Optimization of total discharge permit limit of pollution sources in control units: a case study of Yongding River watershed

Pollutant discharge permit system is widely used in water environment management, which has played an important role in water pollution control and water environment quality improvement. By comparing the allowable discharges based on the water environmental capacity constraints and the discharge standards for the watershed, the management mode of total discharge permit limit of control units and the optimization method of total discharge permit limit of polluting sources were constructed. The empirical study based on the typical control units of Yongding River watershed showed that the typical control unit of Beijing section could meet the needs of water environment quality improvement when implementing the watershed discharge standard, but the typical control unit of Zhangjiakou section still needed to reduce 210.388 t/a of COD_Cr. In the typical control unit of Zhangjiakou section, 0.002 and 32.039 t/a of COD_Cr should be reduced respectively in the two fixed sources whose wastewater was discharged into the sewage treatment plants.