Among several applications of urban wastewater reuse, use of reclaimed wastewater to sustain stream flows has become attractive especially in urban areas. This environmental application of reclaimed wastewater occupies about 32% of total wastewater reuse in Japan. In this paper, residual chlorine in the full-scale project for environmental application in Sapporo was discussed. The field survey of residual chlorine showed that there was the hourly variation of residual chlorine concentration in the river and this variation depended on the hourly variation in composition of chlorine species in the effluent of the WWTP, and that the rate of decline of residual chlorine in the open channel section was greater than that in the pipe flow section. The simple mathematical model was developed for describing the decline process on the basis of the dispersion model including two reactions: interaction of residual chlorine and organic matter in reclaimed water and interaction of chlorine and pipe surface or bio-film. The expressions for these two reaction rates were obtained by lab-scale experiments and these calibration results showed that consumption rate of residual chlorine in the aqueous phase was comparable to the rate on the pipe wall in the pipe flow section, and that residual chlorine in the open channel was consumed mainly by bio-film. The calibrated mathematical model yielded a good estimation of the chlorine concentration profile.

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