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

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.