Mercury removal in municipal wastewater: The role of TSS and DOM in meeting Great Lakes discharge limits

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Mercury removal in municipal wastewater: The role of TSS and DOM in meeting Great Lakes discharge limits

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2021-06

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As efforts to reduce and prevent mercury bioaccumulation in natural environments advance, municipal wastewater facilities are increasingly faced with stringent mercury discharge limits. Limits range from 5 to 10 ng/L in many states, but in the Minnesota Great Lakes watershed, the discharge limit is 1.3 ng/L total mercury. To comply with these low limits, many facilities are evaluating the performance of or implementing new tertiary technologies, such as membrane filters, tertiary clarifiers, and dual media filters; however, little standardized guidance is available to decision makers for low-level mercury removal. Understanding the mechanisms that control both particulate and dissolved mercury movement through a wastewater treatment system may translate to more effective and efficient mercury removal, thus allowing for more informed comparisons of tertiary technologies. We sampled 12 wastewater facilities encompassing a variety of primary, secondary, and tertiary treatment approaches in the Lake Superior watershed, as well as isolated organic matter for elemental analysis on select facilities, in order to investigate how mercury interacts and binds with dissolved and particulate phases under a variety of wastewater conditions and evaluate the implications for mercury removal to low levels. The ratio of mercury concentration per mg particulate (CP) to the mercury concentration in the dissolved phase (HgD), referred to as the effective partitioning coefficient log KD, was used to compare solid-phase and dissolved-phase Hg activity and understand the mechanism of mercury transport within the WWTPs. Most facilities experienced a net increase in Hg binding to the particulate phase compared to the dissolved phase, despite DOC concentrations exceeding TSS concentrations and greater amount of sulfur containing ligands in the dissolved phase. This indicates that the processes defining wastewater’s capacity to carry mercury on particles changes between influent and treated conditions. Similar results were seen when KD was normalized for sulfur and carbon quantity, which suggests that Hg partitioning is not solely dependent on carbon or sulfur quantities. Continued efforts are needed to further understand how Hg partitions in wastewater and why log KD values increase through the treatment process. Our results show that on average, total mercury (HgT) is removed by 90%. The contribution of the total HgT concentration by tertiary technologies varied between technology type, but the dual media filters typically removed less than 6% of influent concentrations. Despite this seemingly low percent removal contribution, this final polishing often allowed facilities to meet discharge limits. The only facilities that were able to meet 1.3 ng/L HgT effluent limits with secondary treatment alone were the pond systems. Both particulate mercury (HgP) and dissolved mercy (HgD) contributed to facilities exceeding discharge limits. In the facilities visited, while TSS was able to be reduced by > 95 % and in some cases to less than 1 mg/L in effluent wastewater, effluent DOC was not able to be removed below 6 mg/L - 10 mg/L. The persistence of HgD quantity due to the presence of DOC, even at low levels (~ 0.5 ng/L HgD) is still a considerable fraction of a 1.3 ng/L HgT limit; meaning that complete, or nearly complete, particulate removal would be required to consistently meet a 1.3 ng/L HgT effluent limit.

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University of Minnesota M.S. thesis. June 2021. Major: Water Resources Science. Advisor: Nathan Johnson. 1 computer file (PDF); iii, 54 pages.

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Spilkia, Geordee. (2021). Mercury removal in municipal wastewater: The role of TSS and DOM in meeting Great Lakes discharge limits. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/224473.

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