SEWAGE
PROS & REBECCA RENNER
ewage sludge is piling up in Sweden. Since October 1999, farmers have refused to put the black goo on their land because they are concerned about the health consequences, even though Sweden has some of the strictest sludge standards in the world. Last October, farmers in California's agricultural heartland also took action over their worries about sludge. But when rural Kern County adopted a plan to restrict the flow of sludge from Los Angeles, the city joined with other Southern California sewage authorities and sued. A fierce le23.1 battle is now raging in California over sludge. Such conflicts reflect the COT! — tinuing and stowing need to dispose of sludge bv applying it to land in the face of persistent concerns about the health and environmental consequences of the practice Now starting from very dif-
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ferent perspectives, U.S. and European scientists and regulators are beginning to examine the science behind their respective regulations. Sewage sludge, also known as biosolids, is what is left behind after water is cleaned in waste treatment works. It is high in organic content and plant nutrients and, in theory, makes good fertilizer. However, most developed countries regulate its use because it also can contain a multitude of metals, organic pollutants, and pathogens. The application of sewage sludge to land, especially on agricultural lands, has been contentious since the late 1980s, when national and international clean water regulations prohibiting the ocean dumping of sludge were first enacted. Advocates enthuse about the natural ability of sludge, like soil, to immobilize potentially toxic metals, and they © 2000 American Chemical Society
SLUDGE The United States and the European nations are far apart on their views of what constitutes safe management.
CONS point to cJeaner water, as well as higher erop yields for farms that use the material. They also pragmatically note that the millions of tons of sewage sludge generated each year {Table 1 (/, 2)) must go somewhere. If not applied to land, most sludge would have to be burned in incinerators or landfilled. U.S. total annual production of sludge is stable or only growing slowly; however, in Western Europe, where tougher clean water laws are beginning to take effect, sludge production is growing significantly, as small communities build and improve waste treatment plants to comply. Although opponents of sludge use have many grievances, one of their main concerns is the longterm buildup of heavy metals in the soil. Over time, they argue, metals such as cadmium, zinc, and copper could build up to levels high enough to dam-
age agricultural soils. Some opponents advocate a full-scale ban on the use of sludge as fertilizer. But for others, who acknowledge its benefits, the question is: At what levels do heavy metals cause harmful effects? Regulatory developments are afoot that will once again call scrutiny to this issue. The European Union (EU) is beginning work on a new sludge directive that will lower permissible limits for heavy metals (3). Another EU directive, which sets absolute values for contaminants in food, could also drive down permitted levels of metals in sludges. Draft standards for some metals taken up by plants, in particular, cadmium in wheat, are set so low that to meet them, sludge cadmium levels would have to be significantly lower than current EU requirements. OCTOBER 1, 2000/ ENVIRONMENTAL SCIENCE AND TECHNOLOGY / NEWS • 4 3 1 A
U.S. regulations for metals in sewage sludge are also slated for scrutiny. EPA is planning to commission a review of the science behind the regulations, according to Alan Hais, associate director of EPA's Health and Ecological Criteria division in Washington, DC {Environ. Sci. Technol. 2000 34 (11), 242A). This action comes in the wake of congressional hearings into allegations that the has bullied and marginalized groups that have criticized the rule including organic farmers environmentalists and scientists The
The United States has the most relaxed standards for metals among developed nations.
a o p n n f s t a n d s l*iAViir»H tY*t*
rule according to Hais but it wants to review the science which is more than 10 years old Standards for heaw metals should come under intense scrutiny in this review United States stands alone When it comes to spreading sludge on agricultural land, the United States has the most relaxed standards for metals among developed nations. Standards for heavy metals are up to 100 times higher than any other country has ever proposed (Table 2, {1,3-7)). To make this comparison, U.S. standards, expressed as cumulative loadings, or total permissible additions of sludge on a metric tons per hectare basis, have to be compared with TABLE 1
Sewage sludge generation rates Large amounts of sewage sludge are generated annually in the United States and western Europe.
Amount Country Austria Belgium Denmark France Germany (West) Greece Ireland Italy Luxembourg Holland Portugal Spain Sweden Switzerland United Kingdom, 1991 United States
Disposal method (%)
(million tons Application Land dry solids/yr) to land filling Incineration Other 320 75 130 700 2500 15 24 800 15 282 200 280 180 215 1107 6900
13 31 37 50 25 3 28 34 81 44 80 10 45 50 55 41
56 56 33 50 63 97 18 55 18 53 13 50 55 30 8 17
31 9 28 0 12 0 0 11 0 3 0 10 0 20 7 22
0 4 2 0 0 0 54 0 1 0 7 30 0 0 30 20
Source: U.S. EPA and Reference (2)-
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European standards, expressed in terms of concentration in treated soil. Although these comparisons require an assumption about how far down into the soil the sludge is mixed, the differences between the standards are so large as to overwhelm any uncertainty in the conversion. Everyone agrees that sludge contains toxic metals, although at what level and when such metals might cause harmful effects are largely unknown. In most cases, the metals are not a problem now, but they could be an issue sometime in the future, between 50 and 500 years hence. Many European scientists favor the low estimate, whereas many U.S. scientists favor the high one. Depending on who is right, farmers could be risking potentially dreadful consequences because once damaged, soil could be almost impossible to fix. Faced with these questions, EPA scientists decided that they already knew enough to mslce some decisions. "We know more than enough to say with confidence that high-quality sludge can be used practiccillv forever on farmland without ciny cidverse effects " Scivs Rufus U S Department of Agriculture soil scientist in Beltsville MD who is one of EPA's principal science advisors and a vigorous champion of the U S aDoroach A question of philosophy Differences in philosophy about environmental protection and in the choices of which organisms to protect explain, in part, the range of metal standards for sewage sludge, according to Steven McGrath. McGrath, a soil scientist who has studied the longterm effects of sludge application to agricultural land for more than 20 years, is based at the Institute of Arable Crops Research in Harpenden, England. Faced with the difficult question of determining what levels of metals are harmful, one approach is to sidestep the issue by minimizing any accumulation. That is what some of the northern European countries have done. They attempt to match the metal inputs from sludge to soil with the small annual losses of metals due to crop removal, soil erosion, and leaching, so that metal concentrations do not exceed background levels. These countries, which include Sweden, Norway, and Denmark, have standards that are stricter than those in the current EU directive. The EU sludge directive, currently under development, will also follow this precautionary approach, according to Luca Marmo, who is leading the revision of the sewage sludge directive at the European Commission Environmental Directorate in Brussels, Belgium. Heavy metal standards will be crafted to minimize accumulation, he says. Another way to select what amount of metals to allow is to perform a risk assessment to determine what heavy metal contamination levels represent an acceptable risk. However, knowledge of the toxicity and environmental interactions of sludge-borne pollutants is incomplete, as is an understanding of their behavior and fate after application. Moreover, the behavior of people and organisms varies greatly. As a result, risk assessments can produce very different conclusions.
The United States and the Netherlands, for example, both used this approach, and ended up with very different standards (Table 2). Both countries conducted comprehensive risk assessments that delineated the pathways of pollutant transfer to selected target organisms and assessed the likelihood of harmful effects that metals may have on specific targets [7-10). But they picked very different targets. The Netherlands placed a high priority on ecological effects, so that protecting soil organisms and microbially mediated soil processes drove the standards down to lower levels [7-9). Because few data are available to describe how soil organisms are affected by metals in sludge-amended soils, die Nemerlands risk assessment incorporated many uncertainty factors. The United States handled the lack of such data by giving soil ecological effects little consideration only copper toxicity effects on earthworms sidered The U S risk assessment estimated the maximum accumulation of each metal that would not cause harmful effects on humans and animals but not soil organisms
like many other pollutants, cannot degrade [and] are retained in soils virtually indefinitely", according to the committee's report. "As a result, there is little opportunity for natural recovery from the consequences of any error in judgment." Applying this philosophy to experimental evidence on the effects of heavy metals on soil microorganisms led the committee to recommend a reduction in the U.K. standard for zinc in sludged soils from 300 mg/kg—the EU standard—to 200 mg/kg, a recommendation accepted by die U.K. government. The comparable U.S. standard for zinc in soils is 1400 mg/kg. Since 1996, further work has demonstrated "The [U.S.] that two species of Rhizobium, one symbiotic with standards ignore clover and another with peas and beans, are adcontradicting versely affected by high scientific zinc concentrations in soil, although soybean symbievidence." onts are not, according to McGrath, whose latest results are to be published in the journal Plant and Murray McBride Soil Science later this year. Cornell University "Not only does Rhizobium have a major imagriculture but it is also a sentinel species demonstrating that the
Adverse effects
Another strategy is to pay particular attention to actual cases of adverse effects on soil organisms, plants, or animals as a result of land application of sewage sludge. That is the approach to heavy metals adopted by the United Kingdom, according to McGrath. The aim of this tactic is to find the concentrations of metals in soil originating from sludge applications that result in adverse environmental effects and then set concentration limits below these, allowing for a safety margin. The United Kingdom put this TABLE 2 approach into action when experimental evidence suggested that soil Heavy metal contaminant standards limits for several metals might not There is no general agreement concerning the maximum allowable concentrations of be adequately protective of soil orvarious metals in sewage sludge. ganisms, soil processes, and fertility. Evidence began to emerge Country Year Cd Cu Cr Ni Pb Zn Hg about 10 years ago that sludgeborne metals could have adverse European 1986 1-3 50-140 100-150 30-75 50-300 150-300 1-1.5 effects on total soil microbial bioC o m m u n i t y (3) mass and on nitrogen fixation by France (J) 1988 2 100 150 50 100 300 1 cyanobacteria and by the nitrogenG e r m a n y " (1) 1992 1.5 60 100 50 100 200 1 fixing bacteria Rhizobium. The evItaly (1) 3 100 150 50 100 300 idence which was not conclusive Spain (J) 1990 1 50 100 30 50 150 1 The Netherlands at that time came from long-term (4): Clean soil experiments at sites where sludge reference values 0.8 36 100 35 85 140 0.3 was repeatedly applied in large Intervention values 12 190 380 210 530 720 10 quantities [11-15) United K i n g d o m (5) 1989 3 135 400 75 300 200 1 This evidence prompted the Denmark)?) 1990 0.5 40 30 15 40 100 0.5 formation of a U.K. independent Finland {1) 1995 0.5 100 200 60 60 150 0.2 scientific review committee in Norway (6) 1 50 100 30 50 150 1 1993 that, despite what was tiien Sweden (1) 0.5 40 30 15 40 100 0.5 United States'(7) 1993 20 750 1500 210 150 1400 8 the inconclusive nature of the evidence, opted for caution and rec"Values are currently being revised. "Values are for soil pHs > 6. At pH 5-6, the Cd and Zn limits are 1.0 and 150 mg/kg, respectively. ommended that the standard for cSoil cleanup levels which also apply to agricultural land amended with sewage sludge. Concentrations less than zinc be reduced in keeping with the clean soil reference are considered clean soil. preliminary experimental results 'Values shown are for soil pHs 6-7. Other values apply at pH 5-6 and >7 (U.K. DoE, 1989)) "Changed following Independent Scientific Committee recommendations (see text). [16). Caution was appropriate for 'Calculated from maximum cumulative pollutant loading limits mixed into soil plow layer. Soil background sewage sludge standards, "particconcentrations are not taken into account. ularly because heavy metals, uns
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Contradictory evidence ignored To a vocal and persistent minority of U.S. scientists, actions such as the failure to incorporate the European Rhizobium research on soil ecological effects into the U.S. assessment typify the U.S. approach to regulating sewage sludge application to land. "The standards ignore contradicting scientific evidence." according to Murray McBride, a soil scientist at Cornell University in Ithaca, NY. McBride has been studying decade-old sludge sites to try to determine whether heavy metals from sludge remain in place or diminish with time. Metals are to some extent, immobilized by soil and sludge, but the effect is not complete, nor does it apply to evmetal he says "My work indicates that the availability of cadmium remains relatively high so for this metal a serious problem could emerge decades after the sludge was applied and this problem would not go awav" In 1999, McBride and colleagues at Cornell published a detailed critique of the U.S. risk assessment [22). The U.S. standards were developed through an extenSewage sludge is prepared for use in a long-term (1942sive risk assessment, they acknowledge. But data gaps present) sludge experiment at Woburn Market Garden, Bedand nonprotective policy choices, they contend, "refordshire, England. sult in regulations that are not adequately protective of human health and the environment." Among heavy metals in sludge are potentially damaging to the the deficiencies in the risk assessment, the Cornell group cites insufficient data to specify transfer along soil ecology," says McGrath. In addition to lowering die allowed zinc concen- pathways; no account for site-specific or regiontration in soils treated with sludgefrom300 mg/kg dry specific factors such as soil depth, acidity, and wt to 200 mg/kg dry wt, the U.K. review committee rec- ricultural practices; and hypothetical exposure sceommended a long-term research program to resolve narios that underestimate consumption of vegetables. uncertainties about soil ecological effects. This stan- "Risk assessments can hide a multitude of assumpdard, although much lower dian U.S. limits, is still tions and bad data and that's what they did" achigher than the numerical limits set by northern Eu- cording to Ellen Harrison director of Cornell's Waste Management Institute ropean countries (100-150 mg/kg) for zinc. In the late 1980s, U.S. researchers also applied But focusing on these aspects of the risk assesssludge to fields of clover and other legumes. These ment misses the point, according to Chaney. For the experiments showed no adverse effects on nitrogen metals of concern, the most stringent loading rates fixation for plants other than clover [1 7-19). But metal were derived from pathways that involved a child diconcentrations in the U.S. experiments were lower rectly ingesting sludge or from pathways involving than the European experiments [20). The U.S. ex- effects on crops. For the crop effects, the standards periments also used a different species of Rhizo- are based on sensitive crops like lettuce and spinbium, which may be more tolerant to metals. Faced ach. The standards assumed that soil acidity could with these conflicting results, the U.S. EPA did not drop as low as pH 5.5, a level at which naturally ocopt for caution. Instead, the agency did not accept curring aluminum begins to cause plant toxicity. experiments that showed an adverse effect on RhiAccording to Chaney, the risk assessment is also zobium {19). EPA's approach was endorsed by 3. Na- supported by the overwhelming weight of evidence tional Academy of Sciences (NAS) 1996 review of the culled from more than 20 years of research. From use of sludge in food crop production The NAS re- tiiese data emerge two important phenomena that view considered the European evidence that heavy limit the effects of heavy metals in soil: soil and sludge metals in sludge adversely affect Rhizobium but de- immobilize metals, and metals are attenuated as they cided that it was inconclusive [21) move up the food chain. 4 3 4 A • OCTOBER 1, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS
Research has shown that an absorption matrix made up of complex hydrous iron- and manganeseoxides coated with phosphates immobilizes contaminants so that they are unavailable to plants, as long as pH remains constant (23). This process works so well that results look promising for the use of sewage sludge to remediate soils containing high levels of lead, Chaney says. Attenuation in the food chain has been demonstrated by numerous feeding studies, Chaney says. In one of these studies, he evaluated two crops that readily accumulate cadmium, Swiss chard and lettuce, in sludged soils. The crops accumulated cadmium, but guinea pigs and mice that ate the vegetables did not show an increase in cadmium in kidney or liver tissues. The researchers attributed this to die simultaneous presence of zinc in the crops, which inhibited cadmium adsorption in die animals. "Standard toxicological tests on cadmium give misleading results," says Chaney, "because the tests alter the ratio of cadmium to elements such as zinc, calcium, and iron which inhibit cadmium absorption in real life." These aspects of the risk assessment, according to die 1996 NAS review, mean that "when sludges are applied to land, according to the EPA rule, there is a built-in safety factor that protects against human exposure to chemical contaminants via human food chain pathways." Despite the controversy, many U.S. soil scientists endorse the U.S. risk assessment. "You may be able to find fault with specific aspects of the risk assessment," comments Albert Page, a soil scientist at the University of California-Riverside. "But there is little in the way of evidence to dispute the reasonableness and objectivity of the approach taken by EPA," says Page, who chaired the 1996 NAS committee on sewage sludge application to agricultural land. He is currently trying to organize an international meeting to discuss differences in sludge metals regulations. So confident are some U.S. scientific supporters inside and outside EPA that diey predict the scientific review will offer no changes. Instead, they see the review as a communications exercise—an opportunity to reassure critics that U.S. standards are sufficiently protective. Meanwhile, across the Adantic, the current effort to tighten EU metals standards is being prompted, in part, by a realization that other new rules going into effect will lead to a significant increase in sludge production. Northern European countries with tighter standards for metals have seen industry make great strides in pollution prevention to meet the tough regulations, and the EU hopes to follow the same path, according to Marmo. Of course, because it will take many decades for metals concentrations in soils to approach regulatory limits, any problems, if they arise, will occur in the future. These long time frames should give some comfort to those who support the U.S. approach, say some soil scientists. "Some profess a case for caution, arguing if we're wrong, the regulations could be loosened. But this goes against the way the
world works," says George O'Connor, a soil scientist at the University of Florida-Gainesville. "It isn't very often that we've seen a major environmental regulation relaxed. Once a number is set, it stays around for a very long time. If the U.S. approach is wrong, there's always time to tighten up the standard," he says.
References (1) McGrath, S. E; et al. Environ. Rev. .994, 2 (4) 108-118. (2) Biosolids Generation, Use and Disposal in the United States; U.S. Enviionmental Protection Agency, EPA530R-99-009, Office of Solid Waste and Emergency Response: Washington, DC, September 1999. (3) Commission of the European Communities. Council directive on the protection of the environment and in particular of the soil, when sewage sludge is used in agriculture; 86/278/EEC; Off. .. Eur. Communities 1981 No. L181, annex 1A, 6-12. (4) Swartjes, F. A. Risk Anal. .199, ,1 (6), 1235-1249. (5) Code ofPractice for the Agricultural Use of Sewage Sludge; U.K. Department of the Environment, HMSO: London, 1989. (6) Forskrift om avepsslam; Miljovemdepartementete Oslo, Norway, 1997. (7) Standards for the use or disposal of sewage sludge. Fed. Regist. .193, 55, 9248-9415. (8) Intervention values snd target values: Soil quality standards; Netherlands Ministry of Housing, Spatial Planning and Environment, Department of Soil Protection: The Hague, Netherlands, 1994. (9) Crommentuijn, G. H. Guidance on the Derivation ofEcotoxicological Criteria for Serious Soil Contamination in View of the Intervention Value for Soil Clean-up; Report No. 955001 003; National Institute of Public Health and the Environment: The Hague, Netherlands, 1994. (10) van den Berg, R.; Denneman, C. A. J.; Roels, J. M. Risk Assessment of Contaminated Soil. In Proposall for Adjusted, Toxicologically Based Dutch Soil Clean-up Criteria in Contaminated Soil; F. Arendt et al., Eds.; Kluwer Academic Publishers: Norwell, MA, 1993; pp. 349-364. (11) Brookes, P. C; McGrath, S. P. /. Soiilci. 1984, 35, 341346. (12) Chaudri, A. M.; et al. Soil Biol. Biochem. 1993, 25, 301309. (13) Giller, K. E.; McGrath, S. P.; Hirsch, E R. Soil Biol. Biochem. 1989, 21, 841-848. (14) Kinkle, B. K.; Angle, JJ ..; Keysyr, H. H. Appl. Environ. Microbioll.987, 53, 315-319. (15) Martensson, A. M.; Wiiter, E. Soil Biol. Biochem. 1990,22, 982-997. (16) Report of the Independent Scientific Committee PB 1561. Review of the rules for rewage sludge application to agricultural land: Soil fertility aspects of potentially toxic elements; United Kingdom Ministry of Agrrcullure, Fisheries and Forests and Department of the Environment, MAFF Publications: London, 1993. (17) Heckman, J. R.; Angle, ,I S.; Chaney, R. . . Biol. Fert. Soils 1986,2, 181-185. (18) Heckman, J. R.; Angle, ,J ..; Chaney, R. L./. Environ. Qual. 1987a, 16, 113-117. (19) Heckman, J. R.; Angle, ,J ..; Chaney, R. L.J. Environ. Qual. 1987b, 16, 118-124. (20) A Guide to the Biosolids Risk Assessments for the EPA Part 503 Rule; EPA/8332//B-93-005; U.S. Environmental Protection Agency, Office of Wastewater Management, U.S. Government Printing Office: Washington, DC, 1995. (21) Committee on the Use of Treated Municipal Wastewater Effluent and Sludge in the Production of Crops for Human Consumption. Use of Reclaimed Water and Sludge in Food Crop Production; Water Science and Technology Board, National Academy of Sciences Press: Washington, DC, 1996. (22) Harrison, E. Z.; McBride, M. B.; Bouldin, D. R. .nt. J. Environ. Poll. 1999, 11 (1), 1-36. (23) Chaney, R. L.; Ryan, I. A. Water Enviion. .echnol. 1992, 4, 36-41. Rebecca Renner is a contributing editor of ES&T. OCTOBER 1, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS • 4 3 5 A
