technical background small scale biogas production

Lettinga Associates Foundation

LeAF

Technical background on small scale anaerobic digestion of food waste

Iemke Bisschops, Els Schuman, Kasia Kujawa, Henri Spanjers

Lettinga Associates Foundation 2LeAF

Anaerobic digestion


What is anaerobic digestion

A chain of biological processes

Carried out by different groups of microorganisms

In absence of oxygen

• Products: biogas and digestate

• Different digester configurations are employed– depending on substrate and local conditions/preferences


Types of digesters

• Completely mixed

• Plug flow

• Batch

• Fed-batch

• Percolation systems (type of batch system)


Slurry digestion

• Digestion of animal manure is the most widespread AD application worldwide. It produces:– Valuable fertiliser – Biogas

• Farm-scale digestion plants– widespread use throughout the world – plants in both developing and

technically advanced countries


Digesters

• Rural communities: typically small-scale units– Nepal 50,000 digesters; China > 8 million.

• These small plants are generally used for providing gas for cooking and lighting for a single household.

(images references in CCN phase 1 report)


Digesters

• Western countries: normally medium- to large-scale

• Germany, Austria and Switzerland are leading countries– Farm-scale digesters: Germany > 2000; Austria ~ 120;

Switzerland > 70.


AD systems - Suitability

• Small plants used in developing countries seem attractive from engineering point of view– relatively simple design, construction, operation

• Not suitable for use in

Western Europe– Regulations to meet– Temperature too low

• At low temperature– Very large systems needed– And/or heating


AD systems - Suitability

• Current systems used for AD of organic solid waste– Large to very large scale systems– Usually batch systems with percolation & recycling of leachate

• Percolation systems not suitable for very small scale– Technically too difficult– Moving parts, risk of clogging,…

• Slurry digester seems most feasible option– Technically quite simple– Experience exists at small scale


Small scale AD interest growing

http://www.patervis.com/sintex.html


Anaerobic digestion of food waste


Food waste

• Main substrate considered in project with CCN

• Weekly production on average 3 kg per household

• Complex substrate – carbohydrates– proteins– lipids– nutrients, minerals

• Usually very concentratedand highly degradable


Anaerobic Conversion of Organic Matter

CH4 / CO2

Methanogenesis

Acetogenesis

Suspended, colloidal organic matter

Hydrolysis

Acidogenesis

volatile fatty acidsalcohol

H2 / CO2acetic acid

proteins carbohydrates lipids

amino acids sugars free fatty acids glycerol

ammonia


Methanogenic capacity exceeded

Poor bufferingcapacity

VFAincrease

pHdecrease

Unionised VFAincrease

Methanogenic toxicityincrease

Overloading

Biological stability - Acidification

Overloading may lead to process deterioration – A vicious circle


Methanogenic capacity exceeded

Poor bufferingcapacity

VFAincrease

pHdecrease

Unionised VFAincrease

Methanogenic toxicityincrease

Biological stability - Acidification

pHConcentration

(mg/l)

5 30

6 300

7 3000

8 30000

50% inhibition by acetic acid


Food waste as AD substrate

• Characteristics of reactor feed and as a result the reactor performance depend on:– composition– particle size– storage time

• Fast acidification, already starting during storage

• Acidification continues inside reactor– Careful process operation is essential


Food waste as AD substrate

• In principle suitable substrate for AD

• Challenging for a small system without advanced monitoring and control

• But:

Normally contains waste from

animal origin

→ regulatory barriers


Examples of regulations

• Animal by-product regulation (ABPR)

• Environmental permitting rules

• Planning regulations

• Regulations for fertiliser use

• Rules on gas safety

• etc…

Important factor!


Legislation - ABPR

• Animal by-product regulation (ABPR) regulates all waste from animal origin including kitchen waste– Classified as “category 3 catering waste”

• Hygienisation of kitchen waste necessary

• Treatment standards for category 3 catering waste:– minimum 57°C for 5 hours, maximum particle size 50 mm– 70°C for 1 hour, maximum particle size 60 mm


Digestate

• Organic nitrogen is converted into ammonia– Directly available to crops– Possibly higher ammonia emissions from the field

• Reduced viability of pathogens and weed seeds

• Legislation – Important to ensure safe use– Probably largest barrier

• Quality protocols– e.g. UK: “Quality Protocol for the Production and Use of Quality Compost

from Source-Segregated Biowaste”– Defines the point at which the material ceases to be a waste and instead

becomes a product.


Digestate use – Practical

• When all regulations are complied with, the digestate can be used as fertiliser

• It should be worked into the soil to prevent ammonia emissions

• Separation in liquid and solid fraction– Solid fraction can be composted together with green waste (might

be more attractive)– Liquid fraction can be used as liquid fertiliser– Will still contain solids and salts, this might limit its application in

drip systems.


Biogas

• Composition (most important compounds)– Methane– Carbon dioxide– Hydrogen sulphide

• Examples of uses:– Heating– Steam production– CHP: electricity and heat– Transport fuel (e.g. car, lift truck, city busses)– Supply to normal natural gas network (so called Green Gas)

• End use determines the needed gas quality


Biogas use on small scale

• In developing countries very common, e.g. cooking and lighting

• In Western Europe not common– gas quality upgrading normally necessary– economy of scale

• But: starting to gain interest


Proposed system concept

for CCN project


Summary of system criteria

• Digester volume fixed at 1 m3

– Amount of waste will be made to fit system size

• Robust and easy to build

• Steel and plastic as preferred building materials

• Focus on kitchen waste as a substrate


Process design

• Biodegradability

• Hydrolysis rate of substrates (particle size!)

• Applied conditions– Temperature (mesophilic / thermophilic)– Retention time– Presence of toxic or inhibitory compounds

• All combined determine overall process performance– Stability of process and products– Biogas production


Proposed system concept

Digester(isolated vessel)

Shredder

Pasteuriser

Biogas holder

Mixer

Sampling port(optional)

Digestate use

Feed storage Digestate storage

Biogas cleaning

Safe

ty valve

Sampling port (optional)

T-I T-RT-C

T-I Temp.indicator / T-R Temp.recorder / T-C Temp.controller

Safe

ty valve

T-I

Check valve

Pump

Hot water boiler

Exhaust gas

Other gas uses

Biogas from digestate storage

Safety valve

Simplified process flow diagram, not to scale.Dashed connecting lines: slurry/liquid flow could be done manually

technical background small scale biogas production

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