Anaerobic Digester perform some pretty basic functions, but also generate electricity and heat from the resultant "bio-gas". Primarily they are a depository for all kinds of waste, from food waste to human waste, and from spoiled crops to animal slurry. From this cocktail a bio-gas is derived which is primarily methane or natural gas. Admittedly dependent on the mix there is CO2 and hydrogen sulphide as well.
From the resultant gas, a Combined Heat and Power plant can be run to give 1 part electricity to 2 parts heat. Now in very hot climates the heat can be utilised to produce chilled water for air conditioning.
In the 1980's Some 6 million anaerobic digesters were set up in China, which became the biogas capitol of the world, attracting many from the developing countries to learn from it. The ‘China dome’ digester became the standard construction to the present day (Fig. 1), especially for small-scale domestic use. But many new types of rural household digesters have also been built based on water pressure, as for example, the plug flow auto-cycle rural digester, the up-flow small scale digester, the fender digester, and recently, the pulse flow anaerobic reactor.