SUBMERGED BIOLOGICAL CONTACTORS

Description

A submerged biological contactor is an aeration device for reducing the amount of BOD and ammoniacal nitrogen in liquid effluents. A fixed film of microorganisms covers the surfaces of the submerged contactor and consumes the pollutants. In one type of submerged biological contactor high-density polyethylene sheets are formed into a cylindrical contactor up to 7 m long. The central horizontal shaft is supported on bearings in a semicylindrical tank so that about 70-90 % of the contactor is submerged. Air is released from diffusers located below the contactor. Some of the rising air migrates through radial and concentric passages in the media while the rest lodges under receptacles to cause a tangential action that turns the contactor on the shaft. The attached biomass on the outer half of the plastic media is alternately submerged in the effluent, where it absorbs BOD, or raised out of the liquid to absorb more oxygen. The central part of the contactor is permanently immersed in the effluent.

In a different design, the media consists of coils of plastic tubing into which smaller diameter tubing has been inserted. The coils are arranged on a reel and immersed and rotated in the effluent. At each revolution, a large bubble of air is trapped in each tube and forced through the system, aerating the biomass that adheres to the inside walls of the tubes and keeping the tube clear.

Submerged rotating contactors impose less load on the shaft bearings than rotating biological contactors. The biomass forms a thinner film than is found on rotating biological contactors, and as the reactor vessel can be deeper, the volume of liquor in contact with the media is greater.

Application

Submerged biological contactors have been used to treat domestic sewage and effluents from the leisure industry and fish farms. Multi-stage systems are available. Submerged biological contactors do not produce odours, and being aerated, can be enclosed without loss of performance. Enclosure will raise the operating temperatures, improve nitrification, and allow exhaust air to be passed through a biofilter.

Sizing

Submerged biological contactor units are available with surface areas up to 25,000 m2 or 240 m2/m3. Submerged rotating filters can accept BOD loadings of up to 8 g/m2/day where BOD removal is required, and up to 4 g/m2/day where nitrification of ammoniacal nitrogen is also required. The maximum loading of ammoniacal nitrogen is about 05-10 g N/m2/day. Sludge production is about 1 kg solids/kg BOD oxidised.