Anaerobic Digestion

Large volumes of wet sludge which are produced in primary and secondary sedimentation tanks may have to be reduced in volume before disposal. This volume of sludge can be reduced by anaerobic digestion. In sludges containing 20,000-60,000 mg dm"3 solid matter, 80% of the degradable matter may be digested, which will reduce the solids content by 50%. During anaerobic digestion acid fermenting bacteria degrade the waste to free volatile fatty acids, mainly acetic and propionic acid, which are then converted to methane (~ 60%) and carbon dioxide (~ 40%). The gas produced (biogas) is a very useful by-product, and can be burnt as a heating fuel, fed to gas engines to generate

Pressure control valve

Waste gas vent

Oxygen supply

Sewage feed

Pressure control valve

Waste gas vent

Oxygen supply

Sewage feed

Multistage Oxygen Aeration System

Surplus activated sludge

Fig. 11.6. Schematic diagram of a multistage UNOX system (Fuggle, 1983).

Surplus activated sludge

Fig. 11.6. Schematic diagram of a multistage UNOX system (Fuggle, 1983).

electricity or used as a vehicle fuel. As well as being used in sludge digestion and conditioning, anaerobic digesters are also used directly in the treatment of many high strength wastewaters, for example from the food and agricultural industries (Gray, 1989).

A number of anaerobic processes have been developed; completely mixed reactors (often these are simply described as anaerobic digesters) with or without sludge recycle, anaerobic filters, up-flow anaerobic sludge blankets (UASB) and anaerobic fluidized beds are the most common. Two stage anaerobic treatment systems utilizing a short retention time completely mixed acidification reactor followed by a UASB methanogenic reactor have been reported (Burgess and Morris, 1984; Ghosh et al., 1985). Benefits claimed

Oxygen supply

Sidestream

Oxygen supply

Sidestream

Pump

Fig. 11.7. Simplified VITOX sidestream aeration system.

Pump

Fig. 11.7. Simplified VITOX sidestream aeration system.

include improved reliability, higher BOD/COD reductions and greater methane productivity.

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