Research On Baffled Biological Aerated Filter For Small Town Sewage Treatment

Currently, urbanization is speeding up in China, as a result, the construction ofsmall towns has made a big development. But the small towns’ infrastructureconstruction is limited, wastewater lacks of treatment. These problems need to besolved instantly. Small town sewage has a good biochemical treatability, concentrationN&P is high, quantity is small, and daily variation is remarkable. And then sewagetreatment equipment with high efficiency, low cost, and easy operation is needed. TheBaffled Biological Aerated Filter Equipment,with huge microorganism quantity, hightreatment loading, small size&investment, is very suitable for small towns.In order to achieve high economical efficiency, constructional sustainability andeasy operation. The Baffled Biological Aerated Filter （BBAF） is integrated in athree-stage series combined wastewater treatment equipment after reasonable structuraldesign and parameter optimization. The productive experiment consist of start-up andoperation performance of BBAF equipment, influence analysis of different operationcondition on treatment and optimal operating condition research. And then, theanalytical research of pollutant concentration change discipline along the direction ofwater flow, biological membrane space distribution characteristics in BBAF equipmentand its technical economic feasibility have been done.The activated sludge was used to inoculate the BBAF equipment for its start-up.14days later, the COD removal rate reached80%, and NH₄⁺N removal rate reached90%.Through microscopic examination, some metazoa such as Rotifer and Eelworm havebeen found in the effluent of BBAF equipment. Under low temperature condition, theequipment has been closing down for21days. The restart-up only needs5days, withthe influent hydraulic loading same to design value, after back-washingWhen BBAF equipment was operating according to the design parameters, theremoval rates of SS、COD、NH₄⁺N and TP were92%,77%,97%,41%and46%respectively. BBAF equipment possesses strong anti-shock loading capability. When theinfluent hydraulic loading was between the range of0.20m³/（m³·h）⁰.30m³/（m³·h）, itspollutant removal efficiency remained well.When the influent hydraulic loading was toohigh，the removal rate of SS and NH₄⁺N were decreased，as well as COD，while it wasgood for the removal rate of TN.The optimal operation parameters of the equipmentthat can be found by the experiment: the gas-water ratio is8:1and the reflux ratio is100%. Under this condition,The removal rate of SS, COD, NH₄⁺N and TN were above90%,90%,80%and50%,respectively. The outlet concentration reached the standard of the GB18918-2002stably.TP could be removed by adding PAC during pretreatment proces.If the TPconcentration of influent is lower than2mg/L, the outlet concentration of TP wouldalso meet the standard。In the process, equipment to remove the SS mainly concentrated in the firstaerobic tank （flow length1.8².7m）, organic matter degradation and TP remove alsomainly concentrated in this stage, ammonia nitrogen nitration are mainly concentratedin the first aerobic tank （flow length1.8².7m） and the second aerobic tank （flowlength3.6⁴.5m）, and TN removal occurred in influent parts （flow length0⁰.9m） andthe effluent parts （flow length4.5⁵.4m）. Generally, the treatment function of eachsection of the equipment can reach the design purpose basically.When BBAF equipment was under the optimal operating conditions, throughmicroscopic examination, it can be found that the level of microorganisms in anoxicfilter is low. The main microorganism is various small-size bacterium. The level ofmicroorganisms in aerobic filter is higher obviously. The main microorganism areflagellates and infusorians protozoa. And some metazoa such as Rotifer and Eelwormcan be found near the outlet. The range of biomass per unit area on ceramics filtermedia surface is0.3⁰.5mg in anoxic filter and the first aerobic filter. The biomass perunit area on ceramics filter media surface in the second aerobic filter is about0.1mg.However, the bioactivity in the second aerobic filter is much higher than it in the firstone.Economic analysis states that the capital investment is1098yuan each tons ofwater, the operating cost is0.46yuan each tons of water. Comparing with average levelof small towns sewage treatment already built projects around Three Gorges ReservoirArea. The capital investment and operating cost have cut down13.3%and11.7%respectively. The structure of BBAF equipment is compact, it just covers an area of0.91m². The operation period of BBAF equipment was2³days., The treatment effiencywas recovered fast after back-washing.About3hours later，all kinds of pollutantconcentration of effluent could meet the first B class discharge standards of "CitiesSewage Treatment Plant Pollutant Discharged Standard"（GB18918-2002）. Sum up, theequipment has outstanding technical economic feasibility.The long-term productive experiment showed that BBAF equipment is effective and stable. The N&P removal rate is high. It possesses strong anti-shock loadingcapability, good technical economic feasibility and well applicability to the small townsewage.