Anaerobic digestion for biogas Introduction

Decaying biomass and animal wastes are broken down naturally to elementary nutrients and soil humus by decomposer organisms, fungi and bacteria. The processes are favoured by wet, warm and dark conditions. The final stages are accomplished by many different species of bacteria classified as either aerobic or anaerobic.

Aerobic bacteria are favoured in the presence of oxygen with the biomass carbon being fully oxidised to CO2. This composting process releases some heat slowly and locally, but is not a useful process for energy supply. To be aerobic, air has to permeate, so a loose 'heap' of biomass is essential. Domestic composting is greatly helped by having layers of rumpled newspaper and cardboard, which allows air pockets and introduces beneficial carbon from the carbohydrate material. Such aerobic digestion has minimal emission of methane, CH4, which, per additional molecule, is about eight times more potent as a greenhouse gas than CO2, see Section 4.6.2.

In closed conditions, with no oxygen available from the environment, anaerobic bacteria exist by breaking down carbohydrate material. The carbon may be ultimately divided between fully oxidised CO2 and fully reduced CH4, see (10.8). Nutrients such as soluble nitrogen compounds remain available in solution, so providing excellent fertilizer and humus. Being accomplished by micro-organisms, the reactions are all classed as fermentations, but in anaerobic conditions the term 'digestion' is preferred.

It is emphasised that both aerobic and anaerobic decompositions are a fundamental processes of natural ecology that affect all biomass irrespective of human involvement. As with all other forms of renewable energy, we are able to interface with the natural process and channel energy and resources for our economy. The decomposed waste should then be released for natural ecological processes to continue.

Biogas is the CH4/CO2 gaseous mix evolved from digesters, including waste and sewage pits; to utilise this gas, the digesters are constructed and controlled to favour methane production and extraction (Figure 11.7). The energy available from the combustion of biogas is between 60 and 90% of the dry matter heat of combustion of the input material. However, the gas is obtainable from slurries of up to 95% water, so in practice the biogas energy is often available where none would otherwise have been obtained. Another, perhaps dominant, benefit is that the digested effluent forms significantly less of a health hazard than the input material. Note, however, that not all parasites and pathogens are destroyed in the digestion.

The economics and general benefit of biogas are always most favourable when the digester is placed in a flow of waste material already present. Examples are sewage systems, piggery washings, cattle shed slurries, abattoir wastes, food processing residues, sewage and municipal refuse landfill

Material Health Renewable EnergyBiodigester FiberBiogas Cow

Figure 11.7 Biogas digesters. (a) Simple oil drum batch digester, with cow to scale (b) Indian 'gobar gas' digester. (c) Chinese 'dome' for small-scale use [adapted from Van Burren]. (d) Accelerated rate farm digester with heating, for use in middle latitudes [adapted from Meynell]. (e) A large system near Aalborg in Denmark, which produces 10000m3 of biogas per day. The biogas store is in the foreground, with the digester tanks visible behind it. The lorry (truck) in front of the white shed indicates the scale. [Photo by courtesy of NIRAS als].

Figure 11.7 Biogas digesters. (a) Simple oil drum batch digester, with cow to scale (b) Indian 'gobar gas' digester. (c) Chinese 'dome' for small-scale use [adapted from Van Burren]. (d) Accelerated rate farm digester with heating, for use in middle latitudes [adapted from Meynell]. (e) A large system near Aalborg in Denmark, which produces 10000m3 of biogas per day. The biogas store is in the foreground, with the digester tanks visible behind it. The lorry (truck) in front of the white shed indicates the scale. [Photo by courtesy of NIRAS als].

Porte Des Fortifications Paris
Figure 11.7 (Continued).

dumps. The economic benefits are that input material does not have to be specially collected, administrative supervision is present, waste disposal is improved, and uses are likely to be available for the biogas and nutrient-rich effluent. However, in high and middle latitudes, tank digesters have to be heated for fast digestion (especially in the winter); usually such heat would come from burning the output gas, hence reducing net yield significantly. Slow digestion does not require such heating. Obviously obtaining biogas from, say, urban landfill waste is a different engineering task than from cattle slurries. Nevertheless the biochemistry is similar. Most of the following refers to tank digesters, but principles apply to other biogas systems.

Biogas generation is suitable for small- to large-scale operation. Several million household-scale systems have been installed in developing countries, especially in China and India, with the gas used for cooking and lighting. However, successful long-term operation requires (a) trained maintenance and repair technicians, (b) the users to perceive benefits and (c) alternative fuels, e.g. kerosene, not to be subsidised.

Biogas systems may be particularly attractive as part of integrated farming, where the aim is to emulate a full ecological cycle on a single farm. Thus plant and animal wastes are digested with the collection of the biogas as a fuel, with the effluent passing for further aerobic digestion in open tanks, before dispersal. The biogas is used for lighting, machines, vehicles, generators, and domestic and process heat. Algae may be grown on the open air tanks and removed for cattle feed. From the aerobic digestion, the treated effluent passes through reed beds, perhaps then to fish tanks and duck ponds before finally being passed to the fields as fertilizer. Success for such schemes depends ultimately on total integrated design, good standards of construction, and the enthusiasm and commitment of the operator, not least for the regular maintenance required.

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