Small-scale household biogas digesters: An option for global warming mitigation or a potential climate bomb?

Abstract

Abstract There are a number of advantages to small-scale biogas production on farms, including savings on firewood or fossil fuels and reductions in odour and greenhouse gas emissions. For these reasons, governments and development aid agencies have supported the installation of biogas digesters. However, biogas digesters are often poorly managed and there is a lack of proper distribution systems for biogas. This results in methane being released inadvertently through leaks in digesters and tubing, and intentionally when production exceeds demand. As methane has a global warming potential 25 times greater than that of carbon dioxide, this compromises the environmental advantages of digesters. Calculations performed in this paper indicate that the break-even point at which the released methane has as great an impact on global warming as the fuel that has been replaced occurs when between 3% and 51% of the produced biogas is released, depending on the type of fuel that has been replaced. The limited information available as regards methane leaking from small-scale biogas digesters in developing countries indicates that emissions may be as high as 40%. With the best estimates of global numbers of small-scale digesters and their biogas production, this corresponds to methane losses of 4.5 Tg yr−1 or about 1% of global emissions or 10% as much as emissions from rice production. Further poliferation of small-scale digesters could therefore contribute significantly to global emissions of methane. It is therefore important that governments and development aid agencies place stricter requirements on digester maintenance and biogas handling before incentives are created and legislation introduced for the installation of small-scale biogas digesters.