Advanced treatment technologies that remove solids from municipal wastewater consistently meet mercury effluent limits.

Abstract

Since many other sources of mercury (Hg) are difficult to control, the removal of Hg from wastewater is an essential component of maintaining low Hg levels in ecosystems. Numerous studies have investigated solid-liquid interactions of Hg at ng/L levels in natural systems, but few studies have translated insights to highly-engineered, organic-rich systems such as municipal wastewater treatment plants. There is a lack of guidance for design engineers concerned with meeting exceptionally low-level Hg limits, particularly the 1.8 ng/L limit applicable to the Laurentian Great Lakes. A historic Minnesota Pollution Control Agency dataset from over 150 municipal wastewater treatment plants (MWWTP) was analyzed and categorized into technology types. Starting with secondary technology types and continuing to tertiary technology types, Hg concentrations from MWWTP in each technology type were analyzed to indicate if there are technologies that reliably reduce HgT in the effluent to below 1.8 ng/L. Additionally, correlations between total suspended solids (TSS) and HgT (total Hg) were evaluated. Certain technology types were better at removing HgT than others. Secondary technologies including conventional activated sludge, MBRs, and polishing ponds as well as tertiary technologies including fine dual media filters, rotating cloth membrane, and deep bed mono-media filters all consistently meet low-level HgT discharge limits. Effluent HgT measurements were correlated with TSS for several secondary technologies with a hydraulic residence time similar to conventional activated sludge, while technologies with very short (attached growth) or very long (ponds) showed little relation between TSS and HgT in effluent water. Nonparametric statistics were used to compare the effluent HgT among technologies and estimate the reliability of meeting mercury limits. The results of this study could help MWWTP determine appropriate technologies to install to meet regional or state mercury discharge limits.