Keywords: anaerobic digestion, biogas, livestock manures, plant nutrients, renewable energy.
Anaerobic digestion comprises a sequence of naturally occurring bacterial processes that breakdown complex, organic compounds in the absence of oxygen. At the final stage of the process, methanogenic bacteria produce a gaseous mixture of methane and carbon dioxide commonly termed biogas. Thus from an industrial and, more recently, an agricultural viewpoint, the process has the advantages of stabilising organic wastes and manures, with potential for reducing risks of pollution, and of yielding a renewable source of energy in the form of biogas. Simple, family sized digesters have been used to provide gas for cooking etc for many years in warmer climates. Large-scale plants in temperate regions, including the UK, were developed for the treatment of municipal sewage. Energy crises, together with concerns over the environmental impacts of intensive farming, have created much interest in on-farm anaerobic digestion plants. Most of these, including the most recent, are of the continuous stirred tank reactor (CSTR) design. An insulated tank is heated and mixed, and feedstock, such as livestock manure, food processing or other organic wastes, or specially grown crops, is fed in to it on a regular basis each day. Digested material (digestate) flows, or is pumped, out at the same rate. Biogas production is related to the composition of the feedstock, the operating temperature and the digester retention time. For most on-farm digesters, temperature is controlled within the range optimum for mesophilic bacteria (20-40 °C) and retention time (the average time feedstock is in the digester) is between 10 and 30 days. Biogas does not readily compress, so storage is difficult, but can be burnt in a boiler for hot water or in an engine to produce power. Combined heat and power units (CHP) provide both heat, via recovery from engine cooling, and electricity via a generator. Digestion reduces chemical and biological oxygen demand (COD and BOD), so reducing the potential for water pollution, and level of odour and pathogens associated with organic feedstocks. It also decreases dry matter content and increases the proportion of nitrogen in inorganic forms available for plant uptake and so increases fertiliser value.
Government incentives, whereby electricity companies are obliged to purchase electricity from renewable supplies in support of policies on climate change, have boosted the economics of anaerobic digestion. High capital cost of plants and regulatory issues still present challenges to developments in the UK.
John Morgan and Brian F Pain, Creedy Associates, Crediton, Devon, UK.
40 pages, 8 figures, 14 tables, 27 references.