Orthophosphate is commonly added as a corrosion inhibitor in drinking water distribution systems (DWDSs). However, there is limited understanding of the interrelationships between its addition, monochloramine decay, and biofilm growth. Further research is needed to evaluate its potential to accelerate monochloramine decay and promote biofilm development. This study examines the impact of orthophosphate doses (0 to 4 mg PO43-/L) on monochloramine decay and biofilm growth using model distribution systems (MDSs) at a 10-day residence time, fed with phosphorus-limited water. Findings showed that, in addition to expected enhanced microbial growth, biofilm formation potential, and metabolic activity (i.e., carbon utilization), orthophosphate addition also increased monochloramine decay. For instance, biofilm growth increased from 2.9 to 3.2 to 5.3–6.3 log CFU/cm2 between 1 and 4 mg PO43-/L, with the most substantial increase observed between 1 and 2 mg PO43-/L (an increase of >2 log units). Around day 52, changes in metabolic activity, biofilm formation potential, and biofilm growth in MDSs with added orthophosphate suggested a shift in the microbial community from early colonizers to bacteria thriving in biofilms. A correlation between biofilm profiles and monochloramine decay was apparent, with significant positive correlations between total chlorine decay and (i) biofilm HPC (R2 = 0.86, p < 0.001), (ii) biofilm formation potential (R2 = 0.73, p < 0.01), and (iii) metabolic activity (R2 = 0.81, p < 0.001). Higher orthophosphate concentrations (2–4 mg PO43-/L) were linked to greater biofilm growth and monochloramine demand, while 1 mg PO43-/L had minimal impact. Total chlorine decay coefficients ranged from 0.0034-0.004 h-1 (control) to 0.0050-0.0072 h-1 (4 PO43-/L) in the phase of further biofilm development. These findings emphasize that orthophosphate usage in DWDSs needs to balance corrosion control aspects with effects on water quality (e.g., biofilm growth and monochloramine stability).
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