Search

Peracetic acid (PAA) may be used in drinking water treatment for pre-oxidation and mussel control at the intake. PAA may exert a downstream chlorine demand, but full details of this reaction have not been reported. There are three possible mechanisms of this demand: (1) PAA may react directly with chlorine; (2) PAA exists in equilibrium with hydrogen peroxide, which is known to react with chlorine; and (3) as H 2 O 2 reacts with chlorine, PAA will hydrolyze to form more H 2 O 2 to re-establish PAA/H 2 O 2 equilibrium, thereby serving as an indirect reservoir of chlorine demand. While the H 2 O 2 reaction with chlorine is well known, the other mechanisms of possible PAA-induced chlorine demand have not previously been investigated. The observed molar stoichiometric ratio of PAA to free chlorine (n) for the presumed direct PAA + free chlorine reaction was determined to be approximately 2, and the corresponding observed reaction rate coefficients at pH 6, 7, 8, and 9 were 2.76, 3.14, 1.61, 10.1 M -n ·s -1 , respectively (at 25 °C). With these estimated values, a kinetic model was built to predict the chlorine demand by PAA. The results suggest that chlorine demand from PAA is likely to be negligible over the course of several days (e.g., < 20% chlorine loss) for most conditions except for high pH (e.g., >8) and high PAA:Cl 2 molar ratios (e.g., >2:1)., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Ltd. All rights reserved.)

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., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

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 dominant radical scavengers during UV/H 2 O 2 and UV/Cl were identified. • The effect of TOC on UV/H 2 O 2 vs. UV/Cl varied with micropollutant type and pH. • pH-TOC regions where either UV/H 2 O 2 or UV/Cl was more efficient were identified. UV/Cl and UV/H 2 O 2 are advanced oxidation processes (AOPs) used for drinking water treatment and water reuse. This work explored the hypothesis that UV/Cl becomes more competitive to UV/H 2 O 2 at neutral-to-high pH as the concentration of total organic carbon (TOC) increases. Lab experiments and kinetic modelling were used to compare initial pseudo first-order contaminant decay rate coefficients between the AOPs at various pH and TOC conditions. The relative effect of increasing TOC concentrations on UV/Cl vs. UV/H 2 O 2 depended on the pH, contaminant, and organic matter reactivity towards radicals. For example, while the reaction rate coefficients during both AOPs generally decreased with increasing TOC, the UV/Cl reaction rate coefficients for the solely •OH-reactive sucralose decreased 41–138% less than the UV/H 2 O 2 coefficients as the TOC concentration was increased from 0 to 5 mg-C L−1. However, UV/Cl was more affected than UV/H 2 O 2 when targeting caffeine (a contaminant reactive to chlorine radicals). The data were used to define TOC-pH conditions for which either AOP would be more energy-efficient, under a set of standard conditions. The results suggest that UV/Cl may be competitive to UV/H 2 O 2 under a wider range of treatment scenarios than has been conventionally thought based on tests in pure water. [Display omitted] [ABSTRACT FROM AUTHOR]