Stormwater runoff pollution loads from an urban catchment with rainy climate in China

An older urban district in Wuhan, China, is transitioning from discharging sewage and stormwater directly into lakes, to directing the sewage to wastewater treatment plants (WWTPs). Dealing with polluted stormwater discharge is a great challenge. Stormwater runoff from an urban catchment with a combined sewer system was sampled and analyzed over a three-year period. Results indicate that wet weather flows account for 66%, 31%, 17%, and 13% of the total load of suspended solids (SS), chemical oxygen demand (COD), total nitrogen, and total phosphorus, respectively. The first flush of COD and SS was significant in all runoff events. More than 50% of the SS and COD loads were transported by the first 30% of runoff volume. Storage and treatment of the first 10 mm from each combined sewer overflow event could reduce more than 70% of the annual COD overflow load. An integrated solution is recommended, consisting of a tank connected to the WWTP and a detention pond, to store and treat the combined sewer overflow (CSO). These results may be helpful in mitigating CSO pollution for many other urban areas in China and other developing countries.