Environ. Sci. Technol. 2000, 34, 2305-2310
Effects of Ozone/Hydrogen Peroxide Pretreatment on Aerobic Biodegradability of Nonionic Surfactants and Polypropylene Glycol M E H M E T K I T I S , † C R A I G D . A D A M S , * ,‡ JOHN KUZHIKANNIL,‡ AND GLEN T. DAIGGER§ Department of Environmental Engineering and Science, Clemson University, 342 Computer Court, Anderson, South Carolina 29625, Department of Civil Engineering, 202 Butler-Carlton Hall, University of MissourisRolla, Rolla, Missouri 65409-0030, and CH2MHill, P.O. Box 241325, Denver, Colorado
Studies were conducted which used the ozone/hydrogen peroxide (O3/H2O2) advanced oxidation process to pretreat three classes of compounds prior to aerobic biological treatment. The study compounds included ethylene oxide/ propylene oxide (EO/PO) block copolymers, polypropylene glycols (PPGs), linear secondary alcoholethoxylates (LSAEs), and alkylphenolethoxylates (APEs). After preoxidation with ozone and hydrogen peroxide (added at their stoichiometric ratio), 300 mg/L as COD samples were bioassayed in aerobic batch bioassays with a mixed liquor suspended solids concentration of 1500 mg/L. It was found that unoxidized polypropylated compounds (EO/PO block copolymers and PPGs) and LSAEs tended to be biorecalcitrant, while alkylphenolethoxylates (APEs) were partially biodegradable. Increasing oxidant dosages (i.e., ozone plus stoichiometric hydrogen peroxide) consistently increased both the rate and extent of biodegradation of these compounds with the exception of NP(EO)5, which initially decreased in biodegradability upon oxidation. Oxidant dosages required to enhance biodegradability varied significantly between and within classes of surfactant. For example, the average oxidant dosages required to reach an 85% DOC removal in the batch bioassays were 0.3 mg O3/mg compound (plus H2O2) for LSAEs, 1.0 mg/mg for EO/ PO and PPGs, and 5.0 mg/mg for APEs, respectively.
Introduction Nonionic surfactants are important due to their ability to retain surfactant properties in hard waters over a wide pH range (1). The primary hydrophilic functionality in nonionic surfactants is a poly(ethylene oxide) chain of varying lengths. The lipophilic functionalities may include polypropylene oxide, alkylphenols, linear alcohols, and secondary alcohols. Ethylene oxide/propylene oxide (EO/PO) block copolymers, linear secondary alcoholethoxylates (LSAEs), and alkylphe* Corresponding author phone: (573)341-4041; fax: (573)341-4729; e-mail:
[email protected]. † Clemson University. ‡ University of MissourisRolla. § CH2MHill. 10.1021/es981228d CCC: $19.00 Published on Web 05/04/2000
2000 American Chemical Society
nolethoxylates (APEs) represent major classes of nonionic surfactants used worldwide (Figure 1). Unfortunately, these classes of surfactants are often biorecalcitrant in many conventional aerobic biological treatment processes. Polypropylene glycol (PPG) of varying lengths is an important nonsurfactant component of many surfactant formulations. PPG alone or in EO/PO block copolymers imparts the biorefractory nature of these compounds due to steric effects on the transport of the compounds through cell membranes (2-4). LSAEs are nonionic surfactants used extensively in a wide range of formulations and applications. In a generic alcohol ethoxylate, the major factors affecting the rate and extent of biodegradability are as follows: (1) the degree of branching in the alkyl chain (the hydrophobe) and (2) the length of the ethylene oxide (EO) chain (the hydrophile) (2). The biodegradability of LSAEs has been shown to decrease with increasing EO chain length (5). Biodegradation studies of partially biodegradable APEs indicate a complex metabolic behavior. Initial biodegradation proceeds via EO-chain cleavage of terminal EO units, thereby decreasing the aqueous solubility and hence biodegradability. This phenomenon results in the characteristic of partial biodegradability often observed in APEs (6-8). Because it may not be feasible to use biologically labile surfactants in all applications, there is a need to identify and develop economically viable treatment technologies to reduce organic content and toxicity in wastewaters containing surfactants. In this research, we studied the effect of O3/ H2O2 pretreatment on the aerobic biodegradability of selected EO/PO block copolymers, PPGs, LSAEs, and APEs. In the O3/H2O2 advanced oxidation process (AOP), the hydroperoxide ion (i.e., the conjugate base of H2O2 or HO2-) reacts with ozone to form radical species which further decompose to hydroxyl radicals in an autodecomposition cycle (9). AOPs have been shown to effectively modify the molecular structures of a variety of surfactants and related compounds, resulting in dissolved organic carbon (DOC) and chemical oxygen demand (COD) reductions, loss of surface active properties (e.g., foaming, hydrophobicity), and affected biodegradability (3, 10-13). The purpose of the research described in this paper was to examine the viability of integrating O3/H2O2 pretreatment of EO/PO block copolymers, PPGs, LSAEs, and APEs with subsequent biological treatment in a conventional activated sludge (AS) process.
Materials and Methods Test Chemicals. The four EO/PO block copolymers examined were Pluronic L31, L35, F38, and P85 (BASF Corp.). The LSAEs examined included Tergitol 15-S-7, 15-S-12, 15-S-20, and 15-S-40 (Union Carbide) where the trailing number indicates the nominal number of ethylene oxide (EO) units present. Three APEs (all nonylphenolethoxylates) were examined with 5, 12, and 40 EO units (NP(EO)5, NP(EO)12, NP(EO)40, respectively) (Rhone-Poulenc). Additionally, two polypropylene glycols were examined, PPG 425 and PPG 725 (where the number corresponds to the nominal molecular weight) (Aldrich Chemical Co.). The generic chemical structure and structural information of each class of compound studied are provided in Figure 1 and Table 1, respectively. All other chemicals used for experiments or analysis were at least reagent grade and were purchased from VWR Scientific Products or Aldrich Chemical Co. Analytical Methods. The laboratory methods used for this research were from either Standard Methods (14), methods developed in previous projects (3), or methods VOL. 34, NO. 11, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Representative structures of study compounds. specified by the manufacturer of an instrument. Chemical oxygen demand (COD) measurements were made using lowand high-range COD ampules (Hach Chemical) with a spectrophotometer (Spectronic 20D, Milton Roy) according to published Hach methods. Dissolved Organic Carbon (DOC) was measured using a Shimadzu TOC-5000 TOC Analyzer with ASI-5000. pH measurements were made using a Corning pH meter (Model 220) and probe (Model 476540). Hydrogen peroxide was measured using a Hach Model HYP-1 titrametric test kit. O3/H2O2 Oxidation Procedure. O3/H2O2-based advanced oxidation was conducted in a 1-L semibatch reactor with both inlet and outlet ports for ozone, hydrogen peroxide feed, temperature monitoring, reactor feed, and sample removal (Figure 2). Ozone was supplied with a Model GTC1B (Griffin Technics Corp.) or a Model OZAT 0 (Ozonia Corp.) ozone generator fed pure oxygen. Gas-phase ozone concentrations in the ozone feed and offgas streams were monitored using a Model HC-12 ozone monitor (PCI Ozone and Control Systems, Inc.). Using mass flow controller data (Tylan FC-280 MFC), the absorbed ozone concentrations were calculated using a Camile 2000 Data Acquisition and Control System (Dow Chemical). Hydrogen peroxide was added via a peristaltic pump at a rate of 0.35 mg of H2O2 per mg of absorbed ozone (i.e, stoichiometric ratio). The solutions that were oxidized contained an average initial surfactant (or PPG) concentration of 1000 ((28 mg/L, R ) 0.05) mg/L as COD. An initial volume of 700 mL was used in the 1-L semibatch reactor to allow enough headspace for potential foaming. The solutions were buffered with 8 mM sodium phosphate to assist in maintaining the pH in the optimum region for the effective hydroxyl radical production. pH was monitored ex situ but not controlled during the oxidation. In these experiments, the pH decreased from an initial pH of 8.8 ((0.5) to 7.3 ((0.2) during oxidation. The reactor temperature was 22 °C ((1) for all the oxidation experiments. Samples were removed at predetermined absorbed ozone dosages over a period ranging from 20 to 40 min for bioassays, DOC, COD, and pH analysis. Immediately after removing a sample, residual H2O2 was quenched by adding a slight excess of sodium sulfite. Foaming in the reactor during ozonation was only a problem for the initial 1 to 3 min and was controlled using a foam trap. Bioassay Procedure. An aerobic sequencing batch reactor (SBR) with a 10-day solids retention time (SRT) and 1.5-day hydraulic retention time (HRT) was continuously operated 2306
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throughout the experiments to generate biomass for batch aerobic bioassays of oxidized and unoxidized samples. The initial seed was obtained from two AS processes at industrial wastewater facilities. The SBR was fed nutrients (e.g., NH4+, Ca2+, Fe3+, Co2+, Zn2+, Mg2+, Mn2+, Cu2+, and Mo3+) as well as a mixture of synthetic organic chemicals including the following: ethylene glycol, poly(ethylene glycol), Alkumuls 0-14, catechol, resorcinol, hydroquinone, trihydroxybenzene, and butyl ether (15). The organic compounds in the feed were selected based on their chemical functionalities potentially being similar to those of anticipated intermediates or byproducts of the O3/H2O2 advanced oxidation. Therefore, the biomass in the SBR was acclimated to these certain chemical structures thereby reducing the associated lag time in the bioassays. The SBR also provided a uniform biomass for the bioassays. The SBR was fed daily with a feed solution, macro- and micronutrients, and deionized water using a Cartridge Pump (Cole-Parmer Inst. Co., Model 7519-00). Potassium phosphate monobasic (KH2PO4) and potassium phosphate dibasic (K2HPO4) were provided in the nutrient solution for buffering purposes as well as a nutrient. Mixing and aeration were provided using activated-carbon filtered laboratory air. The SBR system was automated using a controller (ChronTrol, Model XT). Just before each daily cycle ended (before aeration/mixing was turned off), 1.2 L of Mixed Liquor Suspended Solids (MLSS) was wasted from the reactor to maintain the 10-day SRT and to provide the biomass to be used in the bioassays. After the air was turned off, biomass was allowed to settle for 50 min. Then the treated supernatant was removed using a submerged pump (Little Giant Pump Comp., Model P-AAA-WG) leaving the biomass in the reactor, thus completing one cycle. The next cycle was then initiated by refeeding the reactor. After an initial stabilization period of approximately 20 days, the MLSS averaged 2500 (( 200) mg/L over the following 9 months (15). After the initial startup period, 90-95% COD removals were achieved with 4 h in the SBR during each daily cycle. To assess the extent of biodegradability of the chemically oxidized and unoxidized samples, batch aerobic bioassays were conducted on 200 mL volumes (in 300 mL flasks). The solutions had an initial surfactant concentration of 300 ((15) mg/L as COD. A MLSS concentration of 1500 mg/L was used in the bioassays with the biomass provided from the waste sludge of the SBR. Flasks were shaken on shaker tables (Labline Orbit Shaker) at 180 rpm to provide mixing and aeration. Required micro- and macronutrients and buffering (phosphate) were also provided in the bioassays. All bioassay experiments were conducted with an initial pH of approximately 7.2 ((0.1) and at a temperature of 22 °C ((1). The biodegradable feed solution for the SBR was used as positive control for every bioassay experiment to demonstrate that the bioculture was viable. A blank flask containing only biomass and nutrient solution was also bioassayed to measure the relative DOC contribution of the biomass solution to the original DOC concentrations of contaminant solutions. Samples were periodically removed from the flasks, centrifuged to remove the biomass, and analyzed for DOC. Each of the bioassays was analyzed by plotting the DOC concentrations versus bioassay duration from which the maximum DOC removal could be estimated. The potential for sorptive and volatilization loss of organics in the bioassays was examined using mercuric nitrate (to inhibit biological activity) and found to be negligible (less than 5%). Low volatilization losses are due to low vapor pressures of the study compounds.
Results and Discussion Biodegradability of the Unoxidized Compounds. As a first step in this study, bioassays were conducted to determine
TABLE 1. Molecular Weight and Structural Information for Each Study Compounda
class
av mol wt
EO/PO block copolymer EO/PO block copolymer EO/PO block copolymer EO/PO block copolymer PPG 425 PPG 725 LSAE (15-S-7) LSAE (15-S-12) LSAE (15-S-20) LSAE (15-S-40) APE/NP(EO)5 APE/NP(EO)12 APE/NP(EO)40
1100 1900 4700 4600 424 714 509 729 1081 1961 422 730 1962
no. of EO units
no. of PO units
3 24 96 58
16 16 16 39 7 12
7 12 20 40 5 12 40
CAS no.
trade name
maximum DOC removal (%)
586770 13390 583095 588850 20,230-4 20,231-2 84133-50-6 84133-50-6 84133-50-6 84133-50-6 9016-45-9 9016-45-9 9016-45-9
Pluronic L31 Pluronic L35 Pluronic F38 Pluronic P85 polypropylene glycol polypropylene glycol Tergitol (15-S-7) Tergitol (15-S-12) Tergitol (15-S-20) Tergitol (15-S-20) IGEPAL CO-520 IGEPAL CO-720 IGEPAL CO-890
61 12 8.6 14 17 2.4 47 59 50 21 52 61 76
biodegradability partial biorecalcitrant biorecalcitrant biorecalcitrant biorecalcitrant biorecalcitrant partial partial partial biorecalcitrant partial partial partial
a Identified by class, CAS number, and trade name. Maximum DOC removal (percent) and biodegradability assessment achieved in batch activated sludge bioassays for each compound. “Biorecalcitrant”: ∆DOC < 40%; “partial”: 40 < ∆DOC < 90%.
FIGURE 2. Reactor system used for O3/H2O2 advanced oxidation of surfactant solutions (MFC ) mass flow controller). the aerobic biodegradability of the unoxidized compounds. DOC removals were used to assess the extent of biodegradation of the compounds. Table 1 shows the bioassay results for the unoxidized compounds. Compounds with DOC removals equal to or less than 40%, between 40 and 90%, and equal to or greater than 90% were considered biorecalcitrant, partially biodegradable, and readily biodegradable, respectively. Examination of the results from the bioassays of unoxidized PPGs showed that both PPG 425 and PPG 725 were highly biorecalcitrant with DOC removals of only 17 and 2.4%, respectively. This result can be due either to the presence of a methyl side chain, which can interfere with the transport of the molecule across the cell envelope, or to the folding of the molecule that can make the ends of the chain inaccessible to the biodegradation (4). In any event, it is apparent that high molecular weight may decrease the biodegradability of the unoxidized PPGs. Each member of the Pluronic 30-series (EO/POs) had the same number (16 units) of biorecalcitrant PO units. Examination of the biodegradability results for the EO/PO polymers with 3, 24, and 96 EO units for L31, L35, and F38, respectively, showed that increasing the EO-chain length significantly decreases the biodegradability of the unoxidized surfactants from 61 to 12 to 8.6% DOC removal, respectively (Figure 4). This trend suggests that while PPG may impart biorecalcitrance to specific compounds, the molecular weight of the
FIGURE 3. Normalized DOC removal in batch activated sludge bioassay of EO/PO polymer P85 unoxidized and after O3/H2O2 oxidation (H2O2 dose ) 0.35 mg H2O2/mg O3). (Error bars indicate the 95% confidence intervals.) EO/POs may also play an important role in biorecalcitrance even if the additional structure is simply an EO polymer (Table 1). VOL. 34, NO. 11, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. Maximum DOC removal (percent) achieved in batch activated sludge bioassays for ethylene oxide/propylene oxide (EO/ PO) block copolymers as a function of ozone dose (H2O2 dose ) 0.35 mg H2O2/mg O3). The unoxidized LSAEs with 7, 12, and 20 EO units exhibited partial biodegradability with DOC removals between approximately 47 and 59% (Table 1). The higher molecular weight LSAE with 40 EO units (M Wt 1961) was considerably less biodegradable, with a DOC removal of only 21%, due possibly to its larger size inhibiting transport across the bacterial cell membrane. Nonionic surfactants with longer EO chains (i.e., 40) may exhibit lower biodegradabilities than those with moderate EO chain length due to folding of the molecule inhibiting transcellular transport (2). On the other hand, nonionic surfactants with relatively short EO chains (i.e., 7 units) may exhibit biorecalcitrance due to reduced aqueous solubility (2). Unoxidized NP(EO)5, (NP(EO)12, and (NP(EO)40 demonstrated 52, 61, and 76% DOC removals during biodegradation, respectively. Partial biodegradation of APEs can be due to the formation of APEs with shorter EO units, which are resistant to further microbial degradation (6-8). The observed effect of long EO chains (12 and 40 units) on the nonylphenols was to increase biodegradability. Effects of Advanced Oxidation on Biodegradability. EO/ PO Block Copolymers and PPGs. The results for the advanced oxidation of the EO/PO block copolymers showed that advanced oxidation was effective at enhancing the biodegradability (represented by DOC removal) for each study compound. For example, the DOC removal curve during the shaker table bioassays is presented in Figure 3 for EO/PO P85. These data show that increasing oxidant dosages significantly increased biodegradability (i.e, the DOC removal) in the shaker table bioassays. In fact, greater oxidant dosages resulted in greater DOC removals for all of the EO/ POs (Figure 4). Oxidant dosages of 1 mg O3/mg compound (plus H2O2) resulted in DOC removals of greater than 85% during biodegradation for the EO/PO block copolymers. Similarly, the advanced oxidation was highly effective at enhancing the biodegradability for both PPGs examined. Specifically, Figure 5 shows the normalized DOC results from the shaker table bioassays for the unoxidized and oxidized PPG 425. A slight increase in DOC for the unoxidized PPG 425 is hypothesized to be due to cell lysis and release of intracellular material into solution. Overall, greater oxidant dosages resulted in enhanced biodegradability for both compounds with DOC removals of approximately 92% achieved during biodegradation for both PPGs after application of 1 mg O3/mg compound (plus H2O2) (Figure 6). Hydroxyl radicals react with both EO and PO polymers through hydrogen abstraction mechanisms at any carbon along a chain or terminus (16). Oxidation of the carbon chain leads to polymer cleavage and lower molecular weight byproducts that are much easier for bacteria to metabolize. Hydroxyl radical rate constants for both ethylene oxide (EO) 2308
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FIGURE 5. Normalized DOC removal in batch activated sludge bioassay of PPG 425 unoxidized and after O3/H2O2 oxidation (H2O2 dose ) 0.35 mg H2O2/mg O3). (Error bars indicate the 95% confidence intervals.)
FIGURE 6. Maximum DOC removal (percent) achieved in batch activated sludge bioassays for polypropylene glycols (PPGs) as a function of ozone dose (H2O2 dose ) 0.35 mg H2O2/mg O3). and propylene oxide (PO) monomers are both approximately 1.7(109) L/mol‚s (17). Therefore, it is likely that hydroxyl radicals would have relatively equal propensity toward oxidizing either the EO or PO portions of the EO/PO block structure. EO/PO L31, L35, and F38 have the same number of PO units (sixteen) but different numbers of EO units (i.e., 3, 24, and 96, respectively). Therefore, a randomly cleaved L31 will result in a much higher PO to EO ratio (e.g., approximately 5) than for F38 (e.g., approximately 0.2). Because of decreased solubility and greater branching, POs are less biodegradable than EO monomers or polymers (2). Therefore, a greater DOC removal for the oxidized EO/PO F38 may result than for the L31 at 0.75 to 1.5 mg O3/mg compound (plus H2O2) (Figure 4). LSAEs. For the LSAEs studied, advanced oxidation with very low oxidant dosages was effective at enhancing biodegradability. For example, ozone dosages of 0.25 mg O3/mg compound resulted in percent DOC removal increases of 85, 53, 78, and 370% for LSAEs with 7, 12, 20, and 40 EO units, respectively (Figure 7). While the highest molecular weight LSAE (15-S-40) had the lowest biodegradability for the unoxidized compounds 15-S-40 had the highest biodegradability of any of the LSAEs after the advanced oxidation. Brambilla et al. (10) showed that hydroxyl radical attack of the EO chain was more rapid than for the alkyl chain of the alcohol moiety leading to EO chain cleavage. All four of the
FIGURE 7. Maximum DOC removal (percent) achieved in batch activated sludge bioassays for linear secondary alcohol ethoxylates (LSAEs) as a function of ozone dose (H2O2 dose ) 0.35 mg H2O2/mg O3).
FIGURE 9. Maximum DOC removal (percent) achieved in batch activated sludge bioassays for alkylphenolethoxylates (APEs) as a function of ozone dose (H2O2 dose ) 0.35 mg H2O2/mg O3). studies (3). It is thought that the cause of the decreased biodegradability is that primary ozone- and hydroxyl-radicalbased oxidation pathways for the APEs is EO chain shortening, thereby decreasing an APE’s solubility. Higher oxidant dosages than required to cleave all EO units would result in a variety of other reactions dominating, such as ring and alkyl chain hydroxylation and cleavage, resulting in more labile compounds. In summary, this research examined the viability of enhancing the biodegradability of EO/PO block copolymers, LSAEs, APEs, and PPGs, using an integrated advanced oxidation/biological treatment process. Unoxidized polypropylated surfactants and PPGs were found to be biorecalcitrant, while unoxidized APEs and LSAEs were generally partially biodegradable. The effectiveness of advanced oxidation for biodegradability enhancement of the study compounds appears to be related hydroxyl radical cleavage. Those compounds that cleave to predominantly easily degradable byproducts (e.g., EO units) are most efficiently softened by advanced oxidation pretreatment.
Acknowledgments FIGURE 8. Normalized DOC removal in batch activated sludge bioassay of NP(EO)12 unoxidized and after O3/H2O2 oxidation (H2O2 dose ) 0.35 mg H2O2/mg O3). (Error bars indicate the 95% confidence intervals.) LSAEs examined had the same linear alkyl group. However, the EO-carbon to alkyl carbon ranged from approximately 1.5 for 15-S-7 to 9 for 15-S-40. Therefore, for the higher molecular weight 15-S-40, the vast majority of its carbon is in the form of EO groups that may be readily biodegraded once cleaved via oxidation as seen in Figure 7. On the other hand, 15-S-7 has approximately 35% of its carbon as a more recalcitrant secondary alcohol, leading to lower overall biodegradability even after oxidation (Figure 7). Alkylphenolethoxylates. The bioassay results for unoxidized and oxidized APEs showed advanced oxidation increased the biodegradability of APEs but not nearly as efficiently as with the EO/PO and PPG compounds. The bioassay results for NP(EO)12 is presented in Figure 8 which show that advanced oxidation increased the biodegradation extent, albeit with relatively high oxidant dosages. The biodegradability enhancement achieved for each of the APEs examined are presented as a function of ozone dosage in Figure 9. For example, an oxidant dosage of 7 mg O3/mg compound (plus H2O2) resulted in DOC removals of 85, 81, 95% of APEs with 5, 12, and 40 EO units, respectively. The DOC removals of NP(EO)5 decreased from 52 to 33% after application of 1 mg O3/mg compound (plus stoichiometric H2O2), a result consistent with findings of previous
This research was sponsored by the Hoechst Celanese Corporation and the National Science Foundation (Project BCS-9257625). At the time of this study, Mr. Kitis and Mr. Kuzhikannil were master’s students at Clemson University and the University of MissourisRolla, respectively. Dr. Glen Daigger is Sr. Vice-President of Wastewater Process Engineering at CH2MHill, Corp.
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(12) Adams, C. D.; Cozzens, R.; Kim, B. Water Res. 1997, 31, 26552663. (13) Cline, J. E.; Sullivan, P. F.; Fowler, R.; Lovejoy, M. A.; Collier, J.; Adams, C. D. 89th Annual Meeting of the Air and Waste Management Association, Nashville, TN, March 1996. (14) Standard Methods for the Examination of Water and Wastewater, 18th ed.; American Public Health Assoc.: Washington, DC, 1992. (15) Kitis, M. Master’s Thesis, Clemson University, Clemson, SC, 1996.
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Received for review November 30, 1998. Revised manuscript received February 11, 2000. Accepted March 14, 2000. ES981228D
