Evaluation of the impact of lime softening waste disposal in natural environments
Introduction
Water treatment residues (WTR) are solid wastes from the lime softening of drinking water and often contain trace elements as a result of the constituents removed and the treatment chemicals added (e.g., polymers, alum, ferric coagulants). Water treatment residues can be recycled through land application and offer a number of potential benefits, including a reduction in the nutrient loading of surface water bodies (Ippolito et al., 2011, Oliver et al., 2011). A recent study examined the characteristics of lime softening WTR in Florida, and found that for the most part, trace element concentrations did not vary dramatically among lime WTR when Al and Fe salts were used as a supplement to conventional lime softening (Cheng et al., 2014). This suggests that existing policies related to the beneficial use of lime WTR would apply equally to lime WTR that use small quantities of additional treatment chemicals (alum and ferric salts) to help improve other water quality parameters (such as the removal of the organic chemicals responsible for color) (Cheng et al., 2014). These experiments did find, however, that certain elements (Al, Fe, Mn) were released to a greater extent under chemically-induced reducing conditions (in comparison to the same test where a reducing agent was not utilized).
While the results of the previous work provided guidance to the regulatory community and the utilities producing the WTR, they were conducted using standard practices for assessing the beneficial use of waste materials (e.g., batch leaching tests), and thus trace element behavior under several potential reuse applications remains uncertain. For example, some generators have proposed to use lime-softening WTR as clean fill below the water table, an application that has been demonstrated to have beneficial properties in controlling nutrient loading (Oliver et al., 2011, Spears et al., 2013). Numerous conditions that would be expected to occur under such a management scenario may not be adequately reflected using standardized batch tests, including the development of anaerobic conditions, lower liquid to solid ratios (L/S), and the presence of naturally-occurring organic matter (NOM).
The presence of organic matter in natural water bodies has been demonstrated to affect trace element release from soils (Bauer and Blodau, 2006, Wang and Mulligan, 2006). Leaching of a number of waste materials has been shown to be highly dependent on concentrations of organic matter, either contained as a fraction of the waste itself, or mobilized by landfill leachate or other means (Bauer and Blodau, 2006, Ghosh et al., 2004, Van Zomeren and Comans, 2004, Wang and Mulligan, 2006). In many areas of south Florida, the surficial aquifer is known to contain high concentrations of NOM, if WTR are to be disposed of as a subsurface fill, it is likely that the water in contact with the material would be elevated in NOM content (with respect to the typical extraction fluids used in leaching tests). Therefore, leaching tests designed to evaluate disposal in these types of conditions need to be modified to account for these factors.
The objective of this study is to further examine the leaching of trace elements from lime softening WTR by utilizing experimental procedures that better assess element behavior in the natural environments of question. Several of the procedures employed are from a new suite of leaching protocols developed for the United States Environmental Protection Agency (US-EPA), and patterned after European leaching tests (Garrabrants and Thorneloe-Howard, 2010, Kosson et al., 2002, van der Sloot et al., 1996). As these protocols do not address some conditions that may be encountered in the natural environment [e.g., anaerobic conditions, the presence of NOM], the authors examine possible modifications of standardized protocols to address use-specific conditions. Although this paper provides value to the specific waste reuse question at hand, perhaps of even more importance is the evaluation of how existing leaching protocols can best be applied to the beneficial use, risk assessment process. This paper highlights how the broader suite of recently adopted leaching tests can be used to generate a more thorough assessment of leaching risk, but also identifies limitations within these tests. The use of modified leaching procedures illustrates to the reader additional opportunities for refinement of leaching tests to address site-specific conditions and allow for a better interpretation of results.
Section snippets
Facility description and sample collection
Water treatment residues were collected from five potable water treatment facilities in Florida, US; samples were collected from locations representing the most recently produced WTR available for sampling at each facility. Composite samples were generated by sampling from conveyors or belt presses for a 20-min period, or taking 6–10 subsamples from sludge piles or drying lagoons. After collection, samples were stored for 48 h after which the free water was removed, and the samples were further
Leaching as a function of pH
pH has been shown to be a major factor influencing the leaching of waste materials and is an important component for consideration when evaluating contaminant release in the natural environment (Cappuyns and Swennen, 2005, Cappuyns and Swennen, 2008, Kosson et al., 2014). The results of method 1313 are presented first in order to demonstrate the impact of pH on trace element release from WTR, and so that the data may be used to explain the results present in subsequent sections. For all of the
Conclusion
Utilizing a broader set of leaching tests on lime WTR demonstrated the importance of understanding the intended reuse or disposal scenario and tailoring the testing approach to the intended application. The suite of new leaching test methods developed by the EPA allowed for a more detailed characterization of the WTRs, and demonstrated that certain parameters such as L/S and equilibrium pH played a dramatic role in controlling element leaching. Better understanding these parameters allowed for
Acknowledgements
This work was funded by the Hinkley Center for Solid and Hazardous Waste Management, and the Florida Department of Environmental Protection. Thanks are offered to Dr. Jay Lessel and Dr. Lena Ma from the University of Florida Soil and Water Science Department and to the water treatment facility operators participating in this study. Additional gratitude is owed to Dr. Richard Tedder and Mr. John Coates from the Florida Department of Environmental Protection. The investigator would also like to
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