How to develop a community-led agricultural anaerobic digester
This is an in-depth guide by Local United, bringing together the experience of community groups and social enterprises to offer practical suggestions for other groups to set up their own initiatives. Local United brings together and supports community activists who are setting up social enterprises to address the challenges of peak oil and climate change. We aim to speed up the rate at which good ideas are adopted by community groups motivated to build low-carbon economies.
What is Anaerobic Digestion?
Anaerobic digestion is perhaps one of the least well understood forms of renewable energy but in some ways it is the simplest and one of the oldest (the Babylonians are thought to have first used the technology).
An anaerobic digester can be likened to a cow’s stomach; it uses the same natural processes. Bacteria break down organic materials in an oxygen free environment (large concrete or steel tanks). This results in the production of methane and a liquid called digestate. Methane is used in large engines similar to petrol engines to produce electricity. In doing this, like any engine, they produce large amounts of heat. Thus a 1000 kW anaerobic digester will not only produce 1000kW of electricity, it will also produce a similar amount of heat. This is considered renewable energy because the CO2 released by burning the methane in the engines is then absorbed by the crops grown to feed the digester.
The other product of an Energy Farm is the liquid digestate. More like compost than fresh manure; it is a much better and more effective fertiliser than raw slurry as well as being much less smelly! Around 80 tonnes of digestate are produced for every 100 tonnes of feedstock. This digestate is normally spread back on land as a fertiliser (making the system virtually a closed loop).
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