Pilot UV-AOP Comparison of UV/Hydrogen Peroxide, UV/Free Chlorine, and UV/Monochloramine for the Removal of N-Nitrosodimethylamine (NDMA) and NDMA Precursors

Abstract

Three different ultraviolet-advanced oxidation processes (UV-AOPs) (UV/hydrogen peroxide, UV/free chlorine, and UV/supplemental monochloramine) used in full advanced treatment (FAT) for potable reuse were evaluated at pilot scale using reverse osmosis (RO) permeate from a FAT facility. Oxidant concentrations were varied, and each AOP was assessed for its ability to remove N-nitrosodimethylamine (NDMA) and NDMA precursors. For all AOPs, NDMA was removed well (to near 1.2 ng/L). However, UV/free chlorine exhibited less apparent NDMA destruction, and this level of destruction decreased as the free chlorine concentration increased. Removal of NDMA precursors was variable with no oxidant showing superior performance, ranging widely from 1% to 84% removal. The level of NDMA precursors increased in only two of 38 UV-AOP pilot tests, suggesting that formation of NDMA precursors by UV-AOP is limited. Adjustment of the UV-AOP pH was evaluated as a strategy for controlling NDMA precursors. Increasing the RO permeate pH prior to UV-AOP from 5.5 (ambient pH) to ∼7.0 using NaOH, to decrease the dichloramine concentration, decreased the level of destruction of NDMA precursors (i.e., increased the level of NDMA formation) in the UV/HOCl AOP (likely due to an increased level of radical scavenging by OCl–). Increasing the pH to ∼8.5 via the addition of lime decrease the level of NDMA destruction in the UV/H2O2 AOP due to the lower NDMA quantum yield at higher pH.

N-Nitrosodimethylamine Formation during UV/Hydrogen Peroxide and UV/Chlorine Advanced Oxidation Process Treatment Following Reverse Osmosis for Potable Reuse

“…While our results indicated that net NDMA formation associated with the UV/NH 2 Cl/Cl-DMA pathway is unlikely to be important at 700 mJ/cm 2 , net NDMA formation was important at lower fluence (Figure ). Moreover, recent pilot tests applying 700–1300 mJ/cm 2 at two potable reuse facilities , have indicated that effluent NDMA concentrations are more difficult to control by the UV/HOCl AOP than by the UV/H 2 O 2 AOP. Our results demonstrate that this challenge occurs because chlorine addition promotes NDMA formation by dark breakpoint chlorination reactions over a <10 min time interval.…”

“…30 Pilot-scale comparisons of UV/H 2 O 2 and UV/HOCl AOPs conducted within FAT trains at the Donald C. Tillman facility in Los Angeles (CA) and at the Orange County Water District (CA) demonstrated that both AOPs reduced NDMA concentrations, but that effluent NDMA concentrations were higher for the UV/HOCl AOP. 27,28 Because direct UV photolysis of NDMA should be similar between these AOPs, these observations suggested that NDMA was forming concurrently during UV/HOCl AOP treatment.…”

“…Pathways that form NDMA concurrently during the AOP treatment would increase the UV fluence needed to control NDMA. Recent pilot-scale studies with the UV/chlorine (UV/HOCl) AOP suggest the existence of concurrent NDMA formation pathways during the AOP treatment. , Although the UV/hydrogen peroxide (UV/H 2 O 2 ) AOP has been the most frequently employed AOP for potable reuse, , there has been recent interest in the UV/HOCl AOP. Radical production from HOCl photolysis is higher than from H 2 O 2 because of the higher molar absorption coefficient (62 M –1 cm –1 vs 18.6 M –1 cm –1 ) and quantum yield (0.62 vs 0.5) of HOCl .…”

“…Research at lab-scale and pilot-scale has demonstrated that the UV/HOCl AOP treatment can be more efficient than the UV/H 2 O 2 AOP for degradation of 1,4-dioxane; in addition to controlling NDMA, AOPs in advanced treatment trains typically strive to achieve 0.5 log removal of 1,4-dioxane . Pilot-scale comparisons of UV/H 2 O 2 and UV/HOCl AOPs conducted within FAT trains at the Donald C. Tillman facility in Los Angeles (CA) and at the Orange County Water District (CA) demonstrated that both AOPs reduced NDMA concentrations, but that effluent NDMA concentrations were higher for the UV/HOCl AOP. , Because direct UV photolysis of NDMA should be similar between these AOPs, these observations suggested that NDMA was forming concurrently during UV/HOCl AOP treatment.…”

Exploring Pathways and Mechanisms for Dichloroacetonitrile Formation from Typical Amino Compounds during UV/Chlorine Treatment

“…The UV/chlorine process is an emerging advanced oxidation process (AOP) for oxidation and disinfection in water treatment. − UV photolysis of free chlorine leads to the generation of hydroxyl radicals (HO • ) and reactive chlorine species (RCS), such as Cl • , Cl 2 •– , and ClO • , which are effective for the abatement of refractory micropollutants and inactivation of pathogens. − However, the formation of disinfection byproducts (DBPs) is a major concern. , Compared to chlorination alone, more chlorinated products have been identified during UV/chlorine treatment of natural organic matter (NOM). , Meanwhile, UV/chlorine treatment can lead to increased formation of DBPs, especially nitrogenous DBPs (N-DBPs) with high toxic potency. − Nevertheless, the enhanced formation of many N-DBPs during UV/chlorine treatment is not yet well understood.…”