Comparing the UV/Monochloramine and UV/Free Chlorine Advanced Oxidation Processes (AOPs) to the UV/Hydrogen Peroxide AOP Under Scenarios Relevant to Potable Reuse

Utilities incorporating the potable reuse of municipal wastewater are interested in converting from the UV/H2O2 to the UV/free chlorine advanced oxidation process (AOP). The AOP treatment of reverse osmosis (RO) permeate often includes the de facto UV/chloramine AOP because chloramines applied upstream permeate RO membranes. Models are needed that accurately predict oxidant photolysis and subsequent radical reactions. By combining radical scavengers and kinetic modeling, we have derived quantum yields for radical generation by the UV photolysis of HOCI, OC1(-), and NH2Cl of 0.62, 0.55, and 0.20, respectively, far below previous estimates that incorporated subsequent free chlorine or chloramine scavenging by the Cl-center dot and (OH)-O-center dot daughter radicals. The observed quantum yield for free chlorine loss actually decreased with increasing free chlorine concentration, suggesting scavenging of radicals participating in free chlorine chain decomposition and even free chlorine reformation. Consideration of reactions of (ClO)-Cl-center dot and its daughter products (e.g., CO2-), not included in previous models, were critical for modeling free chlorine loss. Radical reactions (indirect photolysis) accounted for similar to 50% of chloramine decay and similar to 80% of free chlorine loss or reformation. The performance of the UV/chloramine AOP was comparable to the UV/H2O2 AOP for degradation of 1,4-dioxane, benzoate and carbamazepine across pH 5.5-8.3. The UV/free chlorine AOP was more efficient at pH S.5, but only by 30% for 1,4-dioxane. At pH 7.0-8.3, the UV/free chlorine AOP was less efficient. Cl-center dot converts to (OH)-O-center dot. The modeled (CI)-C-center dot:(OH)-O-center dot ratio was similar to 20% for the UV/free chlorine AOP and similar to 35% for the UV/chloramine AOP such that (OH)-O-center dot was generally more important for contaminant degradation.