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. 2025 Aug;97(8):e70158.
doi: 10.1002/wer.70158.

Water Resource Recovery Facilities Meet Low-Level Mercury Limits by Controlling Effluent Suspended Solids

Geordee Spilkia  1 , Scott Kyser  2 , Adrian T Hanson  1   3 , Kelsey Hogan  3 , Nathan W Johnson  1   3
Affiliations

Water Resource Recovery Facilities Meet Low-Level Mercury Limits by Controlling Effluent Suspended Solids

Geordee Spilkia et al. Water Environ Res. 2025 Aug.

Abstract

Hundreds of water resource recovery facilities (WRRFs) in North America have received low-level mercury effluent limits (< 2 ng/L). Although mercury binding to dissolved organic matter (DOM) and particulate matter in natural environments is well understood, guidance about low-level mercury removal at WRRFs is lacking. We collected samples of filter-passing and particulate mercury at 16 WRRFs with a variety of secondary and tertiary particle-control technologies. Particulate mercury in WRRF effluent was covariate with total suspended solids (TSS) at concentrations ranging from < 0.2 to 15 ng/L. Filter-passing (< 0.45 μm) mercury in WRRF effluent was mostly bound to DOM and was typically between 0.3 and 0.8 ng/L. Thermodynamic modeling and sulfur quantities in wastewater TSS and DOM point to a consistent quantity of filter-passing Hg that cannot be removed by typical wastewater technologies and necessitates effective particulate removal to meet low-level mercury limits.

Keywords: TSS removal; dissolved organic matter; low‐level limits; mercury; metals; municipal wastewater; particulate matter control; tertiary filtration.

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Conflict of interest statement

The statements, findings, conclusions, and recommendations are those of the authors and do not necessarily reflect the views of the Minnesota Pollution Control Agency.

The authors declare no conflicts of interest.

Figures

FIGURE 1
FIGURE 1
Total mercury (left column) and TSS (right column) in data from the current study (filled symbols) and historic database (open symbols) for WRRFs included in this study. Mercury and TSS are not available in the historic database for secondary effluent. HgT discharge limits for non–Great Lakes (10 ng/L, dashed) and Great Lakes (1.8 ng/L, solid) watersheds in the state of Minnesota are shown as horizontal lines. Error bars represent ± 1 SD of measurements in each category.
FIGURE 2
FIGURE 2
(a) Dissolved organic carbon and TSS in WRRFs included in this study for influent (squares), secondary effluent (triangles), and tertiary effluent (circles) wastewater samples. (b) Dissolved (< 0.45 μm) and particulate (> 0.45 μm) sulfur concentrations, (c) particulate HgP (> 0.45 μm) versus TSS, and (d) dissolved HgD (< 0.45 μm) versus DOC in wastewater from WRRFs. Samples from plants with DOM‐rich industrial influence are indicated with gray dashes. Pond effluent is indicated with gray triangles. Regional HgT discharge limits for non–Great Lakes (10 ng/L) and Great Lakes (1.8 ng/L) watersheds are shown as horizontal lines.
FIGURE 3
FIGURE 3
CHg,TSS versus TSS in WRRF samples. Samples from facilities with DOM‐rich industrial influence are indicated with gray dashes. Pond effluent samples are indicated with gray triangles.
FIGURE 4
FIGURE 4
(a) SUVA in municipal wastewater influent and effluent. (b) The relation of HgD to ΔSUVA between municipal wastewater influent and effluent.
FIGURE 5
FIGURE 5
(a) Observed fraction HgD (of HgT) versus TSS for wastewater samples. Samples from plants with a DOM‐rich industrial influence are highlighted with gray dashes. Pond effluent samples are indicated with gray triangles. (b) Modeled prediction of the fraction of Hg bound to filter‐passing reduced thiol groups (=Hg[RS]2|DOM) and inorganic sulfide (=HgS2H−) for different wastewater DOC concentrations. All calculations use logK=RSH|DOM = 22.0, [H2S]total = 10–6.6 M, and pH = 7.5; black lines represent an assumed logK=RSH|PM constant of 24.0; gray lines represent an assumed logK=RSH|PM constant of 22.0. Thin lines represent the modeled contribution of organic = Hg(RS)2|DOM to HgD for the constant H2S concentration, a quantity that increases with DOC. Gray bands represent 1–3 mg/L TSS, the threshold below which a majority of Hg in the effluent of nonindustrial wastewater is filter passing.
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MeSH terms