Predicting the Contribution of Chloramines to Contaminant Decay during Ultraviolet/Hydrogen Peroxide Advanced...

Chloramines applied to control membrane biofouling in potable reuse trains pass through reverse osmosis membranes, such that downstream ultraviolet (UV)/H₂O₂ advanced oxidation processes (AOPs) are de facto UV/H₂O₂-chloramine AOPs. Current models for UV/chloramine AOPs, which use inaccurate chloramine quantum yields and ignore the fate of ^•NH₂, are unable to simultaneously predict the loss of chloramines and contaminants, such as 1,4-dioxane. This study determined quantum yields for NH₂Cl (0.35) and NHCl₂ (0.75). Incorporating these quantum yields and the formation from ^•NH₂ of the radical scavengers, ^•NO and NO₂^-, was important for simultaneously modeling the loss of chloramines, H₂O₂, and 1,4-dioxane in the UV/H₂O₂-chloramine AOP. Although the level of radical production was higher for the UV/H₂O₂-chloramine AOP than for the UV/H₂O₂ AOP, the UV/H₂O₂ AOP was at least 2-fold more efficient with respect to 1,4-dioxane degradation, because chloramines efficiently scavenged radicals. At low chloramine concentrations, the UV/chloramine AOP efficiency increased with an increase in chloramine concentration, as the level of radical production increased relative to that of radical scavenging by the dissolved organic carbon in RO permeate. However, the efficiency leveled out at higher chloramine concentrations as radical scavenging by chloramines offset the increased level of radical production. The level of 1,4-dioxane degradation was ∼30-50% lower for the UV/chloramine AOP than for the UV/H₂O₂-chloramine AOP when the concentration of residual chloramines in RO permeate was ∼50 μM (3.3 mg/L as Cl₂). Initial cost estimates indicate that the UV/chloramine AOP using the residual chloramines in RO permeate could be a cost-effective alternative to the current UV/H₂O₂-chloramine AOP in some cases, because the savings in reagent costs offset the ∼30-50% reduction in 1,4-dioxane degradation efficiency.