SUBMERGED AERATED FILTERS

Description

A submerged aerated filter is designed to reduce the amount of BOD and ammoniacal nitrogen in settled sewage and industrial effluents. Aerated effluent is made to flow through submerged media which acts as a physical support for the growth of microorganisms. The media may be high voidage corrugated crossflow PVC, coarse sand, expanded shale, pozzolanic material or buoyant plastic. Submerged aerated filters can be seeded with bacterial cultures to accelerate start-up.

Submerged aerated filters can operate as mixed or plug-flow reactors. The effluent may flow downwards through one or more cells, counter-current to the rising air, or liquid and air may flow co-currently upwards and downwards through alternate cells. Air is introduced from perforated pipes, from nozzles or from fine bubble diffusers located below the media support structure. Submerged aerated filters have no internal moving parts.

With rigid plastic media the biomass is sloughed continuously and a settlement stage or a fine filtration system is required. Where inert granules or buoyant support is used, the effluent is relatively free from suspended solids but the biomass accumulates until a wash cycle is initiated. Then excess biomass is removed by air scouring or water backflushing or both. Wash liquors are returned to the primary settlement stage.

Application

Submerged aerated filters occupy about 40 % of the land area required by equivalent activated sludge plants. Very high concentrations of microorganisms can be established, perhaps 10,000-15,000 mg/l. The effluents produced may have less than 5 mg/l of ammoniacal nitrogen and are therefore appropriate for remote rural communities where direct discharge to a watercourse is intended. Submerged aerated filters are resistant to peak and shock loads and can be used treat industrial wastewaters containing phenolic compounds and thiocyanates.

Sizing

Loading rates of 3-6 kg BOD/m3/day are appropriate where BOD removal is required. Higher loading rates have been used with certain industrial effluents. Where residual ammoniacal nitrogen values of less than 10 mg/l are required loading rates should not be more than 15 kg BOD/m3/day. Loading rates for ammoniacal nitrogen would be about 03-06 kg N/m3/day. Lower flow rates and longer retention times may also be appropriate. The energy requirement for treatment is about 1 kWh/kg BOD removed.