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When treating drinking water using UV/H2O2 advanced oxidation, the H2O2 residual is usually quenched to allow for downstream chlorine stability. In some utilities, monochloramine (NH2Cl) is used for secondary disinfection, and H2O2 may not need to be quenched because NH2Cl and H2O2 can coexist for some time. However, when ammonia and chlorine are applied to form NH2Cl in the H2O2 -containing water, H2O2 will compete with ammonia to react with the applied chlorine, compromising the NH2Cl formation efficiency. This research combined theory and experiments to evaluate NH2Cl formation efficiency in the presence of H2O2 in Lake Ontario water and, after NH2Cl formation, its subsequent decay at different temperatures and pH. The results demonstrated that at many typical temperatures and pH, the presence of H2O2 does not significantly impair the formation of NH2Cl. Furthermore, while H2O2 accelerates NH2Cl decay, the results suggest that under specific conditions, such as short to medium residence times (e.g., less than 48 h), it may be possible for NH2Cl to coexist with as much as 5 mg/L H2O2 without compromising secondary disinfection. [ABSTRACT FROM AUTHOR]

The photolysis of chlorine by UV light leads to the formation of the hydroxyl radicals (OH) as well as reactive chlorine species (RCS) that can be effective as advanced oxidation processes (AOPs) for water treatment. Much of the research to date has been done at laboratory- or bench-scale. This study reports results from a model that demonstrates that the relative effectiveness of the UV/Cl 2 AOP compared to the more traditional UV/H 2 O 2 AOP is a function of optical path length. As such, the relative effectiveness of the two treatment options evaluated at small scale may not reflect the relative performance at full-scale, making results previously obtained at small-scale potentially less scalable. This study therefore compares the performance of UV/Cl 2 to UV/H 2 O 2 at a full-scale water treatment plant, using sucralose and caffeine as spiked surrogates for contaminants that are reactive solely to OH radicals, and to both OH and RCS, respectively. pH was varied between 6.5 and 8.0. The results demonstrated that when using a medium pressure UV lamp, UV/Cl 2 might lead to approximately twice the production of OH radicals as UV/H 2 O 2 at pH 6.5 when using the same molar oxidant concentration, but adding chlorine to the UV reactor at pH 8.0 had a negligible impact on OH radical concentration in comparison to UV alone. The study also confirmed previous small-scale results that RCS can be a major contributor to UV/Cl 2 treatment for compounds such as caffeine that are susceptible to RCS, with UV/Cl 2 effective at both pH 6.5 and 8.0 for such compounds. Disinfection byproducts were monitored, with adsorbable organohalide (AOX) formation increasing by approximately 10 μg-Cl/L due to chlorine photolysis, but only at pH 6.5 and not at pH 8.0. This implies that UV/Cl 2 might increase AOX mostly due to reaction between OH and organic precursors to make them more reactive with chlorine, and not due to RCS. The formation of specific DBPs of current or emerging regulatory interest was minimal under all conditions, except for chlorate. Chlorate yields were in the order of 6–18% of the photolysed chlorine. Image 1 • The relative effectiveness of MP UV/H 2 O 2 versus UV/Cl 2 is light path length dependent. • UV/Cl 2 produced twice as much OH radical as UV/H 2 O 2 at pH 6.5. • The reactive chlorine species generated in UV/Cl 2 at pH 8.0 did not contribute to AOX formation. • Chlorate yields were in the order of 6–18% of the photolysed chlorine in UV/Cl 2. [ABSTRACT FROM AUTHOR]

• CHA degradation was mainly attributed to OH radical attack. • Dividing the chlorine dose into five separated doses increased the removal rate. • Different by-products were identified including two chlorine-containing by-products. • The reaction pathway for CHA degradation during UV/Chlorine treatment was proposed. Degradation kinetics, by-products identification and pathways of a model naphthenic acid, cyclohexanecarboxylic acid (CHA), by the UV/Chlorine process were investigated in this study. Mathematical modeling indicated that the initial CHA decay rate increased rapidly with the chlorine dose when the chlorine dose was lower than 45 mg/L and decreased with further chlorine dose increases. Increasing the chlorine dose from 400 to 800 mg/L resulted in a steady increase in the total removal of CHA after 60 min of UV photolysis. By dividing the 700 mg/L chlorine dose into five separated doses (140 mg/L each) added at 10 min intervals, the total CHA removal increased from 72% to 91%. This implies that the ideal condition of the UV/Chlorine process in degrading CHA is to add chlorine continuously at a constant rate to compensate any chlorine consumption to reduce the radical scavenging effect. It was found that the CHA decay was mainly attributed to the hydroxyl radical (OH) attack and the reactive chlorine species (RCS) contribution was relatively small. Various by-products, including the mono-chlorinated and di-chlorinated by-products, were identified and the reaction pathway for CHA degradation during UV/Chlorine treatment was proposed. [ABSTRACT FROM AUTHOR]

Drinking water treatment using UV/H 2 O 2 advanced oxidation typically results in residual H 2 O 2 that requires quenching to minimize its interference with downstream processes. Chemical quenching using chlorine or bisulfite are options, but there is some uncertainty in the literature about the kinetics of the bisulfite reaction, with some reports quoting the reaction as fast, and others as slow. Part of the contradictory information may be due to interference in H 2 O 2 analysis by bisulfite. An analytical method was developed to avoid this interference, in which monochloramine first selectively quenched bisulfite, and then H 2 O 2 was measured spectrometrically using titanium(IV) oxysulfate for color development. The confirmatory experiments suggested that the bisulfite reaction with H 2 O 2 is actually relatively slow, with a half-life in the order of hours to days depending on the pH and the reagent concentrations. As a result, within the typical pH range of drinking water treatment (e.g., 6–9), chlorine is preferred over bisulfite as the H 2 O 2 quenching agent on the basis of reaction kinetics. However, a decrease in pH will lead to an increase in the bisulfite-H 2 O 2 reaction rate along with a decrease in the Cl 2 –H 2 O 2 reaction rate, such that at pH < 5.7 bisulfite is the faster reagent. Both bisulfite and chlorine were observed to react with H 2 O 2 following a stoichiometric ratio of 1:1 in the natural water matrix tested. Image 1 • Bisulfite quenches H 2 O 2 at too slow a rate to serve as a quenching agent. • Cl 2 quenches H 2 O 2 quickly at neutral and basic pH, but not instantaneously. • A modified titanium(IV) method was developed to quantify H 2 O 2. [ABSTRACT FROM AUTHOR]