UV/H₂O₂ treatment of drinking water increases post-chlorination DBP formation

Abstract

Ultraviolet (UV) irradiation has become popular as a primary disinfectant because it is very effective against Cryptosporidium and does not directly form regulated disinfection by-products. Higher UV doses and UV advanced oxidation (UV/H₂O₂) processes are under consideration for the treatment of trace organic pollutants (e.g. pharmaceuticals, personal care products). Despite the disinfection effectiveness of UV light, a secondary disinfectant capable of maintaining a distribution system residual is required to meet current U.S. regulation. This study investigated changes in disinfection by-product (DBP) formation attributed to UV or UV/H₂O₂ followed by application of free chlorine to quench hydrogen peroxide and provide residual disinfectant. At a UV dose of 1000 mJ/cm², trihalomethane (THM) yield increased by up to 4 μg/mg-C and 13 μg/mg-C when treated with low and medium pressure UV, respectively. With the addition of hydrogen peroxide, THM yield increased by up to 25 μg/mg-C (5 mg-H₂O₂/L) and 37 μg/mg-C (10 mg-H₂O₂/L). Although no changes in DBPs are expected during UV disinfection, application of UV advanced oxidation followed by chlorine addition was assessed with regard to impacts on DBP formation.

Introduction

Many drinking water treatment plants are faced with a variety of source water challenges that may require the application of advanced processes to meet regulatory and local water quality goals. Ultraviolet (UV) irradiation and UV advanced oxidation (UV coupled with hydrogen peroxide [UV/H₂O₂], i.e., an advanced oxidation process [AOP]) has received notable attention for two primary reasons: 1) UV irradiation is a highly effective disinfectant against chlorine-resistant protozoa (e.g. Cryptosporidium parvum and Giardia lamblia) (USEPA, 2006) and 2) UV/H₂O₂ AOP is capable of reducing the concentration of numerous environmentally relevant trace organic pollutants (e.g. estradiol, acetominophen, diclofenac, etc.) (e.g., Snyder et al., 2007). Efficient as a primary disinfectant and for destruction of trace organic pollutants, there has been limited study concerning these UV processes as part of a complete drinking water treatment train. UV alone does not provide a suitable residual disinfectant, and hydrogen peroxide residual post UV/H₂O₂ may be present. Therefore, downstream of any UV process a suitable residual disinfectant (i.e. chlorine or chloramines) may be necessary.

Free chlorine is the most common chemical disinfectant used to maintain a disinfectant residual in North American drinking water. While highly effective against most pathogens, free chlorine reacts with dissolved organic matter (DOM) to form numerous disinfection by-products (DBPs). Hundreds of halogenated and non-halogenated DBPs of varying toxicity form during this reaction (Richardson et al., 2007) but only eleven are currently regulated (i.e. four trihalomethanes [THMs], five haloacetic acids [HAAs], bromate, and chlorite) by the United States Environmental Protection Agency (USEPA) under the 2006 Stage II Disinfectants and Disinfection By-Products Rule (40 CFR Parts 9, 141, and 142). This regulation requires that within the distribution system, the maximum annual running average concentrations of THMs and HAAs cannot exceed 80 μg/L and 60 μg/L, respectively. Some utilities measure total organic halide (TOX) as a broad screening measurement for halogenated DBP formation. For free chlorine, typically 25% of TOX on a molar basis can be accounted for by the four regulated THMs and five HAAs (Krasner et al., 1989).

The treatment efficiency of UV processes correlates directly with the ability of UV photons to penetrate the water. UV processes are typically installed at the end of the treatment train or after primary particle removal as a significant portion of turbidity and UV absorbing DOM has been physically removed. The lack of a residual disinfectant requires that the sequence of UV plus oxidant (e.g. free chlorine) be considered to capture realistic drinking water quality impacts. Free chlorine could be applied upstream of a UV process but will result in the loss of free chlorine and production of chloride (Cl) and hydroxyl (OH) radicals due to UV photolysis (Watts and Linden, 2007). OH radicals react with DOM and destroy trace organic pollutants. In the case of UV/H₂O₂, the hydrogen peroxide forms OH but residual hydrogen peroxide remains after irradiation and must be quenched prior to generation of a chlorine residual. Free chlorine can be used as a quenching agent, quenching hydrogen peroxide at a stoichiometric ratio of 2.09 mg-Cl₂ to 1.0 mg-H₂O₂. With respect to DBP formation, few have studied the effect of the UV/H₂O₂ followed by free chlorine while more have studied UV followed by free chlorine.

Ultraviolet disinfection at doses of 40–150 mJ/cm² followed by chlorination does not significantly change regulated DBP formation compared to the non-UV disinfected water (Malley et al., 1995, Liu et al., 2002a, Linden et al., 2004). However, researchers found that UV disinfection forms small molecular weight aldehydes, carboxylic acids and biologically degradable organic carbon (BDOC) or assimilable organic carbon (AOC) (Shaw et al., 2000, Malley et al., 1995, Linden et al., 2004). Disinfection UV doses are significantly lower than those required for trace organic compound destruction. Higher doses and application of hydrogen peroxide in AOP treatment are likely to have a greater affect on DOM character and potential to yield DBPs when post-chlorinated.

UV doses for trace organic compound destruction vary widely depending on the properties of the target compound. Medium pressure (MP) UV, a polychromatic output, used for treatment of Lake Huron water at doses of 439 mJ/cm² achieved greater than 90 percent removal of compounds such as estradiol, diclofenac, acetaminophen and oxybenzone to name a few (Snyder et al., 2007). Other trace organic compounds required greater UV doses for similar levels of destruction, for example 3%, 57%, 88% and 95% atrazine destruction was observed at doses of 40, 439, 787, and 1318 mJ/cm², respectively. Numerous trace organic compounds were better removed under UV with addition of hydrogen peroxide at less than 7.5 mg/L. While the Snyder and colleagues study was one of the more comprehensive, many others have studied the degradation of trace organic pollutants by UV and UV/H₂O₂ (e.g., Rosenfeldt and Linden, 2004, Vogna et al., 2004).

UV and UV/H₂O₂ processes applied at disinfection and higher doses (40 to < 1000 mJ/cm²) do not typically mineralize DOM, however mineralization depends on DOM structure, which varies widely. 1000 mJ/cm² of UV-C (200–280 nm) did not mineralize the DOM in a reservoir sample, while 10,000 mJ/cm² mineralized nearly 50% of the DOM (Goslan et al., 2006). Alternatively, Buchanan et al. (2005) observed 4% mineralization after an LPUV dose of 23,000 mJ/cm². Fractionation of the treated sample showed decreased concentration of the very hydrophobic acids, stable slightly hydrophobic acids, and increased charged hydrophilic acids and neutrals. UV alone is typically not considered an oxidation process but with the addition of hydrogen peroxide, UV/H₂O₂ has a high oxidation potential. 15% of organic carbon present in a Capilano Reservoir sample was mineralized at a dose of 1500 mJ/cm² and 20 mg-H₂O₂/L (Sarathy and Mohseni, 2007, Sarathy and Mohseni, 2009). High pressure size exclusion chromatography revealed UV/H₂O₂ decreased total chromophoric DOM and apparent molecular weight. While UV and UV/H₂O₂ can both eventually mineralize DOM, the addition of hydrogen peroxide reduces the amount of UV irradiation required.

The effect of UV dose, with and without the application of hydrogen peroxide, suitable for trace organic pollutant destruction has produce varied DBP formation responses when post-chlorinated. Liu and colleagues observed little to no change in THM and HAA formation due to post-chlorination until a UV dose of 1000 mJ/cm² in synthetic waters (Liu et al., 2002a). Venkatesan and colleagues found an increase of up to 20% in THM and HAA formation in natural water with increasing UV dose (400 and 1500 mJ/cm² tested), which corresponded with increased chlorine demand (Venkatesan et al., 2003). Few studies present THM and HAA formation at UV doses greater than that required for disinfection and far fewer present results for UV/H₂O₂ followed by chlorination. Kleiser and Frimmel showed approximately a 20% increase in THM formation potential with increasing UV irradiation in the presence of H₂O₂ until mineralization of greater than 10% of dissolved organic carbon (DOC) occurred (about 200 min of irradiation) in water from the River Ruhr (2000). Further study by Liu et al. (2002b) showed that UV/H₂O₂ with 100 mg-H₂O₂/L resulted in no changes in THM or HAA formation below 500 mJ/cm², and at doses above 500 mJ/cm², THM and HAA formation decreased.

The objective of the present study was to evaluate the impact on formation of THMs, HAAs, and TOX from the sequence of UV or UV/H₂O₂ followed by chlorination with free chlorine. Here chlorine was used as the hydrogen peroxide quenching agent, as would likely be performed at conventional full-scale by WTPs requiring chlorine residual. Two UV lamp types, low pressure monochromatic lamp (LPUV) and a medium pressure polychromatic lamp (MPUV), were tested with and without hydrogen peroxide on two water samples (before and after a post-filter granular activated carbon [GAC]) at UV doses capable of trace organic destruction (500 and 1000 mJ/cm²). The study design was the first to explore the formation of DBPs in treated drinking water after application of UV/H₂O₂ treatment followed by free chlorination to quench residual hydrogen peroxide and provide an oxidant residual. As more WTPs consider the need for oxidation of contaminants of emerging concern, the impact of an AOP process as part of the existing WTP train requiring post-chlorination needs to be explored.