All Great Lakes states, Ontario, numerous tribal nations and non-Great Lakes states such as California and Alabama, have adopted low-level mercury water quality standards to prevent the bioaccumulation of mercury in fish tissue. Hundreds of municipal wastewater treatment plants (MWWTPs) in these jurisdictions have received low-level mercury effluent limits (&lt; 2 ng/L) because they discharge mercury in exceedance of the standard. Despite the widespread need for low-level mercury treatment design information, little research or practical guidance addresses low-level mercury treatment strategies for municipal wastewater.  The lack of guidance is partly due to a limited understanding of how Hg moves and changes binding phases during municipal wastewater treatment.  We collected samples at 15 MWWTPs located in the Great Lakes watershed that have incorporated a variety of tertiary treatment technologies to remove mercury to permitted levels.  The filter-passing (0.45 um) mercury in MWWTP effluent was consistently between 0.4 and 0.8 ng/L, while particulate mercury varied widely and was covariate with TSS in most cases.  Though filter-passing mercury comprises a substantial fraction of the discharge limit, MWWTPs with effective particulate control (&lt; 2 mg/L) consistently met discharge limits of 1.8 ng/L.  In MWW effluent, the sulfur content of filter-passing dissolved organic and inorganic sulfide exceeded the sulfur content of effluent particulates.  Spectrophotometric characterization and elemental composition of dissolved organic matter suggests that the net effect of wastewater treatment may impose a lower limit on the quantity of filter-passing mercury.  Our analysis of dissolved organics, sulfur, and mercury in municipal wastewater effluent across a variety of conditions points to a persistent, but not prohibitive quantity of filter-passing mercury that makes effective particulate removal essential to meeting Great Lakes mercury discharge limits.

Municipal wastewater plants meet low-level mercury limits by controlling effluent suspended solids

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ACS Fall 2022

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