Study On Optimal Retention Rate Of Agricultural And Rural Sewage In Dianchi Interception System

A pollution control system was established from"the ninth five-year plan"to"the thirteenth five-year plan"to achieve the water quality target of Dianchi Basin,one of its goals is to achieve a runoff retention rate of 30%,which has been theoretically verified.In order to minimize the pollution load entering Dianchi basin,an intelligent platform is established based on that retention rate,which can realize the automatic control of intercepted well,to minimize the pollution load from the system overflows,and jointly control operation of two sewage treatment plants.Considering that the target of 30%runoff retention rate is based on quantitative analysis of pollutant load,treatment capacity of treatment facilities,self-purifying ability of Dianchi,but the determination of the retention rate is not combined with the analysis of the specific water quality and without considering the possibility of overflow of the interception system.Therefore,this paper intends to focus on the appropriate control mode of the intercepting well,and based on the simulation of the water quality and quantity in the study area,the optimal retention rate corresponding to the minimum pollutant load from the interception wells and the interception system into the Dianchi lake is studied.The research results can not only provide effective support for the efficient operation of the intelligent platform,but also provide a basis for the design of similar equipment,it is also of great significance for scientific and accurate calculation of the agricultural and rural non-point source pollution load,and further improving the water quality of Dianchi.Firstly,based on the measured runoff data,this study demonstrates whether the current retention rates of rural and agricultural runoff can meet the goal of 30%or not.Then after discussing the relationship between TP flux and runoff,this paper compares the results of four control modes,including the threshold of TP concentration,The threshold of TP flux,Time-The Liquid Level control mode and uncontrolled mode,on the interception of non-point source pollution load in agricultural and rural area,and based on the optimal control mode,the optimal retention rate under typical climatic years and the influence of return period and rainfall peak coefficient on the optimal retention rate are studied.Finally,the agricultural and rural non-point source pollution load is estimated based on the pollutant equivalent method,and the loss rate of agricultural and rural non-point source pollution load is calculated considering the impact of the optimal retention rate.The main conclusions are as follows:(1)In the two rainfall events with rainfall of 43.2mm and 25.9mm,the corresponding retention rates of rural and agricultural runoff are 36.98%and 71.83%respectively;during 1995-2009,the average annual retention rates of typical climatic years,dry,average and rainy,are 61.80%,51.84%and 52.61%respectively.Whether it is individual rainfall or typical climatic year,the retention rate of runoff meets the goal of30%;(2)Results show that there is a good relationship between TP flux and runoff in flower and vegetable greenhouses planting areas.The correlation equation of the flower greenhouse is:y=0.7611x+0.0215,R~2=0.87919;The correlation equation of the vegetable greenhouse is:y=0.8476x+0.0876,R~2=0.84179;(3)In the case of retaining the same runoff,order of the pollution load intercepting efficiency is:The threshold of TP concentration>The threshold of TP flux>Time–The Liquid Level control mode>Uncontrolled mode;(4)Based on the typical pollutant control mode,with the increase of the return period,the optimal retention rate decreases,the average concentration of the intercepted pollutants decreases and the value of threshold decreases;(5)Based on the typical pollutant control mode,with the increase of rainfall peak coefficient,the optimal retention rate changes little and the value of threshold increases;(6)Considering the interception effect of the system,the loss rate of agricultural and rural non-point source pollution load is:in dry,average and rainy years,COD is 0.09,0.14 and 0.23 respectively;TN is 0.10,0.22 and 0.31 respectively;TP is 0.07,0.12 and0.19 respectively.