Methylparaben, as an environmental contaminant, compromises water disinfection under real conditions

Methylparaben (MP), a common preservative in products of daily use, has been detected in drinking water distribution systems (DWDS) due to its recalcitrance to conventional wastewater treatment processes. This study evaluates the effects of environmentally relevant MP exposure (15 μg/L) on dual-species biofilms formed by Acinetobacter calcoaceticus and Stenotrophomonas maltophilia and the implications for drinking water (DW) chlorination. Experiments were conducted under hydrodynamic conditions representative of real DWDS, using the multiple cylinders biofilm reactor (MCBR). MP exposure increased bacterial culturability by one log-fold (CFU/cm²) and reduced biofilm water content by 10 %. Significant decreases in extracellular polymeric substance (EPS) components - 44 % in polysaccharides and 55 % in protein - were observed for MP-exposed biofilms. Bacterial cells from biofilms exposed to MP showed increased tolerance to chlorination, even if these biofilms produced less EPS. Moreover, MP exposure led to higher post-disinfection survival of A. calcoaceticus and S. maltophilia, with 0.7-log increases in non-damaged cells. Furthermore, MP exposure significantly increased the release of non-damaged bacterial cells (by 1.33 log cells/mL) during free chlorine treatment at 5 mg/L and the number of viable but non-culturable (VBNC) cells by 3 log cells/mL under 50 mg/L of free chlorine exposure. This boosted presence of viable cells in the bulk water post-disinfection raises serious concerns about microbial persistence and the risk of downstream contamination in distributed DW, with potential public health implications.