Study: Wastewater Plants Relatively Light on Mercury

Researchers for a recently completed Water Environment Research Foundation (WERF) project on mercury in wastewater treatment plant effluent have determined that bioavailable mercury levels are as low as or lower than those from other common sources such as atmospheric deposition and non-urban runoff, according to WERF.

The research comes at a critical time for wastewater treatment plants, which have been targeted as contributors of mercury, especially bioavailable mercury, to receiving waters.

The project, phase two of Estimation of Bioaccumulation Potential from Wastewater Treatment Plants in Receiving Waters (05WEM1COa), was lead by researchers David Dean of ArcTellus and Robert Mason, Ph.D., of the University of Connecticut.

Based on mercury bioavailability data captured from seven wastewater treatment plants, the research team found that wastewater effluent’s typically low levels of methylmercury makes it one of the lowest among the sources evaluated. In addition, the team determined that due to the effluents' low levels of suspended solids (mercury being present in solids), wastewater treatment plants employing post-secondary treatment should not contribute appreciably to local sediment mercury burdens.

A comparison of WWTP effluent to several common sources is shown in the table below. In this table, bioavailable mercury is taken to be the sum of methylmercury (MeHg) and inorganic bioavailable mercury (IB-Hg).

Water Type/Source

Range of Estimated Bioavailable Hg fraction
in the literature

Atmospheric Deposition

0.2 to > 0.9

Mining Runoff

< 0.1

Impacted Sediment Porewater

0.15 to 0.45

Unimpacted Sediment Porewater

0.2 to 0.45

Urban Runoff

< 0.1

Non-urban Runoff

0.2 to 0.3

Wastewater

< 0.2 (literature)
0.05 - 0.35, average 0.21 (for this study)

A key component of the research, WERF says, was to develop a working definition of bioavailability as it concerns mercury. For this study, the research team defined bioavailable mercury as including those forms of mercury, both inorganic and organic, that are readily transported across biological membranes, either by active or passive processes and subsequently accumulated, and/or biotransformed to a more toxic and bioaccumulative species.

In addition to these findings, the research team developed a tool that will help wastewater treatment plants assess their overall contribution to environmental risk relative to other dischargers in a watershed. The tool allows for mixing an effluent in a receiving water (fresh, estuarine, or marine) to estimate bioavailability in the near- and far-field. The tool accounts for receiving water conditions, specifically pH and dissolved organic carbon, which can have an effect on the conversion of inorganic mercury into methylmercury, which accumulates in fish.

Accompanying the tools is a set of guidelines and protocols to assist treatment plants in conducting a detailed assessment, including sampling, data analysis, and interpretation. This will let wastewater treatment plants make more definitive assessments of the bioavailability of mercury in their effluent. Such an assessment may be warranted if a WWTP is directly involved in a total maximum daily load or similar regulatory action.

Another aspect of effluent mercury covered in the report is the ongoing debate over the presence of higher levels of bioavailable mercury in wastewater treatment plant effluent. The publicly owned treatment works community has argued that the discharge from municipal wastewater facilities should be viewed as a de minimis, or relatively insignificant, source of mercury and, that community resources would be better spent addressing large controllable sources, such as abandoned mines.

Recently the U.S. Environmental Protection Agency and other stakeholders have raised questions regarding concerns that the mercury discharged from WWTPs may contain a larger fraction of bioavailable or methyl mercury than other sources of mercury. This concern is being used to support the position that the discharge from WWTPs should potentially be considered in a different manner than other sources and have more stringent waste load allocations than they otherwise may have based exclusively upon the magnitude of their contribution of total mercury. Advanced treatment to achieve lower levels of mercury in WWTP discharges is capital intensive, which places higher costs on the public.

In addition to the findings above, the research team developed a tool that will help wastewater treatment plants assess their overall contribution to environmental risk relative to other dischargers in a watershed. The tool allows for mixing an effluent in a receiving water (fresh, estuarine, or marine) to estimate bioavailability in the near- and far-field. The tool accounts for receiving water conditions, specifically pH and dissolved organic carbon, which can have an effect on the conversion of inorganic mercury into methylmercury, which accumulates in fish.

Accompanying the tools is a set of guidelines and protocols to assist treatment plants in conducting a detailed assessment, including sampling, data analysis, and interpretation. This will let wastewater treatment plants make more definitive assessments of the bioavailability of mercury in their effluent. Such an assessment may be warranted if a WWTP is directly involved in a total maximum daily load or similar regulatory action.

Another aspect of effluent mercury covered in the report is the ongoing debate over the presence of higher levels of bioavailable mercury in wastewater treatment plant effluent. The publicly owned treatment works community has argued that the discharge from municipal wastewater facilities should be viewed as a de minimis, or relatively insignificant, source of mercury and, that community resources would be better spent addressing large controllable sources, such as abandoned mines.

Recently the U.S. Environmental Protection Agency and other stakeholders have raised questions regarding concerns that the mercury discharged from WWTPs may contain a larger fraction of bioavailable or methyl mercury than other sources of mercury. This concern is being used to support the position that the discharge from WWTPs should potentially be considered in a different manner than other sources and have more stringent waste load allocations than they otherwise may have based exclusively upon the magnitude of their contribution of total mercury. Advanced treatment to achieve lower levels of mercury in WWTP discharges is capital intensive, which places higher costs on the public.

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