evaluated under commercial operating conditions at the Eden Project. It was installed in the Eden Project Waste Neutral compound on 20th April 2005. The composter was evaluated in three ways: i) the physical and biological aspects of the composting process and optimisation of feeding regimes, looking to compliance with UK ABPR and/or EU 208/2006; ii) the biological safety aspects of the composter operation and iii) the quality of the compost product and its compliance with PAS100. Over most of the test period (April 2005-July 2007), the composter produced a reasonable grade of compost, although for a large part of this period it was operating sub-optimally. This was the result of acidic conditions rapidly developing once feed-stocks had been introduced to the composting vessel, causing a delay in the transfer from mesophilic to thermophilic conditions and therefore a slowing in the overall rate of composting. This had the knock-on effect of reducing the volumes of feed-stocks that could be introduced into the vessel. Optimal composting was achieved when food waste was co-composted with shredded green waste from the Eden Project site at a rate of 2:1 by volume. Composting under these conditions, the system was able to deal with up to 150 kg day-1 of waste, achieving a mass reduction of 73.3%, and reaching temperatures in excess of 70oC for short periods. It produced a product that passed two out of three PAS100 assessments and generally performed well in on-site horticultural trials, both as a growing medium supplement and as a soil improver or mulch. The biological safety of the machine was good with virtually no leachates formed, low bioaerosol emissions, and no noxious gas emissions. The study identified that the machine could be improved by modifying the front end of the vessel to handle a wider range of feed-stocks to facilitate co-composting. This was especially relevant to shredded green waste additions. Further improvements in the composting process are possible with adjustments to the feeding regimes that should boost the capacity of the machine. With these improvements consistent compliance with either UK ABPR or EU 208/2006 are anticipated.

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INTRODUCTION

The 1999 EU Landfill Directive (EU; Council of the European Comunities, 1999), defined municipal waste as ‘waste from households, as well as other waste, which, because of its nature or composition, is similar to waste from households’. In the UK, 20 million tonnes of biodegradable municipal waste (BMW) is produced every year. Concern over emissions of the greenhouse gases methane and carbon dioxide, generated by the degradation of this material under landfill conditions was one of the main drivers for the EU Landfill Directive (EU; Council of the European Communities, 1999), which has set member states the target of reducing the amount of this waste sent to landfill to 35% of 1995 levels by 2016 (2020 for the UK).

The majority of the BMW in the UK is still sent to landfill, despite significant reductions (from 79% in 2000/01 down to an estimated 62% in 2005/06; Environmental Services Association, 2007).

As part of the effort to meet the Landfill Directive targets for reducing BMW, alternative treatments for this material need to be found. Data from Poll (2004) indicated that approximately 32% of the household (domestic) waste stream constituted compostable kitchen waste. Composting these waste fractions of BMW would greatly reduce the amount sent to landfill, and, as aerobic composting does not produce methane (25 times more potent as a greenhouse gas than carbon dioxide (Lelieveld et al., 1998 and Ramaswamy et al., 2001)), would also reduce greenhouse gas emissions. In addition, composting to produce growing media or soil improvers would reduce the need for fertiliser treatments by returning nutrients to growing systems in a form that favours long term plant growth, soil health and reduces losses due to leaching.

For the purposes of this study the definition of the composting process provided by Haug (1993) was used: ‘composting is the biological decomposition and stabilisation of organic substrates, under conditions that allow development of thermophilic temperatures as a result of biologically produced heat, to produce a final product that is stable, free of pathogens and plant seeds, and can be applied to land’. The conventional approach to large-scale composting of materials such as horticultural and green wastes is the use of windrow systems. However, the composting of catering wastes (these are defined in EU legislation as all waste food, including used cooking oil, originating in restaurants, catering facilities and kitchens, including central kitchens and household kitchens (European Parliament & Council (2002)) cannot be carried out in these systems since the likely presence of animal by-products (ABP) in these waste streams necessitates the use of containment precautions. This is to prevent the possible spread of animal pathogens (in the U.K., both the 2001 foot-and-mouth disease outbreak (Scudamore, 2002), and the classical swine fever outbreak in 2000 (DEFRA, 2007), are considered to have originated from contaminated catering waste). Treatment of waste containing ABP must comply with the Defra, State Veterinary Service criteria, as set out in the Animal By-Products Regulations (ABPR; HMSO, 2005). However, compliance with ABPR need not be shown for compost produced from catering waste composted on the premises on which it originated, provided no ruminant animals or pigs are kept on the premises, the composting waste is not accessible to birds, and the compost produced is only applied to land at these single premises (HMSO, 2005).

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Outside the UK, the composting of food waste is a widespread and established procedure with a range of systems currently in use world-wide, Haug (1993). An effective approach to composting food wastes on a medium to small scale is the use of composting vessels, generally referred to as compost reactors. Haug (1993) divides reactors into ‘vertical flow’ and ‘horizontal flow’ systems. In Europe there are a number of horizontal flow systems being used to compost food wastes, for example the ‘Rocket’ systems (www.quickcompost.co.uk), the ‘Hotrot’ systems (www.hotrot.co.uk), the ‘Jora’ composter (www.smartsoil.co.uk), the ‘Big Hanna’ composter (www.bighanna.co.uk), plus a range of other, often larger, systems listed by The Composting Association (2004). Of these, the ‘Big Hanna’ Ale Trumman ‘in vessel’ system designed by Torsten Hultin (Wiles, 2006) is of interest as it is designed to produce stable compost within a single contained reactor. With 600 units currently in use, mainly in Scandinavia, Hultin’s philosophy in promoting the use of these reactors to local communities and small businesses was that the provision of a system that could recycle biodegradable waste, such as food waste, to produce a beneficial end product would foster social responsibility towards this waste stream. Certainly, providing the means to deal with biological wastes ‘on-site’ raises awareness of the scale of waste production, and hopefully affects buying behaviour, with ultimately less consumables bought and less waste produced. This social approach to dealing with food waste, and increasing understanding of the issues associated with it, combined with the published success of the ‘Big Hanna’ systems was considered to be strongly in line with the Waste Neutral objectives of Eden Project.

Based on the ‘Big Hanna’ composter, Hultin developed the larger-scale Neter systems. The largest Neter composter constructed so far (the Neter 30) was designed specifically to deal with Eden Project’s catering waste, estimated at between 100 and 700 kg day-1, depending on visitor numbers. It was sited in Eden Project’s ‘Waste Neutral’ compound, where it could be used to demonstrate this developing technology, alongside the other recycling processes currently employed by Eden Project. The composting vessel of the Neter 30 is essentially a stainless steel cylinder 8.5 m long by 2 m diameter, with a holding capacity of 30 m3, and was designed to take up to 1-2 tonnes of organic waste per day. Composting catering waste had not previously been attempted on this scale using a Neter system and the precise parameters of the process were not fully defined. This report describes the technical appraisal of the Neter 30 system established at Eden Project in April 2005.

The aims of this study were to:

• Optimise the composting process by monitoring process variables such as temperature, moisture content, pH, carbon to nitrogen ratio, as well as feed-stock types, quality, and feeding regimes.

• Assess the biological safety aspects of the composting process, for example, assessments of gas emissions, bio-aerosols, leachates and the presence/absence of human pathogens.

• Assess the quality of the compost produced in terms of stability suitability as a growing medium supplement or as a soil improver, and the presence/absence of plant pathogens and weed seeds, (including sending for tests for compliance with PAS 100 (BSI, 2005)).

• Determine whether the composter produces a product that can comply with the current ABPR, or

suggest ways by which this might be achieved.

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MATERIALS & METHODS

Outline of Neter 30 composter:

The Neter 30 in-vessel composter was designed for composting organic wastes, primarily catering waste with added sources of carbon and the optional addition of horticultural waste. It was designed as a single-stage, fully contained, continuous flow-through system, with waste fed via an in-auger into one end and compost automatically removed via an out-auger at the other. The composter consists of a horizontal stainless steel cylinder approximately 8.5m long, by 2m diameter (volume = circa 30m3), which rotates against fixed end walls. It is housed within a stainless steel outer casing 9.5m long, 2.7m wide and 2.6m high. This outer casing acts as a safety barrier for the moving parts and provides some buffering from external temperature fluctuations. The rotation of the cylinder tumbles the material being composted, mixing and aerating it, whilst moving it along the vessel. The periods of rotation (‘run’) and standing (‘wait’) can readily be altered to suit the process. Fixed to the end walls is a spindle that runs through the centre of the vessel. Attached normally to the spindle and spaced at 1.4m intervals, are 5 circular steel plates on each of which 4 temperature sensors and 1 water inlet nozzle (for wetting compost if it becomes too dry) are mounted (positions A-E, Figure 1A&B).

Figure 1: A. Side elevation of the Neter 30 composter showing the locations of sampling-temperature sensor positions A-E, sampling locations X & Y and inspection hatches 1-4. B. Front elevation of the central spindle showing the relative positions of the 4 temperature sensors and water inlet pipe located on plates at positions A-E.

Air is drawn, by fan, across the top of the composting mass during the wait periods and through the mass during the run periods (Figure 2). The speed at which the air is drawn across/through the material in the vessel can be altered to suit the process. Drawing air through the vessel removes the gasses, water vapour and excess heat produced by composting, and replenishes the oxygen levels. The exhaust air expelled from the vessel is passed through a biofilter before being discharged to the atmosphere (Figure 2).

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Figure 2: Axonometric diagram of the Neter 30 composter showing the flow of air through the outer casing, to the cylinder and out via the biofilter.

Compost temperature:

Temperature sensors within the Neter composter allow direct continuous logging of temperatures at the sampling points A to E along the length of the chamber (Figure 1 A&B). In addition to these sensors, two 1.5m long temperature probes were used to provide ‘spot checks’ of temperatures throughout the compost mass, as well as providecalibrations and checks on the accuracy of the composter sensors. Furthermore, at intervals from 28/04/2006 to the present, temperatures of individual ‘batches’ of composting material were monitored throughout their progress through the composter using data-loggers (Egg loggers, Gemini Data Loggers, UK; Palmer(2007)), which were introduced to the compost at position Y through hatch 1 (Figure 1) and collected from the exiting ‘finished’ compost at the out auger.

Compost sampling procedure:

Compost samples were collected from a number of set locations within the composter: the five temperature sensor positions A-E (Figure 1); at the immediate exit from the in-auger (predominantly freshly shredded food-waste, X, Figure 1); at the very front end of the composting vessel where the feed stocks are still mixing (Y, Figure 1); and at the point where finished compost exits the machine (referred to as ‘Out’ at the compost out point, Figure 1). Samples were collected from inside the vessel (at locations A-E & Y) through the access hatches (1-4, Figure1), using a trowel with an extended handle 1.5m long. This was used to scrape away the surface 100mm of material and to collect a sample of approximately 200g. Samples X were collected, via hatch 1, directly from the in-auger before the shredded food-mass fell into the vessel. Samples ‘Out’ were removed from the collection bin situated directly under the out-auger. Samples were either placed in pre-weighed aluminium foil trays for pH and moisture determinations or in polythene bags for immediate transfer to the laboratory for microbial analysis (see relevant sections below).

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Determination of compost pH:

The pH of samples collected from the composter was determined by preparing suspensions of approximately 50g material in 100 ml of deionised water. Suspensions were agitated for ten minutes on an orbital shaker and allowed to settle for 5 minutes prior to measurement of pH using a Jenway 3310 meter.

Moisture content:

The moisture content of samples taken from the composter was determined gravimetrically. Fresh weights of samples of approximately 200g, placed in pre-weighed aluminium foil trays, were recorded before heating in an oven at 105oC to constant dryness (approximately 24-48h, Wilke, 2005). The resultant dry weights were recorded and moisture contents determined by subtraction and expressed as percentages of the fresh weights.

Carbon to Nitrogen (C:N) ratios:

Compost samples (1 kg) were taken at each of the sampling points A-E, X and Y (Figure 1A) and at the out-auger ‘Out’, using the procedure outlined above. These were marked with date, sample number and sampling location and immediately sent by next-day courier to Ecosci, Wolfson Laboratory (Exeter, UK) for C:N analysis.

Measurement of the rate of progress of material through the composter:

A range of markers of various sizes and types were placed in the composter to determine the rate of progress of the composting mass through the machine. Rubber balls of varying sizes and densities, as well as plant labels cut to different lengths (tags), were used to determine whether different size and weight objects would pass through the process at different rates. The larger sizes of ball were also used to assess the risks of damage or loss when passing data-loggers through the composter. The range of marker sizes and types is illustrated in Figures 3A and B. The rubber balls were placed at distances between 1m and 2 m from the discharge end of the vessel. This was to quickly obtain information on the rate of progress and the potential risks to data-loggers. The plant label tags were, and continue to be, placed at various locations in the composter. Tags were colour-coded and their location and date of placement were recorded. Placement of tags consisted of throwing them onto the surface of the composting mass inside the vessel through the relevant hatch to form a cluster of tags as close as possible to the composting material under specific observation.

In addition to the use of markers described above, some feed-stocks were also used to mark the point of change between two treatments, e.g. the large pieces of shredded plant material present in shredded green waste were used to track the progress of compost mixes containing this feed-stock.

Estimation of compost maturity:

The maturity or stability of compost samples was estimated using the Solvita® compost respiration test (Wood End Labs Inc, USA. www.solvita.com). Using the procedures outlined in the Solvita literature, a set volume of the compost under test was reacted with indicator ‘paddles’ to produce indices of carbon

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dioxide and ammonia evolution. These indices were translated, using the table supplied in the kit, to provide an estimation of compost maturity.

Bulk density:

Two litres of fresh compost sample were dispensed into a pre-weighed plastic measuring jug, which was regularly tapped to ensure complete settlement of the sample in the vessel. The weight of the 2 L compost sample was recorded, the ‘wet’ bulk density calculated (wet weight ÷ volume) and a sub-sample taken for determination of the percentage moisture content by the method described above. Using this value the dry weight of the 2 L of compost and the dry bulk density were calculated.

Figure 3: Illustrations of the range of markers used to determine the rate of progress of the composting mass through the Neter composter. (A) Rubber balls of various sizes and densities and (B) plant labels cut into different length tags.

Bioaerosol tests:

Tests were carried out by Ian Moss from the University of Hertfordshire Bioremediation Centre on 28/06/2007 (their log no. 1121). Air was sampled at eight stations in and around the Waste Neutral compound at Eden Project (Table 5) using a RCS High Flow air sampler (Biotest, Denville, NJ, USA). Two air volumes (20 l and either 100 or 200 l) were sampled for each target micro-organism assessed, at each sampling station. The selective agar test strips used in the air sampler were as follows: Tryptic Soy agar for counts of total bacteria; Rose Bengal agar for counts of total filamentous fungi, as well as counts of Aspergillus fumigatus. Exposed test strips were incubated for 2 days at 30oC for bacteria counts, 5 days at 22oC for filamentous fungi and 3 days at 41oC for Aspergillus fumigatus, colonies of which were identified by microscopy. All counts were expressed as colony forming units (cfu) per m3 of sampled air.

Gas emissions:

Gas emissions from the Neter 30 composter were determined by Glyn Leppitt of Leppitt Associates, Waste & Environmental Training & Advisory Service, Bodmin, Cornwall using a gas analyser (GA2000,

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Geotechnical Instruments, UK) with internal datalogger. Tests for the concentrations of oxygen, carbon dioxide, methane, hydrogen sulphide and carbon monoxide were carried out on 07/02/07, 16/02/07 and 25/05/07 and the schedule of testing is currently ongoing. Measurements were taken at an exit valve, located near the fan operating the biofilter and generating the air flow through the composter vessel (see ‘fan’ Figure 2), both when the vessel was rotating and static. Additionally,air was sampled from inside the composting mass itself, using a stainless steel, 20 mm diameter, air sampling tube of approximately 1 m length. Air was also sampled inside the biofilter when the vessel was static. All results were expressed as % volume except for hydrogen sulphide and carbon monoxide concentrations which were expressed in parts per million (ppm).

Micobiological assessments through the composting process:

Coliform bacteria populations:

Samples of compost were collected from points A-E, X, Y and the out-auger (Figure 1), and 3, approximately 1g, sub-samples of each were weighed and suspended in 10ml aliquots of sterile maximum recovery diluent (CM 0733, 9.5g l-1, Oxoid, UK). A fourth sub-sample of approximately 100g was used to calculate the %moisture content as above and therefore the dry weights in each suspension. Suspensions were taken through a dilution series and 0.5ml aliquots of all dilutions were plated onto selective Escherichia coli/coliform chromogenic medium (CM 1046, 28.1g l-1, Oxoid, UK) in 9cm Petri dishes. Aliquots were spread over plate surfaces with sterile plastic spreaders and left open in the flow hood for 10 minutes before incubation in darkness at 30oC for 72h, after which the numbers of purple (E. coli) and pink (other coliforms) colonies were counted. Numbers of bacteria present were presented as colony forming units g-1 dry weight of compost.

Total bacteria, filamentous fungi and thermophiles:

Similar 1g samples were weighed out and suspended in 0.1% agar solution before taking through dilution series in sterile distilled water. Dilutions were plated for total bacteria on one tenth strength tryptic soy agar (0.1 TSA, Oxoid Ltd, UK) and for total fungi on quarter strength potato dextrose agar (PDA), following the procedures of Calvo-Bado et al. (2006) and for thermophiles on 0.1 TSA, after 20 minutes incubation at 60oC (Van Os et al., 2004).

Assessment of presence/absence of plant pathogens:

Samples of finished compost were assessed on three occasions for the presence of pathogenic fungi by the plating procedure of Hunter et al. (2006) using ‘BNPRA’ selective agar for detecting Pythium and Phytophthora spp. (Pettitt & Pegg, 1991) and the semi-selective medium of Parry (1990) for hyphomycetes. In addition to this, seedling bioassays were carried out on two sampling occasions to test for the ‘clubroot’ pathogen Plasmodiophora brassicae and as a back-up test for Pythium spp. using the procedure of Wallenhammar (1996): once with Brassica campestris ssp. pekinensis and once with Lepidium sativum seeds. After visual assessments for clubroot were completed, roots were plated on BNPRA plates to check for presence of pathogenic Pythium spp.

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Effects of compost on plant growth:

Growing medium trials:

The suitability of the finished compost from the Neter composter for use in growing media was assessed ‘in house’ in three trials. The first two trials were with lettuce (Lactuca sativa ‘Little Gem’) and the last was with tomato (Lycopersicon esculentum ‘Gardener’s Delight’). The growth of test plants in 100% Neter compost was compared with growth in a reference medium (EP Mix 1, Vapogro Ltd, UK, the peat-free growing medium used for the majority of propagation and container-growing at Eden Project, see Table 1), and in mixtures containing different ratios by volume of these two materials. The compost, collected from the out-auger of the Neter composter (Figure 1) the day before use in each trial, was hand sorted to remove any large pieces of wood or other matter prior to use. Mixtures of compost and EP Mix 1 were prepared by placing the appropriate volumes of each in a large (50 x 80cm) polyethylene bag and inverting this, with shaking, 10 times.

Table 1: Composition of EP Mix 1 supplied by Vapogro Ltd, UK. (pH = 5.5).

Component Quantity

Melcourt Growbark 400L

Melcourt Sylvafibre 600L

Granular Clay 10kg

Wetting agent None

Base fertiliser (14-16-18) 1kg

Calcium ammonium nitrate 0.65kg

Lettuce seeds were sown in EP Mix 1 at a rate of 100 per seed tray. At the four fully expanded leaves stage of growth, lettuce seedlings were transplanted to test growing media in 130 mm circular pots. Pots were arranged in repeat latin square designs, with six replicates per treatment, on a Mypex-coated floor in a greenhouse with no additional light or heat. Irrigation with mains water was applied daily via over-head sprinklers and no additional mineral nutrition was applied over the duration of the trials. The first trial was started on 25th April 2006 and the second was started on 5th June 2006. The trials ran for 3.5 and 5 weeks respectively.

Tomato seeds were sown, two per cell, in 35 mm standard Ellepots (Vapogro Ltd, UK), and thinned to one per cell at first true leaf emergence. Seedlings were transplanted to test growing media in 90 mm square pots at the four fully expanded leaves stage of growth. Plants were arranged in a randomised block design in a greenhouse set to 19oC with a 22 oC venting temperature, and a 12 h photoperiod maintained by a metal-halide lamp (600W Supernova). Irrigation with deionised water was applied to

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each individual pot by Netafim drippers rated at 2L h-1 with the frequency and duration of irrigations being adjusted throughout the trial according to the plants’ uptake. Plants were arranged on raised supports to allow free drainage. The trial was started on 30/11/2006 and lasted for 10 weeks.

At harvest, plants were photographed and assessed for presence/absence/degree of phytotoxicity symptoms. Roots were gently washed free of growing medium and separated from the shoots by cutting at the hypocotyl. Fresh weights of roots and shoots were recorded, together with dry weights, determined after heating in a drying oven at 80oC to constant dryness (24-48h).

Assessments of air-filled porosity were carried out on compost samples from the tomato trial following the procedure of Bragg and Chambers (1988).

Mulching medium trials:

Pot experiment: Seeds of pepper (Capsicum annuum ‘De Cayene’) and of lettuce (L. sativa ‘ Little Gem’) were sown in ‘John Innes seed’ propagation medium (JI seed, Westland Horticulture Ltd, UK) in 20 mm modules (P286 0224 K, Desch Plantpak, Waalwijk, NL). Pepper seeds were sown on 1/05/2007 and placed on a heated bed in a greenhouse maintained at 20oC with a 25oC venting temperature. Lettuce seeds were sown on 31/05/2007 and were placed in an incubator maintained at 18oC under low energy white fluorescent lamps (MTV B0002 ‘Daylight’) maintaining a 12 h photoperiod until germinated, when they were transferred to the same greenhouse as the peppers.

After approximately 5 weeks (6/06/2007), peppers were transplanted to 9 cm square pots containing JI seed medium adjusted so that each pot, containing medium and seedling, weighed 370 ± 5g. Lettuce seedlings were similarly transplanted, after approximately 2 weeks (13/06/2007), into 11 cm square pots containing JI seed medium, each adjusted to final weight 660 ± 5g.

Freshly produced Neter compost was compared with a proprietary ‘green waste’ compost (‘West Country Compost’, Ecosci) need to check the spelling of this as a mulching medium. Prior to use for mulching, each medium was passed through a 10mm sieve. With 9 replicates of each treatment, aliquots (20g for the peppers and 40g for the lettuce plants) of each medium were compared with untreated controls. Mulch treatments were applied, 7 days after transplanting, to the pepper plants; and 3 days after transplanting, to the lettuce plants. Treatments were arranged in a randomised block design in an unheated greenhouse under mains water sprinkler irrigation, with no supplementary lighting or mineral nutrition. The pepper plants were monitored over 10 weeks for the appearance of phytotoxicity symptoms, with the shoot heights recorded after 6 weeks. The lettuce plants were monitored over 4.5 weeks for the appearance of phytotoxicity symptoms, and ,at the end of that time, the root and shoot weights were determined, (as in the growing medium trials described above), to assess the effects of the mulch treatment on growth.

Field trial: The suitability of the Neter 30 compost for mulching was also assessed in a trial on the main Eden Project site. The area chosen for the assessment was a newly planted mixed hedge in the ‘Spiral Garden’. Comparisons were made between 3 simple treatments; mulching with Neter compost, mulching with Ecosci green waste compost (‘West Country Compost’) and untreated controls. The hedge was divided into twelve 6 m long plots that were randomly allotted to one of the three treatments, giving four replicate plots per treatment. Prior to mulching, samples (200g) of soil were taken

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from all plots. These were pooled and sent, together with a sample of the Ecosci compost, to Yara Analytical Services (Yara, York, UK) for mineral, nitrogen and PTE analysis. At the same time, a sample of the Neter compost was sent to Natural Resources Management (NRM, Bracknall, UK), for PAS100 testing. The placement and depth of mulch used was judged by eye and estimates of the rate of application (both fresh and dry weights per unit area) were obtained by removing all of the placed mulch from sample areas of 0.0225 m2 from two Neter-treated and two Ecosci-treated plots. The performance of the treatments was assessed by recording the increase in plant height in selected species between application on 22/05/07 and 16/07/07, and by observing symptoms of phytotoxicity on all plants. After 8 weeks, soil samples were taken from under the mulching by scraping away the mulch material and removing approximately 200g of the underlying soil. Soils samples from each treatment were pooled and sent to Yara for mineral, nitrogen and PTE analysis.

PAS 100 tests:

To gauge the quality of Neter 30 compost, samples were tested to British Standards Institution Publicly Accessible Specification (BSI PAS 100:2005, generally referred to as ‘PAS 100’ (BSI, 2005)). Samples (7 kg) of compost were taken from the out-auger (Figure 1) on 11/01/07, 26/02/07 and 16/05/07, and sent by courier to NRM for analysis.

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RESULTS

Process variables:

Feed-stocks:

Food waste: The feed-stock referred to as ‘food waste’ in this report consisted of catering waste from the kitchens and food preparation areas as well as scraps from cleared tables, together with spent compostable packaging and wooden eating utensils. Currently, this waste stream contains very little raw vegetable material, as the vast majority of the vegetables cooked on site are delivered ready-prepared (i.e. peeled and washed). This waste is brought to the recycling yard in 90 litre ‘wheely’ bins. When full, these can vary in weight from 25 kg (predominantly bread) to 75 kg (predominantly wet food waste), with the average full bin weighing approximately 40 kg. Food bins are emptied by bin lift onto conveyor belts, where the waste is manually inspected and contaminants (metal, plastics etc.) are removed before passing through a shredder to reduce the particle size to 20 mm. After shredding, the food waste is transferred to the composter vessel by the ‘in-auger’ (Figure 4). The pH of this feed-stock can vary as follows: mainly bread, pH 5.3; mainly sandwiches, pH 5.0; mainly potatoes, pH 5.7; mainly pasties, pH 5.3 and mainly lemons, pH 3.9, but is always acidic. Bins containing a large proportion of citrus material are now avoided!

Figure 4: Shredded food waste entering the composter vessel via the in-auger (see Figure 1)

Biodegradable plastic: Eden Project has moved to using biodegradable plastic (poly-lactic acid; PLA) for some food packaging. This material is source segregated from the main food waste stream in the restaurants on site. It was trialled in the Neter 30 composter and was found to only compost when

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temperatures were consistently above 60oC. This meant that some batches of compost, where this temperature was not achieved, or only achieved briefly, contained visible traces of PLA.

Carbon: A carbon: nitrogen ratio of 30-35:1 is recommended by McGaughey & Gotaas (1985) for rapid composting. The relatively high nitrogen content of the food waste requires the addition of a carbon supplement to achieve this ratio. The carbon source recommended by Susteco, the manufacturers of the Neter 30 composter, is compressed sawdust pellets. This material is not only a source of carbon, but also an effective drying agent; quickly reducing the overall % moisture content of the compost mix once incorporated. The pellets rapidly expand and break up the wet food waste macerate. On the negative side, these pellets needed to be purchased in significant quantity at a cost of approximately £0.30 kg-1, and alternative sources of carbon were therefore investigated and their performance assessed. These included wood shavings as sold for animal bedding, shredded office paper, shredded paper towels, shredded corrugated board, chopped-straw and waste dust collected from a corrugated board-shredding process. The pH of this waste stream can vary from 4.7 and 4.9 for sawdust pellets and wood-shavings respectively, to 5.7 for hemp straw and 8.0 for shredded corrugated board dust. Another very promising possible carbon source was ‘shredded wood waste’, which would have been a good economic and ‘waste neutral’ alternative to sawdust pellets but unfortunately had to be discounted as a result of the high concentrations of lead, detected in mineral analysis (concentrations in mg kg-1: Pb 645, Cd 0.45, Ni 4.8, Cu 45.2, Cr 31.5, As 23.8, Hg 0.03, Zn 361.4, Mo 0.49, B 10.53, Se 0.16, F- 11.4, Stotal 1.6, Mn 0.1, Na 0.8). To date, assessments of alternatives to compressed sawdust pellets as carbon sources have been constrained, especially in the early stages before the addition of the hopper, by the inability of the front end of the Neter 30 composter to deal with them in quantity, necessitating their being loaded into the vessel via hatch 1 (Figure 1). This had several drawbacks: there was a manual handling implication to loading large quantities via hatch 1; the vessel had to be opened to allow access via hatch 1, exposing the composting mass to low external temperatures; mixing with the food-waste was delayed with possible impacts on the composting process (considered in detail later); and introduction of shredded paper and board through hatch 1 did not achieve the rapid and vigorous mixing with the food waste considered necessary to properly wet and incorporate these feed-stocks for effective composting.

Green waste: Freshly-shredded green waste from a number of different locations on the Eden Project site was used as a feed-stock to help with the C:N ratio, to help increase the pH at positions Y and A, and to supply suitable microbial populations to help with the composting process (potentially lacking in the food waste feed-stock). Generally more basic than other waste streams, the pH of green waste can vary enormously, depending on the source plant material being processed, between approximately pH 6 and 8.

‘Finished’ compost collected from the out-auger: Finished compost has occasionally been successfully used as a feed-stock. It performed a number of roles; it was a good drying component, with an approximately 60% lower moisture content than the compost mix measured at position A (Figure 1), it had a high pH of between 7.5 and 8.5, it also contained a high concentration of the thermophilic micro-organisms required for the aerobic composting process, and it marginally increased the C:N ratio.

Carbon to Nitrogen ratios:

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The results of analyses of C:N ratios are shown in Table 2. As would be expected, the ratio declined through the composting process. A comparison between the values at X (largely shredded food waste alone at the times of sampling) and Y show an effective increase in the C:N ratio to the optimal levels (ratios of 30-35:1) recommended by McGaughey & Gotaas (1985) for rapid composting. This was largely achieved by the addition of compressed sawdust pellets added through hatch 1 (Figure 1A). Shredded green waste was also added at this point, but with a relatively low C:N ratio of 22, the higher pH (6.8) and microbial activity in this feed-stock were considered of more consequence.

Table 2: Carbon to nitrogen ratios and pH values for samples of composting mass taken from positions X, Y and A-E down the length of the composting vessel as well as ‘finished’ product (‘Out’, see Figure 1A)

The composting process:

The Neter 30 composter was set up on 20th April 2005 and from this time until July 2007 a total of 33 different treatments, involving different mixtures of feed-stocks and running parameters such as ‘run’ and ‘wait’ times, and the flow of air through the composting vessel, were assessed. These treatments were to determine the optimum operating conditions, and the best available feed-stocks for mixing with Eden Project’s food waste to ensure rapid composting. For the composter to operate optimally, the initial mesophilic stage of the composting process (as indicated by temperatures 20-45oC, Gilbert et al., 2001) should be completed over the distance between sampling points Y and A (Figure 1). At points A and B the process should become thermophilic (Susteco, 2004), consistently attaining temperatures in the range 55-65oC. For this reason, the temperature at point A was regarded as key to the efficacy of the whole process and treatments were grouped, according to their impacts on the temperature at this point, into four categories (1-4,Table 3). The majority of treatments (22 out of 33) fell into the first two categories which essentially represent a non-functional state, with the composting mass still mesophilic at point A and probably largely anaerobic at this point, with a low pH (<5), and temperatures predominantly less than 45oC (ranging between 30-40oC). The remaining 11 treatments fell into categories 3 and 4. Category 4 represents more or less optimal composting conditions, whilst 3 contained treatments at thermophilic but slightly sub-optimal temperatures, some of which were progressing towards optimal conditions (i.e. those where the temperature was ascending, Table 3).

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Whilst achieving thermophilic conditions early in the vessel is desirable for the effective and efficient operation of the Neter 30 composter, considering the temperatures at A in isolation can be misleading with regards the composting process in the machine as a whole. After some initial problems from April to August 2005 (treatments 1-3, Table 3), thermophilic temperatures were achieved at some point in the vessel throughout the trialling period (Figure 5), and often through a large proportion of the vessel (Figure 6). This meant that, whilst the conditions within the vessel were quite variable and at times sub-optimal, compost was being produced over much of the experimental period. The properties, quality and variability of this product are described in the next section. An important observation was that often when temperatures were high at point A, they were thermophilic along the entire length of the vessel (Figure 6). Interestingly, for some 60% of these periods green waste was a significant component of the feed-stocks being used, especially for the two main temperature spikes (12/’05-01/’06 and 01/’07-03/’07, treatments T9/T10 & T26/T27, Figure 6 & Table 3). The acidity of the starting compost appeared to have a strong effect on the rate of change from mesophilic to thermophilic conditions, and for the most effective treatments, where pH data are available, values did not fall below pH 5.9, often being closer to pH 7. The Eden Project food waste materials were generally acidic (in the region of pH 5), and these values rapidly dropped following maceration and deposition in the composting vessel, with values recorded at positions X and Y (Figure 1) often in the range pH 4.0-4.4. These low pHs were often associated with a slow transfer from mesophilic to thermophilic temperatures at points A and B (Table 3). Figure 7 shows the interaction between the pH and moisture content of the composting mix at point A and its temperature. A cluster of low temperature readings is clearly associated with moisture levels of 55-65% and, perhaps more importantly, pH values between 4 and 5. These conditions were always associated either with adding macerated food waste to the vessel with the wrong balance of accompanying feed-stocks (see treatments in categories T1 & T2, Table 3), or with electrical/mechanical breakdowns of the composter, often resulting in the feed-stocks at position Y (Figure 1) not being properly mixed for some time, allowing an acidogenic fermentation to rapidly proceed in the macerated food-waste component. Once established, acidic conditions at Y and A usually proved difficult to reverse, requiring the addition of large quantities of finished compost from the out- auger through hatch 1 (see Figure 1). Alternatively, if no feeds were added for a period of several weeks, the pH would sometimes gradually increase (eg treatment T10, Table 3), although this result could certainly not be guaranteed (e.g. treatments T20 & T22, Table 3).

Over the period from 28/04/2006 to 10/07/2007, nine ‘Egg’ dataloggers were passed through the composter. These provided temperature data for discrete batches of compost material as they passed through the composter, as well as the transit times for these batches (Figure 8 A-I). These data complement the figures obtained from the composter’s sensors (Figure 6) and show that generally the temperatures increased with progress through the composter vessel, except for loggers placed in the machine on 20/02/2007 and 28/02/2007, when thermophilic conditions were attained very quickly and temperatures remained at desirably high levels throughout the composting process (Figure 8, G & H). These loggers were placed in the composter at the start of treatment 27 (Table 3) in which food waste was combined with green-waste at a ratio of 2:1 (w/w), with 15% ‘carbon’ added in the form of compressed sawdust pellets. An important aspect of these temperature data was that there were regular fluctuations in temperature of between 10-20oC (Figure 8). These temperature fluctuations were associated with the composter turning (run) times when recorded temperatures often (although not always) declined. Interestingly, these temperature drops were not seen in the data collected from the composter sensors.

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These were normally near to the centre of the composting material, whilst the data-loggers moved freely throughout this material. Thus, the temperatures recorded by the data-loggers may represent conditions at or near the surface of the composting mass. In addition to these observations, the data-logger traces also illustrated that in general, when temperatures in the vessel were higher, the rate of composting was faster and the transit times of material through the vessel were shorter. One exception to this was the material loaded on 13/05/2007 (Figure 8I). This was a problematic period when the composter was mistakenly loaded with too much water-laden green waste material which led to a drastic reduction in pH at positions A and B (Figure 1) and the wet composting mass adhered to the vessel walls causing weight distribution problems. To deal with this situation, the flow of ‘finished’ material from the out-auger was increased to reduce the weight of material in the vessel, whilst some of this comparatively dry material was placed back in the vessel at the front end to help reduce the overall moisture content and raise the pH. The overall effect of these treatments was to increase the flow of material through the vessel whilst not greatly increasing the compost temperature in the shorter term (i.e. the period of transit for data-logger I, Figure 8). The possibility of localised high temperatures (>100oC) was identified with one data-logger (Figure 8H), and confirmed by the machine’s sensors at position E (Figure 1). This isolated observation was considered of importance as temperatures in excess of 100oC in a compost system carry the risk of chemical heating taking the temperature up to the point of spontaneous combustion.

Progress through the composter vessel:

The mean time for a data-logger to pass through the composter was 95 days (95% confidence limits = ± 18.5 ). This was somewhat faster than the times taken for the marker tags, which, depending on their times of placement, took between 150 and 255 days to pass through the composter vessel. There were no significant differences (P = 0.05) in the passage times of the three sizes (30, 60 and 90 x 25mm) of tag used Table 4). Importantly, there was no indication of reverse movement of composting material within the vessel (i.e. of ‘eddies’ within the composting mass). The main influences on passage time were the volumes of feed-stocks loaded into the vessel and the amounts of finished material removed at the out-auger. This can be seen in Table 4, where the transit times show a large amount of variation, whilst variation in the quantities of feedstock input over these periods was much less (63-96 kg day-1, mean = 78.8 day-1, Table 4). From this average value, an estimated annual feedstock input of 28.8 tonnes can be calculated. This estimated value can be increased, however, if peak performance, as seen in Figure 8 G & H, is assumed. Over this period, the mean feedstock input was 130 kg day-1, giving an annual capacity of 46 tonnes.

Throughput data can also be used to give an idea of the efficiency of the composting process in terms of weight reduction. However, the variation in feedstock treatments and input rates meant that estimates of the percentage weight reduction were only really meaningful if calculated over long periods. Over the period of one year from 01/04/2006 to 31/03/2007, 35.98 tonnes of material were fed into the composter (food waste = 22.91 t; green waste = 5.01 t; wood pellet = 3.07 t; returned finished Neter compost = 4.27 t; wood shavings = 0.2 t; card =0.1 t; paper = 0.08 t; paper towels = 0.03 t; straw = 0.02 t & ‘compostables’ = 0.29 t), whilst over the same period 9.60 tonnes were collected from the out auger, giving an annual weight reduction due to composting of 73.3%.

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Table 4: Transit times through the Neter 30 composter vessel for different sized plastic tags and TinyTalk dataloggers, and the weights of feedstock fed into the machine over these same periods.

Figure 5: Changes in temperatures at point A (Figure 1) over time, in comparison with maximum temperatures recorded for the entire vessel

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Figure 6: Graph showing the point in the vessel when the transfer between mesophilic and thermophilic (>45oC) temperatures occurred, the temperature recorded at this point and the % of the vessel at thermophilic temperature, over time.

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Figure 7: Effects of pH and % moisture content on the temperature of the compost mix at point A (see

Figure 1).

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Figure 8: Temperature profiles of compost ‘runs’ recorded by dataloggers introduced into the Neter composter vessel through Hatch 1 and collected at the out- auger (see Figure 1), at different dates during this study, from A, 28/04/06 to I, 18/05/07.

Bioaerosol tests:

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Counts of total bacteria: The total numbers of bacterial cfu m-3 isolated at all sampling stations, except sample 5, were all less than the nominal reference value of 1000 cfu m-3 (the value that constitutes a risk as proposed by the Environment Agency (Environment Agency, 2001)), and the majority were below 300 cfu m-3 (Table 5). The largest numbers of bacteria were isolated (1780 cfu m-3) at station 5, by the ‘finished’ compost bin with the cover removed, although with the cover in place this number was greatly reduced to an acceptable 167 cfu m-3. The other two stations where comparatively large numbers of bacterial cfu were isolated were 6 and 7, immediately above the food waste conveyor and at hatch 1 respectively, both during a food waste loading operation (Table 5).

Counts of total filamentous fungi: The numbers of filamentous fungal cfu m-3 exceeded the maximum reference value of 1000 cfu m-3 stipulated by the Environment Agency (Environment Agency, 2001) at three sampling stations (Table 5). These were; station 2 near the biofilter (1130 cfu m-

3), station 5, by the ‘finished’ compost bin with the cover removed (1160 cfu m-3), and station 6 above the food waste input (1736 cfu m-3). The station 5 count would appear to be the result of thermophilic fungi contained in the dust rising from the compost and counts were reduced to 430 cfu m-3 when the cover was in place, whilst station 6 count can be accounted for by the likely presence of moulds on bread and similar materials that would readily increase the concentration of the air spora. The high count at station 2, near the biofilter for extracted air was unexpected and might suggest that fungi growing within the biofilter are releasing small numbers of spores. This requires further ongoing assessment. In all cases the concentration of spores was considerably lower than the level of 5 x 104 cfu m-3 which might cause sensitisation, and under conditions of continuous exposure to which, work-related respiratory disorders in workers are very common (Dutkiewicz, 1997). However, the report from the University of Hertfordshire did recommend that it might be a sensible precaution for workers to wear face masks whilst working in the vicinity of the compost collection bin when the cover is off, at the feeding station, and when accessing the biofilter.

Counts of Aspergillus fumigatus: This species was isolated at all sampling stations tested. However, the counts found at all stations (Table 5) were significantly lower than the reference value of 1000 cfu m-3 stipulated by the Environment Agency (Environment Agency, 2001), and were largely well within the normal background range (1-100 spores m-3) seen in the air spora (Swan et al. 2003). The highest count (albeit quite low at 109 cfu m-3) of Aspergillus fumigatus propagules was from station 3, the filter tube. It is unlikely that the fungus could actively grow at the high temperatures and high vapour pressure deficit of the vent system, and this count possibly represents spores from the compost at the front end of the vessel.

Gas emissions:

The concentrations of oxygen and carbon dioxide in the exhaust air from the Neter 30 composter were well within expected and safe levels (Table 6). Minimal hydrogen sulphide was detected (2 ppm on 25/05/07; Table 6), and no methane was detected on any of the sampling dates. This tends to confirm that the composting processes within the Neter vessel were largely aerobic. The concentrations of carbon monoxide detected ranged from 0 – 174 ppm, with the higher levels (>11 ppm) detected only on the second sampling date (16/02/07; Table 6). An assessment of the concentrations of oxygen, carbon dioxide and carbon monoxide present at the hatches and in the compost carried out on 25/05/07 using a sampling tube attached to the gas analyser showed the levels before and after turning (Table 7), and how

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oxygen levels are replenished by turning. The percentages of carbon dioxide and oxygen were inversely proportional (%[CO2] = -0.8574.%[O2] + 19.08; R2 = 0.95) and mixing always increased the concentration of oxygen in the compost. This was illustrated by the higher concentrations seen in the compost below the spindle (the most recently tumbled compost) compared with that above the spindle (Table 7).

Table 5: Results of bioaerosol testing in and around the Waste Neutral site at Eden Project carried out on 28/06/07 to assess counts of propagules of total bacteria, total filamentous fungi and of Aspergillus fumigatus propagules per unit volume of sampled air.

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Table 6: Results of gas emissions tests carried out on the exhaust air from the Neter 30 composter on 7/02/07, 16/02/07 and 25/05/07.

Table 7: Proportions of oxygen and carbon dioxide, and concentrations of carbon monoxide detected within the vessel of the Neter 30 composter, before and after turning.

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Leachates:

There was very little leakage from the composter vessel. This occurred from two locations. The first was slight seepage from the edges of the front end plate of the vessel, and only when the % moisture of the composting mixture was too high. The leachate was contained by stainless steel trays located below the composting vessel, and was readily cleaned by flushing down the foul drain (Figure 9). The second location was not strictly leakage but was due to condensation of the humid air discharged from the composter in the air filtration system. This was drained from the filtration system via a condensation tube to the foul drain. The volume of condensate varied, depending on the moisture content of the composting mass in the vessel, but was approximately 300 ml day-1.

Figure 9: Photograph showing a small amount of compost leachate collected in the steel tray underneath the Neter 30 composter vessel.

Product variables:

Growing medium trials:

The detection of potential phytotoxic effects was a primary aim of the trials assessing the finished compost for use in growing media. No signs of phytotoxicity were found in any of the plant species (L. esculentum, C. annuum, L. sativa, B. campestris ssp. Pekinensis,) assessed in any of the plant trials carried out with Neter compost incorporated in the growing medium (including assessments for the presence of plant pathogens).

Measurements of plant growth in the Neter compost gave variable results. Growth in the L. esculentum trial was reduced by the Neter compost, alone and proportionally to its concentration in mixed media,

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when compared to, and mixed with, the Eden standard (EP mix 1) peat free medium (Figure 10A). However, growth in both L. sativa trials was enhanced by the Neter compost, either alone or in mixed media, (Figure 10B). Throughout all of these experiments, root growth relative to shoots was more or less constant with shoot to root ratios between 0.6 and 1.5. In the L. esculentum trial, root growth appeared strongly affected by the growing medium. Visually there appeared to be some degree of slumping with this particular batch of Neter compost. However, comparisons of AFP measurements at the end of the trial showed these were relatively high with no consistent pattern with the composition of the growing medium (0% Neter compost, AFP = 23.7%; 25% = 31.8%; 50% = 23.0%; 75% = 25.9% & 100% = 37.2% - by this method, Eden soils have on average AFPs between 20 – 30%). One other observation that may have had an impact was the formation of a hard, but thin, ‘skin’ on the surface of media containing this batch of Neter compost, unfortunately this effect was not quantified.

Figure 10: Effects of the amount of Neter compost in growing media consisting of mixtures containing varying proportions of EP mix 1 and Neter compost, on the growth of A) L. esculentum and B) L. sativa seedlings.

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Mulching medium trials:

When used as a mulch treatment, Neter compost was not found to be phytotoxic to any of the wide range of test plant species assessed in both pot and field trials. The species assessed were as follows: a) pot experiments; Capsicum annuum & Lactuca sativa; b) field trial; Prunus spinosa, Euonymus europaeus, Ulmus glabra, Cratægus monogyna, Ilex aquifolium, Ligustrum vulgare, Quercus robur, Rosa canina, Sambucus nigra, Sorbus aucuparia, Viburnum opulus & Acer campestre.

In pot experiments with peppers and lettuce plants no significant differences were seen in shoot growth between Neter-treated and Ecosci-treated plants (Figure 11). Only plant heights were recorded for the pepper plants, but at 0.206 m (± 0.019) for Ecosci-treated and 0.211 m (± 0.023) for the Neter-treated, these were significantly taller than the untreated controls at 0.144 m (± 0.029, 95% confidence limits). Interestingly, in the lettuce trial Neter mulch treatments significantly increased root dry weights in comparison with Ecosci mulches and untreated controls and this effect also made the shoot:root ratio significantly lower than the other two treatments (Figure 11).

Figure 11: Effects of mulching treatments with Neter compost in comparison with Ecosci compost and untreated controls, on lettuce root and shoot growth.

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In the field trial, 3 key species, Prunus spinosa, Euonymus europaeus, and Ulmus glabra, were also assessed for growth by measurements of increase in height between 22/05/07 and 16/07/07 (Table 8). Mean height increases were compared between the three treatments by t-test and found not to be significantly different (P = 0.05). As the species in the treated sections of the ‘Spiral Hedge’ are all hardy and long-lived, the observational work reported here will be ongoing. Mineral analyses of the soil under the spiral hedge were carried out at the start of this experiment and approximately two months later on 20/07/07. These indicated that neither the Neter nor the Ecosci mulches had much impact on the mineral composition of the underlying soil during this short period of investigation (Table 9).

PAS 100 tests:

Three finished compost samples from the Neter 30 composter have so far been sent for testing for PAS 100 compliance. Of these, two have passed (Table 10). The sample that failed the tests, did so because it contained an unacceptable level of contaminant particles (predominantly undegraded shreds of office paper) of size >2mm (1.33%; PAS100 maximum is 0.5%), was not sufficiently stable (21.2 mg CO2 g

-1 Organic Matter day-1; PAS100 maximum is 16 CO2 g

-1 OM day-1), and produced an unacceptably low amount of plant shoot growth (74.7% ; PAS100 minimum is 80%). Importantly, none of the samples sent for testing contained appreciable numbers of propagules of the test human pathogens E. coli and Salmonella spp. There were also no viable weed seeds detected and the levels of potentially toxic elements were well within acceptable limits. In addition to the main test criteria, analysis of the total mineral nutrients in acid extracts showed large variation between the three samples (Table 11). Although no obvious patterns emerged, sample 2 contained markedly higher concentrations of potassium, calcium, magnesium, iron and sodium (Table 11).

Table 8: Increases in plant heights (m) in the newly-planted ‘Spiral Hedge’ at Eden Project, following mulch treatments with either Neter compost or Ecosci composted green waste, in comparison with untreated controls (values in parentheses are 95% confidence limits).

Mulch Treatment Plant species

Control Neter Ecosci

Prunus spinosa 0.25

(± 0.37)

0.30

(± 0.69)

0.23

(-)

Euonymus europaeus 0.35

(± 0.09)

0.34

(± 0.30)

0.30

(± 0.17)

Ulmus glabra 0.48

(± 0.26)

0.40

(± 0.14)

0.46

(± 0.36)

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Simple microbiological assessments of composting material in the vessel:

Dilution-plating assessments of the total numbers of bacteria and fungi in the compost mix as it progressed through the composter vessel showed marked changes at the point in the vessel where temperatures rose about 50-55oC (Table 12). These changes included both a substantial decline in the numbers of cfu recovered per unit dry weight of compost sample together with a substantial shift in the types isolated. This decline was then followed by what appeared to be a re-colonisation by the new dominant populations. Interestingly, even the numbers of thermophilic bacteria appeared to decline in the first set of samples assessed (22/06/06, Table 12), although these organisms were seen to increase by an order of magnitude at temperatures >50oC in the later sample assessed (17/05/07, Table 12). Numbers of coliform cfu were low in samples taken from points X, A, B and C and these organisms were eliminated from the compost once temperatures >50oC were reached (Table 12). E. coli was only detected in one sample at one time, taken from point X, and even in this sample the numbers were very low and most likely originated from traces that might be expected on leaf surfaces in the green-waste feedstock.

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Table 9: Comparison of mineral analyses of soils from under the Spiral Hedge at Eden Project, before and after application of mulch treatments with Ecosci and Neter compost.

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Assessment of presence/absence of plant pathogens:

No plant pathogens have so far been detected in finished Neter compost, in either baiting bioassays or plating assays, nor has any evidence been seen of disease resulting from the compost in any of the growing medium and mulching trials.

Compost maturity:

In addition to the tests for compost maturity carried out for PAS100 compliance, the maturity of a limited number of samples of finished compost collected from the out-auger was assessed using the Solvita test procedure. The maturity of these varied from Solvita index 4 (still moderately ‘active’) on 03/05/07 to 6.5 (more or less ‘finished’ compost) on 16/05/07 (see Table 13). Interestingly, determination of the original feedstock treatments by extrapolation (assuming an average transit time through the composter vessel of 95 days; see Progress through the vessel above) showed all three samples to originate from treatments in category 3 (Table 3). The maturity of the last sample (16/05/07) was confirmed in the PAS 100 tests carried out on compost collected for analysis on this same date (Table 10). A Solvita index of 6.5 is just within the ‘finished compost’ range, whilst its stability in terms of CO2 evolution g-1 organic matter day-1 was 14.9; 1.1 units below the PAS 100 upper limit for stability.

Progress of a sample (2 m3) of compost through a period of static storage in a pile was monitored using Solvita tests to determine whether it was feasible to cure a Neter compost, at Solvita maturity 4, in a static pile with minimal management. Nine tests were carried out on samples taken from the stored static pile of compost over the period 03/05/07 to 12/07/07. During this period, indices for ammonia evolution progressed from 4.5 to 5 (a reduction from low to v. low/no ammonia), but carbon dioxide evolution remained steady at index point 4, giving an unchanged Solvita maturity index of 4 (compost in medium/moderate phase of active decomposition i.e. not mature) throughout the entire test period.

Bulk density:

Routine assessments of the wet bulk density of finished compost from the Neter 30 composter showed this to fall between approximately 350 and 550 kg m-3. Dry bulk density (the usual measure of soil bulk density) gives an impression of the possible reduction in particle size as a result of the composting process (reductions in particle size should give an increase in the dry bulk density), and so, was monitored in samples taken throughout the composter vessel on two sampling dates (Table 14). This showed increases in dry bulk density of approximately 45%, indicating a marked reduction in particle size, whilst the wet bulk density decreased by 20-30%, mainly due to the reduction in compost water content as composting proceeded.

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Table 10: Results of assessments of key parameters of three finished compost samples, from the Neter 30 composter, for compliance with British Standards Institution Publicly Accessible Specification 100:2005 (‘PAS100’).

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Table 11 Results of analysis of the total mineral nutrient concentrations in acid extracts of three finished compost samples, from the Neter 30 composter, carried out in parallel with tests (Table 10) for compliance with British Standards Institution Publicly Accessible Specification 100:2005 (‘PAS100’).

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Table 12: Results of dilution plating of compost samples, taken from points A-E, X and the out-auger (Figure 1), showing total numbers of culturable bacteria, filamentous fungi, thermophilic and coliform bacteria (cfu g-1 dry weight).

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Table 13: Assessments of ‘finished’ Neter compost maturity using the Solvita® compost respiration test.

Table 14: Determinations, made on two separate sampling dates, of bulk density of the compost mix as it progressed through the Neter 30 composter vessel.

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DISCUSSION

With the exception of an initial period of several months’ ‘teething problems’, throughout the majority of the period covered by this report the Neter 30 composter produced a finished compost that could be reasonably considered of ‘horticultural’ quality, being suitable for use in growing media and as either a soil additive or mulch treatment. This product consistently contained no plant pathogens, no E. coli, no weed seeds and minimal concentrations of potentially toxic elements. However, for long periods the composting process, as indicated by the temperatures recorded along the composter vessel, was not optimal. The main problem was a longer period of mesophilic conditions than expected at the front end of the composter (between points A and C, Figure 1). The resultant delay in the transfer to thermophilic conditions had two potential effects: i) a slowing of the overall rate of compost production and/or, ii) an increased risk of producing an un-finished, or immature compost. Of the three finished compost samples tested during this study for compliance with PAS 100, only one failed, but this sample failed on stability (maturity), suboptimal plant top-growth, and the presence of contaminant material in the form of uncomposted paper, possibly also as a result of insufficient time in thermophilic conditions, when cellulose breakdown would be most rapid (de Bertoldi et al., 1983). An additional consideration here is that even when optimal (thermophilic) conditions were attained at position A, the composting mix generally retained thermophilic temperatures along the entire length of the vessel. This indicates that there was still sufficient substrate available to maintain active metabolism/growth of the thermophilic microbiota in the compost at the out-end of the vessel, suggesting a high proportion of volatile solids in the feed-stocks, most likely derived from the food waste component. However, no problems with stability were experienced with the compost produced during these periods of optimal composting, so the maintenance of thermophilic temperatures at the back end of the vessel (position E, Figure 1) might be due to the inability of the heat to rapidly escape from the mixture in the vessel. Nevertheless, a temperature profile similar to that described as optimal by Susteco (2004), with thermophilic conditions rapidly achieved at point A and sustained until point C after which temperatures decline until mesophilic between points D and E, would be desirable from a practical ease-of-operation point of view. This might be achievable by adjusting the ratio of green waste to food waste fed into the composter to effectively reduce the volatile solids fraction. Unfortunately ratios of green waste to food waste greater than 1:2 were impractical in the present study, as the configuration of the front end did not readily allow the loading of large volumes of green waste. Adjusting the front end of the composter to allow feeding of higher proportions of green waste is a strong consideration for future work to optimise composting food waste with the Neter 30.

The pH and moisture content of the composting mixture were probably the most important factors in achieving optimal composting conditions in the Neter 30 system. As can be seen from Figure 7, these two factors are often closely related although the pH has been consistently the most difficult to control. The rapid development of low pH in composting systems is well known (Haug, 1993), especially in those treating food/catering wastes (Beck-Friis et al., 2003; Day et al., 1998; Smårs et al., 2002; Nishino et al., 2003; Sundberg, 2005). Generally this is considered to be the result of increases in the concentrations of volatile fatty acids such as acetic, lactic and butyric acids (Sundberg, 2005), although, as seen in the present study, the starting pH of catering waste can be quite low anyway, often in the region pH 5-5.5 (Day et al., 1998). Eklind et al. (2007) proposed that the increased acidity that rapidly develops in the mesophilic phase at the start of composting food waste is largely the result of the breakdown of

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carbohydrates often in high concentrations in this material. The concentrations of carbohydrates and sugars in the Eden Project catering waste have not been analysed, but concentrations, especially of the former, are likely to be high with large amounts of bread, pastry and pasta often present. Additions of green waste to this material will effectively dilute the carbohydrate concentration in the compost mix and this may be an important factor contributing to the improvements in compost temperature seen when green waste was added in treatments like T26 and T27 (Table 3). This also provides further justification for future attempts to increase the proportion of green waste material in the compost mix as considered above.

The temperature of the compost mix during the initial mesophilic phase is considered to have a strong effect on the duration of the transfer to thermophilic conditions (Smårs et al., 2002; Sundberg et al., 2004). If temperatures exceed 40oC too early, then microbial activity can stagnate or even decline resulting in a prolonged period at low pH with intermediate compost temperatures (35 - 45 oC). To keep temperatures down and possibly help volatilise some of the volatile fatty acid fraction, Smårs et al. (2002) successfully used forced aeration to increase pH and reduce the period of transfer from the mesophilic to the thermophilic phase. Unfortunately, when an increased air flow through the Neter 30 vessel was combined with an increased rate of turning, temperatures declined along the entire length of the vessel and problems occurred with excessive drying of the compost mixture at positions D and E. It was therefore decided that work should concentrate on optimising the mix of feed-stocks to achieve suitable pH conditions.

The pH of acidic compost can be increased by the use of lime, additions of which neutralise fatty acids without strongly affecting their availability for microbial metabolism (Witter & Lopez-Real, 1988). Lime was tested for a short period during this study, but the amounts added were not sufficient to have a strong effect on the pH at position A (Figure1) before additions stopped. The main objections to using lime were: i) cost – although inexpensive, lime still has to be purchased, ii) safety issues associated with storage, handling and dispensing the lime, and possibly most importantly, iii) the use of lime in composts increases nitrogen losses as ammonia, sometimes by as much as 60% (Nakasaki et al., 1993; Witter & Lopez-Real, 1988).

Poor aeration is another possible factor in the development of high acidity in the early stages of composting in the Neter 30, as organic acids can be rapidly generated in an anaerobic composting environment (Miller, 1993). This possibility is raised by the pulp-like appearance of shredded food when introduced into the composter vessel (Figure 4). The shredder face has been set with a 20 mm gap, and produces a food macerate with little or no air porosity. However, the addition of compressed sawdust pellets to this mixture rapidly dries it and, with the expansion that results from re-hydration of the pellets, opens up the mixture to aeration. In addition to this, analyses of gaseous emissions from the composter vessel and directly from the composting mass showed no evidence of anaerobic conditions within the vessel.

The shredder face was originally fitted to the front end of the machine to comply with ABPR (HMSO, 2005), which stipulate that if particle sizes within a closed composting vessel are less than 60 mm, then the compost needs to attain a minimum temperature of 70oC for 1 hour or more followed by a gap and then a second similar treatment or one of two other sets of conditions (either >2days at >60oC in a closed vessel or >8 days at >60oC in a windrow or pile). Whether this is possible within a single closed vessel,

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like the Neter 30, engaged in continuous processing of material, or the composting material needs to be transferred from one system to another for the two periods of high temperature is not clear. Certainly with single-vessel continuous-process reactors there is the possibility of microbially contaminated leachate moving from the mesophilic front end towards the back of the vessel where sanitised compost that has passed through thermophilic conditions resides. However, this is highly unlikely as the Neter 30 system produces little or no leachate, the composting mass gets progressively drier as it passes down the vessel, and if any leachate did move towards sanitised material, it would be very likely to reach thermophilic temperatures of the recommended duration on the way. The small numbers of microbial assessments so far carried out on the contents of the Neter 30 bear this out by showing no evidence of re-inoculations of material that has passed through thermophilic conditions.

An alternative to the two treatments required for ABPR is the single period of ‘sanitising’ thermophilic conditions required for compliance with European Union Commission Regulation (EC) No 208/2006 (EU; Commission of the European Communities, 2006) which stipulates that: ‘material used as raw material in a composting plant must be submitted to the following minimum requirements: (a) maximum particle size before entering the composting reactor. 12 mm; (b) minimum temperature in all material in the reactor at 70oC (all material): 60 minutes’. A potential problem is the statement ‘all material’ which presumably applies to batch processors as opposed to continuous processors like the Neter 30 where it would be impossible to maintain 70oC throughout the entire vessel at one time. The descriptions of particle size in the regulations imply particles of diameter x. However, a clearer explanation of this is given by The Community Composting Network (2004); ‘particle size must be guaranteed but it only has to be met in one direction - for example to meet the 70oC UK (ABPR) standard, material 100 mm long by 150 mm wide is okay as long as it is only 60 mm thick. The shredder face currently fitted to the Neter 30 can easily achieve this standard.

An additional question as to how the ABPR should apply to rotating closed composting vessels is posed by the fact that temperatures can fluctuate markedly at each turn of the vessel (e.g. see temperature logger traces for the Neter 30, Figure 8). Under such conditions it should possible to generate temperatures >70oC for >1 h, but maintaining >60oC for longer than 2 days throughout the compost would be difficult to achieve. However, assessments of the microflora within the compost and records of compost temperatures under optimum conditions in the Neter 30 (e.g. during treatments 26 and 27, Tables 3 & 12) indicate a strong possibility that this type of rotating vessel is capable of achieving the kind of compost sanitation the ABPR were designed to achieve, if optimum composting conditions can be reliably maintained.

Unfortunately, it is not straightforward to verify the sanitary efficacy of systems like the Neter 30. Of the physical parameters of composting, temperature is clearly the most reliable for this purpose (Christensen et al., 2002), but our understanding of how temperatures affect the sanitisation process is still incomplete. Currently, the most reliable overall approach appears to be spot checking samples at various stages within a processor and measuring the populations of ‘indicator’ micro-organisms present (or absent), although the direct approach of spiking systems with consortia of test organisms, usually in pervious containers, to test their survival, can also give valuable information on process dynamics and efficiency (Christensen et al., 2002; Lemunier et al., 2005). In the present study the microbial populations detected in the compost were low compared to other systems (Christensen et al., 2002; Lemunier et al., 2005), especially

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populations of the indicator species E. coli and total populations of coliform bacteria. As might be expected considering the feed-stocks used, the coliform populations were initially low in the Neter 30 system at Eden. They were effectively eliminated during thermophilic conditions with no resurgence in population in the later phases of composting despite, or maybe because of, large overall increases in bacterial numbers seen at this stage, a result in keeping with findings from other systems (Ishii, 2000; Ryckeboer et al., 2003).

Moisture extremes can impede the aerobic composting process. As considered above, excessive moisture inhibits aerobic metabolism by restricting the diffusion of oxygen, but this is reasonably straightforward to avoid by management of feed-stocks. Drying of the compost mixture can occur later in the composting process, indeed in bio-drying systems this process is actively encouraged (Skourides & Smith, 2007), and this can also inhibit the composting process as bacteria are progressively unable to colonise the compost substrate as the matric potential decreases below about -20 kPa (Miller, 1989). In the Neter 30 system, drying of the compost was often observed at positions D and E., and appeared to coincide with sub-optimal conditions at the front end. These sub-optimal conditions necessitated a longer residence time in the vessel, resulting in prolonged exposure to low thermophilic temperatures, air flow and turning. This could be a contributory factor in the instability occasionally seen in the ‘finished’ compost. Moisture contents approaching 20% w/w at the back end of the vessel caused temperatures to fall in this area, with rewetting often raising the temperature substantially. The Neter 30 has a facility for adding water to the compost (see ‘water inlet’, Figure 1B), although the lack of a fail-safe cut-out on this system has meant that its use has been avoided because of the practical consideration of the risk of leaving the tap on! Generally, when the system was running optimally, the transit time for the compost mix through the vessel meant that excessive drying was avoided.

The carbon to nitrogen ratios of the compost mixtures used in this study were well within the limits considered optimal for aerobic composting (30-35:1, McGaughey & Gotaas, 1985). However, as indicated by the rapid development of acidic conditions considered above, the food waste contained a high proportion of readily available carbon. This was partly diluted by the addition of compressed sawdust pellets which also provided considerable improvements to the physical condition of the composting mix. Unfortunately buying-in this feed-stock incurred an extra cost but, so far, no reliable alternative carbon source has been found with the same drying and structural qualities and ease of handling as provided by the pellets. Additions of ‘end product’ or finished compost provided some improvement in the structure and moisture content of the composting mix, and with a pH of 7.5 to 8.5, this material also neutralised some of the fatty acids produced in the food waste. An important consideration with the use of end product as a feed-stock is its potential as a ‘starter culture’ containing large quantities of inoculum of both mesophilic and thermophilic organisms (Nakasaki et al., 1985). This could provide a boost to the low levels of these organisms likely to be present, especially in the food waste and sawdust pellet feed-stocks. In other systems, such a starter culture effect from using end-product type materials has significantly accelerated the mesophilic phase and the transfer to thermophilic composting (Nakasaki & Akiyama, 1988; Nakasaki et al., 1992), especially if inocula of certain groups of organisms could be established within the system, such as acid tolerant thermophiles or acidophilic yeasts (Choi & Park, 1998). Clearly further microbiological study is required to understand the use of end-product as a feedstock in the Neter 30 system more fully, although routine incorporation of end product as a feed-stock has worked well so far and causes no serious operational problems.

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The compost produced by the Neter 30 has been used successfully at Eden as a growing medium supplement and, as a soil improver, it compares well with the Ecosci ‘West Country Compost’ used by Eden Project in the manufacture and mulching of its soils. Generally the mineral composition of the compost agrees with the observations of Tompkins (2006) of food waste-derived composts, with low levels of calcium, magnesium and sulphur and some trace elements such as iron and manganese, and high levels of potassium, sodium and nitrogen. With high levels of nitrogen, the compost needs to be treated with some caution as a soil additive, although it should be useful as a fertiliser substitute once its nitrogen release characteristics are more clearly understood. High concentrations of potassium and sodium could cause problems with ‘lock-out’ of calcium and magnesium and, whilst no deficiency symptoms have been seen in tests so far, this needs to be carefully monitored now that the compost is being used on the main Eden site. Tompkins (2006) recommended co-composting food waste with green waste to ameliorate some of these deficiencies and, possibly with the changes in the proportions of green waste to food waste in the compost mix proposed above, the Neter 30 compost mineral composition will be further improved.

Despite some of the problems with optimisation, the Neter 30 composter produced compost from food waste that was compliant with PAS 100. The process has been demonstrated to be operationally safe, with the minimum of bioaerosol, and no noxious gas emissions. The process, as it currently stands, has provided a significant mass reduction in Eden Project’s waste stream and turns waste into a useful product. However, the machine is still not operating to its full potential. With delays in the transfer from mesophilic to thermophilic conditions at the front end, the overall rate of composting is reduced, meaning that transit times are increased and therefore the amounts of waste processed are reduced. The system still needs some fine tuning, especially with the balance of different feed-stocks in the initial compost mix, and possibly in the way they are prepared. This may mean some alterations to the current front end configuration to allow the introduction of more shredded green waste. The results of the work so far have not resulted in a firm protocol for feeding the machine but if this can be reliably established and the sanitary effects demonstrated, then it will be worth applying the principles of HACCP (Evans, 2003) to its operation and compliance with ABPR should be possible.

ACKNOWLEDGEMENTS

BOC, Viridor, Uncle Tom Cobley etc.

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