Quantifying chlorine-reactive substances to establish a chlorine decay model of reclaimed water using chemical chlorine demands

In order to guarantee the water quality of reclaimed water in pipeline system, a chlorine bulk-decay model is required for simulation and prediction of chlorine profiles in networks. Conventional chlorine decay models of drinking water are not applicable to reclaimed water due to its complex and varying water quality. In this study, the chlorine decay of reclaimed water was investigated under different operational conditions. Based on these results, different chlorine-reactive substances (CRSs) in reclaimed water were quantified by total chlorine demand (TCD), instantaneous chlorine demand (ICD) and lasting chlorine demand (LCD), respectively. A stoichiometric model (CRS model) of chlorine decay of reclaimed water was established using ICD, TCD and reaction rate constant (k) as key independent parameters. The experimental data were fitted to the CRS model with promising results under various initial chlorine concentrations (3-10 mg-Cl-2/L) and temperatures (8-35 degrees C). The ICD, TCD and k of different reclaimed water samples were in the range of 0.23 to 2.85 mg-Cl-2/L, 1.07 to 4.73 mg-Cl-2/L, and 0.04 to 4.06 L/(mg.h), respectively. Furthermore, the ICDs, TCDs and k could be determined directly by measuring the chlorine consumption at 5 min (Delta C-Cl,C-5min) and 8 h (Delta C-Cl,C-8h) after the addition of chlorine into reclaimed water, and the UV254 of reclaimed water, respectively. The relationships between ICD, TCD, k and the corresponding water quality indexes were further validated. In this way, the chlorine decay profile of reclaimed water could be predicted rapidly and precisely by measuring the Delta C-Cl,C-5min, Delta C-Cl,C-8h, and UV254 of reclaimed water.