Envlron. Scl. Technol. 1982, 16, 454-458
Destruction of Pollutants in Water with Ozone in Combination with Ultraviolet Radiation. 2. Natural Trihalomethane Precursors William H. Glaze,” Gary R. Peyton,t Simon Lin, R. Y. Huang, and Jimmie L. Burieson
Department of Chemistry and Institute of Applied Sciences, North Texas State University, Denton, Texas 76203 The simultaneous application of ozone and ultraviolet radiation is appreciably more effective than ozone alone for the destruction of THM precursors in water from two southern U.S. surface-water sources. A mechanism is suggested that involves the destruction of a precursor initially present and the parallel formation and subsequent destruction of a secondary precursor from the natural organic matrix. From each source water the secondary precursor is apparently resistant to oxidation with ozone alone but is more readily destroyed by 03/UV treatment.
Introduction This is the second of a series of papers (1)describing the results of a study (2) in which ozone with and without ultraviolet radiation was evaluated for destruction of organic micropollutants in water. The present paper focuses on the natural organic matrix in one East Texas lake and one Louisiana lake that are high in THM precursors. In a previous study (3),it was shown that this area of the U.S. has several surface reservoirs that are exceedingly high in THM precursors. In the original survey, a small community system using one of these reservoirs showed total THM levels of 900 NUL. Recently Ballard (4) has reported even higher THM levels in other municipalities in the same area. Removal of THM precursors prior to free chlorination has been proposed as one alternative to reducing THM levels (5). Activated-carbon adsorption, improved flocculation, and the use of strong oxidants prior to chlorination are the most commonly proposed treatment possibilities (6). In fact, strong oxidants such as ozone (7)or potassium permanganate (8)are generally not considered to be viable alternatives for this purpose. The use of ozone combined with ultraviolet radiation is more promising, and this and other “catalytic oxidation” processes may be of substantial value in THM control. Here reported are batch-scale kinetic studies that verify the potential of the 03/UV process for this purpose. Experimental Section Ozone/UV Contacting System. Two reactors of the batch sparged type were used in this study. The 3-L quartz reactor was described in an earlier paper (1). The other reactor (HR) was built by Houston Research Inc. (9). This reactor is a 27.8-L (20-L liquid volume) stainless steel vessel equipped with a high-speed stirrer, a sparger for introduction of ozonef oxygen, and two quartz wells for UV lamps. All tubing is either stainless steel or PTFE to ensure resistance to corrosion and contamination. The UV lamps used in the reactor are Conrad-Hanovia 450-W medium-prewure mercury lamps (catalog no. 670A) and/or 200-W medium-pressure lamps (catalog no. 654A). The effective radiation entering the oxygen-sparged stirred tank reactor was measured with ferrioxalate actinometry. The *To whom correspondence should be addressed at: Graduate Program in Environmental Sciences, The University of Texas at Dallas, Richardson, TX 75080. t Present address: SumX Corporation, Austin, TX 78753. 454
Environ. Sci. Technoi., Vol. 16, No. 8, 1982
published output of these lamps coupled with the known quantum efficiency of the reaction can yield the effective radiation transferred in each photochemical system (10). Test Samples. Caddo Lake water was collected from the water supply intake for the City of Marshall, TX, and shipped in 5-gallon glass bottles by private vehicle to Houston, TX. Water was filtered through an S&S no. 588 filter and oxidized in the HR reactor. Some runs were made on unfiltered water. Cross Lake water was collected a t a pilot plant located at the City of Shreveport, LA, water treatment plant. The water was shipped to Denton, TX, and oxidized in the quartz reactor after similar filtration. In each case, runs were made as quickly as possible after collection of the water in order to minimize matrix changes, but in all cases THMFP was observed to fall slowly upon storage. Sampling Procedure: HR Reactor. Four samples were taken from the reactor a t each sampling time and transferred into 120-mL serum bottles (head-space free). One sample was used to measure pH and residual ozone by the iodimetric/thiosulfate titration (11). The results of these titrations were used to determine the quantity of sulfite ion required to quench the ozone. A 50% excess of sulfite was added to the remaining bottles, and they were sealed with PTFE-lined rubber septa and aluminum crimped caps. Samples were stored on ice and shipped by air freight to the laboratory, where they were chlorinated within 16 h. Sampling Procedure: Quartz Reactor. The procedure was similar to that described above, except that samples were taken in 25-mL screw-cap bottles of the type used in the liquid-liquid extraction method for trihalomethanes (12). The samples were chlorinated on site within 1-2 h. Analytical Procedure for THM Formation Potential (THMFP). Samples were chlorinated a t pH 6.5 (phosphate buffer) and at 25 f 2 OC. Samples from Caddo Lake were chlorinated with a dose of chlorine such that the sulfite quench was overcome, and then an additional dose of 15 mg/L added. Cross Lake samples were dosed so as to give a net dose of 20 mg/L after sulfite quench was overcome. Generally, chlorine residual was in the 1.0-2.0 mg/L range after incubation, which was 8 and 5 days for Caddo Lake and Cross Lake Samples, respectively. After the incubation period, excess sulfite was added, and the samples were analyzed for THMs by the liquidliquid extraction procedure. Caddo Lake samples were analyzed by the procedure of Henderson et al. (13) and Cross Lake samples by the modification of Glaze et al. (12). Details of the analytical and calibration conditions are described elsewhere (2). Precision of duplicate THMFP measurements was generally within 10%.
Results Caddo Lake Samples. Caddo Lake is located on the border of Texas and Louisiana in the region of heavy rainfall (40-50 in./year) and dense vegetation. It is one of the few natural lakes in the region and is characterized by high TOC and THM precursor levels. Values of the
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THMFP of samples used in this study ranged from 540 to 700 pg/L, which are not uncommon for waters from this region (3, 4). Six runswere made on these samples with use of the HR reactor. Figure 1 shows a comparison of normalized THMFP values obtained on samples taken from the reactor at various times, by using ozone with no ultraviolet radiation. Figure 2 shows one of these runs plus a run at approximately one-third the ozone dose with UV radiation. In this case, radiation from a 450-W high-pressure mercury lamp and a 200-W medium-pressure mercury lamp was used. In separate experiments, ferrioxalate actinometry indicated the transfer of a total of 6.2 W of radiation or 0.31 W/Liter in the 20-L liquid volume of the reactor. The apparent low transfer efficiency of radiation into the HR reactor may be due to one of two factors: (a) old age of
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Trussel (15),Trussell and Umphries (16),and Rice (17). It is generally agreed that ozonation is capable of oxidizing molecular sites that upon chlorination would produce chloroform and other THMs. However, oxidation of the carbonaceous matrix of natural waters also produces THM precursors. In some cases, direct oxidation results in small increases in THMFP (7, 18). Equations 1-4 suggest a
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mechanism to explain the observations by other workers and the data shown in Figures 1-6. Here P1represents precursor initially present in the water, P2is precursor produced by ozonation of the carbon matrix C, and X is nonprecursor carbon. For Caddo Lake water rate constants kl were estimated from initial slopes of the data shown in Figure 1. The values for the three runs shown in Figure 1were 0.03,0.05, and 0.12 m i d . Figure 7 shows reconstructed experimental decay curves (dotted lines) representing reaction 1for the three cases shown in Figure 1. The values of THM formation potential from these curves were then subtracted from the normalized kinetic data to give the residual curves (solid lines in Figure 7). These curves represent the rise and fall of the concentration of the secondary precursor Pp.Formation of Pzis 458
Environ. Scl. Technol., Vol. 16, No. 8, 1982
so rapid at the highest ozone dose rate that a distinct "hump" is seen in normalized rate curve (Figure 1). One conclusion of this analysis is that the secondary precursor P2is significantly less reactive than PI,indicating that the two types of precursors are of different chemical types. Initial precursors P1are most probably substituted aromatics (19),whereas the secondary precursors P2 may be aliphatic carbonyl compounds of the more classical haloform precursor variety. For mass-transfer-controlled reactions, it can be shown that the value of THM precursor Pzreaches a maximum at a time tm, given by 1
where D is the ozone dose rate and P is the mass-transfer
efficiency constant. Equation 5 predicts that t , will vary inversely with D. Actual values for three cases are approximately 90,60, and 30 min, respectively, for dose rates in the ratio 1:2:3. Figure 8 shows the data for one run on Caddo Lake water plotted in a slightly different manner, Le., as THMFP consumed. A mechanism as suggested in eq 1-4 is indicated by the change in slope after approximately 30 min. Also shown in the figure are values of ozone consumed, that is, ozone does minus ozone in the off gas. It is interesting to note that the rate of ozone consumption in the period from 30 to 180 min is approximately 0.36 pmol of O,/(L min), while THMFP destruction is only about 0.017 pmol of THMFP/(L min). In other words, after a short period when THMFP destruction is relatively rapid, the system reaches a pseudosteady state rate where the net efficiency of ozone utilization for THMFP destruction is only approximately 5%. The majority of the ozone is being utilized for destruction of the matrix, which as noted above is a partially self-defeating process in that more precursors are being formed. Photolytic Ozonation. Figure 2 shows a comparison of ozonation and photolytic ozonation (O,/UV) as means to destroy THM precursors in Caddo Lake water. While this system was investigated only briefly, it is clear from Figure 2 that 03/UV is significantly more effective than ozone alone for THM precursor destruction. A more thorough study of the variables in the ozone/UV process was made with water from Cross Lake, LA. Figures 3 and 4 show a portion of the results (2) plotted as normalized destruction curves. Figure 3 shows the effect of increasing ozone doses a t a given UV intensity, and Figure 4 shows the effect of increasing UV intensity a t a given ozone dose. Both plots show a feature of the data that is of particular interest, i.e., the apparent increase in precursor levels after short durations of treatment in several runs. This effect is well documented within the precision of the data. Moreover, it is significant that Riley et al. have observed a similar effect in static ozonation studies of humic material (7). Referring again to eq 1-4, it is apparent that ozone/UV treatment is producing secondary precursors (Pz)a t a higher rate than precursors (P1+ Pz)are being destroyed. As shown in Figures 5 and 6, this is particularly true a t low ozone doses and/or low UV intensities. One possible explanation of this effect is that certain types of structures are present in the organic matrix that are very rapidly oxidized to byproduct sites more activated for chlorination. For example, rapid hydroxylation of certain aromatic moieties may produce m-dihydroxybenzene systems that are known to chlorinate very rapidly to form trihalomethanes (19). Eventually these secondary precursors would further oxidized as the photolytic ozonation process proceeds, but it is possible that they are also being formed continually during the process. This discussion illustrates that the secondary precursor P2should not be viewed as a single species, and that values of rate constants kl-kl will depend on the specific water source being used. Figures 5, 6, and 9 illustrate another feature of the photolytic ozonation process that has practical implications. Figure 7 in particular shows that TOC values of 2-3 mg/L prevail even after THMFP has been completely dissipated. This is apparently due to the presence of organic substances that are refractory to further oxidation but that do not have the potential to react with aqueous chlorine to form trihalomethanes. Two- and three-carbon polyfunctional organic acids probably account for much
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of the refractive products, and a GC/MS examination of a model aromatic system has shown that such compounds are formed (20). In any case, the data in Figures 5,6, and 9 indicate that TOC is not an appropriate surrogate for THMFP in treatment plants where oxidation is used. Indeed, complete destruction of organic substances is not necessary or economically justified when control of trihalomethanes is the objective.
Prospects for Application of the 03/UV Process Many municipalities are forced to use a water supply that is high in precursor levels, such as the two sources described in this paper. For such cases, two alternatives for control of THMs below the EPA limit of 0.10 mg/L seem most feasible: (a) substitution of an alternative disinfectant; (b) use of advanced treatment methods for removal of precursors before chlorination (5). Alternative b is likely to be the more expensive of the two, a t least if current estimates are accurate (21). Indeed, many municipalities using sources with high precursor levels are actively considering the use of chloramination rather than free chlorination, in order to solve their THM problems (22). As a caveat, we may predict that this “solution” will lead to unacceptable problems when excessive iron and manganese levels and taste and odor problems occur in supply reservoirs, as is often the case annually. As experience in alternative a is gained, many systems will be forced to adopt preoxidation with ozone, permanganate, and chlorine dioxide coupled with postchloramination. The higher costs associated with these alternative oxidants will undoubtedly cause many to look a t alternative b for THM control. Of the advanced treatment methods for removal of precursors, one has been seriously proposed, that is, the use of granular-activated-carbonfiltration (23). With the prospect of combined adsorption and biological degradation (14),this method becomes increasingly attractive, but it is as yet unproven for THM control under conditions as severe as those described here (24). For these cases, ozone with ultraviolet radiation may well become a viable alternative. Prengle and co-workers (2) have estimated that 03/UV treatment of Cross Lake t o remove 90% of the ambient precursors would cost approximately $0.05-0.15/1000 gallons for 10-50 MGD units. These figures are probably low due to assumptions made in the calculations favorable to the process, but they indicate ozone/UV has considerable promise. A pilot-scale evaluation of the 03/UV process for THM precursor removal is currently underway to provide more detailed and realistic estimates of its potential (25). Envlron. Sci. Technol., Vol. 16, No. 8, 1982
457
Conclusions Ozone with ultraviolet radiation is more effective than ozone alone for the destruction of THM precursors in two southern U.S.reservoirs with high TOC levels. In both ozone and ozone/UV applications, kinetic data suggest that secondary precursors of greater refractivity are produced by the oxidation of the natural carbon matrix. In the case of “ozone-only” systems, these secondary precursors may persist for long periods of ozone treatment, but with simultaneous UV treatment they are destroyed more efficiently. Ozone with ultraviolet radiation is seen as a possible alternative or collaborative process to granular activated carbon for control of THM in systems that have high natural precursor levels. Acknowledgments The assistance and advice of J. K. Carswell is sincerely appreciated.
Literature Cited (1) Peyton, G. R.; Huang, F. Y.; Burleson, J. L.; Glaze, W. H. Environ. Sci. Technol., preceding paper in this issue. (2) Glaze, W. H.; Peyton, G. R.; Huang, F. Y.; Burleson, J. L.; Jones P. C. “Oxidation of Water Supply Refractory Species by Ozone and Ultraviolet Radiation”;Final Report, Cooperative Agreement CR-804640, USEPA, Drinking Water Research Division, MERL, Cincinnati, OH, EPA-600/280-110, 1980. (3) Glaze, W. H.; Rawley, R. J.Am. Water Works Assoc. 1979, 71, 509. (4) Ballard, W. T. “Experiences in the Reduction of Total THMs Through Conventional Water Treatment”; Southwest Section,Am. Water Works Assoc., Baton Rouge, LA., October 20-22, 1980. (5) Stevens, A. A.; Symons, J. M. J. Am. Water Works Assoc. 1977, 69, 546. (6) Symons, J. M. “Treatment Techniques for Controlling Trihalomethanes in Drinking Water”; US. Environmental Protection Agency, EPA-600/2-81-156, 1981. (7) Riley, T. L.; Mancy, K. H.; Boettner, E. A. In “Water Chlorination: Environmental Impact and Health Effects”; Jolly, R. L., Gorcher, H., Hamilton, D. H., Jr., Eds.; Ann Arbor Science: Ann Arbor, MI, 1978; Vol. 2, p 543. (8) Colthurst, J. M.; Singer, P. C. “Removal of Trihalomethane Precursors by Permanganate Oxidation and Manganese Dioxide Adsorption”; Annual Conference, Amer. Water Works ASSOC.,Atlanta, GA, June 15-19, 1980. (9) Prengle, H. W., Jr.; Hewes, C. G.; Mauk, C. E. “Proceedings of the 2nd International Symposium on Ozone Technology”; Rice, R. G., Richet, P., Vincent, M. A., Eds.; International
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Ozone Institute: Syracuse, NY, 1975; p 224. 1953,220, 104. (10) Parker, C. A. Proc. R. SOC. (11) “Standard Methods for the Examination of Water and Wastewater”; Public Health Assoc.: New York, 1975; pp 407-416. (12) Glaze, W. H.; Rawley, R.; Burleson, J. L.; Mapel, D.; Scott, D. R. In “Advances in the Identification and Analysis of Organic Pollutants in Water”; Keith, L. H., Ed.; Ann Arbor Science: Ann Arbor, MI, 1981; Chapter 17. (13) Henderson, J. E.; Peyton, G. R.; Glaze, W. H. In “Identification and Analysis of Organic Pollutants in Water”; Keith, L. H., Ed.; Ann Arbor Science: Ann Arbor, MI, 1976; p 105. (14) Sontheimer, H. In “Oxidation Techniques in Drinking Water Treatment”; U. S. Environmental ProtectionAgency, EPA-570/9-79-020, 1979; p 13. (15) Trussell, R. R. In “Organics in Domestic Water Supplies”; Proceedings, California-Nevada Section Forum, Amer. Water Works ASSOC.,Palo Alto, CA, 1978; p 2. (16) Trussell, R. R.; Umphries, M. D. J. Am. Water Works Assoc. 1978, 70, 604. (17) Rice, R. G. Ozone: Sci. Eng. 1980, 2, 75. (18) Lawrence, J. “The Oxidation of Some Haloform Precursors with Ozone”; 3rd International Symposium on Ozone Technology,InternationalOzone Institute, Paris, May, 1977. (19) Rook, J. J. In “Water Colorination: Environmental Impact and Health Effects”; Jolly, R. L., Brungs, W. A., Cumming, R. B., Eds.; Ann Arbor Science: Ann Arbor, MI, 1980;Vol. 3, p 85. (20) Glaze, W. H.; Peyton, G. R.; Saleh, F. Y.; Huang, F. Y. Int. J.Environ. Anal. Chem. 1979, 7, 143. (21) Burke, T. In “Trihalomethanes in Water Seminar, Papers and Proceedings”; Water Research Centre, Medmenham Laboratory, Medmenham,Marlow, Bucks, SL7,2HD, UK, 1980; p 231. (22) Norman, T. S.; Harms, L. L.; Looyenga, R. W. J. Am. Water Works Assoc. 1980, 7, 176. (23) U. S. Environmental Protection Agency, Fed. Regist. 1979, 44,68624. (24) Glaze, W. H.; Wallace, J. L.; Wilcox, P.; Johansson, K. R.; Dickson, K. L.; Scalf, B.; Noack, R.; Busch, A. W. In “Activated Carbon Adsorption“;Suffet I. H., McGuire, M. J., Eds.; Ann Arbor Science: Ann Arbor: MI, 1981,in press. (25) U.S. Environmental Protection Agency, Municipal Environmental Research Laboratory, Cincinnati, OH, Cooperative Agreement CR-808825-01,W. H. Glaze, P. I. Received for review July 13,1981. Revised manuscript received March 12, 1982. Accepted April 9, 1982. This work was supported by Grant No. R-804640 from the U.S. Environmental Protection Agency, Drinking Water Research Division, Cincinnati, OH, J. K. Carswell, Project Director.
