Biogas digesters are nothing more than complex reactor vessels which are sealed off from the air on the outside to present an oxygen-free or anaerobic environment on the inside. Built or manufactured using a variety of methods, shapes and materials, the basic operation is always the same. Organic wastes are introduced into the vessel and the types of bacteria that thrive in anaerobic conditions get to work, breaking the waste down biologically and physically with a valuable by-product – biogas.
Biogas, a mix of mainly methane gas with small amounts of carbon dioxide and traces of hydrogen sulphide. Methane is the same colourless, odourless, combustible gas which constitutes a large proportion of natural gas, a fossil fuel exploited the world over for power generation and for use in the petro-chemical industry.
AGAMA Biogas are leaders in the implementation of small and medium scale biogas projects in Southern Africa. They have built and installed numerous digesters at private homes, schools, wineries and other institutions. “At the residential scale, a small
digester of 6m3 operating optimally in the typical domestic context can be expected to produce between 1 and 2 hours of cooking time per day,” says Greg Austin of AGAMA Biogas. “The addition of agricultural and food waste to such a digester increases energy yield significantly to the equivalent of around 0.8 kg of LP gas per day,” he adds.
However, energy production is but part of the story. In expounding the virtues of biogas technology, there is a general tendency to emphasise the energy-production aspects in disproportion to the other benefits. As a sanitation mechanism, an integrated biogas solution involving an anaerobic treatment and biogas recovery phase coupled to an aerobic treatment phase, is an extremely effective waste water treatment process even at a small scale.
Depending on retention time in the biogas reactor, up to 85% of pathogens requiring oxygen to metabolise are destroyed. The next step is to treat the effluent in an oxygenated aerobic environment which conversely destroys the pathogens (such as e-coli) not dealt with in the anaerobic phase. This can be achieved through algae ponds or constructed reed beds. In this manner, a properly implemented integrated biogas solution satisfies the requirements of the Department of Water Affairs for release of treated effluent into the environment.
Biogas can be used in a number of different ways. The most efficient way of using biogas is as fuel for cooking, heating and light. It can, however, also be purified by a relatively simple scrubbing process resulting in a purer form that can be used to run engines and generate electricity.
Greenways farm in KwaZulu-Natal is a farms that has realised the potential for biogas production in the agricultural sector. At one stage, they sourced enough manure to produce 11 000 kWh of electricity a month, about 7 000 kWh more than was needed to run the entire farm.
Biogas was conspicuously absent at the initial drafting of South Africa’s Industrial Biofuels Strategy in 2007 – much to the chagrin of some of its vocal proponents who expounded the virtues of it as an energy from waste solution and the fact that it sidestepped concerns over crops for fuel and the contentious “energy return on energy invested” debate.
“Many local municipalities in South Africa, while not thinking at the integrative level of some
European cities, are evaluating and planning the use of anaerobic digestion for the treatment of their current sewerage, agro-processing, industrial waste water and wet waste problems,” says Austin.
Read the full feature in the April-May 2011 issue on p34.