ACTIVATED SLUDGE SYSTEMS

Description

An activated sludge system reduces the BOD and ammoniacal nitrogen values in liquid effluents in a continuous cyclical process. The effluent is aerated in a tank in the presence of a sufficiently high population of active aerobic microorganisms. After a treatment time of about 6 hours, the liquor flows into a settlement tank. The microbial bodies and fine solids settle out as a sludge and the treated clarified liquor can be discharged. The settled sludge is returned quickly to the aeration tank so that the microorganisms remain active.

The microorganisms remove dissolved BOD by incorporating it into new cells (the sludge). During respiration the microorganisms also eliminate BOD as carbon dioxide. Normally, an excess of sludge is produced and this is usually dewatered before disposal. Less sludge is produced by maintaining a high level of dissolved oxygen or by encouraging the surplus sludge to digest itself in a separate highly aerated tank

Application

Activated sludge systems are used to treat liquid effluent from the food and beverage, chemical, petroleum, paper and textile industries, and from domestic sewage, agricultural effluents and leachate from landfill sites. More than 90 % of the BOD in an effluent can be removed, generally without odour or fly problems.

The capital cost of activated sludge may be less than other systems but the operating cost may be higher, particularly if sludge production is high. Higher quality effluents and resilience under peak and shock loads may be obtained from extended aeration and sludge production may be minimised by contact stabilisation.

The aeration equipment is a central part of an activated sludge system. Typical systems include fine bubble diffusers, Venturi aerators, surface aerators, and static aerators. Efficient oxygen transfer and mixing is essential to achieve low running costs. Pure oxygen systems are also used, particularly where high oxygen levels are advantageous, as with intractable compounds or where sludge production has to be minimised.

Sizing

Activated sludge is an intensive process and is able to oxidise 0·4-1·7 kg BOD/m3/day of sewage. A typical industrial liquid effluent flow of 20 m3/hour with a BOD value of 1000 mg/l might require about 4 kW of pumping power for delivery into a 200 m3 above-ground aeration tank, about 12 kW of aerator capacity, a settlement tank and a 2·5 kW sludge return pump. A typical oxygen transfer power efficiency could be 1-3 kg O2/kwh, depending upon the particular aerator used.