Effect of residual chlorine on the interaction between bacterial growth and assimilable organic carbon and biodegradable organic carbon in reclaimed water

Reclaimed water can significantly reduce household water consumption. However, microbial growth introduces several problems to reclaimed water, including health concerns, aesthetic deterioration and biofouling. Biological stability refers to the potential of organic matter or nutrients in water to support heterotrophic bacterial growth (HPC). Assimilable organic carbon (AOC) and biodegradable organic carbon (BDOC) are often used as indicators to evaluate the biological stability of water. This study investigated the effect of residual chlorine on the interaction between bacterial regrowth and AOC or BDOC and the variation of AOC after chlorine disinfection in reclaimed water. The results show that the HPC level is insensitive to AOC concentration when residual chlorine is >0.5 mg/L; however, the effects are more pronounced when residual chlorine is <0.5 mg/L. A residual chlorine concentration of >0.5 mg/L therefore maintains biological stability in reclaimed water. When residual chlorine was <0.5 mg/L, HPC levels were found to be limited when AOC was <128 mu g/L or BDOC was <0.25 mg/L; and when residual chlorine was >0.5 mg/L, HPC levels were found to be limited when AOC was <796 mu g/L or BDOC was <0.85 mg/L in reclaimed water. After chlorine disinfection, AOC contents initially increase and then decrease gradually, reaching minimum levels around day 20, and then increase in both greywater reclaimed water and mixed wastewater reclaimed water. Maintaining the chlorine demand and controlling the AOC level is critical for producing an effect on microbial regrowth and stabilising reclaimed water. The results of this study are conducive to the popularisation of reclaimed water use and also provide reference for reuse standards of reclaimed water. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

Reclaimed water can reduce household water consumption significantly, but microbial growth poses health and aesthetic concerns. AOC and BDOC are important indicators for evaluating biological stability of water. Results suggest that maintaining residual chlorine concentration is crucial in maintaining biological stability of reclaimed water, and controlling AOC level is vital for stabilizing reclaimed water.