Using national sewage sludge data for chemical ranking and prioritization
Introduction
Municipal sewage sludge (SS) is an abundant byproduct of wastewater treatment. The SS matrix is rich in organic carbon and nutrients, and hence >50% of the dry mass of municipal SS produced in the United States currently is applied on land for inexpensive disposal and as soil amendment (fertilizer) [1, 2, 3]. Research over several decades has shown that SS, unfortunately, also is rich in pollutants such as toxic metals, pathogens, and organic contaminants [2,4, 5, 6, 7]. Hence, land application of SS has been, and is, considered a controversial practice in the United States and worldwide [8, ∗9, 10, 11, 12]. Looking at the presence of chemical contaminants in SS from a different perspective, one could also say that SS is efficiently capturing and removing toxic, persistent, and highly bioaccumulative chemicals present in reclaimed water during wastewater treatment, which otherwise would be discharged into the environment along with the treated effluent. The presence of elevated levels of captured organic contaminants in SS, in essence, is evidence for the effectiveness of wastewater treatment to remove pollutants from sewage and to prevent the release of sewage-borne contaminants into the aquatic environment.
Chemicals accumulating in SS are the same chemicals used in everyday products for personal care (shampoos, detergents, etc.), health care (prescription drugs), and other household purposes (flame retardation, food preservation, etc.) [2,13]. Although there are many variations of wastewater treatment plants (WWTPs), a conventional treatment plant uses at a minimum the following steps: pretreatment (to remove large objects), primary treatment (sedimentation tanks), secondary treatment (e. g., activated sludge system with secondary sedimentation step), and handling of the solids generated (e. g., anaerobic digestion of SS). The secondary treatment of municipal sewage consists of a biological treatment system that uses a highly complex microbial community optimized to remove most of the organics present in the wastewater. Chemicals that withstand the secondary biological treatment process have to be considered notably resistant to biodegradation and thus have the potential to also persist in the environment upon their release in reclaimed water or SS. Secondary treatment also may be viewed as a large-scale biodegradability test for chemicals. Thus, if a chemical is not significantly attenuated during treatment and still present in the two end products of wastewater treatment, namely treated effluent (reclaimed water) and SS (biosolids), then it must be considered persistent and potentially contaminating for the environment. This concept was introduced in the past, where researchers recognized that WWTPs are observatories to study the environmental fate of chemicals used in commerce [13]. The relative abundance of chemicals in these two end products of WWTP depends on the chemicals' partitioning behavior, i.e., persistent hydrophilic chemicals will be abundant in treated wastewater, whereas persistent hydrophobic chemicals will accumulate in biosolids (Figure 1). Additionally, the levels at which they occur in these two treatment process flows is related to the chemical's production volume in commerce and to the fraction that is disposed of into wastewater [13]. Wastewater-based epidemiology (WBE) enables an estimation of the consumption volume of chemicals from their levels detected in raw wastewater entering the treatment facility [14, 15, 16, 17, ∗18]. Thus, analogous to WBE, data on chemical concentration in SS (e.g., national SS surveys) allow one to identify persistent and potentially bioaccumulative compounds that may represent contaminants of emerging concern (CECs) in the communities served by the respective WWTP.
Section snippets
Chemical prioritization in SS
National SS surveys are a valuable data source for advancing the understanding of what is present in the treated SS and for evaluating the risks posed by sludge-borne, toxic pollutants. In the United States, the US Environmental Protection Agency (US EPA) thus far has conducted four national SS surveys to identify inorganic and organic contaminants of potential regulatory concern [19,20]. The samples from the 2001 and 2007 (the most recent) surveys, now acquired and archived by the National
Decision flowchart for chemical ranking and prioritization using SS data
From the models discussed in the previous section, some common characteristics of chemicals were used for ranking and prioritization: persistence (P), bioaccumulation (B), toxicity (T), and/or long-range transportation potential [26]. These characteristics, especially P and B, are commonly used in risk assessment framework by the US EPA [27] and by other countries [28, 29, 30]. As shown in multiple studies [13,21,23], SS is a useful matrix for identifying chemicals of the P and B category and
Conclusions and future directions
It is estimated that about 2500 new chemicals are introduced each year in the United States, equivalent to a rate of seven new chemicals per day [38]. Identifying problematic chemicals is a time-consuming process and thus developing interventions through policies and laws may take several decades to address risks to humans and the environment. The SS scoring approach proposed here, that makes use of available national SS monitoring data, promises to provide an inventory of priority CECs for
Conflict of interest statement
Nothing declared.
Acknowledgments
This study was supported in part by Award Number R01ES020889 from the National Institute of Environmental Health Sciences (NIEHS) and by award LTR 05/01/12 of the Virginia G. Piper Charitable Trust. The content is solely the responsibility of the authors and does not necessarily represent the official views of the sponsors.
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