Anaerobic Digestion: An Introduction
Anaerobic digestion (AD) is a biological process similar in many ways to composting. It is a natural treatment process and, as in composting, bacteria break down organic matter and reduce its bulk or “mass”.
AD Introduction: Contents
Unlike composting AD is carried out in an oxygen-free environment (known as anaerobic conditions) to allow the presence of bacteria adjusted to these conditions which then multiply and grow, and by so doing achieve the process aims of:
- sanitisation of the feed material and of any liquid discharged;
- a net positive surplus generation of energy as a biofuel to allow power production from methane gas (biogas) produced by the organisms.
Photo: A Greenfinch Anaerobic Digestion Plant which was built during 2004/5 for use on a farm.
Why are we excited about Anaerobic Digestion just now?
A remarkable combination of recent technical, political and regulatory factors is starting to propel AD forward toward the mainstream. Some of these factors are:-
- Rising oil prices: Suddenly traditional energy sources are so much more expensive. In the past AD was a much more expensive energy source, and to be honest it remains so when all capital spend up-front is considered, but the gap is narrowing toward mainstream take-up of AD, and fast.
- Farm waste requires better management: As from June 30, 2006 the EU Waste Management rules apply to farming and agriculture, and their historical exemption has been removed.
- Organic waste must be diverted from landfill: This is the result of the EU landfill Directive and AD is an excellent technology for organic waste landfill diversion.
- The demand for new sustainable technologies investment opportunities powered by individuals but increasingly manifested in actions and investments by large companies and big investors. (January 2007 has seen the big supermarkets in the UK talking about using renewable energy technologies in their stores, including Anaerobic Digestion).
- Rising demand for renewable fuels and bioenergy: This includes government targets and incentives to Anaerobic Digestion which the United Kingdom and other governments provide to help them meet the targets.
- the availability of Carbon Credits to those who avoid Carbon Dioxide emissions: The developing world has been able to apply and obtain CER credits for nearly two years, and under the new US proposals (including the Methane to Markets project) it seems that the US may be soon paying carbon subsidies even to the developed nations where carbon emissions reductions are achieved in carbon neutral, and negative projects such as Anaerobic Digestion.
Main Source Materials for Anaerobic Digestion (Temperate Climates)
- Catering waste from private households
- Food residues
- Restaurant and canteen residues
- Farm manure (e.g. liquid manure, dung)
- Vegetable residues from commerce and trade
- Waste water from food production
- Grease trap fat
- Specially grown raw material - biofuel (e.g. clover) See also our bigger list here.
Products of Anaerobic Digestion
- A gas: Methane - a fuel.
- Solid fibrous material; which is spread without further treatment, or after post composting (maturation), to provide organic matter for improvement of soil quality and fertility (improves soil structure and reduces summer irrigation demand).
- The liquid fraction contains two thirds of the nutrients and can be spread as a fertiliser and sprayed on crops.
- Co-composting on farms - liquid and solid fraction is mostly not separated and is spread as a slurry.
A Comparison of Anaerobic Digestion with Composting
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Anaerobic Digestion
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Composting
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Space requirement (footprint)
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50%
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100%
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Odours
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20%
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100%
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Energy balance
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Energy surplus
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Energy demand
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Biogas production
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100 – 150 m3/Mg
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Nil
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Process time required to produce mature compost
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3 weeks digestion, plus
5 weeks composting
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12 weeks
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Comparison Table From: “Introduction to Anaerobic Digestion”,Wolfgang Muller and Axel Huttner, ORA - Organic Resource Agency Ltd., Malvern Hills Science Park, Geraldine Road, Malvern, Worcestershire WR14 3SZ, and IGW – Ingenieurgemeinschaft Witzenhausen Fricke & Turk, and GmbH, Bischhuser Aue 12, D-37213 Witzenhausen, Germany. Presented at the Biowaste: Digesting the Alternatives Seminar, April 2005, UK.
Consumption of Non-Renewable Fuels and the Climate Change Levy
In an increasing number of countries governments are charging a tax (levy) on the use of non-renewable fuels further increasing the benefits from Anaerobic Digestion. More here..
check out the Energy from Waste swicki at eurekster.com
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