Construction is simple and may be either a reinforced concrete or steel tank. The latter usually showing considerable cost advantages. The tank is frequently situated above ground, so that site excavation costs are minimal.

The plant is controlled by 2 level probes and a series of fully adjustable timers, which are used to control the aeration, fill and settle / decant sequences. Aeration is controlled in conjunction with D.O. probes linked to minimum run and stop timers. Surplussing of sludge from the Bifad is also controlled via a tier and surplussing is initiated during the decant cycle.

Incoming sewage is mixed and aerated with naturally developed and activated sludge in the tank, by means of either mechanical floating aerators or fine bubble diffusers

Sewage treatment using the ‘Bifad’ system is achieved using a fill, aerate and settle / decant function, separated only in time. Only one water retaining structure is used, so this process provides an extremely economical plant for the full treatment of sewage.

A typical ‘Bifad’ cycle would include

• Maximum 20 hours aeration of sewage (including 3 hours minimum of tank fill time)
• One hours settlement
• Three hours decanting of final effluent and sludge surplussing

The treatment of the pollution is a two step process; absorption into the sludge mass followed by enzymic oxidation, and conversion in the bacteria cells. Effluent from the Bifad is decanted using either a telescopic valve or a floating arm.