Advances in Water Treatment and Enviromental Management

DEVELOPMENT OF AERATED FILTER PACKAGE PLANT 79effective grain size. Both treatment and polishing filters were filled with thesame media to depths of 2m and 1.8m respectively.Screened, degritted raw sewage from the main works flow was fed to aprimary sedimentation tank. Settled sewage flowed into a holding tank andwas pumped from there to the inlet of each treatment filter.3. METHODThe programme of work for the pilot plant was divided into 2 operationalphases. During the first period the plant was operated with 3 downflowaerated filters treating the same load with equal air supply, followed by adownflow polishing filter. In the second period the load to 2 of the treatmentfilters was increased, with the third acting as a control on unchanged load.Early results from the pilot plant evaluation showed the feasibility of largersized plant based on an aerated treatment zone followed by an integral highrate polishing filter.As a result a full scale treatment module was designed and installed atChesham sewage treatment works. The performance of the new plant wasmonitored over a period of months.Samples of influent, treatment filter effluent and polishing filter effluentwere taken from the pilot plant, with influent and effluent samples fromthe full-scale plant. The samples were analysed for suspended solids (SS), 5day Allyl Thiourea inhibited biochemical oxygen demand (BOD 5 ), totalkjeldahl nitrogen (TKN), ammoniacal nitrogen (NH 4 -N), nitrite nitrogen (NO 2 -N) and nitrate-nitrogen (N0 3 -N).4. RESULTSStart up of the 3 treatment filters was achieved by initial batch aeration ofsettled sewage followed by a period of continuous low flow. The flow wasincreased from 5.5 1/m to 16.7 1/m over a 1 month period.During the steady state phase the flow to the treatment filters was keptconstant at 16.7 1/m. All of the flow from the 3 treatment filters was passedto the downflow polishing filter giving a total flow of 50.1 1/m. These flowsrelate to hydraulic loading rates (HLRs) of 1.0 m/h and 3.0 m/h for treatmentand filtration respectively.The process air flow rate was kept constant at a rate calculated from thestoichiometric oxygen demand for carbonaceous oxidation and nitrificationand an assumed oxygen transfer efficiency.The average volumetric loading rates (VLR) for this period were 0.86 kgBOD5/m 3 .d, 0.18 kg NH 4 -N/m 3 .d and 0.36 kg TKN/m 3 .d Effluent quality