Formation of oxidation byproducts from ozonation of wastewater

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1 Available at journal homepage: Formation of oxidation byproducts from ozonation of wastewater Eric C. Wert a,, Fernando L. Rosario-Ortiz a, Doug D. Drury b, Shane A. Snyder a a Southern Nevada Water Authority (SNWA), 243 Lakeshore Rd., Boulder City, NV 895, USA b Clark County Water Reclamation District, 5857 E. Flamingo Road, Las Vegas, NV 89122, USA article info Article history: Received 27 July 26 Received in revised form 11 January 27 Accepted 16 January 27 Keywords: Ozone Advanced oxidation process (AOP) Hydroxyl radicals Wastewater Water reuse Disinfection byproducts (DBP) Bromate Aldehydes Carboxylic acids abstract Disinfection byproduct (DBP) formation in tertiary wastewater was examined after ozonation (O 3 ) and advanced oxidation with O 3 and hydrogen peroxide (O 3 /H 2 O 2 ). O 3 and O 3 /H 2 O 2 were applied at multiple dosages to investigate DBP formation during coliform disinfection and trace contaminant oxidation. Results showed O 3 provided superior disinfection of fecal and total coliforms compared to O 3 /H 2 O 2. Color, UV absorbance, and SUVA were reduced by O 3 and O 3 /H 2 O 2, offering wastewater utilities a few potential surrogates to monitor disinfection or trace contaminant oxidation. At equivalent O 3 dosages, O 3 /H 2 O 2 produced greater concentrations of assimilable organic carbon (5 52%), aldehydes (31 47%), and carboxylic acids (12 43%) compared to O 3 alone, indicating that organic DBP formation is largely dependent upon hydroxyl radical exposure. Bromate formation occurred when O 3 dosages exceeded the O 3 demand of the wastewater. Benchscale tests with free chlorine showed O 3 is capable of reducing total organic halide (TOX) formation potential by at least 2%. In summary, O 3 provided superior disinfection compared to O 3 /H 2 O 2 while minimizing DBP concentrations. These are important considerations for water reuse, aquifer storage and recovery, and advanced wastewater treatment applications. & 27 Elsevier Ltd. All rights reserved. 1. Introduction Ozonation has been shown to be highly effective for water disinfection. In drinking water treatment, ozonation is used to meet United States Environmental Protection Agency (USEPA) regulations for the inactivation of viruses, Cryptosporidium, and Giardia (USEPA, 1991, 23). In wastewater treatment, ozonation has been used to meet discharge requirements for coliform and virus inactivation since the 197s (Rice et al., 1981). In recent years, ozonation has gained attention for its ability to oxidize endocrine disrupting chemicals (EDCs) and pharmaceuticals in both drinking water and wastewater (Zwiener and Frimmel, 2; Huber et al., 23, 25; Snyder et al., 26). The combination of microbial disinfection and trace contaminant oxidation make ozonation an attractive alternative for advanced wastewater treatment. During drinking water ozonation, the formation of organic (e.g., assimilable organic carbon (AOC), aldehydes, carboxylic acids, and ketones) and inorganic (e.g., bromate) disinfection byproducts (DBPs) has been well documented (Richardson et al., 1999; Huang et al., 25). Organic DBPs from ozonation have been linked to increased bacterial regrowth in drinking water distribution systems (LeChevallier et al., 1992). Drinking water utilities often employ biological filtration to remove these byproducts creating a biologically stable water prior to distribution (Huck et al., 1991; Krasner et al., 1993). In water reuse applications, AOC has contributed to increased bacterial regrowth and more rapid chlorine decay in distribution Corresponding author. Tel.: ; fax: address: eric.wert@snwa.com (E.C. Wert) /$ - see front matter & 27 Elsevier Ltd. All rights reserved. doi:1.116/j.watres

2 1482 systems (Ryu et al., 25). Currently, bromate is the only ozone (O 3 ) DBP regulated in drinking water by the USEPA, which established a maximum contaminant level (MCL) of 1 mg/l (USEPA, 1998). Dissolved organic carbon (DOC) concentrations are typically greater in wastewater than in surface water, resulting in faster O 3 decomposition rates and increased hydroxyl radical (doh) exposures (Buffle et al., 26a). As a result, higher O 3 dosages are required to meet wastewater treatment goals, potentially leading to increased DBP formation. The organic composition of wastewater is also different from the natural organic matter (NOM) found in surface water supplies and is commonly known as effluent organic matter (EfOM). EfOM is composed of recalcitrant NOM from drinking water, synthetic organic chemicals added during anthropogenic use (including disinfection by-products), and soluble microbial products (Shon et al., 26). Sparse data exist regarding the influence of EfOM on DBP formation. Sirivedhin and Gray (25) studied the impact of EfOM on halogenated DBP formation during chlorination of South Platte River water. They found that locations with greater EfOM influence were less reactive with chlorine on a per carbon basis than locations with minimal EfOM influence. The primary objective of this study was to quantify the formation of known organic and inorganic O 3 DBPs at dosages required for coliform disinfection and trace contaminant destruction in tertiary wastewater. Conventional O 3 and advanced oxidation processes (AOPs) with O 3 and hydrogen peroxide (O 3 /H 2 O 2 ) were evaluated at bench-scale and pilotscale to examine DBP formation. Subsequent chlorination examined the impact of O 3 and O 3 /H 2 O 2 on total organic halide (TOX) formation. These results will determine which ozonation technique is optimal to meet multiple wastewater treatment objectives while minimizing DBP formation. 2. Materials and methods 2.1. Experimental plan Bench-scale and pilot-scale experiments were conducted with tertiary wastewater to quantify the DBP formation during O 3 and O 3 /H 2 O 2 and subsequent TOX formation during chlorination. The testing was conducted in three phases: (1) bench-scale O 3 and O 3 /H 2 O 2, (2) pilot-scale O 3 and O 3 /H 2 O 2, and (3) bench-scale chlorination. Bench-scale O 3 and O 3 /H 2 O 2 tests provided information regarding O 3 demand, decay rate, and doh exposure. Due to the 1 L sample volume limitation during bench-scale testing, pilot-scale experiments were used to gather adequate sample volumes to measure coliform disinfection, DBP formation (AOC, carboxylic acids, aldehydes, and bromate), and trace contaminant removal. Results of trace contaminant testing (i.e., EDCs and pharmaceuticals) are presented elsewhere (Snyder et al., 26). Bench-scale chlorination tests using free chlorine determined the impact of ozonation on TOX formation Wastewater treatment facility Tertiary wastewater was collected from a 416, m 3 /d wastewater treatment plant (WWTP) with nitrification/denitrification operated by the Clark County Water Reclamation District in Las Vegas, Nevada, USA. The WWTP treats wastewater by first adding mg/l of ferric chloride followed by primary clarification and sedimentation, allowing heavier particles to be removed. Then, aerobic and anaerobic biological treatment occurs followed by secondary clarification. During the tertiary treatment stage, 4 13 mg/l of alum is added followed by flocculation and filtration. Ultraviolet light is used to achieve disinfection goals. Samples for this study were collected after filtration, but prior to UV disinfection. Seasonal variations were examined by collecting samples in June 25 and January 26. Table 1 shows the tertiary wastewater quality was very similar during both sampling events Ozonation at bench scale Bench-scale tests were performed using a batch reactor to obtain information about O 3 decomposition, doh exposure, and bromate formation. A sample of Nanopure TM water was placed inside a water-jacketed flask and cooled to 2 1C. Once cooled, 11% gaseous O 3 was diffused into the water using an oxygen-fed generator (model CFS-1A, Ozonia North America Inc., Elmwood Park, NJ USA). O 3 stock solution concentrations and dissolved O 3 residuals were measured according to Standard Methods 45-O 3 (Bader and Hoigne, 1982; APHA et al., 1998). O 3 dosages were administered by injecting an aliquot of the stock solution into a 1-L amber glass container with a repeating pipette dispenser containing the tertiary treated wastewater at room temperature (2 1C). During O 3 / H 2 O 2 experiments, H 2 O 2 was added 3 s prior to the addition of O 3 stock solution. Duplicate experiments were performed with the addition of 1.75 mm (274 mg/l) para-chlorobenzoic acid (pcba), an O 3 -resistant probe compound which reacts selectively with doh (Elovitz and von Gunten, 1999). Dissolved O 3 residual and pcba samples were collected at 1 s intervals during the first minute of reaction and each minute thereafter to investigate O 3 decomposition and doh exposure. Dissolved O 3 residual and pcba samples were collected until the O 3 residual had decayed to less than.5 mg/l, or until a contact time of 15 min was achieved. Bromate samples were collected after each test was completed. Table 1 Water quality summary of tertiary wastewater Water quality parameter Units June 25 January 26 Ammonia mg-n L 1 o.8 o.8 Bromate mg L 1 o.1 o.1 Bromide mg L Nitrate mg-n L ph Units Temperature 1C Total alkalinity mg L Total organic carbon mg L UV254 cm SUVA L mg 1 m

3 Ozonation at pilot scale A 1 L/min bench-top pilot plant (BTPP) consisting of all inert materials including glass, stainless steel, and fluorocarbon polymers was used to conduct the testing (Fig. 1). A peristaltic pump was used to transport the wastewater from a 28-L stainless steel drum into the contactor. During O 3 /H 2 O 2 experiments, H 2 O 2 (3% stock, Fisher Scientific, Pittsburgh, PA USA) was injected into the wastewater flow stream followed by a static mixer prior to entering the contactor. The O 3 contactor consisted of 12 glass chambers each providing 2 min of contact time for a total of 24 min. O 3 feed gas was produced from oxygen gas using a laboratory-scale generator (model LAB2B, Ozonia North America Inc., Elmwood Park, NJ USA). O 3 was added in the first contactor chamber with counter-current flow through a glass-fritted diffuser with bubble size of.1 mm. A mass-flow controller (model AFC26D, Aalborg Instruments and Controls, Inc., Orangeburg, NY, USA) and a feed gas concentration analyzer (model H1-S, IN USA Inc., Needham, MA, USA) were used to calculate and control the O 3 dosage. Due to the column height of.83 m and diameter of.55 m, the transfer efficiency varied between 4% and 7% depending on the desired dose and corresponding gas flow rate. The off-gas was collected from the top of each cell into a central manifold and destroyed by manganese dioxide catalyst. O 3 dosages were selected based upon bench-scale demand tests to evaluate a range of exposures for coliform disinfection and trace contaminant oxidation. O 3 exposure was calculated by integrating the dissolved residual concentration over time (CT) according to the extended integrated CT 1 method (Rakness et al., 25). During each test, the wastewater was maintained at room temperature (2 1C). Dissolved O 3 measurements were collected after 2, 6, 1, 14, and 18 min to examine demand and decay rates. Water quality samples were collected for AOC, carboxylic acids, aldehydes, bromate, total coliforms and fecal coliforms. In order to measure doh exposure, duplicate experiments were performed with 75.6 L Fig. 1 Schematic of bench-top pilot plant (BTPP).

4 1484 of tertiary wastewater spiked with 1 mm (156 mg/l) of pcba. Samples for pcba analysis were collected after 2, 6, 1, 14, and 18 min and quenched with a small aliquot of sodium thiosulfate (Na 2 S 2 O 3 ) Analytical methods Water samples were collected, preserved, and refrigerated at 4 1C until analyzed. Standard methods (SM) were used for the determination of total organic carbon (TOC) (SM 531B), UV absorbance at 254 nm (SM 591), total coliform (SM 9221B), fecal coliform (SM 9221E), and color (SM 212C) (APHA et al., 1998). AOC samples were preserved through pasteurization and measured through a bioassay using Pseudomonas fluorescens strain P-17 and Spirillum strain NOX according to SM Seven aldehydes (acetaldehyde, butanal, formaldehyde, glyoxal, m- glyoxal, pentanal and propanal) were analyzed by gas chromatography with electron capture detection (GC/ECD) according to SM Six carboxylic acids (acetate, formate, ketomalonate, oxalate, propionate and pyruvate) were measured by ion chromatograph by previously established methods (Kuo, 1998; Randtke, 21). TOX concentrations were analyzed by EPA Method 92B. Bromide and bromate concentrations were analyzed by ion-chromatography with inductively coupled plasma-mass spectroscopy detection (IC-ICP/MS)(Quinones et al., 26). Quantification of pcba was achieved using highpressure liquid chromatography (HPLC) equipped with an RP- C18 column with a 45/55 mixture of 1 mm H 3 PO 4 ph 2/MeOH as mobile phase and UV detection at 234 nm. The detection limit was estimated at.2 mm (3.1 mg/l). 3. Results and discussion 3.1. O 3 decomposition Initial O 3 demand and decay from bench-scale tests are shown in Fig. 2. Results demonstrate minimal seasonal changes in O 3 decomposition, which was expected due to similar TOC concentrations. The instantaneous O 3 demand (IOD) was measured by the difference between the transferred dose and the dissolved O 3 residual after 3 s. Results showed the IOD varied between 2 and 4 mg/l depending on the transferred dose. The IOD phase of ozonation has shown similarities to O 3 -based AOPs, resulting in doh exposure (Buffle et al., 26b). After the IOD phase was complete, dissolved O 3 residual decayed according to first-order rate kinetics, as expected (inset of Fig. 2). When targeting disinfection in drinking water applications, a measurable dissolved O 3 residual is required to meet CT guidelines established by the USEPA and assist with O 3 process control. Therefore, pilot-scale wastewater experiments targeted CT values up to 9.9 mg-min/l to evaluate wide range of O 3 exposures (Table 2). During bench-scale O 3 /H 2 O 2 experiments, the O 3 residual rapidly decayed within 1.5 min of reaction time. During pilotscale O 3 /H 2 O 2 experiments, detection of residual O 3 after the initial 2-min mass transfer period indicated that the O 3 /H 2 O 2 reaction was incomplete and a brief period of O 3 oxidation occurred. These results also show that when O 3 dosages exceeded the IOD, H 2 O 2 promoted O 3 decomposition thereby increasing initial doh exposure Hydroxyl radical exposure Oxidation attributed to doh exposure was measured by the depletion of pcba as shown in Fig. 3. During the IOD phase (timeo3 s), results showed pcba depletion was 13 23% greater during O 3 /H 2 O 2 than O 3 when equivalent O 3 dosages were applied. For example, 71% pcba depletion occurred during O 3 /H 2 O 2 versus 48% during O 3 when both processes employed an O 3 dose of approximately 5 mg/l. After 1 min of reaction, pcba depletion was within 7 1% during O 3 and O 3 / H 2 O 2 when similar O 3 dosages were applied. During firstorder O 3 decay, doh formation continued at a slower rate, resulting in comparable doh exposures. These results agree with Acero and von Gunten (21), who demonstrated that the overall doh exposure was similar between O 3 and O 3 / H 2 O 2 in surface waters (DOC ¼ 3.2 mg/l), and that H 2 O 2 addition accelerates the rate of O 3 decomposition into doh. During pilot-scale experiments, pcba depletion was similar to bench-scale results as shown in Table 2. When comparing O 3 and O 3 /H 2 O 2 at similar dosages, pcba was depleted 17 18% more with O 3 /H 2 O 2 than with O 3 during the first 2 min. After 6 min of contact time, pcba depletion was again within 1%, coinciding with bench-scale results. These results confirmed bench-scale findings indicating that the initial doh exposure (timeo3 s) was greater during O 3 /H 2 O 2 than O 3 ; although, overall doh exposure (time46 min) was similar for both processes when applying similar O 3 dosages Color and UV absorbance O 3 and O 3 /H 2 O 2 reduced color and UV absorbance at 254 nm (UV254) as shown in Table 3. Color was reduced from 24 to 5 8 units by O 3 and to 7 units by O 3 /H 2 O 2. Color reduction can be important for public perception during water reuse or aquifer storage and recovery projects. UV254 and SUVA gradually decreased as the O 3 dose increased during O 3 and O 3 /H 2 O 2. The decrease in SUVA demonstrates that the aromatic carbon content decreases as the O 3 dose increases (Weishaar et al., 23). These changes in color, UV254, and SUVA offer wastewater utilities a few potential surrogates to monitor disinfection or trace contaminant oxidation with or without a measurable O 3 residual Coliform disinfection Coliform disinfection was evaluated using both O 3 and O 3 / H 2 O 2 to determine the conditions required to achieve less than 2 fecal coliforms per 1 ml to comply with discharge regulations. Results shown in Table 3 indicate that an O 3 dosage of 2.1 mg/l can achieve effective coliform disinfection in the absence of a stable ozone residual. These results coincide with pilot-scale results reported by Janex et al. (2), who showed that 2-logs of fecal coliform inactivation could be achieved in wastewater before the IOD was met. Significant oxidation of trace contaminants was also observed during the IOD phase as shown in Snyder et al. (26). However, O 3 dosages exceeding the IOD are required to maintain proper

5 1485 Dissolved Ozone Residual (mg/l) O 3 =11.1 mg/l (Jan) O 3 =6.9 mg/l (June) O 3 =4.3 mg/l (June) O 3 =4.7 mg/l (Jan) O 3 =5.1 mg/l, H 2 O 2 =2.5 mg/l (Jan) O 3 =11.2 mg/l, H 2 O 2 =5 mg/l (Jan) ln ([O 3 ]/[O 3 ]o) Time (min) Time (min) Fig. 2 Dissolved O 3 residual decay curves during bench-scale experiments. The inset presents the data to show first-order rate kinetics. Table 2 O 3 residual decay and pcba reduction during pilot scale experiments Date Transferred O 3 dose H 2 O 2 dose Ozone residual (mg/l) Ozone CT (mg-min L 1 ) pcba reduction (%) 2min 6min 2min 6min Jun Jan o o.5 o o o disinfection levels during periodic demand fluctuations and flow rate changes (Rakness et al., 1993). The addition of H 2 O 2 decreased disinfection efficiency. When O 3 /H 2 O 2 was applied using O 3 dosages near the IOD (2.1 mg/l O 3 with 1. mg/l H 2 O 2 ), there was no measurable reduction in total coliform. Less efficient disinfection by O 3 / H 2 O 2 as compared to O 3 can be expected due to a lack of O 3 residual and the quantities of available doh scavengers in wastewater, specifically DOC and alkalinity (Acero and von Gunten, 21; von Gunten, 23) Bromate formation Bromate formation results during bench-scale and pilot-scale testing are shown in Fig. 4. Bromate formation was observed after the IOD of the wastewater was exceeded. At O 3 dosages above 3.1 mg/l, a linear relationship was observed between bromate formation and dose. At O 3 dosages above 4.5 mg/l, bromate formation exceeded the 1 mg/l drinking water MCL; however, there are no US regulations on bromate in wastewater effluents. Additional research is needed regarding the fate of bromate in receiving waters to determine if detection is expected in drinking water supplies influenced by wastewater. During O 3 /H 2 O 2 experiments, bromate formation was not observed at lower O 3 /H 2 O 2 conditions likely due to the lack of a stable O 3 residual and the reduction of hypobromous acid by excess H 2 O 2 (von Gunten and Oliveras, 1998). However, mg/l of bromate was formed at the highest O 3 dose of 7.1 mg/l with 3.5 mg/l of H 2 O 2. Bromate formation during this AOP condition was likely due to the presence of a high initial O 3 residual after the first 2 min of mass transfer.

6 pcba/pcba o O 3 =2.3 mg/l O 3 =2.3 mg/l, H 2 O 2 =2 mg/l O 3 =4.7 mg/l O 3 =5.1 mg/l, H 2 O 2 =2.5 mg/l O 3 =11.1 mg/l O 3 =11.2 mg/l, H 2 O 2 =5 mg/l Time (min) Fig. 3 pcba reduction during bench-scale O 3 and O 3 /H 2 O 2 experiments. Table 3 Coliform disinfection using conventional O 3 and O 3 /H 2 O 2 Test date O 3 dosage H 2 O 2 dose Reaction time (min) Color (units) TOC UV254 (cm 1 ) SUVA (L mg 1 m 1 ) Total coliform (#/ 1 ml) Fecal coliform (#/ 1 ml) Jun- 5 Jan o2 o o2 o o2 o o2 o Organic DBP formation Organic DBP formation during O 3 and O 3 /H 2 O 2 pilot-scale testing is shown in Table 4. Results showed that O 3 /H 2 O 2 formed 5 52% more AOC than O 3. AOC comprised about 14 16% of the TOC after O 3 and 15 25% of the TOC after O 3 / H 2 O 2. AOC was further investigated by identifying aldehyde and carboxylic acid formation. Minimal data exist regarding the natural attenuation of these organic DBPs in receiving waters. Total aldehyde concentrations showed that O 3 /H 2 O 2 formed 31 47% more aldehydes than O 3. Butanal and pentanal were not detected during any of the testing conditions. During O 3 / H 2 O 2 conditions, propanal was detected at concentrations between 2.9 and 3.9 mg/l. Formation of the remaining four aldehydes is shown in Fig. 5. Formaldehyde was formed at greater concentrations than the other aldehydes. Since O 3 / H 2 O 2 produced greater concentrations of formaldehyde than O 3, formation can be attributed mostly to doh exposure. However, the results in Fig. 3 indicated that the overall doh exposures were within 1% after O 3 and O 3 /H 2 O 2. Therefore, it can be deduced that the majority of formaldehyde formation occurred during the greater initial doh exposure within the IOD phase (timeo3 s) of O 3 decomposition.

7 BTPP (June 25) BS (Jan 26) BTTP (Jan 26) Bromate (µg/l) 4 3 y = x R 2 = Ozone Dose (mg/l) Fig. 4 Relationship between bromate formation and O 3 dose during bench scale (BS) and bench-top pilot plant (BTPP) experiments. Table 4 Summary of biodegradable byproducts formed during pilot scale ozonation O 3 Dosage H 2 O 2 dose Reaction time (min) AOC (mgl 1 ) AOC/TOC (%) Total aldehydes (mgl 1 ) Total carboxylic acids (mgl 1 ) Carboxylic acids were formed at much greater concentrations than aldehydes. Total carboxylic acid concentrations showed that O 3 /H 2 O 2 formed 12 43% more carboxylic acids than O 3. Of the six carboxylic acids analyzed, ketomalonate and propionate were not detected under any testing conditions. Formation of the four remaining carboxylic acids is shown in Figs. 6 and 7. Formate, oxalate, and acetate formation appear to be related to the O 3 dose. Formate accounted for 55 6% of all carboxylic acid formation during both O 3 and O 3 /H 2 O 2, and may be the result of formaldehyde oxidation by O 3 (Can and Gurol, 23). Results also indicate that O 3 may have oxidized pyruvate, while O 3 /H 2 O 2 (doh exposure) may have formed pyruvate. Due to the detection limit of 2 mg/l, it is difficult to determine whether these relationships are valid or due to analytical variability. These results coincide with aldehyde results indicating that carboxylic acid formation occurs during the greater initial doh exposure within the IOD phase of ozonation TOX Simulated distribution system (SDS) testing was conducted by adding free chlorine to ozonated and non-ozonated samples and holding for 24 h as shown in Table 5. Each O 3 condition reduced the 1-h chlorine demand by at least 1 mg/l and reduced TOX formation by 2%. The chlorine residual information shows the chlorine exposure (CT) was higher in the ozonated samples. As a result, even greater TOX reduction can be expected when lowering the chlorine dosages to target a constant chlorine CT. A TOX reduction was expected due to the lower SUVA values after ozonation as shown in Table 3. SUVA has been shown to be a good indicator of TOX formation during the chlorination of surface waters (Korshin et al., 1997). TOX results from O 3 /H 2 O 2 testing are not shown due to variable chlorine demand observed, likely due to the presence of excess H 2 O 2.

8 Formaldehyde Glyoxal M-Glyoxal Acetaldehyde 146 Concentration (µg/l) Tertiary Effluent 3 O 3 =3.6 mg/l O 3 =3.6 mg/l H 2 O 2 =2. mg/l O 3 =7. mg/l O 3 =7.1 mg/l H 2 O 2 =3.5 mg/l Fig. 5 Aldehyde formation during O 3 and O 3 /H 2 O 2 pilot-scale experiments. 6 Concentration (µg/l) Formate Oxalate Acetate Pyruvate <2 <2 Tertiary Effluent O 3 =2.1 mg/l O 3 =3.6 mg/l O 3 =7. mg/l Fig. 6 Carboxylic acid formation during O 3 pilot-scale experiments. 4. Conclusions O 3 /H 2 O 2 resulted in 13 23% greater doh exposure than O 3 during the IOD phase (3 s). However, the overall doh exposure was within 7 1% after the first-order O 3 decay was completed. These results show that H 2 O 2 accelerated the rate of O 3 decomposition into doh, but did not enhance the overall doh exposure during ozonation. Coliform disinfection was more effective using O 3 as compared to O 3 /H 2 O 2 when similar O 3 dosages are applied. Sufficient disinfection was achieved with O 3 when applying dosages at or above the IOD. O 3 dosages above the IOD are recommended for process control during full-scale application.

9 Concentration (µg/l) Formate Oxalate Acetate Pyruvate <2 <2 Tertiary Effluent O 3 =2.1 mg/l H 2 O 2 =1. mg/l O 3 =3.6 mg/l H 2 O 2 =2. mg/l O 3 =7.1 mg/l H 2 O 2 =3.5 mg/l Fig. 7 Carboxylic acid formation during O 3 /H 2 O 2 pilot scale experiments. Table 5 Summary of free chlorine residual decay and subsequent TOX formation O 3 dose Free chlorine dose Free chlorine residual (mg/l) 2 min 6 min 96 min TOX (mgl 1 ) o Color was reduced from 24 units to 5 8 units by either O 3 or O 3 /H 2 O 2 at the dosages tested. Color reduction can be important for public perception for water reuse or aquifer storage and recovery projects. UV254 and SUVA gradually decreased as the O 3 dose increased. These changes offer wastewater utilities a few potential surrogates to monitor disinfection or trace contaminant oxidation with or without a measurable O 3 residual. Bromate formation occurred when applying O 3 dosages greater than the IOD. The lack of O 3 residual coupled with the presence of excess H 2 O 2 inhibited bromate formation during the O 3 /H 2 O 2 process. While not regulated in wastewater effluents, bromate could be a concern during indirect potable reuse applications, or during the treatment of surface water heavily influenced by wastewater effluent. O 3 /H 2 O 2 produced greater concentrations of AOC (5 52%), aldehydes (31 47%), and carboxylic acids (12 43%) than O 3 at similar dosages, indicating their formation is largely dependent upon doh exposure. Since doh exposure was similar after 1 min of reaction, the majority of organic DBP formation likely occurred during the initial doh exposure within the IOD phase (timeo3 s) of O 3 decomposition. O 3 reduced TOX potential by 2% after chlorination. TOX reduction would benefit water reuse systems by reducing trihalomethane and haloacetic acid formation. As a result of these findings, O 3 may be preferred to O 3 / H 2 O 2 in wastewater applications. In addition, O 3 /H 2 O 2 is prone to leaving a H 2 O 2 residual in water. If subsequent chlorination or chloramination is required, the H 2 O 2 will quench these oxidants resulting in increased chlorine dosages. O 3 provided optimal coliform disinfection and trace contaminant removal (Snyder et al., 26) while minimizing DBP formation. Acknowledgements The authors thank Oscar Quiñones and Janie Holady-Zeigler at the Southern Nevada Water Authority s Water Quality Research and Development Division for their valuable analytical support and assistance conducting bench-scale and pilot-scale experiments. The authors also thank Devon Morgan at the Clark County Water Reclamation District for providing the wastewater and performing the coliform analysis. R E F E R E N C E S Acero, J.L., von Gunten, U., 21. Characterization of oxidation processes: ozonation and the AOP O 3 /H 2 O 2. J. Am. Water Works Assoc. 93 (1), 9 1. APHA, AWWA, WEF, Standard Methods for the Examination of Water and Wastewater. American Public Health Associa-

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