Technical Background Small Scale Biogas production
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The document provides a technical overview of small-scale anaerobic digestion (AD) of food waste, detailing types of digesters, suitable conditions, and challenges in various regions. It emphasizes the importance of regulatory barriers and legislation, particularly regarding animal-origin waste and digestate use as fertilizer. Additionally, it discusses the biogas production process and its applications, especially in developing countries for cooking and lighting.
Technical Background Small Scale Biogas production
- 1. Technical background on small scale anaerobic digestion of food waste Iemke Bisschops, Els Schuman, Kasia Kujawa, Henri Spanjers
- 3. 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
- 4. Types of digesters Completely mixed Plug flow Batch Fed-batch Percolation systems (type of batch system)
- 5. 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
- 6. 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)
- 7. Digesters Western countries: normally medium- to large-scale Germany, Austria and Switzerland are leading countries Farm-scale digesters: Germany > 2000; Austria ~ 120; Switzerland > 70.
- 8. 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
- 9. 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
- 10. Small scale AD interest growing http://www.patervis.com/sintex.html
- 11. Anaerobic digestion of food waste
- 12. 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 concentrated and highly degradable
- 13. Anaerobic Conversion of Organic Matter CH 4 / CO 2 Methanogenesis Acetogenesis Suspended, colloidal organic matter Hydrolysis Acidogenesis volatile fatty acids alcohol H 2 / CO 2 acetic acid proteins carbohydrates lipids amino acids sugars free fatty acids glycerol ammonia
- 14. Biological stability - Acidification Overloading may lead to process deterioration – A vicious circle Overloading Methanogenic capacity exceeded Poor buffering capacity VFA increase pH decrease Unionised VFA increase Methanogenic toxicity increase
- 15. Biological stability - Acidification 50% inhibition by acetic acid Methanogenic capacity exceeded Poor buffering capacity VFA increase pH decrease Unionised VFA increase Methanogenic toxicity increase 30000 8 3000 7 300 6 30 5 Concentration (mg/l) pH
- 16. 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
- 17. 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
- 18. Examples of regulations Animal by-product regulation (ABPR) Environmental permitting rules Planning regulations Regulations for fertiliser use Rules on gas safety etc… Important factor!
- 19. 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
- 20. 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.
- 21. 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.
- 22. 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
- 23. 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
- 24. Proposed system concept for CCN project
- 25. Summary of system criteria Digester volume fixed at 1 m 3 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
- 26. 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
- 27. Proposed system concept Simplified process flow diagram, not to scale. Dashed connecting lines: slurry/liquid flow could be done manually