Compliance Analysis Of Water Quality Monitoring Section In Zhuzhou Baishigang Based On SWMM Simulation

With the development of urbanization in China,the proportion of impermeable surface and the coefficient of runoff are increasing,which aggravates the pollution of rainfall runoff and combined overflow.In this paper,the near-term and long-term carrying capacity of Baishigang watershed in Zhuzhou was calculated by taking COD,NH₃-N,and TP as control factors;The runoff pollution model and overflow pollution model of Baishigang basin were constructed based on SWMM,and the variation law of pollution load with or without sponge city facilities was discussed.The main work and conclusions of this paper are as follows:（1）The study area sub-catchment,drainage network and river cross-sections were generalised,and the model parameters were set with reference to relevant literature and previous experience,the rainfall runoff,overflow pollution model and river water quality model based on SWMM were constructed.The results of the parameter calibration showed that the mean values of the modelled runoff coefficients under the three common rainfall intensities differ from the actual integrated runoff coefficients by only 1.38%,and the E_NS of water quality data were all greater than 0.7,which met the calibration requirements.（2）The pollution loads of runoff and overflow were simulated under different return periods.Under the current situation,the pollution load of COD,NH₃-N and TP runoff in the diversion system area could reach up to 44.626 kg/ha,1.545 kg/ha and0.204 kg/ha;The pollution load of COD,NH₃-N and TP in the combined system area can reach up to 60.424 kg/ha,2.161 kg/ha and 0.258 kg/ha.The simulation results of different design hydrological years show that the total pollution load in the study area is the largest under the condition of abundant hydrological year,in which the runoff pollution load of COD,NH₃-N and TP in the diversion system area reaches 659.99 t/a,20.80 t/a and 3.60 t/a respectively;The overflow pollution load of COD,NH₃-N and TP in the combined system reaches 496.98 t/a,35.39 t/a and 5.24 t/a respectively.（3）Under the present situation,the main sources of COD pollution load in Baishigang river are rainfall runoff,combined system overflow and sewage plant tail water,which account for 34.37%,25.88%and 18.37%respectively;NH₃-N pollution load mainly comes from combined system overflow and direct sewage discharge,contributing 27.90%and 27.79%respectively;The highest contribution of TP pollution load is from the tailwater of the sewage plant with 29.50%,followed by the combined system overflow with a percentage contribution of 23.00%.Rainfall runoff and combined overflow pollutants are the main pollution sources of Baishigang river.The water quality of the river section cannot reach the standard without any pollution control measures and with the implementation of point source pollution control plan only.（4）The non-point source pollution load in the study area can be reduced by five sponge measures:bio-retention facilities,green roofs,permeable pavement,sunken green space and rainwater storage tank.The simulation results under the conditions of abundant hydrological year show that after the implementation of the long-term sponge scheme,the runoff pollution loads of COD,NH₃-N and TP in the diversion system area will be reduced to 224.20t/a,6.58t/a and 1.07t/a respectively,and the overflow pollution loads of COD,NH₃-N and TP in the combined system area will be reduced to 139.28t/a,11.34t/a and 1.68t/a;After the implementation of the near-term sponge scheme,the runoff pollution loads of COD,NH₃-N and TP in the diversion system area will be reduced to 550.79t/a,16.48t/a and 2.72t/a respectively,and the overflow pollution loads of COD,NH₃-N and TP in the combined system area will be reduced to 283.06t/a,21.70t/a and 3.23t/a respectively.（5）Summarize the total pollution load into the river after the implementation of the sponge scheme and point source pollution control measures,carry out the calculation of water quality index reachability of the river monitoring section,and use SWMM to simulate the water quality change process of the river monitoring section under the conditions of abundant,flat and low hydrological years.The results show that under the conditions of different hydrological years,the monthly average concentrations of COD,NH₃-N and TP in the monitoring section are all below the surface V water standard in the near-term planning horizon year and below the surface III water standard in the long-term planning horizon year.