Quantifying first flush loadings of stormwater runoff from residential catchment

Stormwater runoff is the main contributor to non-point source (NPS) pollution in urban areas. This issue is extremely important in tropical environment because of its high intensity and frequent storms. The objective of this study was to investigate the fraction of pollutant mass loading transported by the first 30% of runoff volume (FF₃₀). Stormwater samples and flow rate data were manually collected from a residential catchment during 18 storm events, with mostly return period less than 3 month (33 mm/hr for a 1-hour duration storm). The mean of FF30 values for 5-day biochemical oxygen demand (BOD5), chemical oxygen demand (COD), total suspended solids (TSS), total phosphorus (TP), ammonia nitrogen (NH₃-N) and oil and grease (O&G) are 44%, 45%, 46%, 44%, 52% and 40%, respectively. Correlation analysis showed that antecedent dry day was likely to result in a greater pollutant loading transported in the early 30% of runoff volume. Intercepting the first 30% of runoff volume is able to remove an average of 45% of total pollutant loading during the frequent storms occurred in tropical residential catchment.