Climate Change Impacts Of Food Waste Diversion To Anaerobic Digesters1

Evans, T. D.

TIM EVANS ENVIRONMENT, Stonecroft, Park Lane, Ashtead, KT211EU England

tim@timevansenvironment.com

ABSTRACT

Treating food waste (FW) by anaerobic digestion to recover biogas and biofertiliser is a fantastic

opportunity. The biogas is renewable, base-load, non-fossil energy. Using the digestate on land

as biofertiliser completes nutrient cycles and conserves soil organic matter. The Part-503 rules

protect health and the environment. Existing sludge digesters can be „turbo-charged‟ by

retrofitting thermal hydrolysis, which allows the solids loading to be doubled or trebled, the

specific biogas yield to be increased and the digestate, which will be Class-A, to be dewatered

better.

FW amounts to about 20% of municipal waste. It has a high moisture content (~70%),

biodegrades rapidly and has weak structural strength. When it is stored it therefore tends to

slump, ooze leachate, attract vermin and become smelly. When FW is mixed with other [dry]

wastes it sticks to them and contaminates them, which compromises mechanical sorting and

separating for recovering dry recyclables. Because of the high moisture content, FW reduces the

calorific value and energy recovery from incineration; it also increases the volume of emissions

that have to be treated. Curbside collections have to be frequent in warm weather if odor

complaints are to be avoided. Centralized anaerobic digestion has a better global warming

potential (GWP) than centralized composting. Home composting is a good solution but only a

small proportion of households are willing or able to practice it. The GWP of food waste

disposers (FWD) with sludge digestion is similar to centralized digestion. FWD are

commonplace in the USA and New Zealand (where 50% and 34% of households have them

respectively), whereas in the EU (European Union) fewer than 5% of households have them.

Where they are unfamiliar, FWD are contentious because of concerns about water use, energy

use, influence on FOG accumulation and effect on wastewater treatment and biosolids

production. None of the research published on FWD substantiates these concerns. The

additional carbonaceous load helps feed biological nutrient removal. The additional cost of

wastewater management is less than the saving on solid waste management, but there is a

question of fair reimbursement. This paper reviews the options for managing FW, their costs

and their environmental footprints. It includes the benefits of recovering N and P fertilizer from

dewatering liquor by physic-chemical stripping.

KEYWORDS Ammonia stripping, anaerobic digestion, biofertilizer, biogas, biosolids, composting, digestate,

FOG, food waste disposer, global warming potential, incineration, pasteurization, landfill, peak

phosphorus, recycling, struvite, thermal hydrolysis

1 Proc. WEF Residuals & Biosolids Conference, Portland, OR, USA May 2009

INTRODUCTION

In 1800 the world‟s population was about 1 billion people. By 1900 it had increased to 1.6 bn; in

1950 it was 2.5 bn; today it is 6 bn and in 2050 it is expected to stabilize at about 9bn (UN, 2004)

Figure 1. For the first time in history, more people now live in urban areas than in rural ones.

Not only has the world‟s population grown rapidly, but those in countries with growing

economies (Brazil, Russia, India, China, etc.) want more food and more animal products.

Producing animal products is inefficient in both energy and nutrient conservation. The cereals to

meat conversion ratios in intensive animal husbandry are 3:1 for poultry, 4.5:1 for pork and 6:1

for red meat (Steen, 1998). All of this means that we shall need to grow more food, and this

food with its embodied nutrients and carbon, will be transferred from rural soils to urban

population centers and food processing facilities. At the same time as we shall be demanding

more from agriculture, the area of farmable land is expected to decrease because of climate

change and one of the three major plant nutrients (phosphorus) is running out.

Figure 1 World human population - millions (UN, 2004)

Peak phosphorus

In the 19th

Century, Europe‟s agricultural potential was limited by phosphate, because its soils

had been farmed for centuries. Much of the food was imported from the “newly discovered”

continents where it was grown on the fertility accumulated over centuries under natural

vegetation.

“The phosphorus content of our land, following generations of cultivation, has greatly

diminished. It needs replenishing. I cannot over-emphasize the importance of

phosphorus not only to agriculture and soil conservation, but also the physical health

and economic security of the people of the nation. Many of our soil deposits are deficient

in phosphorus, thus causing low yield and poor quality of crops and pastures…”

President Franklin D. Roosevelt (1938) quoted by Barnard (2007)

0

1000

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5000

6000

7000

8000

9000

10000

1750 1800 1850 1900 1950 1999 2050 2150

Oceania

Northern

America

Latin

America &

CaribbeanEurope

Asia

Africa

Today there are no new worlds from which we can milk fertility and mineral fertilizer supplies

the gap between crop-offtake and the sum of returns, fixation and mineralization. 80% of the

atmosphere is nitrogen, so it is not in short supply and there is the real possibility of engineering

symbiotic N-fixation into crop plants other than legumes. Phosphorus is the least abundant of

the major plant nutrients (Table 1Error! Reference source not found.); there is already talk of

“peak phosphorus” (Cefic, 2008) and predictions that today‟s resources will be exhausted in 65

years at the current rate of exploitation; we might find another 200 years of reserves (Heffer et al.

2006). Phosphate is essential for all living cells.

“…life can multiply until all the phosphorus is gone, and then there is an inexorable halt

which nothing can prevent…. We may be able to substitute nuclear power for coal, and

plastics for wood, and yeast for meat, and friendliness for isolation - but for phosphorus

there is neither substitute nor replacement.”

Isaac Asimov, “Asimov on chemistry” (June 1974) Doubleday Company, New York

Table 1 Abundance of some elements in the earth's crust* (CRC, 2005)

Element Concentration, mg/kg m/m % of crust

1 Oxygen O 4.65 x 105 46.500

2 Silicon Si 2.82 x 105 28.200

3 Aluminum Al 8.23 x 104 8.230

4 Iron Fe 5.63 x 104 5.630

5 Calcium Ca 4.15 x 104 4.150

6 Sodium Na 2.36 x 104 2.360

7 Magnesium Mg 2.33 x 104 2.330

8 Potassium K 2.09 x 104 2.090

SUB-TOTAL 99.5%

9 Titanium Ti 5.65 x 103 0.565

10 Hydrogen H 1.40 x 103 0.140

11 Phosphorus P 1.05 x 103 0.105

12 Manganese Mn 9.50 x 102 0.095

13 Fluorine F 5.85 x 102 0.059

14 Barium Ba 4.25 x 102 0.043

15 Strontium Sr 3.70 x 102 0.037

16 Sulfur S 3.50 x 102 0.035

17 Carbon C 2.00 x 102 0.020

* Note these are the composition in and percentages of the crust not including the atmosphere

Phosphorus economics

Like farm animals, humans are inefficient users of dietary phosphate; we retain 2% and excrete

about 98% of the P in our diets, which is about 1.2-1.4 gP/capita.day, to this we can add 1.3-1.8

gP/capita.day from other household and urban sources (Smil, 2000). The combined average

appears to be about 1 kgP/capita.year; world production is about 15 million tP/year, which for 6

billion people is 2.5 kgP/capita.year. The P in farm animal manure in the UK equates to about

1.6 kgP/capita.year (from Chambers and Chadwick, 2008). Wastewater treatment can recover

95% of the P from urban wastewater and concentrate it into the sewage sludge that, after

appropriate treatment, can be applied to land as nutrient-rich soil improver (biosolids). 80% of

the mined phosphate rock is used as fertilizer, so it seems to make sense to use the P recovered

from wastewater as fertilizer, where we can, rather than trying to refine it further so that it can

replace the other uses of P (detergents 12%, animal feeds 5%). The exception is incineration and

other destructive processes in which case the P should be recovered or the ash, etc. stored for P-

recovery when resources are even more exhausted. Phosphorus is too precious to squander.

More people wanting more per-capita agricultural production from a declining area of farmable

land and a dwindling supply of one of the major plant nutrients sounds apocalyptic. It is a clear

sign that as part of our coping strategy we need to complete the nutrient cycle and return

phosphate in manure, food waste, sewage sludge and other organic residuals to the areas where

crops are grown.

Food Waste

Food waste (FW) biodegrades readily. Generally, it has a high moisture content (typically >70%

moisture) and a weak structure. Thus, it tends to slump, become anaerobic and putrid if stored.

Frequently, it is attractive to birds, animals and insects (gulls, crows, rats, foxes, flies, etc.),

which can be vectors for spreading disease. There are reports that storing FW indoors poses a

respiratory risk to susceptible individuals (Wouters, 2000) and has even been attributed to an

increase in botulism (Böhnel, 2002).

Fat, oil, grease and so-called “brown grease” from traps and interceptors (FOG) could be

included in the category of food waste but the properties and treatment requirements for these

hydrophobic materials are very different from the general run of food wastes. FOG has special

characteristics that mean that it should be managed separately as far as is practicable. It can be

converted to fuel oil (diesel) or (via anaerobic digestion, AD) to biogas with such high energy

yields that these are the best routes, financially and for Global Warming Potential, GWP (Figure

2). In Denmark, where biogas from co-digestion has been part of the energy strategy for several

years, biogas plants welcome FOG to such an extent that some pay to receive it, because of the

high biogas yield (Evans et al., 2002). We do not want FOG in sewers, if there was a FOG

removal and treatment service (which need not be free of charge), catering facilities would have

less difficulty keeping FOG out of sewers and AD operators would benefit from the biogas – an

all win deal.

Figure 2 Biogas yield - Nm3 per tonne of substrate (from Yeatman, 2007)

GWP is expressed as carbon dioxide equivalent (CO2e) over 100 years. Methane (CH4) is

estimated to have a GWP of 25, where carbon dioxide (CO2) is 1; nitrous oxide (N2O) has a

GWP of 298 (IPCC, 2007). N2O is a “leakage product” from two steps in the nitrogen cycle:

nitrification (ammonium to nitrate) and denitrification (nitrate to nitrogen gas). This paper does

not include N2O when estimating GWP because generic emission factors are uncertain and errors

multiplied by 298 could have a disproportionate and distorting effect.

When considering the carbon footprint, the direct CO2 evolution from treating food waste is of

no consequence to GWP because it is short-cycle CO2, i.e. it was atmospheric CO2 in the recent

past, before it was fixed by plants, and therefore returning it as CO2 as a result of treating FW

has no effect. However, escape of CH4 from whatever source does have GWP because it has

been transformed from the CO2 that was fixed by plants, into CH4, which was not a constituent

of the original atmosphere. CO2 from fossil fuels contributes to GWP because the carbon has not

been in the earth‟s atmosphere for millions of years.

This paper uses the EU-average electricity generation emission factor which (cited by Smith et

al., 2001) is 0.45 kg CO2e /kWh (range coal = 0.95 to wind = 0.009 kg CO2e /kWh). The large

proportion of nuclear power in France influences this average strongly.

Biosolids (Treated Sewage Sludge)

The main theme of this paper concerns climate change impacts of diverting FW to AD and since

many wastewater treatment works (WwTW) have AD and co-digestion of sewage sludge with

FW is one of the options, it is appropriate to consider biosolids briefly with up to date

information.

The principle of treating urban wastewater is to produce water, which is safe to return to the

environment, and biosolids, which contain the solids, biodegradable matter and nutrients.

Inevitably, traces of nearly all the chemicals used in society can be found in biosolids, but as

Paracelsus said 500 years ago, it is the dose that makes the poison, not the mere presence of a

substance. In the absence of a pathway to deliver a substance to a receptor or if the dose

delivered is less than the harmful dose, there is no risk.

Chemical contaminants from industrial sources have been controlled at source by co-operation

between industry and wastewater operators, facilitated by legislation (Figure 3 left). Source

control has meant that biosolids are typically 30-50 mgCd/kgP2O5, i.e. comparable to mineral

fertilizer; it is also worth noting that the Cd in biosolids is already in the anthropogenic cycle, not

a new introduction. Other chemicals have been eliminated by dangerous substances legislation,

e.g. CEC, 1985 (Figure 3 right). “Heavy metals” and persistent organic pollutants might have

been a problem in the past, though no adverse health effects have been found even in farm

families exposed to prolonged use (Sherlock, 1982 and USEPA,1985), but the risks from today‟s

biosolids used according to today‟s rules (CEC, 1986 and USEPA, 1993) are very small (about

10-6

).

Endocrine active substances (EAS) are “substances of concern”, but in the context of agricultural

soils, manures can have much greater concentrations than biosolids (Hanselman et al., 2003).

Legumes are significant contributors to the agricultural background of EAS because they are rich

in phytoestrogens, especially soya, which coincidentally is promoted as a „natural‟ alternative to

pharmaceutical hormone replacement therapy because of its large concentration of

phytoestrogen.

The food industry has been plagued by an on-going succession of food related illnesses that have

resulted in harm [occasionally death] to customers, product recalls, litigation and financial

collapse of some major companies. E. coli O157:H7 has been the most frequent and notorious,

but there have also been large Salmonella, Listeria and Campylobacter occurrences. At the time

of writing, sales of peanut products were down 25% because of Salmonella contamination at a

factory in GA. Mostly these events, especially E. coli O157, are traced to manure. None has

been traced to biosolids.

Biosolids are valuable sources of N, P and S with useful amounts of K, Mg and trace elements.

The risk of accumulating pollutants in soil and transferring them to crops, animals and the

environment has been controlled by limiting inputs at source and by regulating application rates.

The risk of disseminating disease is controlled by the dual barriers of post-application cropping

rules and/or treatment processes. The USEPA and the European Commission have both said

repeatedly that there have been no cases of adverse effect where biosolids have been used on

farmland according to the rules. Using biosolids on land is the best way to recycle P in

wastewater and conserve the planet‟s P-resource.

GLOBAL WARMING POTENTIALS OF ROUTES FOR FOOD WASTE

When estimating global warming potentials the boundaries have to be drawn somewhere; in this

paper the carbon embedded in the „infrastructures‟ have not been included. For example the

carbon embedded in the materials and manufacture of FW bins, disposers, digesters, trucks, etc.

has not been included. Had it been considered, there would have been the added complication of

whether and how much of the embed C would be recovered if elements of this infrastructure

were recycled.

As explained earlier, the very powerful greenhouse gas nitrous oxide (N2O) was omitted from

the estimates because the quantities are small relative to CO2 and CH4 but the uncertainties are

very large and multiplying these by the 298 weighting factor could have been very misleading.

Some N2O is released whenever ammonium is biologically oxidized to nitrate or nitrate is

biologically reduced to nitrogen gas, i.e. when solids are composted, wastewater is nitrified or

denitrified, soil is plowed or becomes waterlogged, ammonia or urea fertilizer is used. It is a

limitation but probably a relatively trivial one that will not change the overall ranking.

Home Composting

Home composting and use of the compost on site has the benefit of proximity but the variability

of performance of the practice means that estimating GWP is probably meaningless. The GWP

of curbside collection and of centralized treatment are avoided, as are their costs. Composting

transforms the most readily biodegradable carbon to short-cycle CO2 (mainly) but there is also

some CH4. CH4 might be oxidized at the surface of the compost heap/bin, depending on the

Figure 3 Changes in the concentrations of zinc and cadmium (left) and dioxins and

furans (right) in the biosolids of a large WwTW in West London

quality of composting. Nutrients are retained, apart from about half the nitrogen, which is lost as

ammonia (mainly) and some N2O (when nitrification and denitrification occur).

Good composting (i.e. aerobic) requires an adequate ratio of carbon to nitrogen in the feed

material, and open structure to allow the passage of air and adequate moisture to maintain the

biomass. Many people in urban areas either do not have the facilities (no gardens or

carbonaceous bulking agents) or inclination to practice home composting. An alternative is

needed for these people.

Frederickson, et al. (2006) reported the GWP of vermistabilization after windrow composting

was 508 g CO2e / t.day whereas the windrow composting had been only 17 g CO2e / t.day. They

attributed the large GWP of vermistabilization mainly to N2O released by the worms. The

stabilized organic matter feeds soil biomass and promotes soil structure; there is some net

sequestering of carbon in soil.

The efficiency of aerating home composting systems varies from site to site. Undoubtedly, some

home composting will be oxygen deficient and consequently will emit CH4 and N2O.

Frequently, compost worms are abundant in the later stages of home composting and they are

emitting N2O, presumably. These unintended consequences of home composting are impossible

to quantify with certainty.

Landfill

Modern efficient landfills capture a lot of the landfill gas (which is 50-65% CH4) and use it as

renewable fuel. The CO2 from burning this CH4 is short-cycle. However, there is some leakage

from the capped area and from the working area of the landfill before it is capped and managed

for landfill gas capture. The conventionally accepted CH4 leakage (Smith et al. 2001) has been

used in this paper, though it could be disputed.

As a generalization, the GWP of curbside collection of FW as part of mixed waste can be

assumed to be less for landfill than for separate collection for composting or AD. The following

discussion applies to a modern landfill site that has been constructed and managed to best

practice standards with efficient landfill gas collection and use of that landfill gas for electricity

generation. When biodegradable (putrescible) waste is placed in a landfill, the first stage of

degradation is aerobic; this releases short-cycle CO2, which has no GWP.

When the available oxygen has been used, degradation becomes anaerobic; initially the pH

decreases because of VFA (volatile fatty acid) production, this mobilizes metals. Later,

methanogenic bacteria develop and convert the VFAs to landfill gas; metals are re-precipitated

as the pH increases.

Even the best techniques of landfill construction and landfill gas pumping result in some landfill

gas leakage, and since this is 50-65% CH4 by volume the GWP is very significant. On the

positive side, landfills sequester significant amounts of carbon. The components of GWP from

landfilling biodegradable waste are shown in Table 2.

Table 2 GWP components of curbside collecting FW as part of mixed waste and disposing

of it in landfill (after Smith et al., 2001)

Component GWP kgCO2e/t FW

mixed waste collection and conveyance +14

electricity generation from landfill gas -32

short-cycle carbon sequestration -272

fuel use within the landfilling operations +8

methane from leaking landfill gas +1114

GWP for the landfilling route +832

In addition to sequestering carbon, landfills sequester/squander the nutrients embodied in the

input materials. It is doubtful whether the concentration of P in mixed landfill will ever be

economically viable to mine but eventually it would probably be viable to recover P if ash were

mono-filled, i.e. stored pending the day when we have drawn the earth‟s resources down

sufficiently low.

Incineration

Incineration is attractive to householders and industry because of its practicability. It is not

subject to the problems of physical contaminants (plastics, glass, metals, rags, etc.) that are

significant for the other routes. The cities of Aarhus in Denmark and Rotterdam in the

Netherlands both decided in 2006 to stop composting separately collected domestic and

supermarket FW because of the unacceptably high contents of physical contaminants; they

decided to collect and incinerate the mixed wastes instead. Whilst Danish and Dutch citizens

accept incineration and appear satisfied that emissions are controlled adequately, this is not the

case in the UK where a significant proportion of the public is opposed to incineration; I think it is

similar in North America.

The cost of operating waste incinerators and their emission controls is expensive; water vapor

from FW adds to the volume of emission. The value of electricity and heat from burning [wet]

FW (or sewage sludge) is relatively trivial. Smith et al. (2001) found incineration was one of the

more expensive options for whole MSW (municipal solid waste); the putrescible fraction has the

lowest net calorific value of any of the combustible fractions confirming that offset income from

energy generation would be negligible. In contrast, dry MSW, without FW, has a very useful

energy yield.

Table 3 GWP components of curbside collecting FW as part of mixed waste and disposing

of it via incineration and landfilling the ash (after Smith et al., 2001)

Component GWP kgCO2e/t FW

mixed waste collection and conveyance +14

incineration incl. emission clean-up, offset by energy

use -2

delivery of ash to landfill +1

GWP for the mixed collection and incineration route +13

Centralized Composting

Separate curbside collection of FW has a larger GWP than mixed waste collection because of

longer collection rounds and/or lower payloads and the imperative of more frequent collection to

avoid odor nuisance. Composting curbside collected FW in the EU must conform to the Animal

By Products Regulation (CEC, 2002) [ABPR]. ABPR were designed to prevent the transmission

of diseases (e.g. foot-and-mouth disease also called hoof-and-mouth disease [Aphtae

epizooticae]) from wastes to farm animals.

A survey of 16 in-vessel composting plants in NL (Wannholt, 1998 cited by Smith et al., 2001)

found the energy consumption of in-vessel composting (not ABPR) to be 40 kWhe per tonne of

waste, i.e. 18 kg CO2e/tonne at the EU-average power emission factor. This average of the 16

plants surveyed includes the use of gas cleaning systems to remove odor emissions as well as the

electricity used for blowing air to aerate the piles and maintain correct temperature and humidity.

The additional requirements of ABPR would probably result in somewhat greater energy use

because ABPR defines pre-shredding and two stages of treatment to prevent by-pass. Methane

produced in microzones in the compost is oxidized to CO2 in the surface layers of the

composting material or in the biofilter, thus emission is not significant for practical purposes.

The question of occupational health issues related to composting has been debated for several

years. Bünger et al. (2007) reported significant impairment of lung function etc. of compost

workers, compared with office workers; they attributed this to exposure to dust and bioaerosols

containing pathogens, glucans and allergens. This reinforces the advice to monitor workers

subject to such occupational exposure for the sake of their own health and to protect employers

from possible claims for industrial injury.

Smith et al. (2001) estimated that allowing for the decay of compost added to soil over 100 years

(the conventional time scale for GWP calculation) the use of compost on land would sequester

the equivalent of 22 kg short-cycle CO2 /t FW treated by composting. They also estimated 36

kg CO2e avoided /t waste for the fertilizer replacement value of the compost; they probably

overestimated the nitrogen replacement value but it is good enough for this exercise.

Curbside collection of garden waste with FW has been shown in many municipalities in many

countries to increase the total mass of waste collected because it disincentivizes home

composting. Compost can be manufactured into competitive growing media (Evans and

Rainbow, 1998) but it is never going to earn a lot of money because of the cost of blending,

bagging, distribution, marketing etc..

Table 4 GWP components of curbside collecting source-segregated FW, treating it by

composting and using the compost on farmland

Component GWP kgCO2e/t FW

separate curbside collection and conveyance +24

in-vessel composting incl. electricity +18

delivery from composting site to land application +2

carbon-sequestration in soil -22

fertilizer offset -36

GWP for separate collection and centralized composting -14

Centralized Anaerobic Digestion

AD has several practical and revenue advantages for separately collected FW:

a) Whereas composting converts biodegradable carbon to CO2 (using energy), AD converts

it to biogas, which is about 65% CH4 and 34% CO2 with traces of other gases. The CH4 is

contained and can be used as renewable energy, i.e. it has a negative GWP contribution

(because of offsetting fossil fuel) and a significant income generation potential from sales

of electricity or biomethane. Mesophilic anaerobic digestion (MAD) at 33 to 40 °C is

stable and reliable.

b) Operators in Denmark, Germany and Norway have al found that physical contaminants

(glass, metal, plastic) are inevitable in source-segregated, separately-collected FW.

Riedel (2008) reported that when separate collection was introduced in Germany,

household FW contained only 4% contamination, but, “now that the initial enthusiasm

has worn off”, this has deteriorated to 20%. The same pattern besets centralized

composting of FW.

c) With the cooperation of wastewater operators, it would be possible to “turbocharge” the

AD infrastructure that already exists at the larger wastewater treatment works (WwTW),

which would obviate many of the planning issues of developing a treatment site de novo.

The factors that might make this interesting to wastewater operators could be financial

and transforming the biosolids to Class-A, plus better dewatering, but regulatory issues

might make it unnecessarily difficult.

The yield of biogas depends on the makeup of the material being digested (Figure 2). Thermal

hydrolysis (TH) pressure-cooks the feed at 160 °C for 30 minutes, which increases the

digestibility of the organic matter, sterilizes the feed and reduces its viscosity to such an extent

that the solids loading can be trebled and the digesters continue to be fully mixed, i.e. the

capacity of existing digesters could be trebled by retrofitting TH (Evans, 2003). TH exceeds the

time-temperature requirements for ABPR. The digestate from TH + MAD dewaters much better

than from other MAD configurations (Evans, 2006). The combined effect of increasing organic

matter conversion and better dewatering means that, for the same feed solids, the mass of cake is

halved compared with „conventional‟ MAD. Expressed another way, the solids fed to a digester

could be doubled by retrofitting thermal hydrolysis, the useable biogas would increase by 240%

but the mass of cake would remain the same, though dryer and more stackable. There is an

entirely wrong assumption that the digestate has to be composted before it can be used on land.

It is counter-productive to compost digestate from an ABPR plant because firstly, the readily

degradable carbon has already been stabilized and there is therefore no necessity to use more

energy to create short-cycle CO2 when this carbon would be better feeding soil biomass as soil

improver. Secondly, ABPR requires that feed is pre-sanitized prior to AD and therefore post-

composting would have no additional hygienic value. Thirdly post-composting volatilizes

ammonia, which is a waste of valuable fertilizer-replacement nitrogen.

Table 5 shows an estimate of the revenue from electricity and gate fees and the GWP for

curbside collected FW treated by ABPR compliant AD. The prices have been converted to US$

at the rate of exchange current at the time of writing the paper but this currency conversion is for

ease of understanding it is not a prediction of actual prices in the USA.

Table 5 GWP components of curbside collecting source-segregated FW, treating it by

pasteurization+MAD or thermal hydrolysis+MAD and using the digestate on farmland (£ prices converted to US$ at 9

th Feb 2009 rate)

Description 70°C+AD TH+AD unit

electricity income (incl. ROCs) @ $177/MWh $57.80 S71.94 S/t FW

FW gate fee (letsrecycle.com Jan 2009 data) $52 to $78 $52 to $78 S/t FW

separate curbside collection and conveyance +24 +24 kg CO2e/t FW

treatment (including electricity generated) -132 -183 kg CO2e/t FW

delivery from AD site to land application +4 +2 kg CO2e/t FW

carbon-sequestration in soil -22 -22 kg CO2e/t FW

fertilizer offset -36 -36 kg CO2e/t FW

GWP - centralized AD, electricity generation and

land application of dewatered digestate -162 -215 kg CO2e/t FW

The UK government incentivizes generation of electricity from renewable sources by the

“Renewables Obligation”, which requires electricity suppliers to source increasing percentages

of the electricity from renewable sources. The level for 2008/09 is 9.1% rising to 15.4% by

2015/16. Eligible renewable generators receive Renewables Obligation Certificates (ROCs) for

each MWh of electricity generated. ROCs can then be sold to suppliers, in order to fulfill their

obligation. Suppliers who cannot present enough ROCs to cover the required percentage of their

output must pay the Regulator a „buyout‟ price for the shortfall. All proceeds from buyout

payments are recycled to suppliers in proportion to the number of ROCs they present. The

Renewables Obligation was designed as a market mechanism to increase the uptake of renewable

electricity. Other countries have their own “green energy” incentives. Table 5 shows income

from electricity inclusive of ROCs and gate fee (which would be similar for composting) the

GWP offset for MAD preceded by ABPR-compliant „pasteurization‟ or TH.

Food Waste Disposers

Food waste disposers (FWD) enable separation of FW at source without the hygiene issues, odor

and inconvenience of storing it on site pending curbside collection. In countries where FWD are

uncommon, sewerage professionals often regard FWD as controversial but, when the technical

and scientific publications are reviewed (e.g. Galil and Yaacov, 2000; CIWEM, 2003; Davis et

al, 2004; Evans, 2007), FWD have been shown repeatedly to have no adverse effect on water

resources, sewerage or wastewater treatment and that, in general, they provide a good solution

for those unwilling or unable to home-compost.

A FWD is an electro-mechanical device that fits in the drain line from a kitchen sink. It

comprises a grind chamber the floor of which is a spinning disc on which two or more blunt

impellers are mounted, these spin the food waste onto an abrasive ring that forms the wall of the

chamber. The ring is slotted so that when particles are small enough they can escape the

chamber (98% of particles are smaller than 2 mm diameter). The chamber is flushed with cold

water. These fine particles join the wastewater collection and treatment system, which is

designed to convey and treat [biodegradable] material suspended in water. The particles are too

small to be detectable by rats. FWD grind rather than smash so glass, stones and metal do not

splinter and plastic film remains in the grind chamber. Thus, FWD separate FW at source and

exclude physical contaminants. It is fair that the operators of the wastewater system should be

reimbursed for their extra costs. The cold water used for flushing coalesces fat onto the other

particles and thus avoids deposition on sewer walls; also, it cools the electric motor.

Around 50% of households in the USA have FWDs; they are used with both mains drainage and

septic tanks. 34% of households in the New Zealand have FWDs. The percentage of households

with FWDs installed in Europe is much less than in the USA. In the UK, which has the greatest

use, only 5% of households have a FWD. In 2008 Stockholm removed the fee for installing

FWD because the city and Stockholm Water decided that more FW would produce more biogas

and it would also feed the biological steps of wastewater treatment that remove nitrogen and

phosphorus from the water.

Field studies in several countries of water use by domestic FWD have in some cases found a

reduction in water use and in others a small increase. Reviewing all of the published studies, the

amount of water used is related to the number of starts (times a FWD is used) and not to the

number of people in the household, this is because use is related to food preparation events. The

number of starts averages 2.2 /day and water use averages 3.1 liters/start, i.e. 6.8 liters/FWD.day

– approximately one lavatory flushes per household. This would have minimal impact on water

resources or on the hydraulic loading on wastewater management. Electricity use has been

measured in field studies to average around 3 kWh/FWD.year.

The output of FWD (ground FW suspended in water) is very biodegradable and carried very

easily at the design self-cleansing velocity of sewers (Kegebein et al., 2001).

Surahammar in Sweden is an interesting case study. It is served by a single WwTW, which has

diffuse-air activated sludge and MAD. Differential charging for biodegradable municipal waste

(BMW) was introduced in May 1997. The lowest annual charge was for home composters;

householders had to purchase and maintain their own compost bins. The highest charge was for

households that chose curbside collection of source-segregated BMW, householders were

provided with an additional wheeled bin which was collected weekly, or twice a week in hot

weather. The third option was an 8-year contract to lease a FWD. To qualify for the leasing

contract the municipality inspected the sewer lateral connecting the property to the main sewer

using closed circuit television (CCTV). Sometimes this revealed problems. When the pipework

was suitable the householder was eligible to have a FWD installed by the municipality as part of

an 8-year leasing agreement during which the municipality repairs any faults. After 8 years the

FWD becomes the property of the householder, whose waste collection charge reverts to that of a

home composter; alternatively the householder can have a new FWD and start another 8-year

contract. The approximate annual costs are home-composting zero, FWD leasing $40 and

curbside collection $310.

The uptake of FWD was rapid. By October 1998, 1100 of the 3700 households contributing to

the WwTW had FWD installed but there were few parameters that changed significantly at the

WwTW (Karlberg and Norin, 1999). The BOD7:N increased from 3.7 to 4.5 mg/l; this is a

desirable change at WwTW where there is biological nutrient removal (BNR) because the extra

BOD (biological oxygen demand) is needed to feed the process. Many WwTW buy

supplemental carbon; this is typically methanol to feed denitrification and/or acetic acid to feed

phosphate removal. Electricity consumption for secondary treatment (air pumps) did not change

measurably. 4-monthly biogas increased from about 340 to about 420 m3/d.

I visited Surahammar in 2008 and found the terrain is gently undulating. Sewer gradients are

typically 0.4%. It is not atypical as regards its sewer network or its wastewater treatment. In

2008 50% of Surahammar‟s households had FWD and 30% had home-composting. Two

sections of the municipality are connected by a rising main but the extra organic loading has not

increased septicity. The municipality has no increase in sewer blockages, septicity, odor or

wastewater treatment difficulties, but they do have less N in the wastewater (presumably

denitrification in the sewers) and 20% more biogas. The pest control contractor for the

municipality had noticed an increase in rats but this was associated with the compost bins, not

with the sewers. As a result of its policies, Surahammar has reduced its tonnage of waste to

landfill from 3600 tonnes/year in 1996 to 1400 tonnes/year in 2007, it considers FWD have made

important contributions to this success.

The data reported here are based on the audited household waste data from Herefordshire and

Worcestershire (population 727,100), which is one of the regional authorities for municipal

waste management in England (Figure 4). In 2005, H&W began encouraging householders and

builders to install FWD by offering cashbacks against the cost of purchase and installation. In

2006 H&W decided it was time for an independent assessment of the implications and

commissioned TIM EVANS ENVIRONMENT to examine the financial and environmental implications of

the options for FW; the data presented here are derived from H&W‟s audited 2005/06 „Best

Value Performance Indicator‟ data (BV); the County Surveyors‟ Society contributed to funding

the research. In the UK, the best estimate in 2006 was that FW comprised 17.6% of household

waste and that nationally the average annual household FW is 216 kg. Residents in H&W are

somewhat less wasteful and 17.6% of their household waste equaled 180 kg/hhd.year. H&W‟s

pro rata BV showed the cost for FW via the curbside solid waste route the cost was

$27.44 /hhd.year. The cost for collecting this FW separately would have been even greater

because of the cost of running extra collection trucks and because treatment would have been

more expensive.

In the case of Herefordshire and Worcestershire, all of the sewage sludge is treated by anaerobic

digestion, either at the WwTW where it arises or at sludge treatment centers to which it is

transported from smaller WwTW for digestion. The biogas is used as renewable energy and the

[Class-B] biosolids are dewatered and applied to farmland as nutrient-rich soil improver thus

completing nutrient cycles and conserving organic matter. I estimated the additional cost to the

water company attributable to FWD would be $1.00 /hhd.year.

The conversion of FW to biogas affects the prediction of the amount of biosolids for recycling as

well as the income and GWP offset from electricity generation. The estimates in Table 6 are

derived by back-calculating from the data presented by Karlberg and Norin (1999) for

Surahammar, which were in turn similar to the predictions of Kegebein et al. (2001).

Figure 4 Map of England showing the counties with Herefordshire and Worcestershire

outlined in red

100 miles

As Table 6 shows, the GWP of the FWD > AD > recycling route is equivalent to the source-

segregated > curbside-collected > AD > recycling route but the overall cost is $26 /hhd.year less.

Table 6 GWP components of FW separated at source by FWD and treated by MAD with

electricity generated from the biogas and using the dewatered biosolids on farmland

Component GWP kgCO2e/t FW

conveyance (electricity use, etc.) +6

treatment including electricity generated offset -150

delivery from WwTW to land application +3

carbon-sequestration in soil -22

fertilizer offset -36

GWP - FWD + sludge treatment by AD with electricity

generation and land application of dewatered cake -199

NUTRIENT CONSERVATION

The critical importance of conserving, and not squandering, phosphorus was discussed in the

Introduction. The conventional approach at WwTW has been to recycle dewatering liquors to

the head of the treatment works and remove the ammonium and phosphate again; however this

takes energy, wastes the nitrogen fertilizer value and requires treatment capacity, which could

entail capital expenditure. Even where the plant is only treating „indigenous‟ sludge, dewatering

liquor can be 20% of the N and P load on the works; for a regional sludge treatment centre it can

be considerably more. Chemical stripping leading to recovery of the fertilizer is emerging as a

viable alternative.

AD releases ammonium, from the peptide links in proteins, and solubilizes some of the

biologically-bound phosphate from the biomass. By bubbling air to strip the dissolved CO2 and

adjusting the magnesium concentration, struvite (magnesium ammonium phosphate) can be

crystallized; this happens best at pH 8.5-9.0. Unintended precipitation of struvite in pipelines

can be a major operational problem, so stripping it out in a reactor intentionally has a second

benefit that it does not block pipes. Struvite reactors have been operated from more than 10

years in Japan; they are reliable and can remove 90% of the P. Struvite is a good phosphate

fertilizer (Johnston and Richards, 2003). Struvite removal partners very nicely with air-stripping

ammonia at pH 10 and recovering it with sulfuric acid and ammonium sulfate fertilizer (Figure

5). Ammonia stripping from alkaline dewatering liquor has been operated at Oslo, Norway for

more than 10 years with 99% plant availability and >90% removal efficiency. The capital cost

of such a combined plant for 500,000 population equivalent dewatering liquor treatment should

be not more than US$ 3 million.

The price of fertilizers trebled in recent years because of increased demand and inelastic supply

as a result of restricted investment in production facilities for years. The December 2008 unit

prices of fertilizers delivered to farms in lots of at least 20 tonnes in the UK were £1.13 /kg N,

£1.38 /kg P2O5, £0.93 /kg K2O and £1.03 /kg MgO (Farmers Weekly) – [$1.66, $2.03, $1.37 and

$1.52 respectively]. This price hike makes recovery rather than wasting N and P all the more

attractive. In addition it increases the value of MAD cake delivered and spread as being more

than £10.87/t ($16) in terms of its fertilizer replacement value [the fertilizer replacement value in

the first year is N:P2O5:K2O:MgO:SO3 2:4.5:1:1:3 kg/t (wet wt)].

Figure 5 Schematic of a combined struvite and ammonium sulfate recovery plant for

treating dewatering liquor

For all practical purposes the rate of biosolids application has been limited by the nitrogen dose

for several years because the concentrations of potential pollutants have been reduced so

effectively. Increasingly the agronomic phosphate requirement is becoming the application rate

limit in many areas. Keeping P in the biosolids or taking it out as struvite can materially affect

the cost of the biosolids program.

Table 7 and Table 8 show operational cost data reported by wastewater treatment operators to TIM

EVANS ENVIRONMENT as part of a research project for Cefic (the European Chemicals Industry).

The differences in prices reflect the local differences of economies of scale; Göteborg is a single

large works whereas the UK companies have a mix of larger and smaller WwTW. The operating

cost of keeping P from dewatering liquor in the system by treating it conventionally amounts to

approximately $1500 /t P2O5 (the sum of Table 7 and Table 8).

dewatering

liquor

Mg

alkali

Alk

ali C

O2

rem

oval

Stripped liquorcould go to THP Pulper for diluting

cake to correct viscosity and

capture Mg in digester as struvite

struvite

H2SO4

(NH4)2SO4pH 9

pH 10stirred

40°C

alkali

air Key

liquids

gases

densitymeter

pH

Table 7 Costs reported by wastewater operators for chemical-P removal from wastewater

Wastewater treatment operator US$/kg P US$/tonne P2O5

Göteborg, Sweden 1.36 590

UK water company A 1.58 685

UK water company B 2.52 1095

Table 8 Costs reported by wastewater operators for land-applying the P in biosolids

Wastewater treatment operator US$/kg P US$/tonne P2O5

Göteborg, Sweden 0.81 352

UK water company A 1.78 775

UK water company B 1.21 525

In contrast, the operating cost (chemicals, labor, maintenance, drying etc.) of removing P from

dewatering liquor as struvite would be approximately $1200 /t P2O5. However the struvite is a

valuable fertilizer and even selling it (e.g. to a fertilizer company) at only half the farm gate price

it would be worth $1600 /t P2O5. The net “contribution” of recovering struvite (before capital)

would thus be $1900 /t P2O5.

Treating the N in dewatering liquor conventionally involves to nitrify ammonia to nitrate, which

requires 10 kgO2/kgN. Activated sludge operates at about 2 kgO2/kWh; if the O2 transfer is

100% efficient the requirement is 5 kWh/kgN. If electricity costs S0.09/kWh the cost of

nitrification is $0.45/kgN. If the WwTW has a total-N limit there is additional cost of

denitrification.

In contrast, the operating cost (chemicals, labor, maintenance, etc.) of removing N from

dewatering liquor as ammonium sulfate solution would be $630 /t N. However the ammonium

sulfate is a valuable fertilizer and even selling it at only one-third the farm gate price it would be

worth $590 /t N. The net “contribution” of recovering ammonium sulfate (before capital) would

thus be $410 /t N.

CONCLUSIONS

We are putting massive demands on the earth and the combined effects of population growth and

climate change are going to increase the pressure inevitably. There is a compelling case for

regarding FW and well as other organic residuals as potential resources for recovering energy

and biofertiliser, which can complete nutrient cycles and conserve soil organic matter.

There is no single best option for FW because people live in different types of properties and

have differing degrees of willingness to participate in FW management. People with gardens

might have the opportunity to compost but some might not be willing. People living in

apartment buildings might have the opportunity of communal FW bins but be unwilling to store

FW and carry it down to the bins. Adapting existing infrastructure avoids some of the problems

of permitting and public acceptance that beset new-build treatment facilities.

This paper has found that anaerobic digestion (biofertiliser plants) and food waste disposers

delivering to WwTW where sludge is digested have nearly equivalent GWP and better than the

alternatives such as landfill, incineration or centralized composting. AD is also more

conservative of nitrogen fertilizer value in the residuals. Physico-chemical stripping of N and P

from digestate dewatering liquors appears to have considerable cost benefits compared with

biological treatments that aim to waste the soluble N and (re)capture the P into the biosolids.

At present we have a somewhat „fractured‟ approach to using organic resources on land.

Completing nutrient cycles and conserving organic matter is good for soils and food security but

we do not have a consistent approach to environmental protection. The biosolids use in

agriculture model has worked well for more than 30 years with no evidence of adverse effect and

it seems unnecessary to invent different paradigms for organic matter from other sources or for

other uses. Undoubtedly FWD could save local authorities a lot of money and it is only fair that

wastewater operators are reimbursed for their extra costs.

ACKNOWLEDGMENTS

I am grateful to Worcestershire County Council, the County Surveyors‟ Society Research Fund

and Cefic the European Chemical Industry Council all of whom contributed towards funding

elements of the research on which this paper has been based.

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