Study On A/A/O Oxidation Ditch Process Of Advanced Nitrogen Removal Synergistic Biological Phosphorus Removal

There are some difficulties in treating municipal wastewater in Three GorgesReservoir Area of China because of the low carbon concentration, comparative strongnitrogen concentration, seasonal changing quality and quantity, the effluent quality cannot be stable to reach the first level A criteria of the Discharge Standard of Pollutantsfor Municipal Wastewater Treatment Plant（GB18918-2002）. To solve the problem theanaerobic-anoxic-aerobic oxidation ditch（AAO oxidation ditch） technology wasinvented. The research focus on real municipal wastewater treatment in a pilot scalesystem with capacity of25m3/d and a productive regulation project with capacity of10000m3/d.The effluent of sewage treatment plant can not be stable to reach the first level Bcriteria was mainly reflected in the effluent TN concentration can not meet first level Bcriteria before regulation, which mainly due to weak carbon concentration,excessiveaeration, a single control technology and poor activated sludge traits.The influentconcentratons of COD, NH₄⁺-N, TN and TP of pilot system were215mg/L、26.5mg/L、38.8mg/L、4.95mg/L respectly, the ratio of C/N and C/P was5.5and43.4respectly. Theneeded carbon for nitrogen and phosphorus removal was lacking because of the lowC/N and C/P ratios. The influent quality fluctuates with the alternation of season and thepollutant concentration in rainy weather was about68%of that in dry weather. So thebiological reactor must be adjusted accoding to the changes of temperature, influentquality and precipitation for making the most of nitrogen and phosphorus removal withthe limited carbon resource.The impact of the internal reflux ratio, sludge reflux ratio, sludge age （SRT）, theaverage DO concentration in aerobic zone of a pilot scale system AAO oxidation ditchto the nitrogen and phosphorus removal performance have been studied systematically.The changes of the pollutants removal performance and pollutant removal commitmentof the various stages of the AAO oxidation ditch process have been studied by adjustinga range of operating parameters: internal reflux ratio, sludge reflux ratio, SRT andaverage DO concentration in a aerobic zone, a reasonable parameters have beenpreferred.The effect of DO on the performance of a pilot scale system AAO oxidation ditchprocess was in-depth studied by setting the concentration of DO at4.0,3.0,2.0,1.0 mg/L. It was observed that the effect of DO concentration on COD removal wasinvisible, whereas the removal rate of TN increased with the decrease of DO, and theefficiency of SND of the system gradually improved simultaneously. The AAOoxidation ditch system could run in low DO condition when high nitrogen removalefficiency is required only. The concentration of DO has a great influence onphosphorus removal, the system keep high biological activity on phosphorus removalwhen the concentration of DO was not less than1.0mg/L. While when theconcentration of DO was approximately1.0mg/L, the effluent phosphorus can not meetfirst level B criteria, chemical phosphorus removal was needed to complete thephosphorus removal performance of the system. On the whole, in order to achieveexcellent nitrogen and phosphorus removal efficiency, the mean concentration of DOshould not be lower than1.0mg/L,2.0mg/L around has been recommend.On the basis of the single factor experiments of a pilot scale system AAOoxidation ditch,the study involves the pollutants form and removal efficiency, the CODdistribution in different processes. With these studies the operating modes of the AAOoxidation ditch reactor in different seasons were confirmed.By comparing differentprocesses in the same period, following operting mode of AAO oxidation ditch reactorwas recommend: the conventional AAO oxidation ditch process fits on spring period,low DO/conventional DO AAO oxidation ditch process was recommend in dry weatherin summer period, step feed low DO/conventional DO process was adopted in rainyweather in summer period, the conventional AAO oxidation ditch process wasappropriate in autumn period, and step feed step reflux AAO oxidation ditch processwas better in winter period. The analysis of the organic matter, nitrogen and phosphorusremoval rule and the carbon resource distribution shows that the preferable process canbetterly distribute and utilize the limited carbon resource.The methodology to strengthen nitrogen removal efficacy by SND of the aerobiczone has been studied, the material equation about the natural partition of aerobic andanoxic between the two aeration turntables has been proposed, the theoreticalsignificance and application value of the equation have been discussed.Based on this,regulatory test of the DO control has been carried in the sewage treatment plant, whichobtained better nitrification reaction effect and denitrification effect through acombination of overall partition and partial partition, based on the local nitrate whichproduction and consumption obtained mass balance, the advantage of the overallnitrogen removal has been focused, so the nitrogen removal has ben enhanced significantly,the effluent dissolved TN of the aerobic zone decreased from16.0mg/Lbefore regulation to9.5mg/L,which achieved the depth of denitrification.The stability of the effluent quality of sewage treatment plant has been enhancedafter regulation, the effluent has fully reached the first level A criteria, the removalefficiency of TN has been significantly improved, the effluent TP become more stable,the carbon utilization has become more reasonable, and achieve energy efficiency.