Optimization And Combination Of Advanced Treatment Technology Of Secondary Effluent From Municipal Wastewater Treatment Plants

With the continuous expansion of the scale of urban wastewater treatment in China,wastewater discharge standards become increasingly stringent in order to reduce the impacts of tailwater discharge from wastewater treatment plants on the water environment.The construction of advanced treatment technology for tailtwater is of vital importance for improvement of quality and efficiency for wastewater treatment plant.To solve the problems of poor adaptability,low utilization rate and high energy consumption for the current advanced treatment technology,the key factors affecting the operation effect of high-efficiency sedimentation tank（HST）,denitrification biofilter（DNBF）,ozonation process（O₃）and biological aerated filter（BAF）were firstly investigated and analyzed in this study.Based on the investigation and analysis,the optimal operating parameter of single advanced treatment technology and the pollutants removal capability by different combined processes were explored in pilot tests.The suggestions on the selection of combined processes of advanced treatment technology for different influent conditions and effluent quality requirements were proposed.The main research conclusions are as follows:The influence of coagulant dosage,flocculant dosage and sludge reflux ratio on the operation effect of high-efficiency sedimentation tank was investigated.The influence of the types and dosage of carbon sources as well as the types and particle size of filter media on the operation effect of denitrification biofilter was investigated.The influence of ozone dosage and ozone contact time on the operation effect of ozone technology was investigated.The influence of hydraulic loading and air to water ratio on the operation effect of biological aerated filter was investigated.The recommended design and operating parameters of the four advanced treatment technologies were proposed.By researching the single technology and combined process of HST and DNBF,it was found that the optimal sludge reflux ratio of HST was 3%,and the average volume removal load of SS was 488.76 g/（m³·d）.The optimal dosage of external carbon source for DNBF was C/N=4,and the average effluent TN concentration was 1.78 mg/L.The effluent TN concentration could be reduced to 1.65 mg/L by the combined HST-DNBF process.By researching the single technology and combined process of O₃ and BAF,it was found that the optimal ozone contact concentration was 1.07 mg/L.The NH₄⁺-N removal efficiency of BAF could be improved by the increase of the air to water ratio,and the optimal air to water ratio of BAF was 3:1.The effluent COD concentration could be reduced to 12.01 mg/L by the combined O₃-BAF process,which was lower than the COD limit of classⅢsurface water standard（20 mg/L）.The average NH₄⁺-N removal rate by O₃-BAF combined process was 59.67%when the influent NH₄⁺-N concentration was higher than 0.97 mg/L.The operating conditions of the DNBF-O₃-BAF combined process were as follows:the dosage of external carbon source was C/N=4,the ozone contact concentration was 1.07 mg/L,and the air to water ratio of BAF was 3:1.The average removal rates of COD,NH₄⁺-N and TN by the combined process were 25.22%,23.95%and 43.44%,respectively.The average effluent concentrations of COD,NH₄⁺-N and TN were 16.35 mg/L,0.24 mg/L and 3.19 mg/L,respectively.Analysis of three-dimensional fluorescence spectrum showed that ozonation process could effectively remove soluble microbial metabolites,fulvic acid and humic organic compounds in secondary effluent.Analysis of microbial community structure showed that Proteobacteria was the dominant bacteria in the denitrification biofilter at the phylum level.The denitrification biofilter could maintain better anaerobic and alkaline environment and promote the denitrification process.The pollutants removal effect of the three combined processes was evaluated in this study.Meanwhile,the process selection criteria and operating parameters were put forward and the engineering investment cost was estimated.This study provides basis for improvement of quality and efficiency for wastewater treatment plant.