Flush Effect of Initial Overflows from Combined Sewer Systems in Mountainous Cities

A typical overflow outlet in a mountainous city,the Chengwuxiao(No:YW22-3J) in Fuling,was selected to study the flush effect of initial overflows from a combined sewer system.The results showed that the EMCs including TSS=477 mg/L,TCOD=530 mg/L,TP=3.11 mg/L and TN=26.2 mg/L were higher than the Ⅴ criteria specified in the Environmental Quality Standards for Surface Water.During massive rainfall,TCOD,TSS,TN and TP showed markable first flush effects.The flush effect of initial overflows from a combined sewer system is influenced by amount of rainfall,rainfall intensity,rainfall duration,number of dry days before rain,accumulation of surface sediments,and pipeline sediment.The large terrain height difference and large elevation drop of sewer flow strengthened the flush effect of initial overflows from the combined sewer system and increased contaminant diffusion,which caused more serious pollution in surrounding water.