Optimization Research On Online Monitoring And Automatic Control Of A/A/O Rural Domestic Sewage Treatment Facilities

Rural domestic sewage treatment facilities（RDSTFs）are numerous,decentralized but difficult to operate and maintain.A/A/O as one of the most widely used treatment processes in rural areas,is good at nitrogen and phosphorus removal when the facility is well operated and managed in a stable condition.Installation of online monitoring and automatic control equipment could effectively improve the stable operation of A/A/O facilities.However,there are lack of low-cost and durable online monitoring system for RDSTFs whose water quantity and quality vary greatly.As a result,it is difficult for RDSTFs to adjust control strategies timely.In this study,the water quality of rural domestic sewage in Jiaxing,Hangzhou and Zhoushan was investigated on the spot.By studying the correlation between the pollutant indexes in the sewage,a low-cost and durable online monitoring method was established to indicate the concentration of rural domestic sewage by using a single index of conductivity.On this basis,an automatic control strategy is developed to adjust the volumetric load of A/A/O facilities by controlling the opening time of the inlet pump,so as to improve the pollutant removal efficiency.The main research results are as follows:（1）The influent and effluent quality of 30,44 and 42 rural domestic sewage facilities were investigated in Jiaxing,Hangzhou and Zhoushan.The results showed that the influent concentration of Jiaxing were the highest,those of Zhoushan were the lowest,and those of Hangzhou were the middle.The COD removal rates of the facilities in the three regions were all good and higher than 50%,while the removal rates of TN were poor with numerical values of 22.00%,18.44%and 18.21%,respectively.The removal rates of NH3-N were 55.00%,50.53%and 89.37%,respectively.（2）The correlation between pollutant indexes of rural domestic sewage was studied.For 30 RDSTFs in Jiaxing,the correlation coefficients R²between influent conductivity and NH₃-N,TN and TP were 0.9327,0.9219 and 0.8831,respectively.The cumulative judgment times with error less than 20%accounted for 88.89%,88.89%and 85.19%of the total judgment times,respectively.The above linear relationship also existed for 44 RDSTFs in Hangzhou.However,the applicability of Zhoushan City was poor,mainly due to the high salinity background value of rural domestic sewage in the island area,so that the conductivity changes caused by pollutants such as nitrogen and phosphorus were no longer easy to identify by conductivity.The source apportionment of influent EC was carried out and found that the main inorganic ions(Na⁺、K⁺、Ca²⁺、Cl^-)and pollutants（NH₄⁺、NO₃^-、H₂PO₄^-）contributed approximately 70%of the total value,however,there was no single ion in rural domestic sewage dominated the EC value.Furthermore,the simulation of rural domestic sewage diluted by background water were performed by a dilution model,according to the universal leakage of sewer reticulation systems in rural areas.Influent TN/NH₃-N/TP concentrations that strongly and linearly correlated with influent EC values were confirmed and simulated by the dilution model.The mechanism of correlations was attributed to the dilutions by background water,such as groundwater and agricultural irrigation water.Particularly,the dilution coefficient in Fuyang was speculated to obey a Poisson distribution withλ=4.（3）A low-cost and durable on-line monitoring method for comprehensive indication of rural domestic sewage concentration by single conductivity index was established.Continuous inflow water quality monitoring was carried out on 30RDSTFs in Jiaxing City,and it was found that the proportion of indication error less than 20%in spring,autumn and winter was 80.77%,88%and 90.74%,respectively,while that in summer was only 49.30%,indicating that the method of on-line monitoring of rural domestic sewage water quality based on conductivity had good applicability in spring,autumn and winter,but poor applicability in summer.This was mainly due to the summer rainfall,water use habits and high groundwater level,resulting in a large number of rainwater and groundwater entering the sewage treatment system,affecting the correlation between the influent conductivity and pollutant concentration.（4）A method was developed to quickly determine the pollutant removal efficiency of RDSTFs based on the ratio of effluent conductivity to influent conductivity.This method can quickly screen out facilities that have no removal effect on pollutants,and the recognition accuracy was as high as 78%,which was conducive to improving the efficiency of operation and maintenance supervision and reducing the cost of water quality detection.（5）An automatic control strategy was developed to adjust the volumetric load of the A/A/O facility by controlling the opening time of the inlet pump,so as to improve the pollutant removal efficiency.It was applied in the three facilities of Beizhoubangnan,Lijiawan and Dangwan.After control,the ammonia nitrogen removal rates of the three facilities increased from 27.64%,41.98%and 32.01%without control to 81.29%,73.02%and 72.41%,respectively,indicating the effectiveness of automatic monitoring for the stable operation of A/A/O facilities.This result indicated that high instantaneous volumetric load of influent caused low removal rate of ammonia nitrogen in winter.By reducing the volumetric load of ammonia nitrogen in influent,the removal rate of ammonia nitrogen in facilities can be effectively improved.