| With the continuous promotion of rural revitalization and beautiful countryside construction,the development of rural environmental governance has fallen behind the pace of agricultural and rural modernization,and has long been unable to meet the new needs of the development of The Times.The 13 th Five-Year Plan period is a critical period for rural environmental protection,driven by the strategic goal of "world-class ecological island",Chongming has built more than 17000 new rural sewage treatment facilities,but in the long running process,most of the water processing facilities belowstandard total nitrogen,ammonia nitrogen,total phosphorus,low C/N,but poor biodegradability.In this paper,through field investigation,collection,calculation,analysis of Chongming rural average household population,discharge coefficient,treatment device operation status and other data and information,on the basis of this preliminary proposed sewage treatment system technical reform scheme.In addition,the author also conducted long-term follow-up monitoring on the water quality of the five sets of low-C /N integrated treatment units for rural domestic sewage that were already in operation in the technical reform scheme.Based on the treatment efficiency,operation cost,environmental impact and other factors,a set of comprehensive evaluation system was established to select the best wastewater treatment process.According to the evaluation results,the process conditions were further adjusted and optimized,and the optimized pollutant removal efficiency,sludge properties and microbial distribution were analyzed and studied.The main conclusions are as follows:(1)Current situation of Chongming rural domestic sewage treatment facilitiesA total of 17,250 treatment units are distributed in the seven towns of Chongming District involved in the project,among which 14,996 units of 1 ton/day are small and scattered,accounting for 86.9%.In QL village,GY Town,Chongming District,according to certain rules,the effluent from 35 sets of rural sewage treatment devices was randomly collected.The results showed that the effluent was characterized by high nitrogen concentration,SS exceed serious standards,the success rate of total phosphorus is low,and relatively low C/N.The average concentrations of total nitrogen,ammonia nitrogen,total phosphorus,SS and COD were 17.63 mg/L,10.86 mg/L,0.96mg/L,43 mg/L and 21 mg/L,respectively.228 villagers in the study area were visited,and the results showed that the average household population calculated was 2.03 people,far lower than the value of 3 people/family originally designed by the processing system.In terms of water consumption,per capita water consumption and per capita displacement in the study area were 73.35 L/(person day)and 44.3 L/(person day),41% of the original design standard.It is calculated that the sewage discharge coefficient of rural domestic sewage in the study area is about 0.6,which is quite different from 0.8～0.95 provided in the design manual.This is related to the fact that there are a large number of elderly people or children left behind in Chongming,and most of the miscellaneous water is taken from the surrounding rivers.It also indicates that the initial design water amount of the treatment device is not consistent with the actual situation in Chongming.After comprehensive analysis,it is decided to collect the effluent from the existing decentralized rural sewage treatment unit through pressure pipe transportation,and finally to the integrated sewage treatment unit for centralized treatment.(2)Analysis of tracking monitoring results of integrated treatment device for low C/N rural domestic sewageFrom December 2020 to April 2021,the water quality of five low-C /N integrated wastewater treatment units A,B,C,D and E was tracked and monitored.The results showed that,under the condition that the influent total nitrogen was lower than 70mg/L and COD was lower than 140mg/L,the removal rate of main pollutant indexes was as follows: Average removal rate of total nitrogen: B(60.1%)> E(58.4%)> A(54.8%)>D(52.9%)> C(39.3%);Average removal rate of ammonia nitrogen: B(85.2%)> E(82.8%)> A(75.2%)> D(72.5%)> C(45.9%);The average removal rate of total phosphorus: B(78.3%)> E(78.1%)> A(67.5%)> D(65.8%)> C(55.8%);The average removal rate of SS: E(69.4%)> B(63.2%)> D(62.1%)> A(56.1%)> C(48.5%).From the aspects of success rate: Success rate of total nitrogen: B(97.3%)>E(93.9%)> C(89.2%)> A(63.7%)> D(52.9%).Success rate of ammonia nitrogen:B(97.3%)= C(97.3%)> E(87.8%)> A(72.7%)= D(72.7%).Success rate of total phosphorus: E(96.9%)> B(91.9%)> A(66.7%)> C(64.9%)> D(36.4%).Success rate of SS: B(91.9%)> E(81.8%)> A(78.1%)> C(64.9%)> D(39.4%).Under normal conditions,the integrated unit B has the best effect on pollutant removal.(3)Comprehensive evaluation of low C/N rural domestic sewage treatment processIn this study,the mathematical model of fuzzy entropy weight method was used to evaluate the processing technology.The results showed that the comprehensive order of economic performance was C > D > B > A > E.In terms of technical performance,the comprehensive ranking is: B > E > C > A > D;The comprehensive ranking of environmental impact was as follows: B = C > A > E> D;The comprehensive ranking of management effectiveness is: B > E > A > C> D;The ranking of comprehensive performance evaluation is: B >C >E >A > D.Therefore,integrated device B is the best choice.(4)Study on the operation of the optimized deviceThe process and working conditions of the optimized integrated unit B were further optimized.The study found that the system could achieve the best operation effect when the hydraulic retention time of anoxic tank and aerobic tank were 2 h and6 h,the dissolved oxygen of aerobic tank was 3 mg/L,and the sludge reflux ratio and nitrifying liquid reflux ratio were set at 100% and 300%,respectively.The study shows that C/N=8 is the most economical operating condition.After optimization,the average removal rates of total nitrogen,ammonia nitrogen,total phosphorus and SS were77.66%,85.48%,80.43% and 69.62%,respectively,which were 17.56%,1.28%,1.57%and 6.42% higher than those before optimization.The analysis of pore structure distribution characteristics of sludge shows that the sludge is mainly mesoporous,and the average pore size of anaerobic tank and aerobic tank is 3.39 nm and 4.31 nm,respectively.The specific surface areas were 67.92 m2/g and 31.28 m2/g,respectively.The Ace,Chaol and Shannon indexes of July’s sludge samples were significantly higher than those of April’s,while the Simpson index was on the contrary.At the gate level,the microbial community structure of sludge samples in April mainly included Proteobacteria,Patescibacteria,Actinomycetes and Bacteroide.At the subordinate level,there are mainly Micropruina,Propioniclava and Saccharimonadales.In July,Chloroflexi,Patescibacteria,Nitrospira and Proteobacteria were the main bacteria in the distribution of sludge samples.There are Saccharimonadales,Micropruina,Nitrospira and TM7a. |
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