Advances in Water Treatment and Enviromental Management

76 WATER TREATMENTPublic awareness of both environmental and financial issues, and recentlegislation has reinforced the need for high-rate, prefabricated, packagetreatment plant as alternatives to conventional process options. Most currentsewage treatment options in the United Kingdom are based on aerobicprocesses in use since the early 1900’s ( 1,2 ).Development has led to increases in efficiency and quality, however large“aeration” tank volumes, final settlement and tertiary treatment are normallyrequired to produce good quality fully nitrified effluents. As a result theiruse in prefabricated package plants is limited by size.A comparatively recent development is the Biological Aerated Filter. Firstpatented in North America in the early 1970’s (3), a number of commercialvariants are available (Biocarbone(4), Biofor(5)) and plants are operationalin France, Japan and North America.Work in the UK has been limited. A pilot plant evaluation for carbonaceousoxidation of settled sewage and tertiary nitrification of secondary effluent hasbeen carried out by the Water Research Centre(6). Further pilot scale work hasbeen carried out by Severn Trent Water plc(7) into the production of highquality effluents and a large scale evaluation for complete treatment on a 1250population equivalent plant has been carried out by Thames Water(8).The process comprises a submerged packed bed of granular media. Air isintroduced at or near the base with wastewater flowing upwards ordownwards through the bed.The large specific surface area of the media allows the growth of a highconcentration of attached biomass which is responsible for treatment,typically 4 to 5 times that found in suspended growth systems. As a resultthe process requires aeration tank volumes significantly smaller thanconventional process options.Table 1 shows a comparison of 5 day Biochemical oxygen demand volumetricloading rates (VLR) for conventional treatment options and the biologicalaerated filter. For an equivalent effluent quality, the aerated filter requiresan aeration tank volume approximately 3 times smaller than an activatedsludge plant and 20 times smaller than a percolating filter.In addition to the relatively small aeration tank size, suspended solidsremoval is accomplished throughout the bed and effluent quality is oftengood enough to discharge directly to the receiving watercourse without finalsettlement.Where good quality effluents are required final effluent polishing can beachieved by a static portion of the bed below an intermediate aeration grid,as in the Biocarbone process, or by a separate high rate filter.