A Novel Perspective on Breakpoint Chlorination: The Optimal Practices for Inactivating Fungal Spores in Peak Chloramination

Abstract

This study investigates the breakpoint chlorination process and its impact on fungal spore and bacterial inactivation, focusing on the dynamic role of chloramines. Using Aspergillus niger and Bacillus subtilis as model microorganisms, a three-stage inactivation pattern driven by varying Cl2/N ratios was revealed. As Cl2/N increases, the overall disinfection efficiency improves, with free chlorine dominating bacterial inactivation beyond the breakpoint. However, for fungal spores, monochloramine (NH2Cl) remains the primary inactivating agent even as Cl2/N approaches and surpasses the breakpoint. At the peak chloramination stage, NH2Cl contributes 94% of fungal inactivation, exploiting its superior ability to penetrate the robust, multilayered spore wall, compared to only 71% for bacteria. In contrast, the oxidative potential of free chlorine is more effective against the simpler bacterial cell wall. These findings emphasize the pivotal role of peak chloramination in fungal control, as NH2Cl demonstrates superior cost-efficiency and inactivation performance during this stage. Although free chlorine provides broad-spectrum pathogen coverage beyond the breakpoint, targeting fungal spores effectively requires leveraging the unique advantages of NH2Cl. This study provides valuable insights for optimizing disinfection strategies by balancing Cl2/N ratios to enhance microbial inactivation while minimizing operational costs and disinfection byproduct risks.

Keywords: monochloramine, breakpoint chlorination, fungal spores, Cl2/N ratio, disinfection mechanism