Balancing bromate formation, organics oxidation, and pathogen inactivation: the impact of bromate suppression techniques on ozonation system performance in reuse waters
The ozonation process is becoming a critical step in potable reuse systems that do not rely on membranes. By providing a barrier to pathogens, increasing biodegradable carbon for downstream biofiltration, and oxidizing contaminants of emerging concern, the ozonation process can also form the disinfection byproducts, bromate and N-nitrosodimethylamine (NDMA). Bromate formation mitigation techniques including free ammonia addition, monochloramination, and the chlorine-ammonia process have been used in the past with varying degrees of success. However, the impact of these suppression methods has not been studied on NDMA formation, disinfection, organic oxidation, and bromate formation simultaneously. This study found that both preformed monochloramination and the chlorine-ammonia process were effective in controlling bromate formation, whereas free ammonia addition was found to be ineffective. The addition of preformed monochloramine and the chlorine-ammonia process was able to reduce bromate formation by up to 80%. Additionally, the chlorine-ammonia process was able to increase the attainable disinfection credits for the system by reducing ozone demand through preoxidation, reducing the required ozone dose for treatment objectives by 50%. NDMA was found to plateau after a residual ozone concentration was detected. Additionally, the chlorine-ammonia process formed slightly less NDMA than other suppression techniques, suggesting the preoxidation of precursor compounds prior to ozonation and decreasing NDMA formation potential. In terms of bromate formation, organics oxidation, and pathogenic inactivation, the chlorine-ammonia process was found to be the optimal treatment technique if implemented properly.
Background of Ozone
Ozone was introduced in drinking water treatment as a disinfectant in the 1970’s. This treatment process has also proven itself in the field of water reuse as a disinfection and oxidation step. While providing a formidable barrier for pathogens, ozonation also effectively converts refractory organic carbon to assimilable, ideal for systems with downstream biofiltration (von Gunten 2003a). Ozonation has multiple advantages when compared with other advanced oxidation processes for these applications including low cost, replicability, and limited production of disinfection by-products (DBPs). Bromate, however, is a key DBP unique to the ozonation system and has an EPA, EU and WHO MCL of 10µg/L (U.S. Environmental Protection Agency 1998). For this reason, it is imperative to balance ozone oxidation and disinfection with bromate formation for maximum system efficiency.
Reactions of Ozone in Water
Upon application to process water, aqueous ozone decays into hydroxyl radicals (OH•) through autocatalytic decay or through reactions with organic moieties. Direct reactions between ozone and organics greatly increase the bioavailable fraction of dissolved organic carbon. Some of these reactions, typically with electron-rich moieties (ERMs) which have fast reaction rates with ozone, generate relatively large amounts of OH• (Buffle et al. 2004; von Gunten 2003a). This process of generating radicals through ozone-DOM reactions (OH• initiation) in turn allow for a greater number of organics to be oxidized by OH•, some of which are ozone-refractory. These organics can either generate further radicals (OH• propagation) or end the radical chain reactions (OH• termination) (von Gunten and Buffle 2006). Though OH• can oxidize more organic compounds than ozone due to its higher redox potential, the short-lived existence and low concentration of OH• makes its impact on disinfection and bulk organic oxidation minimal (von Gunten and Buffle 2006). Trace organic constituents (TOrCs), such as certain pharmaceuticals and personal care products (PPCPs) and contaminants of emerging concern (CECs), often react only slowly with ozone, or may even be completely refractory. In order to oxidize ozone-resistant organics such as 1,4-dioxane, advanced oxidation through OH• is required. Unfortunately, both OH• and O3 exposures result in the conversion of bromide to bromate. Figure 4 summarizes the actions of ozone and OH• in process water (Buffle et al. 2004, 2006b).
Ozone Decomposition Kinetics
While aqueous ozone decay is often modeled through first order regression, this approximation does not hold true for the entire reaction. Ozone has been described to have “biphasic” kinetics in water: a fast-initial decay phase followed by pseudo-first order decay. During the initial decomposition of ozone, often called instantaneous ozone demand (IOD), moieties with fast reactions with ozone (k>108 M-1s-1) cause a chain reaction resulting in radical proliferation (Buffle et al. 2004). In ozonation processes with sources high in organics, such as treated wastewater effluent, this IOD can consume more than 50% of the transferred ozone dose. During IOD, the ratio of the exposure of OH• to ozone exposure (RCT) is much higher than later in the reaction, where first-order decay dominates. Ozonation in wastewater effluents is often considered an inherent advanced oxidation process (AOP) due to these elevated OH• exposures during the initial phase (Buffle et al. 2006b). During phase 2 of ozone decay, the OH• exposure decreases, and ozone decay stabilizes. This is a result of the most reactive moieties reacting to completion with ozone and less OH• is generated as a result (von Gunten 2003a; b). RCT during this second phase decreases, and is generally considered to be constant throughout (RCT ~10-8) (Buffle et al. 2004). It should be noted, however, that changes in the transferred ozone dose will yield different first order decay rate constants for the same water quality during this phase. This is due to more reactions going to completion during the first phase of ozonation. However, the differences in decay constants between phase 1 and phase 2 are not currently well interpreted mathematically, with the function of ozone decay not truly fitting a pseudo-first order regression (Buffle et al. 2006b).
OH• Exposure Measurement Methods
A OH•, generated during the ozonation process, is an extremely short-lived molecule which exists at very low concentrations. Consequently, OH• exposures (the product of concentration and time) in ozonated water are extremely low (~10-10 M*s) (Buffle et al. 2004). These low exposures are due to the high reactivity of OH• with compounds in the water matrix. The measurement of OH• is vital to discern reaction kinetics as these species have major impacts on both organics oxidation and bromate formation. Due to their short half-life and reactivity, coupled with their low concentrations with respect to other oxidants present in the water, direct measurement of OH• in aqueous samples is effectively impossible. Therefore, probe compounds have been used to estimate total OH• exposures during ozonation processes. These compounds must be carefully selected, such that ozone does not directly react with them resulting in interference. Traditionally, para-chlorobenzoic acid (pCBA) has been used as a OH• probe compound (Gerrity et al. 2012; von Gunten and Buffle 2006). This is performed by measuring the amount of pCBA oxidized during ozonation. The formula for calculating OH• with pCBA oxidation may be seen in equation 8 (Gerrity et al. 2012). Due to method requirements such as high-pressure liquid chromatography (HPLC) and high capital cost, many municipalities do not have the capability to measure OH• exposures in ozonation systems through use of pCBA. Therefore, alternative methods are sought to increase the availability of OH• exposures. It is hypothesized that 1,4-dioxane would be a desirable alternative OH• probe compound due to the use of a GC-MS/MS Triple Quadrupole rather than HPLC, with the added benefit of gleaning contaminant abatement data. Oftentimes, 1,4-dioxane is also monitored in treatment processes in order to ensure the maintenance of water quality goals.
According to Buffle et al. 2004, bromate can be formed during the ozonation process through two primary reaction pathways: directly oxidized by ozone, and indirectly oxidized by radical reactions. The first step of the pathway can either be driven to Br• by hydroxyl radicals, or directly to HOBr/OBr– by ozone. Following the radical-driven pathway, Br• can be further oxidized to BrO• through ozone. This compound typically reacts to form two products: OBr– and BrO2–. The product OBr– and it’s conjugate acid HOBr can be considered key intermediates in this pathway: OBr– can react with ozone to form BrO2–, the final step prior to forming bromate, while HOBr reacts slow enough for the reaction to be discounted (k=0.01 M-1s-1). The bromate formation pathway is illustrated in Figure 5 below (Buffle et al. 2004). A variety of water quality parameters have an impact on bromate formation. Bromide, from sources such as salt/brackish water and landfill leachate, is the main consideration for bromate formation potential. Waters with elevated pH (>8) increase bromate formation due to the generation of more OH• and the shift in equilibrium of HOBr/OBr–. Temperature fluctuations also have a considerable impact on bromate formation, but whether there is a positive or negative impact depends on other water quality parameters as well as process design. NOM also has an impact on bromate formation but is specific to NOM reaction pathways. NOM may act either as a OH• terminator or initiator/propagator, as well as a sink for bromide-bromate intermediates. In waters with elevated alkalinity, OH• exposures decrease due to carbonate and bicarbonate ions acting as OH• sinks.These factors which affect bromate formation are summarized Table 1.
Bromate Suppression Methods
Along with tighter control of ozone doses, residuals, and exposures, chemical suppression of bromate is widely practiced. All forms of bromate suppression techniques can be categorized into four separate groups, consisting of: ozone exposure limitation, pH depression, intermediate formation and subsequent masking, and hydroxyl suppression. Some commonly-practiced techniques for bromate suppression in waters with elevated bromide concentrations include free ammonia addition, monochloramine addition, and the chlorine-ammonia process (Buffle et al. 2004). As depicted in Figure 6, ammonia can react with HOBr leading to the formation of bromamines. This intermediate product removes HOBr from the bromate formation pathway and remains relatively stable. Bromamine is oxidized slowly by ozone, forming bromide and nitrate. Bromamines can also react with organic matter in the water forming brominated organics. While these organic compounds sequester bromine throughout the rest of the ozonation process, little is known about their toxicity (Buffle et al. 2004). According to Buffle et al., 2004, the premise of the chlorine-ammonia process is to add free chlorine followed by free ammonia to the water source prior to ozonation in order to sequester bromine as an intermediate product, bromamine, thus slowing the formation of bromate. The free chlorine preoxidizes bromide ions into hypobromous acid to allow for a reaction between free ammonia and hypobromous acid to form bromamine. In this process, the excess free chlorine residual also reacts with the free ammonia to form monochloramine. It was noted in previous studies that having a free chlorine residual during ozonation leads to an increased formation rate of bromate, therefore excess free ammonia is critical to this process (Buffle et al. 2004). The impacts of di- and tri- chloramines are currently unknown in the bromate formation scheme.
1.1 Project Motivation and Objectives
2. Literature Review
2.1 Ozonation in Water Treatment
2.2 Reactions of Ozone in Water
2.3 Ozone Decomposition in Water
2.4 Reactions of ozone and hydroxyl radicals with organics
2.5 Hydroxyl Radical Measurement Methods
2.6 Absorption spectrum of ozone-treated process water
2.8 Chlorinated and brominated disinfection byproducts (Haloacetic Acids and Trihalomethanes)
2.9 NDMA Formation
2.10 Bromate Formation
2.11 Bromate Suppression
2.12 Effects of preoxidation on ozone processes
2.13 Ozone control strategies
3. Manuscript 1: Balancing bromate formation, organics oxidation, and pathogen inactivation: the impact of bromate suppression techniques on ozonation system performance in reuse waters
3.3 Materials and Methods
3.4 Results and Discussion