ANAEROBIC DIGESTERS

Description

An anaerobic digester is a tank or vessel which excludes oxygen and in which a sludge or a liquid effluent is modified by the action of anaerobic bacteria. Organic material in the effluent is converted to methane and carbon dioxide (biogas) and the BOD and COD value of the effluent is reduced. The biogas can be used to heat the effluent to the optimum temperature of 35°C, or to generate electricity.

Sewage sludge is anaerobically digested to remove volatile organic material and render it stable and suitable for land spreading. The process can take 12-30 days and the digesters are large. The sludge is kept mixed by gas recirculation or stirring.

Industrial wastewaters are treated for 4-24 hours in reactors that are partly filled with plastic media or with fluidised sand or sludge on which the anaerobic bacteria grow. Alternatively, effluents can be treated in shallow rectangular lagoons that are made airtight by floating flexible fabric seals. The effluent from anaerobic digesters would normally be discharged to sewer, or to a watercourse after an aerobic stage.

Application

Anaerobic digesters are used to digest sewage sludges, cattle and pig slurry, and high strength warm industrial wastewaters. Suitable wastewaters arising in the food industry may range from sugar in solution to semi-solid organic wastes such as potato and vegetable peelings.

Anaerobic digestion is also used in the beverage industries and in pharmaceuticals, pulp and paper and petrochemicals industries. Biogas may be used to generate electricity on site and CHP systems would be able to supply heat to the effluent at the same time. Such schemes involve high capitalisation. Anaerobic systems are susceptible to some toxic substances and individual wastewaters should be tested in pilot studies. Anaerobic systems can be tolerant of extended shutdown.

Sizing

Sewage sludge digesters may be 500-2000 m3 capacity and treat sludge with about 5% dry solids. Industrial digesters will treat waste streams with COD values from 1000-100,000 mg/l and would typically remove 90-95% of the BOD.

Loading rates can be as high as 10-20 kg COD/m3/day or up to 50 tonnes of COD/day. Sludge production is low. Methane production is 0·35 m3/kg COD destroyed, and the biogas has a calorific value of 22·5 MJ/m3.

Anaerobic digestion has an electrical requirement of about 1·5 kWh/m3 of effluent for pumping and stirring. Capital costs of anaerobic digestion are higher than those of aerobic systems but the operating costs are less.