BIOLOGICAL FILTERS

Description

A biological filter is a system for the treatment of relatively small flows of sewage or industrial wastewater to remove BOD, particulate material and suspended solids. Ammoniacal nitrogen may also be oxidised. Discharge of the treated effluent may be to a subsoil distribution system or to a watercourse. The treatment system typically comprises a settlement stage or a septic tank in which the bulk of the solid material is retained. The partially clarified liquor is then distributed by nozzles or splash plates over a bed of plastic media on which a film of microorganisms develops. The partially treated effluent drains to a sump from which it will be recycled by a pump over the media effluent several times before discharge.

The outer surfaces of the microbial films are in contact with air and the microorganisms there are aerobic. Aerobic conditions in the media may be assisted by fan ventilation of the biofilter. The microorganisms that lie well below the film surface, next to the plastic surfaces, may be anaerobic in nature but will also assist in the reduction of the BOD. The objectives of removing BOD and ammoniacal nitrogen are tackled successively. The BOD value must be low and the effluent well oxidised before the nitrifying bacteria will begin to oxidise ammoniacal nitrogen to the less toxic nitrate.

The microbial film builds up until it is sloughed off by the flow of effluent. This biological sludge and trapped fine solids settle out and is then pumped intermittently by a small centrifugal pump or by an airlift system back to the septic tank. The septic tank is usually sized to be emptied by vacuum tanker at regular intervals of perhaps 3-6 months. The clarified liquor is often of adequate quality to be discharged to field drains or, if ammoniacal nitrogen is sufficiently low, directly to a watercourse.

Application

Biological filters are particularly appropriate where connection to a mains sewer would be expensive or impractical. Typical applications would include small factories, service stations, schools, golf course clubhouses and nursing homes. Systems are available that are capable of treating the effluent from 5-300 people or the equivalent small industrial discharge.

Sizing

A typical biological filter treating an average flow of 2 m3/hour would require a 2 kW recirculation pump, a 2 kW sludge pump, a 1 kW aerator, and a 0ยท75 kW transfer pump. Treating the typical daily flow would require about 75 kWh.