Monochloramine effects on biofilm growth with/without orthophosphate in high-residence time drinking water model distribution systems

Orthophosphate is a common corrosion inhibitor used to control lead release in drinking water distribution systems (DWDSs). It may enhance monochloramine decay and promote biofilm growth. This study assessed the impact of two monochloramine dosages on biofilm growth and monochloramine decay in model distribution systems (MDSs), with/without orthophosphate, at a 10-day residence time. Four bench-scale MDSs were run for 15 weeks: two tested a low monochloramine dose (2 mg Cl₂/L), with or without orthophosphate, and two a higher dose (3 mg Cl₂/L), with and without orthophosphate. The feedwater was phosphorus-limited, which may have amplified microbial response to orthophosphate addition. At both monochloramine dosages tested, orthophosphate addition increased viable cell count, biofilm reformation potential, and metabolic activity, while biofilm genetic diversity was influenced by both orthophosphate and monochloramine dose. Communities subjected to high monochloramine without orthophosphate were the least diverse, whereas those exposed to low monochloramine with orthophosphate showed the greatest diversity. These findings suggest that high monochloramine exerts selective pressure, reducing diversity, while orthophosphate enhances it. First-order total chlorine decay coefficients were higher in MDSs with lower monochloramine doses, and at the same dose, MDSs with orthophosphate showed slightly higher monochloramine decay than those without orthophosphate.