Assessment of Combined Sewer Overflow impacts on receiving water: A case study of Bangkok

摘要

The effect of combined sewer overflow on receiving water body in tropical rain climate was investigated in Rattanakosin, Bangkok. Rattanakosin is historic and tourism area, and is surrounded by Chao Phraya River and canals. The area is serviced by a combined wastewater collection and treatment system. Most of the combined sewer overflow (CSO) in rainfall event is discharged to the canals. A CSO chamber located in the study area, with catchment area of 0.50 ha was selected for the monitoring of the inflow, overflow of sewage and quality of sewage in rain events. The monitoring was conducted on three rain events, dated on June 4, July 9, and July 12, 2003 and the average intensity of rainfall were 52 mm/hr, 13 mm/hr and 37 mm/hr respectively. The overflow from CSO chambers on June 4 and July 9 were measured to be 132 m~3/ha and 26 m~3/ha respectively and no overflow on July 12. The behavior of soluble COD variation was almost similar in all cases that initial runoff had higher concentrations due to wash-off of contaminants from the ground surface and accumulated matters in sewer pipes. It was found that the peak of organic load per unit area came several minutes after the peak of soluble COD load concentration. The amount of organic pollutants discharged to the environment on the first and second events were calculated to be 13.4 kg/ha and 0.69 kg/ha respectively. Based on the monitoring results, the estimated COD load to the canals from CSO was 29.2 ton/month while COD treated by the treatment plant was estimated to be 160 ton/month. While wastewater collection and treatment system is contributing a lot to reduce the COD load to the canals, there could be room for improvement about CSO.