Sintex's digester uses bacteria to break down waste into sludge, much like a septic tank. In the process, the bacteria emit gases, mostly methane. But instead of being vented into the air, they are piped into a storage canister.
A one-cubic-meter digester, primed with cow dung to provide bacteria, can convert the waste generated by a four-person family into enough gas to cook all its meals and provide sludge for fertilizer
A model this size costs about $425 but will pay for itself in energy savings in less than two years
source: http://money.cnn.com/2008/02/26/news/international/kahn_biogas.fortune/index.htm
