Chlorine Dose Determines Bacterial Community Structure of Subsequent Regrowth in Reclaimed Water

Abstract

Bacterial regrowth in reclaimed water, specifically observed when residual chlorine concentration declines along the distribution system, causes undesirable changes in water quality and hampers its acceptability. To study the impact of chlorination on regrowth and bacterial community structure, unchlorinated tertiary treated reclaimed water was collected and dosed with chlorine such that the initial doses were 1, 3, 5 mg-Cl₂/L before being stored at ambient temperature under dark condition. Chlorine measurement, cell counts and bacterial community profiling were carried out at regular intervals for 21 days. Addition of chlorine caused rapid decline in intact cell concentration and no regrowth was observed until free chlorine decayed below detection limit (0.03 mg-Cl₂/L). Upon regrowth, intact cell concentrations reached the initial level except in the case of 5 mg-Cl₂/L where the intact cell concentration was lower by 1-log₁₀. The dominant species that regrew under each condition were distinct, based on their capacity to withstand chlorine. The most chlorine-tolerant groups belonged to the order Sphingomonadales and Rhizobiales, which have been previously reported to initiate biofilms. This study demonstrates that chlorination selects specific bacterial groups which have the potential to regrow in the distribution network.