Determination of discharge permit limits for indirect discharge enterprises with equivalent mass method: a case study in Changzhou City

The discharge permit system is an important means to control the total pollutant load and improve the watershed environmental quality. The indirect discharge enterprises (IDEs) are also included in the discharge permit system. It is a difficult problem on how to determine the discharge permit limit (DPL) for the IDEs, and there is no such a uniform regulation at the national level at present. By referring to the NPDES of the United States, the Equivalent Mass Method (EMM) was formulated to estimate the DPL for the IDEs. The EMM was successfully used to estimate the DPL for six IDEs, which were located in Changzhou City, Jiangsu Province and with the wastewater being discharged into one wastewater treatment plant. The EMM was used to calculate the equivalent mass load limits for the 6 enterprises and, in combination with the current discharge concentration standards, the COD_Cr DPLs were determined. Thus, the EMM is an effective method to estimate the discharge permit limits for the IDEs, and can be applied flexibly to the enterprises with different discharge characteristics, including newly built enterprises discharging into the pipeline network, existing enterprises stopping to discharge into the pipeline network, and under the circumstance of enhancing the discharge standards, etc.