Pollution load and the first flush effect of BOD₅ and COD in urban runoff of Wenzhou City

Four typical rainfalls were monitored in two different research areas of Wenzhou Municipality. Concentrations of BOD₅ and COD in six different urban runoffs were measured. In addition the event mean concentration (EMC), M (V) curve and BOD₅/COD of pollutant were calculated. The results showed that concentrations of BOD₅ and COD in different urban runoffs of Wenzhou ranged from ND to 69.21 mg·L^-1 and ND to 636 mg·L^-1. Concentrations of BOD₅ and COD in different urban runoffs were decreasing over time, so it is greatly significant to manage the initial runoff for reducing organic pollution. Judged by EMC of BOD₅ and COD in these five rainfalls, concentrations of pollutant in some urban runoffs were out of the integrated wastewater discharge standard. If these runoffs flowed into river, it would cause environmental pressure to the next level receiving water bodies. According to the M (V) curve, the first flush effect of COD in most urban runoffs was common; while the first flush effect of BOD₅ was same as that of COD. The result also showed that organic pollution was serious at the beginning of runoff. The underlying surface type could affect the concentration of BOD₅ and COD in urban runoff. While the results of BOD₅/COD also suggested that biodegradation was considered as one of the effective ways to decrease the pollution load of organics in urban runoff, and the best management plans (BMPs) should be selected for various urban runoff types for the treatment of organic pollution.