Empirical Models for Predicting the Occurrence and Concentration of

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Chapter 19

Empirical Models for Predicting the Occurrence and Concentration of Organic Chemicals in Biosolids Randhir P. Deo and Rolf U. Halden* Center for Environmental Biotechnology, The Biodesign Institute at Arizona State University, Tempe, AZ 85287 *Corresponding author: School of Sustainable Engineering and the Built Environment, Arizona State University, 1001 South McAllister Avenue, P.O. Box 875701, Tempe, AZ 85287-5701. E-mail: [email protected].

Tens of thousands of organic chemicals are used by consumers on a routine basis for multiple applications. Despite an increasing number of publications in this area, not enough is known about their fates during wastewater treatment and upon release into the environment. For example, hydrophobic organic compounds have been shown to persist in digested municipal sludge (biosolids), an abundant by-product of wastewater treatment. The sequestration of persistent contaminants in biosolids is of concern due to the widespread practice of land application of these materials for inexpensive disposal or for use as a soil conditioner and fertilizer. Land application of biosolids represents a potential pathway for the contamination of agricultural soils, uptake into food crops, bioaccumulation in terrestrial ecosystems and human exposure. Several thousand high production volume (HPV) chemicals are produced or imported in the U.S. each year at rates exceeding 450 000 kg (1 million pounds) per chemical. Since monitoring at wastewater treatment plants of such a large number of compounds is impractical and cost-prohibitive, modeling approaches have been proposed to estimate the likely behavior of compounds during wastewater treatment. In this literature review, we present an overview of recent empirical modeling approaches for estimating both the occurrence and concentration of © 2010 American Chemical Society In Contaminants of Emerging Concern in the Environment: Ecological and Human Health Considerations; Halden, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

hydrophobic organic compounds (HOCs) in biosolids destined for application on land.


Introduction A large number of organic chemicals are used by consumers on a routine basis for multiple applications and, following disposal into domestic sewage, are conveyed to wastewater treatment plants (WWTPs) and may enter the environment contained in treated effluent. An alternative pathway for persistent wastewater contaminants to reach the environment is their accumulation in digested sewage sludge (biosolids), and subsequent application on land of this abundant by-product of conventional wastewater treatment. Hydrophobic organic chemicals (HOCs) are particularly prone to become sequestered and persist in biosolids during municipal wastewater treatment (1–3). Sequestration of persistent contaminants in digested sewage sludge is problematic because approximately 49% of the 6.9 million metric tons of sewage sludge produced annually in the U.S. is applied on land for inexpensive disposal and as a fertilizer and soil conditioner (4). A growing body of literature indicates the occurrence of toxic or potentially harmful organic compounds in biosolids, and their subsequent transfer to agricultural soils, tile drainage and terrestrial biota (5–12). Thus, application of biosolids on land represents a potential pathway for the contamination of agricultural soils, uptake into food crops, bioaccumulation in terrestrial ecosystems, contamination of aquatic systems through surface runoff, and harmful exposure of animals and humans to biosolids-borne compounds (5, 13). From a risk management perspective, of particular concern are high production volume (HPV) chemicals that are produced in quantities exceeding 450,000 kg (1 million pounds) per year (14). Their massive use and release can lead to adverse effects even if these compounds have only moderate or low toxicity. Monitoring at wastewater treatment plants of the thousands of HPV chemicals currently in use may be highly desirable but ultimately is impractical (15). Instead, modeling has been suggested as an alternative approach that can be informative and economically attractive for predicting the fate of problematic organic compounds during wastewater treatment and their accumulation in biosolids. This literature review presents an overview of contemporary empirical approaches for predicting the occurrence and concentration of organic chemicals in biosolids destined for land application.

Models for Predicting the Occurrence of Organic Chemicals in Biosolids Modeling has been suggested as an economical and technically sound approach for assessing the fate of hydrophobic organic compounds (HOCs) in biosolids (2). Available models rely on one or more of the following physicochemical properties of HOCs: volatility, organic carbon partition 386 In Contaminants of Emerging Concern in the Environment: Ecological and Human Health Considerations; Halden, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


coefficient (KOC), hydrolysis, etc. (1). Other models consider parameters specific to individual WWTP (2, 15–18), including specific rate of biodegradation (19). In situations where such detailed information on a specific WWTP is not available, universally applicable empirical models can provide value. This was demonstrated in a meta-analysis of mass balances conducted at WWTPs around the world (15). The study produced a simple empirical model relying on readily available information as input, for predicting the fraction of the total mass loading of a given HOC expected to persist in biosolids after treatment (15):

where fbiosolid is the mass fraction of HOC expected to persist in biosolids, KOW is the HOC’s n-octanol-water partition coefficient, and pfit is a dimensionless fitting parameter. The pfit value was obtained by fitting equation 1 to the actual mass fractions of organic compounds accumulated in biosolids against the respective compound’s log KOW, which gives a value of 6.51× 10-6. Where pH is important, the individual compounds’ log DOW value is used. For example, at pH 7.5 and using log DOW values, the new pfit value is 1.76×10-6 (14). Since the pfit value is an empirical parameter determined from observations at full-scale sewage treatment plants, it reflects the combined effects of all relevant removal processes including biodegradation. Use of such models is most useful for predicting the fate during wastewater treatment of compounds for which no measured information is available. For example, the empirical relationship shown in Eq. 1 was recently applied to predict the mass fractions of 207 selected HPV chemicals destined to accumulate in biosolids (20). Modeling results suggested that two thirds of the HPV chemicals examined in the study were destined to persist in biosolids with mass fractions of at least 50% relative to their initial mass loading to the WWTPs. The corresponding log KOW threshold value was ~5.2. For chemicals exceeding this critical threshold value, more than half of their mass loading to the plant was predicted to persist in biosolids (20). The model in Eq. 1 can be easily adapted to account for the effect of pH on hydrophobicity. This is done by replacing the parameter for hydrophobicity, namely KOW, with the pH-dependent n-octanol water partition coefficient, i.e., DOW. The resulant pfit value for use of Eq. 1 with DOW substituted for KOW was estimated to equal 1.76 × 10-6 at pH 7.5 (14), which is the typical pH of domestic raw wastewater in the USA (21). The model was further developed to enable the prediction of concentration of organic chemicals destined to accumulate in biosolids (14). The model’s sole input requirements included the fitting parameter (pfit), and a given organic chemical’s concentration in raw wastewater as well as its DOW value. The model then takes the form as shown below:

387 In Contaminants of Emerging Concern in the Environment: Ecological and Human Health Considerations; Halden, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


where Cbiosolids represents an organic chemical’s concentration in biosolids, CT(influent) its total concentration in influent wastewater, and Y the characteristic average yield of biosolids per volume of raw wastewater treated, i.e., 1.296×10-4 kg L-1 (15). In instances where only the dissolved concentration (but not the particle-associated, sorbed mass of an organic chemical in raw wastewater is available, the desired total concentration can be estimated using the following relationship (14):

where CT(influent) and Caq are the organic chemical’s total and dissolved concentrations, respectively, in influent wastewater, CSS is the concentration of suspended solids in influent wastewater, fOC is the fraction of organic carbon in suspended solids, and DOC is the organic carbon normalized sorption coefficient.

Table 1. Compound names, Chemical Abstract Service (CAS) registry number, log DOW values, total influent concentrations and concentrations predicted for biosolids on a dry weight basisa Compound

Benzophenone

Isobutylparaben

Menthol

Oxybenzone

Propylparaben

Simazine Traseolide a

CAS #

Log DOW (at pH=7.5)

Ref.

Total Influent (ng L-1)

Predicted Conc. in Biosolids (µg kg-1)

119-61-9

3.18

25

72

2

2,686

55

93

2

438

8

3,628

78

17,010

365

367

10

2,812

76

810

7

2,131

19

4247-02-3

89-78-1

131-57-7

94-13-3

122-34-9 68857-95-4

3.15

3.20

3.30

2.81

2.28 6.14

24

24

24

24

24 24

5

0.2) is deemed unacceptable (34). In Figure 1, this threshold of 20% sorption corresponds to a CSS value of at least 363 mg/L. Thus samples containing suspended solids in exess of this value should be subjected to chemical analysis of both the dissolved and sorbed mass for all target organic chemicals featuring log DOC values of ≥3.2. The significance of accurate quantification of organic chemicals in influent wastewater becomes apparent when considering the various uses of monitoring data. Besides estimating the fate of organic chemicals during wastewater treatment (more specifically, the mass fractions and concentrations in biosolids), influent wastewater concentrations also are used for tracking rates of consumption and discharge loads from point sources, for estimating mass loadings to WWTPs, for calculating organic chemicals’ removal efficiencies in WWTP, and for estimating ecological and/or human health risks of the amount of organic chemicals expected to persist after treatment (27). For example, a 2010 study (27) estimated the impact of using dissolved concentrations instead of total concentrations of organic compounds when calculating removal efficiencies in WWTPs. The authors used the following relationship for estimating the absolute removal efficiency:

where Mdiss and Mpart are the masses of organic chemical’s (diss – dissolved phase, part – particulate (sorbed) phase) in influent (inf) and effluent (effl). However, when considering only masses present in the dissolved phase, the removal efficiency equation is simplified to:

After adjusting measured dissolved concentrations to total concentrations, and using these two equations, the calculated removal efficiencies for select organic chemicals differed by as much as 20% for the very same chemical and plant examined (27). The authors concluded that the removal efficiency of a WWTP is not a fixed number but can take different values as a function of the method of mass accounting. From their secondary data analyses, the authors pointed to potential implications in data interpretation and formulated the following recommendations to ensure proper use of environmental monitoring data (27, 28): 391 In Contaminants of Emerging Concern in the Environment: Ecological and Human Health Considerations; Halden, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


(a) To provide an accurate estimate of the total mass of a given HOC arriving at a WWTPs in raw sewage, mass loadings of sorptive substances should be determined in unfiltered samples or by processing the sample via liquid/liquid extraction, to capture the sum of both the dissolved and sorbed mass present. (b) To enhance transparency in data presentation, measured concentrations or masses of compounds determined in WWTP influent should be reported as either dissolved or total levels, consistent with the nature of sample preparation. For example, legends of Figures should state “concentrations and mass flows of target analytes determined via analysis of filtered [or unfiltered] samples.” (c) To eliminate uncertainty and inadvertent misinterpretation of monitoring data, removal efficiencies at WWTPs should be reported either as absolute or as aqueous removal efficiencies, consistent with the nature of sample preparation and analysis strategy (for example, filtration of samples or liquid–liquid extraction of unfiltered samples). Unambiguous reporting of environmental monitoring data is key because once the data are entered into databases, the critical link between sample processing information and reported value typically is broken (27). Failure to follow these guidelines for reporting potentially can result in unscientific use in the regulatory context of scientifically sound data that were obtained with considerable investment of time and financial resources.

Conclusions A review of empirical models predicting the fate of HOCs during wastewater treatment shows the utility and limitations of these approaches. Empirical models can serve to narrow down the list of potentially problematic organic chemicals earmarked for monitoring due to their presumed tendency to accumulate and persist in biosolids. Although being valuable for in silico screening, model prediction should be followed up with actual measurements performed on biosolids from WWTPs of interest. While model predictions can be an important tool in determining the fate of organic chemicals during wastewater treatment, the proceeses in WWTPs are complex, and thus plant-specific parameters should be used whenever available. Further identified was the need for improvements in the reporting of environmental monitoring data to eliminate uncertainty and preempt misinterpretation of collected data. This applies particularly to the reporting of concentrations of organic chemicals present in the dissolved phase when indeed a significant fraction of the compound is expected to be sorbed to particulate matter. For compounds exhibiting significant sorption potential (i.e., log DOC of ≥3.2), chemical analyses should be performed on the dissolved and sorbed phases of influent wastewater, such that total concentrations can be reported that reflect the entire mass of the analyte of interest. Secondary data analyses have suggested that the removal efficiencies of organic compounds in WWTPs may vary by as 392 In Contaminants of Emerging Concern in the Environment: Ecological and Human Health Considerations; Halden, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


much as 20% when considering dissolved concentrations only instead of the total chemical mass contained in a sample volume in both the dissolved and sorbed phase. The implications of this review is that modeling can be a powerful tool in prescreening problematic organic chemicals before their actual measurement in biosolids. However, the quality of modeling results is a function of several factors, including the quality of the data used as model input. Formulae provided here also can be applied in secondary data analyses to estimate the total concentration of sorptive compounds that were measured and reported for the dissolved phase only.

Acknowledgments The project described was supported in part by Award Number R01ES015445 from the National Institute of Environmental Health Sciences (NIEHS) and by the Johns Hopkins University Center for a Livable Future. The content is solely the responsibility of the author(s) and does not necessarily represent the official views of the NIEHS or the National Institutes of Health.

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Empirical Models for Predicting the Occurrence and Concentration of

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