ARTICLE IN PRESS
WATER RESEARCH 41 (2007) 1481– 1490
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/watres
Formation of oxidation byproducts from ozonation of wastewater
Eric C. Werta,Ã, Fernando L. Rosario-Ortiza, Doug D. Druryb, Shane A. Snydera aSouthern Nevada Water Authority (SNWA), 243 Lakeshore Rd., Boulder City, NV 89005, USA bClark County Water Reclamation District, 5857 E. Flamingo Road, Las Vegas, NV 89122, USA article info abstract
Article history: Disinfection byproduct (DBP) formation in tertiary wastewater was examined after
Received 27 July 2006 ozonation (O3) and advanced oxidation with O3 and hydrogen peroxide (O3/H2O2). O3 and
Received in revised form O3/H2O2 were applied at multiple dosages to investigate DBP formation during coliform
11 January 2007 disinfection and trace contaminant oxidation. Results showed O3 provided superior
Accepted 16 January 2007 disinfection of fecal and total coliforms compared to O3/H2O2. Color, UV absorbance, and
SUVA were reduced by O3 and O3/H2O2, offering wastewater utilities a few potential Keywords: surrogates to monitor disinfection or trace contaminant oxidation. At equivalent O3 Ozone dosages, O3/H2O2 produced greater concentrations of assimilable organic carbon (5–52%), Advanced oxidation process (AOP) aldehydes (31–47%), and carboxylic acids (12–43%) compared to O3 alone, indicating that Hydroxyl radicals organic DBP formation is largely dependent upon hydroxyl radical exposure. Bromate Wastewater formation occurred when O3 dosages exceeded the O3 demand of the wastewater. Bench- Water reuse scale tests with free chlorine showed O3 is capable of reducing total organic halide (TOX) Disinfection byproducts (DBP) formation potential by at least 20%. In summary, O3 provided superior disinfection Bromate compared to O3/H2O2 while minimizing DBP concentrations. These are important Aldehydes considerations for water reuse, aquifer storage and recovery, and advanced wastewater Carboxylic acids treatment applications. & 2007 Elsevier Ltd. All rights reserved.
1. Introduction trace contaminant oxidation make ozonation an attractive alternative for advanced wastewater treatment. Ozonation has been shown to be highly effective for water During drinking water ozonation, the formation of organic disinfection. In drinking water treatment, ozonation is used to (e.g., assimilable organic carbon (AOC), aldehydes, carboxylic meet United States Environmental Protection Agency (USEPA) acids, and ketones) and inorganic (e.g., bromate) disinfection regulations for the inactivation of viruses, Cryptosporidium,and byproducts (DBPs) has been well documented (Richardson Giardia (USEPA, 1991, 2003). In wastewater treatment, ozona- et al., 1999; Huang et al., 2005). Organic DBPs from ozonation tion has been used to meet discharge requirements for have been linked to increased bacterial regrowth in drinking coliform and virus inactivation since the 1970s (Rice et al., water distribution systems (LeChevallier et al., 1992). Drinking 1981). In recent years, ozonation has gained attention for its water utilities often employ biological filtration to remove ability to oxidize endocrine disrupting chemicals (EDCs) and these byproducts creating a biologically stable water prior to pharmaceuticals in both drinking water and wastewater distribution (Huck et al., 1991; Krasner et al., 1993). In water (Zwiener and Frimmel, 2000; Huber et al., 2003, 2005; Snyder reuse applications, AOC has contributed to increased bacter- et al., 2006). The combination of microbial disinfection and ial regrowth and more rapid chlorine decay in distribution
ÃCorresponding author. Tel.:+1 702 567 2306; fax:+1 702 567 2085. E-mail address: [email protected] (E.C. Wert). 0043-1354/$ - see front matter & 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2007.01.020 ARTICLE IN PRESS
1482 WATER RESEARCH 41 (2007) 1481– 1490
systems (Ryu et al., 2005). Currently, bromate is the only tion operated by the Clark County Water Reclamation District ozone (O3) DBP regulated in drinking water by the USEPA, in Las Vegas, Nevada, USA. The WWTP treats wastewater which established a maximum contaminant level (MCL) of by first adding 14–16 mg/L of ferric chloride followed by 10 mg/L (USEPA, 1998). primary clarification and sedimentation, allowing heavier Dissolved organic carbon (DOC) concentrations are typically particles to be removed. Then, aerobic and anaerobic greater in wastewater than in surface water, resulting in faster biological treatment occurs followed by secondary clarifica-
O3 decomposition rates and increased hydroxyl radical (dOH) tion. During the tertiary treatment stage, 4–13 mg/L of alum is exposures (Buffle et al., 2006a). As a result, higher O3 dosages added followed by flocculation and filtration. Ultraviolet are required to meet wastewater treatment goals, potentially light is used to achieve disinfection goals. Samples for this leading to increased DBP formation. The organic composition study were collected after filtration, but prior to UV disinfec- of wastewater is also different from the natural organic matter tion. Seasonal variations were examined by collecting sam- (NOM) found in surface water supplies and is commonly ples in June 2005 and January 2006. Table 1 shows the tertiary known as effluent organic matter (EfOM). EfOM is composed of wastewater quality was very similar during both sampling recalcitrant NOM from drinking water, synthetic organic events. chemicals added during anthropogenic use (including disin- fection by-products), and soluble microbial products (Shon et 2.3. Ozonation at bench scale al., 2006). Sparse data exist regarding the influence of EfOM on DBP formation. Sirivedhin and Gray (2005) studied the impact Bench-scale tests were performed using a batch reactor to of EfOM on halogenated DBP formation during chlorination of obtain information about O3 decomposition, dOH exposure, South Platte River water. They found that locations with and bromate formation. A sample of NanopureTM water was greater EfOM influence were less reactive with chlorine on a placed inside a water-jacketed flask and cooled to 2 1C. Once per carbon basis than locations with minimal EfOM influence. cooled, 11% gaseous O3 was diffused into the water using an The primary objective of this study was to quantify the oxygen-fed generator (model CFS-1A, Ozonia North America formation of known organic and inorganic O3 DBPs at dosages Inc., Elmwood Park, NJ USA). O3 stock solution concentrations required for coliform disinfection and trace contaminant and dissolved O3 residuals were measured according to destruction in tertiary wastewater. Conventional O3 and Standard Methods 4500-O3 (Bader and Hoigne, 1982; APHA advanced oxidation processes (AOPs) with O3 and hydrogen et al., 1998). O3 dosages were administered by injecting an peroxide (O3/H2O2) were evaluated at bench-scale and pilot- aliquot of the stock solution into a 1-L amber glass container scale to examine DBP formation. Subsequent chlorination with a repeating pipette dispenser containing the tertiary examined the impact of O3 and O3/H2O2 on total organic treated wastewater at room temperature (20 1C). During O3/ halide (TOX) formation. These results will determine which H2O2 experiments, H2O2 was added 30 s prior to the addition ozonation technique is optimal to meet multiple wastewater of O3 stock solution. Duplicate experiments were performed treatment objectives while minimizing DBP formation. with the addition of 1.75 mM (274 mg/L) para-chlorobenzoic
acid (pCBA), an O3-resistant probe compound which reacts selectively with dOH (Elovitz and von Gunten, 1999). Dis-
2. Materials and methods solved O3 residual and pCBA samples were collected at 10 s intervals during the first minute of reaction and each minute
2.1. Experimental plan thereafter to investigate O3 decomposition and dOH exposure.
Dissolved O3 residual and pCBA samples were collected until
Bench-scale and pilot-scale experiments were conducted the O3 residual had decayed to less than 0.05 mg/L, or until a with tertiary wastewater to quantify the DBP formation contact time of 15 min was achieved. Bromate samples were during O3 and O3/H2O2 and subsequent TOX formation during collected after each test was completed. chlorination. The testing was conducted in three phases: (1) bench-scale O3 and O3/H2O2, (2) pilot-scale O3 and O3/H2O2, and (3) bench-scale chlorination. Bench-scale O3 and O3/H2O2 tests provided information regarding O3 demand, decay rate, Table 1 – Water quality summary of tertiary wastewater and dOH exposure. Due to the 1 L sample volume limitation during bench-scale testing, pilot-scale experiments were used Water quality Units June January to gather adequate sample volumes to measure coliform parameter 2005 2006 disinfection, DBP formation (AOC, carboxylic acids, alde- hydes, and bromate), and trace contaminant removal. Results Ammonia mg-N L 1 o 0.08 o 0.08 of trace contaminant testing (i.e., EDCs and pharmaceuticals) Bromate mg L 1 o 0.001 o 0.001 1 are presented elsewhere (Snyder et al., 2006). Bench-scale Bromide mg L 0.28 0.21 1 chlorination tests using free chlorine determined the impact Nitrate mg-N L 13.6 13.8 pH Units 6.92 7.02 of ozonation on TOX formation. Temperature 1C 27.2 20.4 Total alkalinity mg L 1 130 110 2.2. Wastewater treatment facility Total organic carbon mg L 1 7.10 7.20 UV254 cm 1 0.128 0.119 1 1 Tertiary wastewater was collected from a 416,000 m3/d waste- SUVA L mg m 1.80 1.65 water treatment plant (WWTP) with nitrification/denitrifica- ARTICLE IN PRESS
WATER RESEARCH 41 (2007) 1481– 1490 1483
2.4. Ozonation at pilot scale H1-S, IN USA Inc., Needham, MA, USA) were used to calculate
and control the O3 dosage. Due to the column height of 0.83 m A 1 L/min bench-top pilot plant (BTPP) consisting of all inert and diameter of 0.055 m, the transfer efficiency varied materials including glass, stainless steel, and fluorocarbon between 40% and 70% depending on the desired dose and polymers was used to conduct the testing (Fig. 1). A peristaltic corresponding gas flow rate. The off-gas was collected from pump was used to transport the wastewater from a 208-L the top of each cell into a central manifold and destroyed by stainless steel drum into the contactor. During O3/H2O2 manganese dioxide catalyst. experiments, H2O2 (3% stock, Fisher Scientific, Pittsburgh, PA O3 dosages were selected based upon bench-scale demand USA) was injected into the wastewater flow stream followed tests to evaluate a range of exposures for coliform disinfec- by a static mixer prior to entering the contactor. The O3 tion and trace contaminant oxidation. O3 exposure was contactor consisted of 12 glass chambers each providing calculated by integrating the dissolved residual concentration
2 min of contact time for a total of 24 min. O3 feed gas was over time (CT) according to the extended integrated CT10 produced from oxygen gas using a laboratory-scale generator method (Rakness et al., 2005). During each test, the waste- (model LAB2B, Ozonia North America Inc., Elmwood Park, NJ water was maintained at room temperature (20 1C). Dissolved
USA). O3 was added in the first contactor chamber with O3 measurements were collected after 2, 6, 10, 14, and 18 min counter-current flow through a glass-fritted diffuser with to examine demand and decay rates. Water quality samples bubble size of 0.1 mm. A mass-flow controller (model were collected for AOC, carboxylic acids, aldehydes, bromate, AFC2600D, Aalborg Instruments and Controls, Inc., Orange- total coliforms and fecal coliforms. In order to measure dOH burg, NY, USA) and a feed gas concentration analyzer (model exposure, duplicate experiments were performed with 75.6 L
Fig. 1 – Schematic of bench-top pilot plant (BTPP). ARTICLE IN PRESS
1484 WATER RESEARCH 41 (2007) 1481– 1490
of tertiary wastewater spiked with 1 mM (156 mg/L) of pCBA. exceeded the IOD, H2O2 promoted O3 decomposition thereby Samples for pCBA analysis were collected after 2, 6, 10, 14, increasing initial dOH exposure. and 18 min and quenched with a small aliquot of sodium thiosulfate (Na2S2O3). 3.2. Hydroxyl radical exposure
2.5. Analytical methods Oxidation attributed to dOH exposure was measured by the depletion of pCBA as shown in Fig. 3. During the IOD phase Water samples were collected, preserved, and refrigerated at (timeo30 s), results showed pCBA depletion was 13–23% 4 1C until analyzed. Standard methods (SM) were used for the greater during O3/H2O2 than O3 when equivalent O3 dosages determination of total organic carbon (TOC) (SM 5310B), UV were applied. For example, 71% pCBA depletion occurred absorbance at 254 nm (SM 5910), total coliform (SM 9221B), fecal during O3/H2O2 versus 48% during O3 when both processes coliform (SM 9221E), and color (SM 2120C) (APHA et al., 1998). employed an O3 dose of approximately 5 mg/L. After 10 min of AOC samples were preserved through pasteurization and reaction, pCBA depletion was within 7–10% during O3 and O3/ measured through a bioassay using Pseudomonas fluorescens H2O2 when similar O3 dosages were applied. During first- d strain P-17 and Spirillum strain NOX according to SM 9217. Seven order O3 decay, OH formation continued at a slower rate, aldehydes (acetaldehyde, butanal, formaldehyde, glyoxal, m- resulting in comparable dOH exposures. These results agree glyoxal, pentanal and propanal) were analyzed by gas chroma- with Acero and von Gunten (2001), who demonstrated that d tography with electron capture detection (GC/ECD) according to the overall OH exposure was similar between O3 and O3/ SM 6252. Six carboxylic acids (acetate, formate, ketomalonate, H2O2 in surface waters (DOC ¼ 3.2 mg/L), and that H2O2 d oxalate, propionate and pyruvate) were measured by ion addition accelerates the rate of O3 decomposition into OH. chromatograph by previously established methods (Kuo, 1998; During pilot-scale experiments, pCBA depletion was similar Randtke, 2001). TOX concentrations were analyzed by EPA to bench-scale results as shown in Table 2. When comparing Method 9020B. Bromide and bromate concentrations were O3 and O3/H2O2 at similar dosages, pCBA was depleted 17–18% analyzed by ion-chromatography with inductively coupled more with O3/H2O2 than with O3 during the first 2 min. After plasma-mass spectroscopy detection (IC-ICP/MS)(Quinones et 6 min of contact time, pCBA depletion was again within 10%, al., 2006). Quantification of pCBA was achieved using high- coinciding with bench-scale results. These results confirmed pressure liquid chromatography (HPLC) equipped with an RP- bench-scale findings indicating that the initial dOH exposure
C18 column with a 45/55 mixture of 10 mM H3PO4 pH 2/MeOH as (timeo30 s) was greater during O3/H2O2 than O3; although, mobile phase and UV detection at 234 nm. The detection limit overall dOH exposure (time46 min) was similar for both was estimated at 0.02 mM(3.1mg/L). processes when applying similar O3 dosages.
3.3. Color and UV absorbance 3. Results and discussion
O3 and O3/H2O2 reduced color and UV absorbance at 254 nm
3.1. O3 decomposition (UV254) as shown in Table 3. Color was reduced from 24 to 5–8
units by O3 and to 7 units by O3/H2O2. Color reduction can be
Initial O3 demand and decay from bench-scale tests are important for public perception during water reuse or aquifer shown in Fig. 2. Results demonstrate minimal seasonal storage and recovery projects. UV254 and SUVA gradually changes in O3 decomposition, which was expected due to decreased as the O3 dose increased during O3 and O3/H2O2. similar TOC concentrations. The instantaneous O3 demand The decrease in SUVA demonstrates that the aromatic carbon
(IOD) was measured by the difference between the trans- content decreases as the O3 dose increases (Weishaar et al., ferred dose and the dissolved O3 residual after 30 s. Results 2003). These changes in color, UV254, and SUVA offer waste- showed the IOD varied between 2 and 4 mg/L depending on water utilities a few potential surrogates to monitor disinfec- the transferred dose. The IOD phase of ozonation has shown tion or trace contaminant oxidation with or without a similarities to O3-based AOPs, resulting in dOH exposure measurable O3 residual. (Buffle et al., 2006b).
After the IOD phase was complete, dissolved O3 residual 3.4. Coliform disinfection decayed according to first-order rate kinetics, as expected
(inset of Fig. 2). When targeting disinfection in drinking water Coliform disinfection was evaluated using both O3 and O3/ applications, a measurable dissolved O3 residual is required to H2O2 to determine the conditions required to achieve less meet CT guidelines established by the USEPA and assist with than 200 fecal coliforms per 100 mL to comply with discharge
O3 process control. Therefore, pilot-scale wastewater experi- regulations. Results shown in Table 3 indicate that an O3 ments targeted CT values up to 9.90 mg-min/L to evaluate dosage of 2.1 mg/L can achieve effective coliform disinfection wide range of O3 exposures (Table 2). in the absence of a stable ozone residual. These results
During bench-scale O3/H2O2 experiments, the O3 residual coincide with pilot-scale results reported by Janex et al. (2000), rapidly decayed within 1.5 min of reaction time. During pilot- who showed that 2-logs of fecal coliform inactivation could be scale O3/H2O2 experiments, detection of residual O3 after the achieved in wastewater before the IOD was met. Significant initial 2-min mass transfer period indicated that the O3/H2O2 oxidation of trace contaminants was also observed during the reaction was incomplete and a brief period of O3 oxidation IOD phase as shown in Snyder et al. (2006).However,O3 occurred. These results also show that when O3 dosages dosages exceeding the IOD are required to maintain proper ARTICLE IN PRESS
WATER RESEARCH 41 (2007) 1481– 1490 1485
8 0 O3=11.1 mg/L (Jan) O3=6.9 mg/L (June) 7 O3=4.3 mg/L (June) -1
O3=4.7 mg/L (Jan) ]o) 3 O3=5.1 mg/L, H2O2=2.5 mg/L (Jan) -2
6 O =11.2 mg/L, H O =5 mg/L (Jan) ]/[O
3 2 2 3 -3
5 ln ([O -4
-5 4 0246810121416 Time (min) 3
2 Dissolved Ozone Residual (mg/L)
1
0 0 2 4 6 8 10121416 Time (min)
Fig. 2 – Dissolved O3 residual decay curves during bench-scale experiments. The inset presents the data to show first-order rate kinetics.
Table 2 – O3 residual decay and pCBA reduction during pilot scale experiments