Sewage Sludge, Pros & Cons
Rebecca Renner / Environmental Science & Technology V.34 - I.19 1oct00
The United States and the European nations are far apart on their views of what constitutes safe management.
Sewage 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 legal battle is now raging in California over sludge. Such conflicts reflect the continuing and growing need to dispose of sludge by applying it to land in the face of persistent concerns about the health and environmental consequences of the practice. Now, starting from very different 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 point to cleaner water, as well as higher crop yields for farms that use the material.
Sewage sludge is prepared for use in a long-term (1942-present) sludge experiment at Woburn Market Garden, Bedfordshire, England. Steven McGrath, Harpenden, England
They also pragmatically note that the millions of tons of sewage sludge generated each year (Table 1 (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 long-term 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 damage 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.
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 agency has bullied and marginalized groups that have criticized the rule, including organic farmers, environmentalists, and scientists. The agency stands behind the rule, according to Hais, but it wants to review the science, which is more than 10 years old. Standards for heavy metals should come under intense scrutiny in this review.
Source: U.S. EPA and Reference (2)
Large amounts of sewage sludge are generated annually in the United States and western Europe.
(million tons dry solids/yr)
|Disposal method (%)|
|United Kingdom, 1991||1107||55||8||7||30|
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 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 make some decisions. “We know more than enough to say with confidence that high-quality sludge can be used practically forever on farmland without any adverse effects,” says Rufus Chaney, a 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. approach.
There is no general agreement concerning the maximum allowable concentrations of various metals in sewage sludge.
|European Communitya (3)||1986||1–3||50–140||100–150a||30–75||50–300||150–300||1–1.5|
|The Netherlandsc (4):|
|Clean soil reference values||0.8||36||100||35||85||140||0.3|
|United Kingdomd (5)||1989||3||135||400a||75||300||200 e||1|
|United Statesf (7)||1993||20||750||1500||210||150||1400||8|
currently being revised.
b Values are for soil pHs > 6. At pH 5-6, the Cd and Zn limits are 1.0 and 150 mg/kg, respectively.
c Soil cleanup levels which also apply to agricultural land amended with sewage sludge. Concentrations less than the clean soil reference are considered clean soil.
d Values shown are for soil pHs 6-7. Other values apply at pH 5-6 and >7 (U.K. DoE, 1989).
e Changed following Independent Scientific Committee recommendations (see text).
f Calculated from maximum cumulative pollutant loading limits mixed into soil plow layer. Soil background concentrations are not taken into account.
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 long-term 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, the Netherlands 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 were considered. 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.
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 approach into action when experimental evidence suggested that soil limits for several metals might not be adequately protective of soil organisms, soil processes, and fertility. Evidence began to emerge about 10 years ago that sludge-borne metals could have adverse effects on total soil microbial biomass and on nitrogen fixation by cyanobacteria and by the nitrogen-fixing bacteria Rhizobium. The evidence, which was not conclusive at that time, came from long-term experiments at sites where sludge was repeatedly applied in large quantities (11-15).
This evidence prompted the formation of a U.K. independent scientific review committee in 1993 that, despite what was then the inconclusive nature of the evidence, opted for caution and recommended that the standard for zinc be reduced in keeping with preliminary experimental results (16). Caution was appropriate for sewage sludge standards, “particularly because heavy metals, unlike 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 the U.K. government. The comparable U.S. standard for zinc in soils is 1400 mg/kg.
Since 1996, further work has demonstrated that two species of Rhizobium, one symbiotic with clover and another with peas and beans, are adversely affected by high zinc concentrations in soil, although soybean symbionts are not, according to McGrath, whose latest results are to be published in the journal Plant and Soil Science later this year. “Not only does Rhizobium have a major impact on agriculture, but it is also a sentinel species, demonstrating that the heavy metals in sludge are potentially damaging to the soil ecology,” says McGrath.
In addition to lowering the allowed zinc concentration in soils treated with sludge from 300 mg/kg dry wt to 200 mg/kg dry wt, the U.K. review committee recommended a long-term research program to resolve uncertainties about soil ecological effects. This standard, although much lower than U.S. limits, is still higher than the numerical limits set by northern European countries (100-150 mg/kg) for zinc.
In the late 1980s, U.S. researchers also applied sludge to fields of clover and other legumes. These experiments showed no adverse effects on nitrogen fixation for plants other than clover (17-19). But metal concentrations in the U.S. experiments were lower than the European experiments (20). The U.S. experiments also used a different species of Rhizobium, which may be more tolerant to metals. Faced with these conflicting results, the U.S. EPA did not opt for caution. Instead, the agency did not accept experiments that showed an adverse effect on Rhizobium (19). EPA's approach was endorsed by a National Academy of Sciences (NAS) 1996 review of the use of sludge in food crop production. The NAS review considered the European evidence that heavy metals in sludge adversely affect Rhizobium but decided that it was inconclusive (21).
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 every metal, 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 away.”
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 extensive risk assessment, they acknowledge. But data gaps and nonprotective policy choices, they contend, “result in regulations that are not adequately protective of human health and the environment.” Among the deficiencies in the risk assessment, the Cornell group cites insufficient data to specify transfer along pathways; no account for site-specific or region-specific factors such as soil depth, acidity, and agricultural practices; and hypothetical exposure scenarios that underestimate consumption of vegetables. “Risk assessments can hide a multitude of assumptions and bad data, and that's what they did,” according to Ellen Harrison, director of Cornell's Waste Management Institute.
But focusing on these aspects of the risk assessment misses the point, according to Chaney. For the metals of concern, the most stringent loading rates were derived from pathways that involved a child directly ingesting sludge or from pathways involving effects on crops. For the crop effects, the standards are based on sensitive crops like lettuce and spinach. The standards assumed that soil acidity could drop as low as pH 5.5, a level at which naturally occurring aluminum begins to cause plant toxicity.
According to Chaney, the risk assessment is also supported by the overwhelming weight of evidence culled from more than 20 years of research. From these data emerge two important phenomena that limit the effects of heavy metals in soil: soil and sludge immobilize metals, and metals are attenuated as they move up the food chain.
Research has shown that an absorption matrix made up of complex hydrous iron- and manganese-oxides 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 the simultaneous presence of zinc in the crops, which inhibited cadmium adsorption in the 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 the 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 they 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 Atlantic, 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.
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. J. Eur. Communities 1986 No. L181, annex 1A, 6-12.
Crommentuijn, G. H. Guidance on the Derivation of Ecotoxicological 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.
van den Berg, R.; Denneman, C. A. J.; Roels, J. M. Risk Assessment of Contaminated Soil. In Proposals 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.
Report of the Independent Scientific Committee PB 1561.Review of the rules for sewage sludge application to agricultural land: Soil fertility aspects of potentially toxic elements; United Kingdom Ministry of Agriculture, Fisheries and Forests and Department of the Environment, MAFF Publications: London, 1993.
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.
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.
Rebecca Renner is a contributing editor of ES&T.
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