Qu: Would it be possible to put waste, i.e. bio-degradable rubbish and sewage into an air-tight container, let it produce methane gas, siphon it off to cook with and power gas-fired central heating, filter off the water, and be left with compost to put on gardens?
Yes – very possible, in fact it’s already being done. It’s called Anaerobic Digestion and the process is widely used to treat wastewater sludges and organic wastes. Anaerobic digestion can reduce the emission of harmful landfill gases into the atmosphere and is a renewable energy source because the process produces a methane and carbon dioxide rich biogas suitable for energy production. Also, as the question suggests, the nutrient-rich solids left after digestion can be used as fertiliser.
So why isn’t this process used everywhere? Anaerobic digesters require a high level of technical expertise to maintain as careful control of the digestion temperature, pH, quality of input and loading rates is crucial. Because of these complexities, despite being recognised by United Nations Development Programme as one of the most useful decentralised sources of energy supply and being less expensive to run than large powerplants, anaerobic digesters are not widely used in industry. (source)
Anaerobic Bioreactors, a type of landfill, can also transform decomposing material into an energy source. By removing oxygen and pumping lechtate (water which has collected waste products from the decomposing matter) and other liquids around the bioreactor the waste produces methane, which can be collected and used instead of fossil fuels. (source)
Perhaps the most exciting and accessible use of the anaerobic production of methane from plant waste is the ‘pyrolysis unit’ – invented for domestic use in India by Ravi Kumar. A family collects waste plants, dries then and puts them into the unit’s circular casing. This casing is sealed to stop oxygen combining with CO2. A small fire is started in the central void; this heats up the plants and causes them to emit gases through small holes in the casing. The gases ignite and burn for long enough to cook dinner (about an hour). Then, brilliantly, the remaining charcoal can be raked out and used as fertiliser – what’s more the soil’s subsequent ability to lock in carbon dioxide also increases. (James Bruges – What About China?- p19)
Hope that helps,