ROTATING BIOLOGICAL CONTACTORS

Description

A rotating biological contactor (RBC) is an aeration device for reducing the BOD value of a liquid effluent. A fixed film of microorganisms is established on a contactor which is constructed from sets of discs made of corrugated GRP, high-density polyethylene or polypropylene fixed to a central horizontal shaft. The shaft may be 3-7 m long and is supported on bearings in a semicircular steel, glass reinforced plastic or concrete tank so that about 40 % of the contactor lies below the surface of the effluent to be treated. The shaft is turned slowly and the attached biomass is alternately submerged in the effluent where it absorbs BOD or raised out of the liquid so that the microorganisms absorb oxygen.

RBCs for small flows are constructed as packaged systems comprising a primary settlement zone or septic tank in which gross solids settle. The partially clarified effluent passes forward into the semi-circular aeration tank. The tank can be subdivided so that different microbiological populations develop on different parts of the contactor. The first sections process BOD and ammoniacal nitrogen may be oxidised in later sections. Excess biomass is continually sloughed from the rotating contactor and is settled in a third compartment. The clarified liquor is then discharged from the plant and the settled sludge may be pumped to the septic or primary settlement tank. For larger flows the three stages can be separated.

An RBC can maintain a higher biological population than in a conventional activated sludge plant. As a result an RBC can be relatively small, and the effluent discharged from an RBC may have a BOD value of less than 20 mg/l. Flow balancing and management systems are advisable if high quality effluents and, in particular, if low ammoniacal nitrogen values are to be achieved routinely.

Application

RBCs are used for the treatment of domestic sewage, and for the effluents from small industrial estates, motorway service areas and from the leisure industry. RBCs are frequently installed where there is a requirement for a low visual impact and no fly nuisance. RBCs may be designed either to reduce the BOD value of the discharged effluent or to also provide a high quality nitrified effluent.

Sizing

RBCs come in a range of sizes with disc diameters from 1-36 m with loading rates of 5-10 g BOD/m2/day. The media for BOD removal may have a surface area of 125-175 m2/m3. Package units are made for population equivalents of 5 to 400 people and typically treat up to 25 kg/day of BOD in flows up to 80 m3/day. Units with three separate stages are available for loadings of 60 kg/day of BOD, equivalent to 1000 people. Such a unit would require a 22 kW motor to power the rotor. The electrical consumption is about 1 kWh/kg of BOD oxidised.