Control of emerging organic contaminants (EOCs) and degradation of antibiotic resistance genes (ARGs) are two emerging issues in protecting the aquatic environment and supplying safe drinking water. The breakpoint chlorination process, which generates reactive species such as HNO, ONOOH and HO^• in ammonia-containing water, is a promising process tackling in such emerging issues because the reactive species can effectively degrade chlorine-refractory EOCs and may serve as an additional barrier to ARG dissemination. However, there are two critical challenges in applying the breakpoint chlorination process: the complexity of the nature of reactive species generated through breakpoint chlorination reactions and the formation of undesirable disinfection byproducts (DBPs) from natural organic...[ Read more ]

Control of emerging organic contaminants (EOCs) and degradation of antibiotic resistance genes (ARGs) are two emerging issues in protecting the aquatic environment and supplying safe drinking water. The breakpoint chlorination process, which generates reactive species such as HNO, ONOOH and HO^• in ammonia-containing water, is a promising process tackling in such emerging issues because the reactive species can effectively degrade chlorine-refractory EOCs and may serve as an additional barrier to ARG dissemination. However, there are two critical challenges in applying the breakpoint chlorination process: the complexity of the nature of reactive species generated through breakpoint chlorination reactions and the formation of undesirable disinfection byproducts (DBPs) from natural organic matter. This research was undertaken to provide a thorough investigation of the generation of reactive species and their roles in EOC degradation in the breakpoint chlorination process and to establish a novel multi-step breakpoint chlorination process to balance the risks posed by EOCs, ARGs, and DBPs.

The pathway of HO^• generation in the breakpoint chlorination process was investigated. The decomposition of ONOOH only contributed approximately 7% to the formation of HO^•, suggesting the dominant role of other pathways, such as the decomposition of dimerized HNO (i.e., HO-N=N-OH) in HO^• formation in the breakpoint chlorination. The generation of reactive chlorine species (RCS, i.e., Cl^•, ClO^• and Cl₂^•‒ ) and reactive nitrogen species (RNS, e.g., HNO and ONOOH) in breakpoint chlorination was demonstrated and their contributions to the degradation of EOCs were quantified. In addition, the chlorine to nitrogen (Cl/N) ratio and pH greatly affected the reactive species generation and EOC degradation. At pH 7.2, an optimum Cl/N ratio for HO^•, Cl₂^•‒, and RNS generation occurred at 9, while the radical speciation shifted towards ClO^• due to the scavenging effect of excessive free chlorine at a Cl/N ratio above 9. Increasing the pH from 7.2 to 9.5 decreased the degradation of EOCs by HO^• and RNS but significantly increased that by RCS, especially ClO^•, due to the strong scavenging of HO^• by OCl^‒.

The impacts of three operational parameters, the number of chlorine dosing steps (3-step, 5-step, and continuous dosing), the Cl/N ratio of the initial dosing step (starting from an initial Cl/N mass ratio of 5.0 and dosing evenly), and the time interval between each dosing (3, 5, 7, and 9 min) on EOC degradation and reactive species generation were evaluated. The optimum chlorine dosing procedure to degrade five representative EOCs was determined to be 5-step chlorine dosing starting from an initial Cl/N mass ratio of 5 with a dosing time interval of 5 min. Compared to single-step breakpoint chlorination, the optimized multi-step breakpoint chlorination fostered HO^• generation and enhanced the EOC degradation. Although the single-step and multi-step breakpoint chlorination achieved a comparable level of ARG degradation, the latter scenario increased the relative contribution of the reactive species, rather than the chlor(am)ine species, to ARG reduction and retained a higher level of residual chlorine. In addition, a significantly decreased formation of carbonaceous and nitrogenous DBPs was observed by using the multi-step chlorine dosing approach.