Ultraviolet disinfixtion of potable water Current technology and research needs
Roy L. Wnlfe Metropolitan ffiter District of Southern California Los Angela. CA Po054 Because of upcoming surface and groundwater regulations regarding the control of microbiological and chemical contaminants, there is a need to evaluate the feasibility and effective ness of ultraviolet 0radiation for primary disinfection of potable water supplies. Topics addressed in this assessment include the regulatory motivation for investigating alternative disinfectants; the background and theory of W application; the disinfection performance of UV against bacteria, viIUS-, and protozoan cysts; factors that affect inactivation efficacy; new developments in W technology;the formation of undesirable by-products; cost analysis; and remnunendations for future research. In addition, special emphasis has been placed on assessing the feasibility of UV treatment for groundwater in light of the upcoming groundwater disinfection regulations and disinfection by-product @BP) regulations.
Regulatory issues In the next several years, EPA will promulgate a number of new regulations for the control of microbiological and chemical contaminants in drinking water. Amendments to the 1986 Safe Drinking Water Act require EPA to set maximum contaminant levels (MCLs) for 83 contaminants by 1991 and to set additional MCLs at regular intervals thewher. In addition, many surface water utilities will be rrpuired to install 6ltration systems and meet specific disinfection requirements for the control of Giarah and enteric viruses as part of the Surface Water Treatment Rule (SWTR). EPA is also considering a re788
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I quirement for mandatory disinfection of all groundwater supplies to ensure protection from enteric viruses. In addition to disinfection regulations, EPA will promulgate regulations for the control of DBPs by 1991, including lowering the current MCL for trihalomethanes. These regulations will undoubtedly significantly alter disinfection practices in the United States. To meet the proposed regulations, utilities will have to balance the disinfection process so as to minimize the formation of DBPs without compromising the inactivation of waterborne pathogens. Because of the reactivity of chlorine with naturally occurring organics, producing myriad halogenated byproducts, the use of chlorine as a disinfectant will probably be greatly curtailed and restricted to utilities that use source waters with low levels of DBP
precursors. Thus, a search for suitable, cost-effectivealternative primary disinfectant agents for surface waters and groundwaters must be actively pursued.
Microbial concerns Significant differences in the microbial quality of surface waters and groundwaters necessitate different approaches to treatment and disinfection. For example, surface waters are typically exposed to more sources of microbial contamination than groundwaters and are subject to wide fluctuations in physical and chemical qualities such as temperature and turbidity. Accordingly, surface waters are typically treated with several processes, including coagulation, flocculation, sediientation, filtration, and disinfection. Groundwaters, on the other hand,
0013-936x190109240768$02.5010 Q 1990 American Chemical Society
have historically been presumed to be relatively free of microbial contaminants as a result of filtration through soil. Consequently, most groundwaters are either untreated or marginally chlorinated before distribution. Although groundwaters may be devoid of large microorganisms (e.g., algae, protozoans, and helminths), this is not the case for smaller microorganisms (e.g., viruses). A number of investigations have shown that enteric viruses can be transported through subsurface soils into drinking water aquifers (1-3). Moreover, the analytical technology for detecting viruses is improving so rapidly that inexpensive, sensitive, accurate monitoring systems such as gene probes may soon be available to the water industry (4, 5).
Disinfection of groundwaters Groundwaters account for many of the outbreaks of waterborne disease in the United States. Approximately half of the 502 waterborne outbreaks (approximately 111,000 cases of illness) reported between 1971 and 1985 were attributed to contaminated groundwater (6). The primary sources of groundwater contamination are septic tanks, cesspools, and leakage from municipal sewer systems and treatment lagoons. Collectively, these sources contribute more than a trillion gallons of domestic wastes to groundwater each year (7). In many cases, outbreaks of contaminated water are caused by lack of-or inadequate-disinfection. The primary etiologic agents of waterborne disease in groundwaters are enteric viruses and Shigella (8). In approximately 50%of waterborne disease outbreaks, no causative agent is identified. However, many of these outbreaks are believed to be caused by enteric viruses. In a recent survey, C. r! Gerba (personal communication, 1989) detected enteric viruses in approximately 19% of 112 samples collected from groundwaters in Arizona. This percentage may overestimate the true percentage of virus-contaminated groundwaters, as samples were collected from sites with a greater than normal probability of contamination. It is significant that coliforms were detected in only l %of the samples, suggesting that coliform bacteria are inadequate indicators of viral contamination in groundwaters. Such surveys could be greatly facilitated by the use of gene probe technology. Groundwater disinfection options For surface waters, a number of alternative disinfectants are available, the most common of which are ozone, chlorine dioxide, and chloramines. Some of these disinfectants may be in-
appropriate for use in groundwaters, however. For example, chloramines are weak virucides and would be unlikely to meet primary disinfection requirements. The use of free chlorine will have to be limited to high-quality groundwaters in order to meet the DBP regulations. Chlorination of colored groundwaters or groundwaters that are high in total organic carbon (TOC) would probably produce unacceptable levels of trihalomethanes and other DBPs. Because of the SWTR disinfection and residual maintenance requirements, it is likely that a combination of chloramines and ozone will be used by many utilities in treating surface water. The application of ozone would effectively inactivate enteric viruses but could also produce undesirable byproducts, especially in high-TOC water. For example, it is well documented that ozonation produces considerable amounts of assimilable organic carbon (AOC), which can result in extensive bacterial regrowth in the distribution system (9).If ozone is used, then filtration through biologically active filters may be necessary to remove the AOC; the cost of ozonation and filtration, however, is likely to be prohibitive for many small utilities. Ozone is too short-lived to provide a residual for the distribution system, so addition of a postdisinfectant is necessary when using ozone. Moreover, little is now known about the identity, occurrence, and health impact of ozonation by-products in potable water. Chlorine dioxide is also an effective virucide, but there is growing toxicological concern over its use in drinking water. In addition, chlorine dioxide must be generated on site and is one of the most expensive disinfectants. Another potential disinfectant for small utilities treating surface or groundwaters is UV radiation. For UV to be considered a viable primary potable-water disinfectant,it must be evaluated with respect to several criteria: effectiveness against bacterial, viral, and protozoan pathogens (for groundwaters, only removal of bacteria and viruses is necessary); the accuracy and reliability with which the process can be monitored and controlled; production of undesirable by-products in water; and economic considerations. The next several sections of this article will review and discuss UV technology in relation to the above criteria.
Fundamentals of UV disinfection UV has been used commercially for many years in the pharmaceutical, cosmetic, beverage, and electronic industries. It has also been successfully applied in wastewater treatment. UV was first used on drinking water in the early
1900s but was abandoned shortly thereafter for a variety of reasons, including high operating costs, poor equipment reliability, maintenance problems, and the advent of chlorination, which was found to be more efficient and reliable. However, UV technology is gaining popularity, particularly in European countries, because of improvements in equipment reliability and reduction of undesirable by-products. Approximately 2,000 treatment plants in Europe currently use UV (IO). Electromagnetic radiation is an effective agent for microorganism inactivation (Figure 1) in the wavelengths ranging from 240 to 280 nm, which kill microorganisms by causing irreparable damage to their nucleic acid. The most potent wavelength for DNA damage is approximately 260 nm (Figure 1). The inactivation of microorganisms by UV is proportional to the intensity (watts per square centimeter, or W/cm2) multiplied by the time of exposure(s). The product of the intensity and the contact time is termed the UV dose and is reported in units of (W.s)/cm2 or joules (J)/cm2, where 1 J/s is equivalent to 1 W. The actual UV dose received by the organism, however, depends on a number of factors, such as the flow rate of the water through the UV system, the transmission efficiency of the water, and the geometry of the UV radiation chamber. Most bacteria and viruses require relatively low UV dosages for inactivation (Table 1)-usually in the range of 2,000 to 6,000 (pW*s)/cm2 for 90% kill. However, protozoan cysts appear to be considerably more resistant to UV inactivation than other microorganisms. Rice and Hoff (16) showed that less than 80% of G. lamblia cysts were inactivated at UV dosages up to 63,000 (pW*s)/cm2.Carlson et al. (13) demonstrated that 90% of G. muris cysts were inactivated when the dosage was increased to approximately 82,000 &W*s)/cm2. These results are significant in light of the fact that the maximum designed dose of many commercially available UV units is 25,000 to 35,000 &W.s)/cm*. Although no information on the inactivation of Cryptosporidium by UV could be found in the literature, it is likely that the dosages would be higher than for Giardia, given the extreme resistance of Cryptosporidium to chlorine. Based on the bacterial and viral results, the U.S. Department of Health, Education, and Welfare issued a 1966 policy statement in which the criteria for the acceptability of UV disinfecting units were stated as a minimum dosage of 16,000 (pW*s)/cm2and a maximum water depth of approximately 7.5 cm (19). Environ. Sci. Technol., Vol. 24, No. 6, 1990 769
Disinfection studies have also been conducted to evaluate the field performance of UV systems. Slade, Harris, and Chishohn (20), comparing well-head disinfection of virus-contaminated groundwater by free chlorine and W, found that UV was a more potent virucide than free chlorine, even when the chlorine residual was increased to 1.25 mglL at a contact time of 18 min. The UV dosage in their study was set at 25,000 (pW.s)/cm*. Kruithof et al. (21) found that UV was comparable to chlorination for inactivation of heterotrophic-plate-countbacteria in water that had been treated with granular activated carbon. It is important to recognize, however, that in order to maintain bacteriological integrity in the distribution system, and to comply with the residual disinfectant requirements of the SWTR, a postdisinfectant will be necessary for disinfding surface waters by W. The need for a postdisinfectant in groundwaters will depend on the potential for bacterial regrowth and cross-contamination.
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pressure lamps so that higher treatment capacity is possible (22,23). Several problems with W systems are fresuently cited. One is shortcircuiting through the UV chambers. Although early models had this problem, many modern units are designed to prevent short-circuiting. Another r e ported problem is biofouling of the lamp surface, which can reduce effective intensity. Most modern units, however, contain built-in devices for easy, rapid lamp cleaning. Perhaps the most important problem in the use of UV is an inability to determine the W dose reliably and consistently. The reliability of W systems is of the utmost importance for ensuring microbiologically safe drinking water, as no residual is produced. This is particularly important for groundwaters if a postdisinfectant is not added. For most other disinfectants, dosages
and residual concentrations can be easily and rapidly monitored to document process performance. Recent advances in W system technology suggest that the reliability of W monitors is improving (18,24). Systems are being developed that continuously monitor W intensity and automatically control the operation of the system, including alarms, backup systems, and bypass capability (18). Another potential problem with W disinfection is that bacterial regrowth may occur. When exposed to visible light, bacterial cells that have been injured by UV light can repair their DNA damage in a process known as photoreactivation. Although this may be of concem in wastewater and surface waters where W-treated water in basins is exposed to sunlight, it is not likely to present a problem in groundwaters unless open reservoirs are present.
W disinfection eficiency Because W light must be absorbed into the microorganisms to achieve inactivation, anything that prevents the UV light from reacting with microorganisms will impair disinfection. Conditions and materials that interfere with this process include chemical and biological films that develop on the surfaces of W lamps; clumping or aggregation of microorganisms, which has a protective effect; turbidity; color; dissolved organics and inorganics; and short-circuiting in water flowing through the exposure chamber. Disinfectant activity using UV, however, appears to be relatively independent of temperature and pH. By-products of W radiation Undesirable DBPs from UV radiation in water can be classified into two general groups-those that are harmful to health and those that affect the water’s aesthetic quality. The available literature indicates that UV treatment does not contribute to either DBP category. Regarding the first category, several studies have shown that UV radiation did not produce increased mutagenic activity in the water (21, 26, 27), whereas one study showed only a slight increase in mutagenicity (28). Also, Kmithof et al. (21) showed that W treatment of carbon-filtered drinking water did not produce AOC compounds. This gives UV a distinct advantage over omnation, especially in cases where the water is not filtered. However, additional AOC studies should be conducted in a variety of water quality conditions to determine whether this finding holds up under different circumstances. Regarding the second category of DBPs, a number of anecdotal comments by researchers testing UV have indicated that it produces no tastes and odors (2529). This should probably be verified by means of flavor protile analysis (30).A newly recognized benefit is the production of high-energy, shortlived radicals when UV is combined
with ozone or hydrogen peroxide. These W-based advanced oxidation processes have been shown to be effective in breaking down selected taste and odor compounds and groundwater contaminants (31-33, 35, 36). However, recent research has demonstrated that combining W with ozone is less effective than using ozone alone for inactivating microorganisms (37).
C0St.S Several cost analyses have indicated that the costs of UV treatment are comparable to or slightly higher than the costs of chlorination and lower than those of ownation and chlorine dioxide disinfection (17, 34). The most recent detailed cost comparison was conducted in 1984 by Gumeran, Bunis, and Hansen (38). For this study, their estimates were updated based on 1989 construction cost indexes, and total annualized costs were calculated by using an interest rate of 8% over a 20-year period.
As shown in Table 2, the capital costs for 0.5- and 1.0-MGD UV facilities are greater than those for chlorine, chlorine dioxide, or ozone facilities. However, the annual operation and maintenance costs for W are lower than for any of the other disinfectants except chlorine. Because of UV’s relatively low operation and maintenance reqnirements, the total annual cost for UV is less than that for chlorine dioxide or ozone. For a 1.OMGD facility, the estimated total costs for disinfection by chlorine, UV, ozone, and chlorine dioxide are 1.7, 5.3, 6.6, and 8.6 U 1 1 , W gal, respectively (Table 3). It should be noted that treatment costs for W wiU be higher if a postdisinfectant is added.
Research recommendations The use of UV by small utilities to disinfect surface waters and groundwaters appears promising. However, more information is needed in a halfdozen areas to better predict the performance capability of UV technology.
TABLE 1
Approximate dosages f microorganisms by UV Mkmcug~nlrun
Bacterfa
Environ. Scl.Teohnol., VOl. 24, No.6, 1990 771
UV disinfection should be evaluated for a variety of sonrce water qualitiesto better determine the effects of color, turbidity, particle sue, minerals, and dissolved organics. More information is needed on the resistance of microorganisms, particularly enteric viruses. Currently available data are based on only a few studies, conducted under water quality conditions that did not mimic conditions in the field. Additional experiments should be conducted on the inactivation of coliphages, which are potential indicators of enteric viruses. A comprehensive evaluation of the process reliability of recently installed UV systems is needed. This will allow a better determination of commonly cited UV problems such as shortcircuiting and reliability of new W dose sensors. Additional research should be performed on the formation of off-flavors and odors, AOC, and other undesirable by-products. AOC production may limit W s potential in groundwaters unless additional processes are provided. The need for a postdisinfectant in groundwaters disinfected by UV should be evaluated. The impact of coliform photoreactivation in systems with open reservoirs should be assessed. Moreover, to assess the extent of groundwater contamination and the need for alternative disinfectants such as W, additional information is needed on the occurrence of pathogens, particularly viruses, in groundwaters.
Acknowledgment This article was reviewed for suitability as an ESdrTfeaIure by Charles A. Haas, Illinois Institute of Technology, Chicago, IL 60616, and William H. Beauman, Everpure, Inc., Westmont, IL 60559.
References (1) lansons, J. et al. Water Res. 1989, 23(3),
293-99. 772
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dvantages and disadvantages of UV disinfect
I I V has the following Umlted informatio
I (2) Drewry, W. A.; Eliassen, R. J. Wafer Pollut. Control Fed. Res. Suppl. 1968, WE),Part2, R257-71. (3) Lance, 1. C.; Gerba, C. p. J. Environ. Qual. 1980, 9, 31-34. (4) Margolin, A. B. et al. In Biohorordr of Drinking Water Environment; Larson, R. A., Ed.; Lewis: Chelsea, MI, 1988; pp. 265-71. (5) Gerba, C. P; Margolin, A. B.; Hewlett, M. I. Woter Sci. Technol. 1989, 2J(3), 147-59. (6) Craun, G. E J. Am. Water Works Assoc. 1988,80(2), 40-52. (7) Keeley, 1. W. In Public Policy on Ground Water Quality Protection: Kerns, W. R., Ed.; Virginia Water Resources Center: Blacksburg, VA, 1911; pp. 2-9. (8) Craun, 0 . E In Uhterbornc Diseases in the United States; Craun, G . E, Ed.; CRC Press: Boca Raton, FL, 1986; pp. 71-1hIl .”” (91 van der Kooq. 0 . Hipen. W A M Krutthof. J C Prureedmgs uf the Etghth Ozone World Connresr. 1987. lnternational Ozone Ass&iation: Zurich, 1987; Vnl 1 n no6 .’. r ’ (IO1 Gclzhaurer, G . P Summunrr. WASSER BERLIN ‘89: International Ozone Asrocialion. European Committee: Paris.
.-
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1989; pp. m-5-1-13, (11) Meulemans, C.C.E.
(12)
(13)
(14) (15)
(16) (17) (18)
(19)
(20)
Ozone: Sci. Eng. 1987,9.299-314. Sobsey, M. D. “Quarterly Report: Detection and Chlorine Disinfection of Hepatitis A in Water”; US. Environmental Protection Agency: Cincinnati, OH, 1989; EPA CR-813424. Carlson, D. A. et al. ”Project Summary: Ultraviolet Disinfection of Water for Small Water Supplies”; Office of Research and Development, U.S. Environmental Protection Agency: Cincinnati, OH, 1985; EPA/600/S2-85/092. Angehm, M. Aqua 1984,2, 109-15. Chang, J.C.H. et al. +pi. Environ. Microbiol. 1985,49(6), 1361-65. Rice, E. W.; Hoff, I. C. Appl. Environ. Microbiol. 1981, 42(3), 546-47. Combs, R. E; McGuire, P 1. Ultrapure Water 1989, 6(3), 62-68. Zinnbauer, E E.; Conacher, 1. C. 1987 Annual AWWA Conference Proceedings. Kansas City,MO: American Water Works Association: Denver, CO, 1987; pp. 32733. National Academy of Sciences. Drinking Waterand Health, Vol. 2 ; National Academy Press: Washington, DC, 1980. Slade, 1. S.; Harris, N. R.; Chisholm,
R. G . Water Sci. Technol. 1986, 18(10), 115-23. (21) Kruithof, J . C. et al. Summaries. WASSER BERLIN ‘89: International Ozone
Association. European Committee: Paris, 1989: pp. Ill-3-1-15, (221 Egberts, G . Summaries. WASSER BERLIN ‘89; International Ozone Association. European Committee: Paris, 1989: pp. 1V-2-l-lO. (23) Stein, H . 1.; Lazarevic, M. Summaries, WASSER BERLIN ‘89: International Ozone Association. European Committee: Paris, 1989; pp. IV-3-I-10. (24) Qualls. R. G . ; Dorfman. M. H.; lohnson. J . D. Wnrer Res. 1989, 23(8), 317-
HERE IT IS! THE BRAND NEW EDITION OF THIS “MUST-HAVE” REFERENCE!
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(25) Tobin, R. S. et al. J. Am. Water Works Assoc. 1983, 75(9), 481-84. (26) Zoeteman, B.C.J. et al. Environ. Health Perspecr. 1982.46, 197-203. (27) Kaol, H . J . . Kreijl. C. F.;Hrubec, J. In Water Chlorination: Chemistry. Environmenral lmpocr and Health Effects, Vol. 5 : Jolley. R . L. et al. Eds.; Lewis: Chelsea. MI, 1985: pp. 187-205. (28) Jolley, R. L. et al. In Worer Chlorinarion: Environmental Impact and Heolrh Effecrs, Vol. 4: Jolley, R. L. et a l . , Eds.; Ann Arbor Science: Ann Arbor, MI. 1983: pp. 449-523. (29) SMC Martin Inc. “Microorganism Rcmoval for Small Water Systems”; Office of Drinking Water, U.S. Environmental Prolection Agency: Washington, DC, 1983; EPA 57019-83-012. (30) Krasner. S. W.: McGuire. M. 1.; Ferguson, V. B. J . Am. Water Works ASSOC. 1985, 77(3), 34-39. (31) y o n Sonntag, C. Summnn‘es, WASSER BERLIN ‘89: International Ozone ASSOciation, European Committee: Paris, 1989; pp. V-I-1-11, (32) Underbrink, T. K . et al. Summaries. WASSER BERLIN ‘89: International Ozone Association, European Committee: Paris, 1989; pp. V-4-1-IO. (33) Peyton. G . R. et al. Environ. Sci. Techno/. 1982, 16. 448-53. (341 LFgan,~R.-W. Indusr. Warer Eng. 1982, IY(Z1, 12-25.
Peyton, G. R.; Glaze, W. H. Environ. Sci. Teerhnol. 1988,22(7). 761-64. Glaze, W. H. el al. J . Am. Worer Works Assoc. 1990,82(5), 79-84. Duguet, J. P et al. Summories, WASSER BERLIN ‘89: International Ozone ASSOciation. European Committee: Paris, 1989:... DO. V-5-1-10. Gumeran, R. C . ; Burris, B. E.;Hansen, S . P “Estimation o f Small System Water Treatment Costs”; Office of Research and Develooment. U.S. Envirnnmenfil Pro-
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Roy L. Wove i s principal microbiologist at the Metropoliran Water Disrrict of Sourhern California. He joined Metropolitan in 1985, a j e r receiving a Ph.D. from rhe Universiry of California, Imine. He has spenr rhe pasr 10 years studying the microbiology ofporable water and, in particular, disinfecrion processes.
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