Study on Residual Chlorine Transport in Receiving Waters from Sewage Treatment under Seasonal Operation

In some sewage treatment plants in Japan, treated water with increased ammonia concentration is discharged to supply for the lack of nutrients in the seaweed farms. However, chlorinating treated water, including ammonia, reduces the disinfection effect and generates secondary products. In this study, the behavior of residual chlorine was investigated in a tidal river receiving effluents by seasonal operation. The findings indicated that the seasonal operation increased NH4+-N, which acted as a chlorine consumer, consequently increasing the proportion of combined chlorine as a secondary product. In the field survey, the residual chlorine in the river channel under the seasonal operation demonstrated a tendency to disperse in tidal areas by the tides. Additionally, it is suggested that there is a possibility of a decrease in total residual chlorine concentration originating from the effluent and the increase in concentration due to lateral inflow from the numerical simulation result of a planar two-dimensional model that accounts for the temperature dependence of water in winter. Then, the residual chlorine decay coefficient r0 was roughly estimated to be 3 × 10−5 1/s. Finally, it was shown that the residual chlorine from discharged water under seasonal operation does not necessarily significantly affect the receiving water.