Mechanistic insights into endotoxin release from biofilms in drinking water pipeline network: Contrasting chlorine...

Bacterial endotoxins originating from Gram-negative biofilm bacteria are emerging concerns in drinking water due to their pyrogenic and inflammatory health effects. This study systematically compared the effects of chlorine and chlorine dioxide (ClO₂) on endotoxin release, using planktonic E. coli for mechanistic analysis and biofilms to verify relevance under pipeline conditions. Biofilms were cultivated in a rotating disk reactor under hydrodynamic conditions simulating real distribution systems and exposed to disinfectants at concentrations ranging from 0.2 to 5 mg/L. Results indicated that chlorine induced rapid bacterial inactivation accompanied by significant endotoxin release, reaching about 3.23 EU/mL, while ClO₂ produced a similar peak level (about 3.00 EU/mL). The main difference between the two disinfectants lies in the post-release fate of endotoxin, as ClO₂ promoted partial attenuation of endotoxin activity, whereas chlorine showed limited subsequent degradation. Bacterial density, pH, and temperature are key factors influencing endotoxin release. Low bacterial density enhanced endotoxin release, while pH affected chlorine and ClO₂ differently, with stronger effects under acidic and alkaline conditions, respectively. Flow cytometry and scanning electron microscopy (SEM) revealed distinct bactericidal mechanisms between the two disinfectants, correlating with their endotoxin release profiles. The findings highlight that ClO₂ can more effectively control endotoxin accumulation than chlorine, suggesting its potential advantage in drinking water systems serving vulnerable populations or where aerosol exposure is a concern.