| According to historical records, until2011, the national Wastewater TreatmentPlants(WWTP) were mainly consists of four types of biological nutrient removaltechnology: the oxidation ditch process(28.6%), the traditional A/O process or A2/Oprocess(27.1%), the SBR process(16.5%) and conventional activated sludge process(27.8%). However, the above processes due to the presence of the drawbacks inherentin a single sludge growth system, resulting in more than90%of WWTP could notachieve first level A discharge standards in China(GB18918-2002). Especially for theweak wastewater with low C/N, which was considered to be a "bottleneck" of WWTP,and it was difficult for the conventional biological treatment processes to achieveefficient and stable removal of nitrogen and phosphorus. Therefor, it is very importantfor traditional BNR that how to use the organic carbon reasonably in order toimprove the efficiency of nitrogen and phosphorus removal, and which couldeffectively curb the further deterioration of the slow-flow water eutrophication.The continuous flow step-feed technology had been extensively studied andapplied at home and abroad because of higher mixed liquor suspended solids(MLSS),shorter hydraulic retention time(HRT), higher utilization of carbon sources, nitrogenand phosphorus removal stable and efficient, saving internal reflux et al. Therefore,the step-feed technology can provide a new way up to first level A discharge standardsfor weak wastewater with low C/N. But so far, the step-feed technology was studiedbased primarily on two aspects: one was the laboratory simulated wastewater, theother was the higher municipal sewage with higher C/N, so there is not any researchor application reports about using step-feed process to treat weak actual municipalwastewater with low C/N at home and abroad.In order to improve the development and the wider application of step-feedprocess，and provide scientific reference for the operation of sewage treatment planttreating weak wastewater(COD≤200mg.L-1,NH4+-N≤40mg.L-1) with low influent C/N,a pilot modified Anaerobic/Anoxic/Oxic(A2/O) step-feed process was applied forsimultaneous biological nitrogen(N) and phosphorus(P) removal. During theexperiment, the effluent of swirl grit tank as the research object from TianyuQingyuan Wastewater Treatment Plant(WWTP)(40,000m3.d-1) employing the OrbalOxidation Ditch process (OD) in Yang ZhouCity, Jiangsu Province, China. And the influent COD, NH4+-N, TN, TP, COD/TN and COD/TP were146mg.L-1,30.8mg.L-1,31.4mg.L-1,2.9mg.L-1,4.77and52.8, respectively. Under steady-state andunsteady-state sinusoidal influent conditions, and the different influence factorsaffecting denitrification and phosphorus removal performance and optimal operationstrategies were investigated. In the case of steady-state influent, this paper haddiscussed the optimization and operating strategy of different influent flowdistribution ratio, volume ratio, HRT and DO, and how to strengthen simultaneousnitrification and denitrification(SND) and nitritation-denitritation of the system to bestudied. In the case of unsteady-state sinusoidal influent, that the different amplitudesaffect pollutant removal efficiency and effluent variation as well as DO controloptimization strategy were closer to the actual operating conditions of sewagetreatment plant, which could provide scientific reference for the design and follow-upoperation. After these main elements of systems research into this subject, and thefollowing conclusions were obtained.（1）When the influent was in steady-state mode, this experiment had studiedand optimized operation control parameters, and the effluent effects of pollutants werecompared at different conditions. At last we identified several different operatingmodels of modified A2/O step-feed process:①Nitrogen and phosphorus removal only by activated sludge process andachieved first level A discharge standards in China(GB18918-2002), the optimum andmost economical operating parameters can be set as following: the inflow distributionratio was20:35:35:10%with HRT of7h and SRT of15d, the volume ratio ofanaerobic/anoxic/aerobic was4:8:10and the volume ratio of anoxic/oxic zone inevery step was1:1, sludge reflux ratio was75%, and DO concentration in every oxiczones were controlled between0.8mg.L-1to1.2mg.L-1; Under this condition, theaverage effluent of COD, ammonia nitrogen, total nitrogen, total phosphorus were27.43mg.L-1、1.57mg.L-1、14.3mg.L-1and0.25mg.L-1respectively, and the effluentconcentration was up to Grade A discharging standard of WWTP (GB18918-2002).The majority of COD in raw water was used as carbon source for anaerobicphosphorus release and denitrification, and more than70%of carbon sources wasutilized effectively. In addition, HRT could be reduced to6h when the DOconcentration in aerobic zones were controlled between1.0-1.5mg.L-1and addingsuspended carriers in aerobic zones, the effluent performance of COD, NH4+-N, TNand TP were25.92,1.98,14.5and0.47mg.L-1respectively; and with this operation control condition, SND performance could be enhanced while reducing the demandfor carbon sources.②The effluent achieved first level A discharge standards took advantage ofactivated sludge process combine with chemical method, and this study proposed theoptimum operating parameters: the inflow distribution ratio was20:35:35:10%withHRT of7h and SRT of15d, the volume ratio of anaerobic/anoxic/aerobic was5:9:8and the volume ratio of anoxic/oxic zone in every step was2:1, sludge reflux ratiowas75%, and DO concentration in every oxic zones were controlled between0.3mg.L-1to0.5mg.L-1; Under this condition, the average effluent of COD, ammonianitrogen, total nitrogen, total phosphorus were28mg.L-1、0.33mg.L-1、8.69mg.L-1and0.65mg.L-1respectively. Wherein the nitrogen removal in the system mainly throughnitritation-denitritation, but phosphorus removal needed further chemical method.And the control strategy and the limiting factors of nitritation and denitritation instep-feed system were studied that lower DO concentration and shorter HRT inaerobic zones were key factors for step-feed process to accumulate nitrite. Nitritationwas achieved through a combination of short aerobic hydraulic retention time(HRT=2.5h) and low dissolved oxygen (DO) levels (0.3-0.5mg.L-1). The nitriteaccumulation rate was about58%and ammonia removal efficiency was over95%. Atthe same, the advantages and disadvantages of this model were analyzed. Theadvantages of second operating model is very suitable for low C/N wastewater andgreatly reduce aeration energy consumption, but the main disadvantage is that toachieve nitritation and denitritation and enrich large amount of Ammonia-oxidizingbacteria(AOB), which need very strict controlled conditions and will take a long time.And it is a big challenge for the actual WWTP to operate.（2）When the influent was in unsteady-state mode, this paper had discussed therelationship between unsteady sinusoidal influent and pollutants removal, variation instep-feed process, and obtain the optimal DO control parameters in aerobic zones. Theresults were as follows:①Under the controlled condition: HRT was8.7h at unsteady influent equilibriumposition, influent flow distribution ratio was20:35:35:10%, and sludge reflux ratiowas75%. When Sinusoidal influent amplitude was increased from±25%to±75%,there was little effect on COD removal, but the effluent NH4+-N, TN and TPconcentration were gradually increased.②The amplitudes of Sinusoidal influent were±25%,±50%and±75%under two operation modes of periodic variable aeration and constant aeration, effluentpollutants showed the same variation respectively: the effluent COD exhibit avariation of piecewise function, but the effluent NH4+-N, TN and TP show withsinusoidal function. When influent at peaks and troughs, effluent NH4+-Nconcentration of three different influent amplitudes appeared the peaks and troughswere delayed2h,3h and5h, respectively; effluent NO3-were delayed8h,9h and11h,respectively; effluent TN were delayed8h,9h and5h, respectively; effluent TP weredelayed4h,3h and2h, respectively.③Two operation modes of periodic variable aeration and constant aeration, thathad no significantly impact on COD and TP remove, but periodic variable aerationwas conducive to improve ammonia and TN removal efficiency; and when influent atpeaks and troughs that influent carbon source effective utilization was increased undervariable aeration mode compared to the constant aeration mode.④From up to level A emission standard measured, when the system wasoperated with constant aeration mode, influent fluctuation amplitude only was±25%and DO concentration was controlled of2.0mg·L-1in aerobic zones at unsteadyinfluent equilibrium position, the effluent of COD、NH4+-N、TN and TP were21.82,0.59,11.87and0.26mg·L-1, respectively. However, when the system was operatedwith periodic variable aeration mode, influent fluctuation amplitude could beincreased to±50%, and the effluent of COD、NH4+-N、TN and TP were23.19、1.50、13.88and0.48mg.L-1, respectively; and compared to the constant aeration mode,periodic variable aeration mode could save9.2%aeration energy consumption. |
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