When organic material is allowed to decay naturally but in the absence of oxygen one of the products of the process is a gas which is rich in methane. The process, called anaerobic decomposition, occurs when urban waste is buried in landfill sites. Significant quantities of methane are released from such sites and this gas can be collected and burned in a gas engine to generate electricity. (In many countries the collection of this gas is now mandatory since methane is a more potent greenhouse gas than carbon dioxide.)
Animal wastes from dairy and pig farms can be treated in a specially designed digester to achieve similar results, anaerobic decomposition and a methane-rich gas. The economic viability of such a scheme depends on the size of the farming operation but if the latter is large enough it can generate enough revenue to more than offset investment cost. Sewage farms which deal with human waste can also utilise similar technology.
All these applications are relatively small scale with generating units rarely larger than 100 kW, though landfill gas power plants may reach 20-30 MW. The cost effectiveness will depend in part of prevailing environmental regulations. Where these require wastes, effluents or the landfill methane gas to be collected and treated, the additional cost of a generating unit will easily be offset by the value of the power it produces. If treatment is not mandatory, other forms of disposal may prove more cost effective.
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