The Optimization Research Of Cass Process In Urban Sewage Treatment

CASS process (Cyclic Activated Sludge System) is an advanced activatedsludge process which has good biological denitrification and phosphorusremoval performance of discharging wastewater, it has many advantages, forexample it covers an area of saving, no need to establish the first and the secondsedimentation pond, saving capital, has strong ability to resist impact loading, itis convenient to operate and manage. CASS process has been more and moreused in the sewage treatment industry in our country in recent years.A CASS process sewage treatment plant which deals with urban sewage inXian yang was up to the requirement of the standard of level B in class I ofDischarge Standard of Pollutants for Municipal Wastewater Treatment Plant（GB18918—2002）before. The removal efficiency of COD, BOD5, SS, NH3-N washigh, but removal effect of TN and TP was not good, especially the removalefficient of TP was the lowest. Aimed at the fact of the situation of thewastewater treatment plant that a small test was used to adjust the removalefficient of COD, NH3-N, TN and TP. Studying the effect of five parameters offlooding time, aeration time, aeration quantity, sludge refluxing ratio and sludgeretention time on the CASS process.When the small experiment was designed and was successfully formed,inoculation method was used to cultivate and domesticate activated sludge.Practice showed that after vaccination and closed aeration time in3days, thecultivation and domestication time in20days, operation at full loading in25days, the average removal rate of COD was90.7%, the average effluentconcentration of COD was27.0mg/L; the average removal rate of NH3-N was94.3%, the average effluent concentration of NH3-N was1.1mg/L; the averageremoval rate of TN was61%, the average effluent concentration of TN was7.8mg/L; the average removal rate of TP was77.7%; the average effluentconcentration of TP was0.76mg/L. Not only the removal effect of COD andNH3-N were good but also the removal rate of COD and NH3-N were high. The removal rate of TP and TN were low. In addition to the TP, the effluentconcentration of COD、NH3-N and TN reached the requirement of the standardof level A in class I of Discharge Standard of Pollutants for MunicipalWastewater Treatment Plant（GB18918—2002）. As a result, improving theremoval rate of TN and TP was the key point of process optimization.After four months of parameter optimization test in process, the test resultshowed that when the cycle is4h, influent time of1h, aeration time of2h,precipitation time of1h, water draining time of1h, water influent and aerationtime of1h, the effluent water indicators of CASS process were good, it wasdetermined to be the best cycle. When dissolved oxygen aeration in the end wereselected among under2.0mg/L,2.0to3.0mg/L,3.0to4.0mg/L, the averageremoval rate of COD, NH3-N, TN, TP, respectively were87.37%,91.61%,65.95%,91.61%;97.21%,98.83%,68.77%,98.83%;97.91%,94.67%,64.37%,94.67%, therefore, to choose2.0-3.0mg/L as the best dissolved oxygen aerationin the end; The removal rate of TN and TP were greatly influenced by the sludgereflux ratio, the sludge reflux ratio was30%, the effect of treatment in CASSprocess was the best; Sludge retention time is an important factor to the removaleffect of denitrification and dephosphorization in CASS process, biologicaldenitrification and dephosphorization are the two conflicting process, therefore,the selection of a suitable sludge retention time is the key to denitrification andphosphorus removal in the CASS process, in small experiment when sludgeretention time was12d, denitrification and phosphorus removal effect was thebest in CASS process.The result of the test was applied to the practical engineering, afterdebugging in1month, effluent water quality in sewage treatment plant wasremarkablely improved, the average removal rate of COD, NH3-N, TN and TPwere92.74%,96.55%,70.15%,96.55%; the average effluent concentration ofCOD, NH3-N, TN and TP were:23.82mg/L,0.88mg/L,13.04mg/L,0.23mg/L.The effluent indexes were up to the requirement of the standard of level A inclass I of Discharge Standard of Pollutants for Municipal Wastewater TreatmentPlant（GB18918—2002）.