Impacts of biofilm on monochloramine decay in storm sewer systems: Direct reactions or AOB cometabolism

Monochloramine (NH2Cl), with its long-lasting disinfection effect, has been widely used for drinking water secondary disinfection. However, NH2Cl can enter storm sewers and ultimately fresh water sources (e.g., rivers or lakes) through outdoor tap water uses, and total active chlorine levels as high as 0.77 mg/L have been detected in Edmonton stormwater. To evaluate the potential harmful impact of NH2Cl on aquatic environment, it is crucial to elucidate NH2Cl dissipation mechanisms in storm sewers. The objective of this research is to investigate the effect of biofilm colonized on storm sewers on NH2Cl decay. Storm sewer biofilm was cultured in annular reactors on PVC and cement materials under operational conditions that mimicked the storm sewer environment. Biofilm microbial community analysis detected a higher abundance of total bacteria, ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria functional gene copies in biofilm formed on cement surfaces, as compared to that on PVC materials. Three NH2Cl dissipation mechanisms in the presence of stormwater biofilm were evaluated in batch experiments, including, (i) the direct NH2Cl reaction with biofilm, (ii) AOB cometabolism, and (iii) the NH2Cl reaction with AOB utilized associated products produced during AOB metabolism process. In this research, a kinetic model was developed to characterize these dissipation processes in storm sewer biofilm. The modeled results agreed well with the experimental data, which suggests that this model can be used to describe the NH2Cl dissipation under various stormwater conditions. It is believed that this is the first study to simulate the NH2Cl decay in the presence of storm sewer biofilm, thus contributing to the regulation of chloraminated water discharge in storm sewer systems.