Correspondence/Rebuttal pubs.acs.org/est
Response to Comment on “Determining the Ecological Impacts of Organic Contaminants in Biosolids Using a High-Throughput Colorimetric Denitrification Assay: A Case Study with Antimicrobial Agents”
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presents challenges. For instance, spiking dissolved triclosan directly into the soil does not take into account differences in bioavailability for the compound as compared to when it has been aged through the wastewater/digestion treatment process, which can lead to greater sorption of the organic contaminant in the solids. In a critical review, Alexander10 showed that bioavailability, and therefore the toxicity of a chemical is greatly impacted by the time a chemical was partitioned to solids. Thus, the bioavailability of a chemical in wastewater treatment plant biosolids, which can take months to produce, would be greatly different than the bioavailability of dissolved chemical spiked into biosolids and mixed with soil within a few minutes to hours.7,8 Finally, the other compounds present in the biosolids to which the chemical of interest is spiked would again make determining the individual impact of a chemical on microbial processes difficult. As an example of the complex interactions between multiple compounds, in previous research we have shown that individual compounds can have antagonistic effects when present in mixtures.11 One potential method to isolate individual impacts while maintaining environmental complexity would be to simulate a wastewater treatment and digestion process to generate biosolids with and without the specific compound(s) of interest, essentially by preparing a “biosolid standard material”. These biosolids could then be used in a simulated land application process in parallel and their relative impacts ascertained. However, this process would still require an extensive amount of time as well as materials and may not be warranted, especially if initial tests such as the simple assay described in our paper showed that the chemicals of interest did not significantly affect specific ecological end points. In conclusion, the high-throughput denitrification assay described in our paper can provide an initial screening and indication of potential ecotoxicity of contaminants found in biosolids. Compounds found to impact denitrification with a LOAEC below or near concentrations measured in biosolids could then be flagged for follow-up analysis, with more intensive methods such as the approach described above. Beyond this study, however, it is critical that other cost-effective and high-throughput methods be developed to screen for potential ecological impacts of emerging contaminants in biosolids so that those data can be better incorporated into life cycle analyses. In addition, this highlights the need for the development of biosolids standard materials. These lines of research will ensure the safe use of an economically sustainable resource.
e appreciate Professor Ormeci for writing a comment on our recently published paper. The main criticism raised in her comment was the lack of incorporating biosolids in our assay. Potential improvements to the assay, including this specific criticism, are discussed in our original paper1 and we elaborate on the rationale for our assay below. The primary objective of our study was to develop a simple high-throughput method, which relied on a functional end point ecologically relevant to land application and could be used to provide an initial screening of emerging organic contaminants found in biosolids. To date, more than 500 different organic compounds have been measured in biosolids, including many pharmaceuticals and personal care products for which limited ecological data are available.2,3 Because of the sheer number of compounds found in biosolids, it is challenging to determine the impacts of individual compounds in an actual land application treatment scenario where chemical and microbial complexity is tremendous and highly variable between sites. While using simplified assays may not provide an exact representation of environmental conditions, a wider range of environmentally relevant variables (i.e., concentrations, mixtures, etc.) can be tested. Furthermore, these measurements provide valuable preliminary data and suggest logical research next steps when a positive signal is obtained. Ideally, an assay, which utilizes a range of standard biosolids materials and soils should be developed as this approach would most accurately represent environmental complexity. However, biosolids and soil characteristics differ by location and, thus, the addition of other variables (e.g., organic carbon content, cation exchange capacity, etc.) would also need to be tested. The general practice for measuring impacts on microbial communities has consisted of two approaches. The first method (and that suggested by Ormeci) consists of considering all contaminants in a biosolid amendment in aggregate. Microbial community as well as their corresponding functional activity changes can then be correlated to contaminant concentration profiles. This approach is less than ideal as a full chemical profile for all contaminants in that sample would need to be obtained or impacts may be attributed to specific contaminants incorrectly. Finally, the lack of change in a microbial community profile may be misinterpreted as an absence of environmental impact when in fact the microbial community may have a priori developed resistance to the contaminant of interest. Using a simple assay with a nonresistant strain (such as that described in our paper) would flag this issue and help in proper data interpretation. The second approach consists of either spiking the contaminant directly into soil or by spiking the chemical into biosolids and then mixing with soil.4−9 In both cases, microbial impacts can then be measured subsequently. This method also © 2014 American Chemical Society
R. M. Holzem† H. M. Stapleton‡
Published: October 1, 2014 12470
dx.doi.org/10.1021/es5036305 | Environ. Sci. Technol. 2014, 48, 12470−12471
Environmental Science & Technology
Correspondence/Rebuttal
C. K. Gunsch*,† †
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Department of Civil and Environmental Engineering, Duke University, Durham, North Carolina 27708, United States ‡ Nicholas School of the Environment, Duke University, Durham, North Carolina 27708, United States
AUTHOR INFORMATION
Corresponding Author
*Phone: (919) 660-5208. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Holzem, R. M.; Stapleton, H. M.; Gunsch, C. K. Determining the ecological impacts of organic contaminants in biosolids using a highthroughput colorimetric denitrification assay: A case study with antimicrobial agents. Environ. Sci. Technol. 2014, 48, 1646−1655. (2) Reiss, R.; et al. An ecological risk assessment for triclosan in lotic systems following discharge from wastewater treatment plants in the United States. Environ. Toxicol. Chem. 2002, 21 (11), 2483−2492. (3) Harrison, E. Z.; et al. Organic chemicals in sewage sludges. Sci. Total Environ. 2006, 367 (2−3), 481−497. (4) Waller, N. J.; Kookana, R. S. Effect of triclosan on microbial activity in australian soils. Environ. Toxicol. Chem. 2009, 28 (1), 65−70. (5) Liu, F.; et al. Terrestrial ecotoxicological effects of the antimicrobial agent triclosan. Ecotoxicol. Environ. Saf. 2009, 72 (1), 86−92. (6) Butler, E.; et al. Effects of triclosan on soil microbial respiration. Environ. Toxicol. Chem. 2011, 30 (2), 360−366. (7) Pannu, M. W.; O’Connor, G. A.; Toor, G. S. Toxicity and bioaccumulation of biosolids-borne triclosan in terrestrial organisms. Environ. Toxicol. Chem. 2012, 31 (3), 646−653. (8) Park, I.; et al. Effects of Triclosan and Biosolids on Microbial Community Composition in an Agricultural Soil. Water Environ. Res. 2013, 85 (12), 2237−2242. (9) Harrow, D. I., Felker, J.M.; , Baker, K.H. Impacts of triclosan in greywater on soil microorganisms. Appl. Environ. Soil Sci., 2011. 2011. (10) Alexander, M. Aging, Bioavailability, and Overestimation of Risk from Environmental Pollutants. Environ. Sci. Technol. 2000, 34 (20), 4259−4265. (11) Wang, S.; Gunsch, C. K. Effects of selected pharmaceutically active compounds on the ammonia oxidizing bacterium Nitrosomonas europaea. Chemosphere 2011, 82 (4), 565−572.
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