The effect of mixing conditions on anaerobic digester performance

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The basic principle of aerobic wastewater treatment is the conversion of some parts of organic matters to biomass and this causes production of excessive sludge in aerobic processes. This excessive sludge is withdrawn from time to time to prevent accumulation of biomass in aeration basin. In most cases, this sludge requires the stabilization before the dewatering and disposal. Different processes have been used for stabilisation of waste activated sludge (WAS) produced in wastewater treatment plants. Anaerobic digestion has become the most widespread method for the stabilisation due to the energy recovery as biogas and development in heating and mixing equipment. Anaerobic digestion has also been applied to treat high strength organic wastes, just like food and agriculture wastes. The advantages of anaerobic processes are: high degree of waste stabilisation, low production of waste biological sludge, low nutrient requirements, reduction in pathogens, no oxygen requirements and production of biogas as a useful end product (McCarty, 1964). The major disadvantages are relatively high temperatures requirement for optimum operation and difficulty in start up due to the slow rate of growth of the methane producing bacteria. Dilute wastes may not produce sufficient methane for heating the reactor and this may represent a major limitation. Anaerobic digestion process is composed of three basic steps: hydrolysis, acidogenesis and methanogenesis (Fig. 1). Different bacteria and microorganisms with different environmental requirements carry out each stage. The methanogenic reaction has been considered to be rate-limiting step in anaerobic process. However, the rate of degradation of WAS is especially low and in this case the hydrolysis reaction may be considered to be the ratelimiting step in overall anaerobic digestion process (Li and Noike, 1992).

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