The impact of monochloramines and dichloramines on reverse osmosis membranes in wastewater potable reuse process trains: a pilot-scale study†
Water reuse is an important strategy to optimize water management. Microfiltration (MF) followed by reverse osmosis (RO) and ultraviolet (UV) based advanced oxidation process (AOP) is a proven treatment train for delivering potable recycled water. Chloramines are typically applied as disinfectants to mitigate biofouling of MF and RO membranes. While monochloramine is the intended oxidant, dichloramine is also produced. This study investigated mono- and dichloramine permeability across pilot-scale RO membranes, their oxidative effects on polyamide RO membranes, and their implications for downstream UV treatment. Permeability of mono- and dichloramine was 91% and 96% in the pilot system, respectively. The conversion of dichloramine to monochloramine was observed across the RO membrane suggesting that membrane-rejected ammonia promoted monochloramine speciation. At the RO membrane surface, dichloramine exhibited stronger oxidation of membranes as compared with monochloramine. Interestingly, when exposing polyamide membranes to dichloramine, flux decreased by 36%, suggesting that strong oxidation causes structural collapse of the polyamide thin film. Finally, a greater reduction of downstream UV transmittance (UVT) was observed by the permeation of monochloramine through RO membranes. This study suggests that any technoeconomic analyses of potable reuse processes should consider the impacts of both monochloramine and dichloramine on the capital cost associated with RO membrane lifetime and the operating costs associated with the UVT of UV processes.