BIOLOGICAL AERATED FILTER (BAF)

The basic operational principle of a biological aerated filter is that of a conventional biological filter operating in a submerged mode. A biological aerated filter comprises a reactor vessel, aerated by sub-surface fine bubble diffusers with associated air blowers. The reaction vessel contains an inert media (usually random) that is used as a support for an attached biomass. The media is usually granular in appearance with a size of between 3 and 6 mm and has a rough porous surface structure to increase the surface area available for attached growth. Due to the media size a BAF will operate as a filter and remove non-biodegradable solids from the waste stream however, it is essential to provide fine screening (< 3mm) or primary sedimentation of the wastewater prior to the BAF to avoid blockages. Biological aerated filters do not require a secondary clarifier as excess biomass is removed by periodic backwashing washing of the media bed (at least once per day and usually automatic based on a headloss value), the backwash liquor being returned to the head of the works for further treatment. During the backwashing phase of the operational cycle the filter will be unavailable for duty, a minimum of two filters are therefore required, each capable of accommodating 100% of the hydraulic and organic load. The basic principle and design of a BAF is amenable to a cellular approach to treatment and for this reason a single BAF plant, with the appropriate sub-divisions, may be used for carbonaceous oxidation, nitrification and denitrification. There are a number of different types of BAF available, usually classified according to the biomass carrier type (media) and the direction of the wastewater flow.

SUBMERGED AERATED FILTER (SAF)

Although referred to as a filter, the basic operational principle of a submerged aerated filter is that of a conventional biological filter operating in a submerged mode and does not therefore fulfil any solids/liquid separation function. A biological aerated filter comprises a reactor vessel, aerated by sub-surface fine bubble diffusers with associated air blowers. The reaction vessel contains a plastic media (structured or random) that is used as a support for an attached biomass and as with all media is designed to have a large surface area to support the maximum biological population and maximum voidage to permit the free flow of liquid/air through the media. Although the media is designed to achieve the maximum voidage possible, the open space between the media is often very small (to permit surface contact of as much media as possible). It is therefore essential to provide fine screening (< 3mm) or primary sedimentation of the wastewater prior to the SAF to avoid blockages. As with any fixed film reactor excess biomass (the equivalent of surplus sludge in an activated sludge plant) is continually sloughed from the support media (humus) and treated effluent from a SAF must always be discharged to a secondary clarifier to settle entrained solids prior to the discharge of final effluent from the plant. As with a BAF plant, the basic principle and design of a SAF is amenable to a cellular approach to treatment and for this reason a single SAF plant, with the appropriate sub-divisions, may be used for carbonaceous oxidation, nitrification and denitrification.

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