ReviewOrganic chemicals in sewage sludges
Introduction
Sewage sludges are residues generated at centralized wastewater treatment plants (WWTPs) as a result of the treatment of wastes released from a variety of sources including homes, industries, medical facilities, street runoff and businesses The use of these sludges as soil amendments is widely practiced in the U.S., where more than 60% of the 6.2 million dry metric tons (MT) of sludge produced annually are applied to land (U.S. Environmental Protection Agency, 1999). Since 1991 when ocean dumping was banned, both the quantity produced and the percentage land-applied have increased (U.S. Environmental Protection Agency, 1999).
Sewage sludges contain nutrients and organic matter that can provide soil benefits, but they also contain contaminants including metals, pathogens, and organic pollutants. The fate of chemical contaminants entering a WWTP depends on both the nature of the chemical and the treatment processes (Zitomer and Speece, 1993). Organic chemicals may be volatilized, degraded (through biotic and/or abiotic processes), sorbed to sludge, or discharged in the aqueous effluent. Degradation results in the creation of breakdown products that can be either more or less toxic than the original compound.
For many hydrophobic organic chemicals, sorption to the sewage sludge solids is the primary pathway for their removal from wastewater. This is especially true of persistent, bioaccumulative toxics that may enter the waste stream (Petrasek et al., 1983). Even volatile chemicals, such as benzene, are commonly found in sewage sludges as a result of sorption to organic substances in the sludge matrix (Wild et al., 1992). After they have been separated from wastewater, land-applied sludges must be treated to reduce pathogens through one of a number of processes including anaerobic digestion, lime stabilization, or composting. Each of these processes has effects on the fate of both pathogens and the organic contaminants in the sludge (Rogers, 1996).
The information available on the concentration of organic chemicals in sewage sludges arises largely from academic reports or from the national sewage sludge survey (NSSS) which was conducted by the U.S. Environmental Protection Agency (EPA) in 1988 (U.S. Environmental Protection Agency, 1990). The NSSS was performed by analyzing samples of the final sludge product collected from approximately 180 wastewater plants for the presence of 411 chemicals. This survey was used in the development of the U.S. regulations (U.S. Environmental Protection Agency, 1996).
Very few countries have rules limiting the concentration of any organic chemicals in sewage sludges (Beck et al., 1995). The European Union is considering establishing limits for a handful of organic chemicals. Under the Clean Water Act, (CFR Section 405 (d)), the rules regarding the concentration of pollutants permitted in land-applied sewage sludges in the U.S. are mandated to be protective of human health and the environment. A biennial review is called for to determine if there are additional chemicals that might pose a risk and should thus be subject to regulatory review.
To date, EPA has not established regulations for any organic chemicals and there is no federal requirement to monitor the type or concentration of organic chemicals in sludges. When promulgating the original rules in 1993 (CFR 40 Part 503), the EPA declined to include any organic contaminants. There were three criteria that led to the elimination of all of those considered: 1. the chemical was no longer in use in the U.S.; 2. the chemical was detected in 5% or fewer of the sludges tested in the NSSS; or 3. a hazard screening showed the chemical to have a hazard index of one or greater (Beck et al., 1995). Where sufficient data were lacking to evaluate the hazard, for example the lack of fate and transport data, that chemical and pathway were also eliminated from further consideration (U.S. Environmental Protection Agency, 1996).
Concerns with this process include the persistence of some chemicals in the environment despite their elimination in commerce, the high detection limits for some chemicals, and the potential risks posed by chemicals that were eliminated from consideration merely due to a lack of data (National Research Council, 2002). In a court-ordered review of additional contaminants, the EPA reconsidered regulation of some organic chemicals. In that review, it eliminated chemicals that were detected in 10% or fewer of the sludges in the NSSS. Of the 411 analytes in the NSSS 269 were not detected and 69 were detected in fewer than 10% of the sludges. Fifteen of the 73 remaining chemicals were eliminated due to lack of toxicity data (U.S. Environmental Protection Agency, 1996). Hazard screening analysis was conducted on the remaining chemicals. Dioxins, furans and co-planar PCBs were the only organic chemicals that remained and a risk assessment was then conducted (U.S. Environmental Protection Agency, 2002). Based on the assessment, EPA decided not to extend regulation to dioxins or any other organic pollutant (U.S. Environmental Protection Agency, 2003a). The Round 2 review conducted by the EPA in 2003 was not limited to the chemicals analyzed in the NSSS. It considered 803 chemicals and resulted in the selection of 15 chemicals as candidates for regulation based on available human health or ecological risk end points but not on concentration data from sludges. Among those were 9 organic chemicals (U.S. Environmental Protection Agency, 2003b).
The National Research Council of the U.S. Academy of Sciences (NRC) conducted two reviews of the land application of biosolids (National Research Council, 1996, National Research Council, 2002). Their 2002 report included a comparison of the limits of detection for samples analyzed in the NSSS to EPA soil screening limits (SSLs) and pointed out that high limits of detection for many chemicals in the NSSS were a concern. The SSLs are conservative risk-based soil concentrations of selected industrial pollutants (93 organic and 16 inorganic compounds) that are used in determining whether a site specific risk assessment is required at a Superfund site (U.S. Environmental Protection Agency Superfund, 1996).
The SSLs were used by the NRC as an indicator of concentrations that might pose a risk requiring remediation. For 5 of 8 organic chemicals examined in the NRC report, most sludge samples analyzed in the NSSS had limits of detection that were higher than the EPA-established SSLs. Thus the NSSS results were not sensitive enough to detect pollutant concentrations that, if present in soil at a Superfund site, would have triggered a risk assessment. For example, in the case of hexachlorobenzene (HCB), the NSSS did not detect HCB in any of the 176 samples tested, thus prompting EPA to exclude it from regulatory consideration. The NSSS limits of detection exceeded 5 mg/kg for the majority of samples and was greater than 100 mg/kg for 4 samples (National Research Council, 2002). Depending on the pathway of exposure being considered, the SSLs for HCB range from 0.1 to 2 mg/kg. Only one of the NSSS samples reached a limit of detection of 0.1 mg/kg. Analysis of the data compiled in this paper revealed that 9 of the 13 reports of HCB concentrations in sewage sludges exceeded 0.1 mg/kg and 3 exceeded 2 mg/kg. Thus the majority of samples exceeded an SSL for HCB.
In addition to concerns regarding analytical limitations, the introduction of new chemicals into commerce, suggests that there is a need for a new survey in order to better characterize sludges with respect to the presence and concentration of contemporary organic chemicals. Flame retardants, surfactants, chlorinated paraffins, nitro and polycyclic musks, pharmaceuticals, odorants, as well as chemicals used in treating sludges (such as dewatering agents) are among the chemical categories suggested by the NRC as compounds requiring additional data collection and consideration in future risk assessments (National Research Council, 2002).
Although the EPA conducted a limited survey of sludges in 2001 to determine the concentration of dioxins, furans and co-planar PCBs, and plans to conduct a survey of sludges to test for the 9 organic chemicals being considered for regulation, it is not proposing a broader survey of organic chemicals in sludges (U.S. Environmental Protection Agency, 2003b).
Section snippets
Methods
To help fill the gaps in knowledge regarding the presence and concentration of organic chemicals in sewage sludges, we examined the peer-reviewed literature and official governmental reports to compile available data on the concentration of organic chemicals reported in sludges. In some cases sources did not contain sufficient information to permit comparison of chemical concentrations as a function of sludge dry weight and were therefore not included. One hundred and thirteen usable data sets
Results and discussion
Tens of thousands of organic chemicals are currently in use, however sludge concentration data could only be found for 516 organic chemicals in the peer reviewed literature and official government reports (supporting information 1). Table 2 shows the number of compounds in each of the 15 classes for which concentration data were found, and the number of studies from which these data were obtained.
Ninety of the 111 organic priority pollutants and 101 of the 143 target compounds were reported in
Conclusion
More data are needed on the chemicals that are in sludges today and on the temporal trends for those chemicals. Relying on existing lists of chemicals such as priority pollutants will not identify many chemicals of current concern.
To make more informed assessments about the impact of sludge processing on chemical concentrations, more information on the type of treatment (both of the wastewater and the sludge) and the sludge residence time as well as the nature of significant non-domestic inputs
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