Understanding
Micro
Increasing attention is being given t o the detection, treatment, and removal of problematic effluent-derived contaminants.
contaminants
in Recycled Water
D A V I D L. S E D L A K ,
T
J A M E S L. GRAY, A N D K A R E N E.
hroughout history, intentionally or not, people have recycled water. As a result, it is not unusual for rivers to receive discharges of municipal, agricultural, and industrial wastes from upstream communities while water supplies are being drawn downstream. This has occurred to such a degree that, for example, during the dry season, the water in California's Santa Ana River largely consists of wastewater effluent. Despite this situation, effluent-dominated waters provide habitat for aquatic organisms and are often used to supply water. The extent of such unplanned water recycling is likely to increase as restoration and maintenance of aquatic ecosystems, urban expansion, and agricultural production exert growing pressures on limited water supplies. The introduction of wastewater effluent into drinking water aquifers and surface waters as a deliberately planned activity is becoming more common throughout the world. Such undertakings are already an important element of water resource planning in some regions of the United States (see Figure 1). There are several schemes for using recycled water. Nonpotable water recycling supports activities such as highway landscaping and irrigation of golf © 2000 American Chemical Society
PINKSTON
courses and is publicly accepted. Indirect potable reuse involves the introduction of recycled water into a system where it may eventually be used as a potable water source. Although indirect potable water recycling is less common, numerous projects are operating or are being planned in the United States. But while an increased reliance on such water recycling may be necessary to stretch a scarce resource, the potential health and ecological effects of exposure to chemical contaminants in recycled water are not well understood. Concerns about the presence of effluent-derived microcontaminants in recycled water are raising questions about existing practices and impeding new water recycling projects. To address these uncertainties, it is necessary to identify the chemicals likely to be present in recycled water, evaluate their potential effects on humans and aquatic ecosystems, and assess approaches for minimizing their release. In most locations, the primary source of recycled water is municipal wastewater effluent. Until recently, concerns associated with municipal wastewater recycling focused on whether h u m a n pathogens could be present in water after conventional and advanced treatment. These issues are being adDECEMBER 1, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY/ NEWS • 5 0 9 A
FIGURE 1
Potable recycled water programs In the United States, water recycling programs are most common in California, Florida, and Arizona.
dressed through research on pathogen fate in wastewater treatment systems and the installation of multiple treatment barriers. Considerable attention also has been given to detecting human health effects from chemical contaminants. Although research conducted during the 1980s and early 1990s demonstrated that recycled water can elicit adverse responses in bioassays, such as the Ames test, the causative agents have not been identified (1). Quantification of chemical contaminants has been limited to priority pollutants and several well-studied compounds, such as disinfection byproducts and certain industrial products, including alkylphenol polyethoxylate detergents. Concerns have been raised periodically about the possible presence of contaminants that are not routinely monitored, but little information about their occurrence has been available. A 1998 National Academy of Sciences (NAS) report articulated some of these issues: "The ability to evaluate and manage risks [associated with chemical contaminants] is greatest for minerals and trace inorganic chemicals, less for identifiable organic compounds and disinfection byproducts, and minimal for the unidentified mix that comprises the majority of organics in the water (2)." Municipal wastewater microcontaminants Previously undetected effluent-derived microcontaminants have been identified in numerous wastewater effluents and in surface waters subjected to wastewater discharges {2-7) since the publication of the NAS report; in one case, they have also been found in drinking water (4). Detection has been facilitated by improvements in analytical techniques capable of identifying polar organic contaminants in complex matrixes. In addition, recent reports of adverse effects following exposure to extremely low concentrations of endocrine-disrupting contaminants (8, 9) have focused the scientific community's attention on these formerly ignored contaminants. Their 5 1 0 A • DECEMBER 1, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS
occurrence and detection are challenges to analytical chemists, environmental engineers, and environmental scientists. Their presence in recycled waters also raises broader questions about the risks and benefits of water recycling and our approaches for anticipating the emergence of new contaminants. Much of the recent concern about trace levels of chemical contaminants can be traced to the early 1990s, when scientists in the United Kingdom observed endocrine disruption in fish exposed to wastewater effluents. Early studies documented the development of female characteristics, such as the production of vitellogenin, an egg sac protein, when male fish were exposed to undiluted wastewater effluent (8). Follow-up bioassays, conducted to identify the causative agent for endocrine disruption, implicated estrogenic hormones, such as 17(3-estradiol (a natural estrogen present in urine) and ethinyl estradiol (an ingredient of birth control pills) (9). These findings were complemented by exposure studies demonstrating feminization of male fish at hormone concentrations as low as several parts per trillion (a few nanograms per liter). Initial attempts at using chemical analyses to verify the presence of estrogenic hormones were hampered by interference from organic matter present in wastewater. Analysis of trace concentrations of estrogenic hormones is more difficult than other trace analyses because the phenol and alcohol functional groups on the hormones limit the use of cleanup steps frequently employed—such as use of silica gel columns—to separate hydrophobic organic contaminants from organic matter. Standard environmental analytical techniques for compounds having structures similar to steroid hormones have detection limits between 50 and 1000 ng/L. These techniques are not sensitive enough to detect estrogenic hormones, which are expected to be present in wastewater influent at concentrations of 30-60 ng/L. Attempts to modify standard analytical techniques by increasing the preconcentration factor are usually unsuccessful because the interference from organic matter is also amplified. Reliable methods for quantifying estrogenic compounds in wastewater at concentrations as low as 1 ng/L were finally developed through the application of gas chromatography/tandem mass spectrometry (GC/MS/MS) (5-7). GC/MS/MS offers a more selective mode of detecting organic compounds in the presence of interfering organic matter (see Figure 2). The use of GC/MS/MS is notable because, until recently, instruments capable of tandem mass spectrometry were uncommon in environmental laboratories. However, the development of low-cost ion trap mass spectrometers has made the technique accessible to many environmental researchers.
New developments in immunochemistry also have led to simple, relatively inexpensive techniques that can detect hormones at even lower concentrations than those detected using GC/MS/MS (-0.1 ng/L) (7). Immunochemical techniques recently have become popular as tools for screening water samples for contaminants such as pesticides and nitroaromatic compounds. One of the simplest immunochemical techniques, the enzyme-linked immunosorbent assay, is available in premade kits for several important hormones. Immunoassays are such sensitive detectors that they typically are used to measure environmentally relevant concentrations of contaminants without any sample preconcentration. However, they cannot detect the extremely low concentrations of hormones present in wastewater effluents without sample preconcentration and cleanup (to remove artifacts attributable to organic matter). Concurrent with the detection of estrogenic hormones, scientists in Germany and Switzerland have identified a suite of pharmaceuticals and their metabolites in municipal wastewater effluents at concentrations as high as 10 ug/L {2-4). Adverse effects attributable to exposure to trace concentrations of pharmaceutically active compounds (PhACs) in wastewater effluent have not been documented. Nonetheless, some scientists have hypothesized that the compounds could have subtle effects on wildlife and humans {10). For example, fluvoxamine elicits spawning of male mussels at concentrations of approximately 0.3 ug/L {11). These effects could be exacerbated when the aquatic organisms are exposed to mixtures of PhACs over extended periods. Adverse effects of PhACs on human health are less likely because doses administered to humans are many orders of magnitude higher than those received from drinking water, and drugs undergo extensive human testing before registration. Initial attempts to detect PhACs in wastewater were unsuccessful because the hydrophilic nature of many pharmaceuticals precluded their analysis with conventional analytical techniques. Most pharmaceuticals and their metabolites are extremely hydrophilic and cannot be isolated efficiently by liquidliquid extraction, which, until recently, was the preconcentration technique of choice for environmental analyses. Even after the compounds are concentrated by solid-phase extraction, coeluting organic matter interferes with the detection of low concentrations of PhACs when the samples are analyzed by conventional techniques such as GC/MS or high-performance liquid chromatography/mass spectrometry (HPLC/MS). Just as with estrogenic hormones, quantification of pharmaceuticals in wastewater effluents has been facilitated by the increased availability of tandem mass spectrometry. Compared with GC/MS/MS, the coupling of liquid
chromatography with tandem mass spectrometry (HPLC/MS/MS) provides an even more convenient means of quantifying polar compounds, such as pharmaceuticals, by eliminating laborious derivatization steps. HPLC/MS/MS also shows promise for the analysis of thermolabile and nonvolatile compounds that cannot be analyzed by GC. However, HPLC/MS/MS systems are not yet as popular in the environmental community as GC/MS/MS systems. As HPLC/MS/MS systems become less expensive and easier to operate, additional polar organic contaminants may be identified in recycled water. A final example of an effluent-derived contaminant is AT-nitrosodimethylamine (NDMA). NDMA is a potent carcinogen present in lubricants, pesti-
FIGURE 2
Using tandem mass spectrometry to identify the painkiller nabumetone To analyze trace concentrations of effluent-derived contaminants, samples often must be concentrated by a factor of 1000 or more. Analysis of a 200 ng/L wastewater effluent sample is greatly facilitated by tandem mass spectrometry. As a result of the many organic compounds in the extract, (a) the total ion chromatogram cannot be used to detect the contaminant. The conventional approach for detecting contaminants in such complicated matrices involves the use of (b) selected ion monitoring. However, when wastewater effluent samples are preconcentrated before analysis, peaks other than the analyte frequently are observed. The use of GC/MS/MS removes much of this interference, resulting in (c) a MS/MS chromatogram that can be used for quantification without artifacts attributable to coeluting compounds (lower chromatogram).
DECEMBER 1, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS • 5 1 1 A
FIGURE 3
Organic compound removal Predicted removal of organic compounds by adsorption onto organic phases of particles as a function of the octanol-water partition coefficient (/(^J of the compound. The three lines depict different conditions encountered in the environment soil aquifer treatment systems (100 g/L particulate organic matter); activated sludge municipal wastewater treatment systems (5 g/L particulate organic matter); and surface waters (0.01 g/L particulate organic matter).
cides, and liquid rocket fuels. It also has been detected at low concentrations in foods and beverages. Although NDMA is a priority pollutant under the Clean Water Act, a maximum contaminant level (MCL) has not been established under the Safe Drinking Water Act. On the basis of toxicological data and U.S. EPA's protocol for setting MCLs, the likely MCL for NDMA would be near 1 ng/L. NDMA has not been a significant concern as a drinking water pollutant because it is rarely detected in environmental samples analyzed by the applicable U.S. EPA standard method (method 625), which has a method detection limit of approximately 1000 ng/L. Recently, analytical chemists have developed alternative techniques for measuring NDMA that are significantly more sensitive than the U.S. EPA standard method. The new methods, which use chemical ionization GC/MS/MS, can detect NDMA at concentrations as low as 1 ng/L {12). Results of these analyses indicate that NDMA is present at low concentrations in numerous water sources, including recycled water. Of particular interest is the reported formation of as much as 1000 ng/L of NDMA when secondary wastewater effluent is chlorinated {13). Toxicological data suggest that exposure to low concentrations of NDMA represents a significant hu5 1 2 A • DECEMBER 1, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS
man health risk. As a result, NDMA is currently raising concerns about the safety of using chlorine disinfection when recycled water is used as a potable water source. Treating chemical contaminants Potential ecological and human health effects of these previously ignored contaminants may necessitate additional regulations and/or further treatment of secondary wastewater effluents. For example, if effluentderived microcontaminants cause fish downstream of an effluent discharge point to undergo feminization, it may be inappropriate to discharge secondary wastewater effluent to surface waters unless significant dilution takes place. Relatively high concentrations of NDMA in chlorinated secondary wastewater effluent could pose threats to water supplies if the wastewater effluent is used to recharge drinking water aquifers. Adverse effects of effluentderived chemical contaminants could be minimized by optimizing existing treatment systems; replacing chlorine disinfection systems with alternative disinfection processes, such as UV disinfection; or installing advanced wastewater treatment systems, such as microfiltration or reverse osmosis. Predicting which of these treatment processes will be most effective for a particular contaminant requires a fundamental understanding of contaminant removal mechanisms in these treatment systems. Removal of dissolved contaminants by sorption onto particulate organic matter is an important mechanism of removal for many nonpolar contaminants. Because most effluent-derived microcontaminants are relatively polar, they have a low affinity for particulate organic matter. A simple illustration of the relationship between polarity (hydrophobicity) and contaminant partitioning in different treatment systems is depicted in Figure 3. In this figure, the fraction of the contaminant xe-j moved by partitioning onto particulate organic matter is plotted as a function of the hydrophobicity of the contaminant, represented by the logarithm of the octanol-water partition coefficient (log Kow). As the hydrophobicity of the compound increases, the fraction associated with particulate organic mat-' ter increases. To account for the possible association of hydrophobic compounds with humic substances, we have included a fixed concentration of 10 mg/L of humic substances that exhibit* partitioning similar to that of particulate organic matter. Hydrophobic contaminants, such as polynuclear aromatic hydrocarbons and polychlorinated biphenyls, have log Klm values between 4.5 and 7. In contrast, most effluent-derived chemical contaminants have much lower log AT()W values (see Table 1). The hormones, which have log Kov/ values between 4.0 and 4.5, are a notable exception to
TABLE 1
Examples of effluent-derived microcontaminants Most effluent-derived chemical contaminants have much lower log Kow values than hydrophobic contaminants, such as polynuclear aromatic hydrocarbons and polychlorinated biphenyls, which have log Kow values between 4.5 and 7.
this generalization. Their behavior is similar to that of moderately hydrophobic contaminants such as trichlorobenzene. With respect to removal by adsorption onto particulate organic matter, it is instructive to consider losses of moderately hydrophobic contaminants, such as hormones, that could occur in different types of treatment systems. In an activated sludge wastewater treatment system (red line in Figure 3), sorption of organic compounds onto organic matter-rich sludge particles could result in contaminant removal during clarification. Assuming a particulate organic matter concentration of 5 g/L, we predict mat approximately 90% of the hormone will be adsorbed, which is consistent with available data from municipal wastewater treatment plants that indicate removals ranging from 70 to 95% for 17f5-estradiol {5-7). Because almost the entire mass of contaminant will be dissolved, hormones should be strongly retarded in soil aquifer
treatment systems, where a particulate organic matter concentration of 100 g/L is typical (blue line in Figure 3). In contrast, little removal should occur in effluent-dominated surface waters, where particulate organic matter concentrations are much lower (—0.01 g/L) and little of the overall mass is associated with particles (green line in Figure 3). Repeating this exercise for more polar contaminants (e.g., PhACs and NDMA) suggests that sorption will be a relatively unimportant removal mechanism in all systems other than groundwater recharge, in which a limited amount of retardation may occur. Other treatment processes that exploit phase partitioning to remove contaminants will probably not be very effective in the removal of effluentderived contaminants. Because tertiary treatment processes such as microfiltration and nanofiltration mainly remove colloids and particles, and little of the overall contaminant mass will be present in these phases, these processes will not be very efDECEMBER 1, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS " 5 1 3 A
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fective. Activated carbon might be effective at removing moderately hydrophobic compounds, such as hormones, but it will not remove polar contaminants like pharmaceuticals and NDMA. Transformation during disinfection or advanced oxidation also could be an important removal mechanism for effluent-derived microcontaminants. Because little information is available on chemical transformation pathways, it is difficult to quantify the importance of these reactions. In the absence of kinetic data, it is useful to consider the rates of reactions necessary for transformation under the conditions expected in treatment systems. For example, some effluent-derived contaminants could be transformed during chlorine disinfection. When free chlorine (hypochlorous acid/hypochlorite) is used to disinfect drinking water, a typical concentration of 8.5 x 10"5 M (3 mg/L) is maintained for 45 min. Under these conditions, a second-order rate constant of 3 M"1 • s"1 is required to transform half of the contaminant during disinfection. Because their rate constants for reaction with free chlorine species are typically greater than this value, compounds with phenol and amine functional groups should be transformed to an appreciable degree during drinking water chlorination. Additional research is needed to assess whether and to what degree the products of these reactions retain their biological activity. Although higher chlorine doses are used during chlorine disinfection of wastewater, the potential for contaminant transformation is lower because competitive reactions of hypochlorous acid with wastewater constituents such as ammonia result in the formation of less reactive species of reduced chlorine, including NH2C1. Alternative disinfection methods—using UV light or ozone—should minimize the formation of disinfection byproducts, such as NDMA, and could transform other effluent-derived microcontaminants. Improvements in UV disinfection have lowered the cost of these systems to the point that they are competitive with chlorine disinfection. Although the conditions in UV disinfection systems are insufficient to enhance the transformation of all but the most photoreactive species, it might be practical under some circumstances to extend contact times to transform contaminants. Ozonation, which is commonly used to disinfect surface water, groundwater, and other waters with relatively low ozone demand (such as recycled water after membrane treatment), could oxidize the effluent-derived contaminants with reactive functional groups (such as phenols and unsubstituted alkenes). The high ozone demand of organic matter usually precludes its use for disinfection of secondary wastewater effluent. Advanced oxidation processes would remove most effluent-derived microcontaminants, through oxi5 1 4 A • DECEMBER 1 , 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS
dation by hydroxyl radicals. However, the high reactivity of hydroxyl radicals with h u m i c substances and bicarbonate greatly increases the cost of treatment. Fate of microcontaminants After passing through conventional and advanced treatment systems, municipal wastewater is discharged to the aquatic environment. The rate of attenuation of contaminants in recycled waters determines the extent of the area affected by the effluent discharge. If the contaminants are quickly removed by natural processes, their effect on ecological systems or human health will be minimal. In many circumstances, it may be appropriate to exploit these natural removal processes by engineering the environment adjacent to wastewater treatment plants, such as by installing engineered wedands or soil aquifer treatment systems. Little is known about the fate of effluent-derived microcontaminants in the aquatic environment because the means of measuring many of these contaminants have been developed only recendy. However, data from wastewater treatment plants and studies of other organic contaminants in engineered ecosystems suggest that natural processes provide an effective means of removing many microcontaminants. Biotransformation is probably the most important removal mechanism in engineered ecosystems. As evidenced by their disappearance during secondary wastewater treatment and the subsequent appearance of known metabolites, some contaminants are readily transformed by bacteria. For example, despite their low affinity for surfaces, most analgesics (painkillers) are removed during conventional wastewater treatment (2). There is a strong possibility that such reactive compounds will be transformed further in engineered wetlands and soil aquifer treatment systems, where long residence times and high biological activity offer opportunities for biotransformation. Other effluent-derived microcontaminants appear to be resistant to biotransformation, despite the existence of bacteria capable of metabolizing the compounds in activated sludge. For example, the estrogenic hormone 17p-estradiol is quickly metabolized when it is added to activated sludge at concentrations above 1 ug/L (5), but data from wastewater treatment plants indicate that metabolism is less important at lower, ecologically relevant, concentrations. This discrepancy could indicate that extremely low concentrations of effluent-derived microcontaminants are insufficient to induce enzyme expression. Additional research is needed to assess the biotransformation of contaminants at the relatively low concentrations detected in recycled water.
Anticipating emerging contaminants The effluent-derived contaminants that are currently being discussed in the scientific community account for a small fraction of the organic compounds in recycled water. As analytical techniques and our understanding of aquatic and human toxicology improve, it is likely that other chemical contaminants and disinfection byproducts will be detected in recycled water. Because either unintentional or planned water recycling will be practiced in most communities, the risks and benefits associated with chemical contaminants in recycled water must be quantified. If additional treatment is required, research will be needed to develop cost-effective, reliable treatment processes. The current approach used to minimize the risks associated with chemical contaminants in recycled water involves control of surrogate parameters, such as total organic carbon. For example, California requires that recycled water used for groundwater recharge contain less than 1 mg/L of organic carbon of wastewater origin (i). Although this approach is expedient, it implicitly assumes that effluentderived contaminants are removed by natural attenuation at the same rate as other components of the dissolved organic carbon, such as humic substances. This simplification is inappropriate for effluent-derived contaminants that resist biotransformation. Furthermore, much of the organic carbon in wastewater effluent may be derived from the source water. The fate of effluent-derived organic carbon may be determined more by the reactivity of humic substances in the source water than by the effluent-derived microcontaminants. An alternative approach would be to use technologies, such as reverse osmosis, that are designed to remove nearly all chemical contaminants from water. Although this approach may be feasible in cost-insensitive markets, many communities will be unable to afford this level of treatment. Furthermore, disposal of reverse-osmosis brines presents an array of challenges, including the environmental impacts of salts and the presence of high concentrations of metals and organic contaminants in the brines. The uncertainties associated with recycled water also can be addressed through fundamental and applied research. Because we will never be able to quantify all of the organic compounds present in recycled water, future research should consider those compounds most likely to cause adverse effects in humans and aquatic ecosystems. As discussed in this article, research is needed to identify contaminants of concern and to evaluate and optimize treatment systems. Given the range of physical and chemical properties of these contaminants, a feasible treatment strategy might consist of multiple chemical bar-
riers to contaminants—such as membrane treatment or UV disinfection followed by soil aquifer treatment—in concert with a comprehensive monitoring program. Such monitoring programs should be accompanied by periodic reviews of contaminants potentially present in wastewater effluents. Acknowledgment The authors gratefully acknowledge support form the American Water Works Association Research Foundation, the National Science Foundation, and the National Water Research Institute. References (1) National Academy of Sciences. Issues in Potable Reuse: The Viability ofAugmenting Drinking Water Supplies With Reclaimed Water; National Academy Press: Washington, DC, 1998. (2) Ternes, T. A. Occurrence of drugs in German sewage treatment plants and rivers. Water Res. 1998, 32 (11), 32453260. (3) Buser, H. R.; Miiller, M. D.; Theobald, N. Occurrence of the pharmaceutical drug clofibric acid and the herbicide mecoprop in various Swiss lakes and in the North Sea. Environ. Sci. Technol. 1998, 32 (1), 188-192. (4) Stan, H. J.; Heberer, T.; Linkerhaegner, M. Occurrence of clofibric acid in the aquatic system. Is the use in human medical care the source of the contamination of surface, groundwater, and drinking water? (in German). Vom Wasser 1994, 83, 57-68. (5) Belfroid, A. C. et al. Analysis and occurrence of estrogenic hormones and their glucuronides in surface water and waste water in the Netherlands. Sci. Total Environ. 1999, 225 (1,2), 101-108. (6) Ternes T. A. et al. Behavior and occurrence of estrogens in municipal sewage treatment plants. I. Investigations in Germany, Canada, and Brazil. Sci. Total Environ. 1999, 225 (1,2), 81-90. (7) Huang, C. H.; Sedlak, D. L. Analysis of estrogenic hormones in municipal wastewater effluent and surface water using ELISA and GC/MS/MS. Environ. Toxicol. Chem. 2000, 20 (15), in press. (8) Purdom, C. E. et al. Estrogenic effects of effluents from sewage treatment works. Chem. Ecol. 1994, 8 (4), 275-285. (9) Desbrow, C. et al. Identification of estrogenic chemicals in STW effluent. 1. Chemical fractionation and in vitro biological screening. Environ. Sci. Technol. 1998, 32 (11), 1549-1558. (10) Daughton, C. G.; Ternes, T. A. Pharmaceuticals and personal care products in the environment: Agents of subtle change? Environ. Health Perspect. 1999,107 (S6), 907938. (11) Fong, P. P. Zebra mussel spawning is induced in low concentrations of putative serotonin reuptake inhibitors. Biol. Bull. 1998, 194 (2), 143-149. (12) Plomley, J. B.; Koester, C. J.; March, R. E. Determination of JV-nitrosodimethylamine in complex environmental matrixes by quadrupole ion storage tandem mass spectrometry enhanced by unidirectional ion ejection. Anal. Chem. 1994, 66 (24), 4437-4443. (13) Hultquist, R. H. California Department of Health Services, Sacramento, CA. Personal communication, 2000.
David L. Sedlak is an associate professor, James L. Gray is a doctoral candidate, and Karen E. Pinkston is a doctoral candidate, all of whom work in the Department of Civil and Environmental Engineering, University of California-Berkeley.
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