Biological aerated filter

Biological aerated filtering

The biological aerated filter (BAF) has a 2-3m deep bed of relatively small size filter media, to provide a high surface area, on which to grow a biomass. The filter bed is submerged and a settled wastewater is pumped either upwards or downwards through the filter. A blower sends air through a diffuser at the bottom of the bed which generates bubbles which then rise through the filter providing a steady stream of oxygen for the biomass to support the oxidation process.

Trapped solids and a growing biomass gradually block the filter pathways and these obstructions are cleared from time-to-time by air-scouring or back-washing with treated effluent. The waste backwash water is held in a well and gradually returned either to the inlet of the BAF plant or to a primary sedimentation tank, if one is being employed.

Biological Aerated Filter

Package plant variants

The package plants use very lightweight plastic particulates that float to the top of the tank and which are restrained from breaking to the surface by a mesh near the top of the tank. This necessitates only air scouring to re-establish the surface area of the bed and keep the drainage path clear, removing the need for backwashing.

The sludge settles to the bottom of the tank and is drained back to the head of the works. Larger plants use a structured medium.