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Mercury in Dental Facilities

This report presents the results of a study by the Massachusetts Water Resources Authority (MWRA), which characterizes discharges to the sewer from dental facilities. MWRA undertook this study primarily because of concerns about mercury in the effluent from its sewage treatment plants and in its residuals. The U.S. Environmental Protection Agency (EPA) and the Massachusetts Department of Environmental Protection (DEP) enforce strict regulatory limits for mercury. MWRA's effluent and the fertilizer pellets produced from MWRA's residuals meet EPA and DEP limits; however, MWRA is studying sources of mercury in its system and potential control measures to ensure future compliance, and, if possible, to further reduce its mercury load.

This report presents the results of analyses of samples of wastewater from fifteen dental facilities, including three large dental schools, two hospital clinics, one medium-sized clinic, and six small general practice dental offices. Part 1 discusses the regulatory background for MWRA's mercury control program, information on dental use of mercury, and studies of dental discharges in other sewage treatment systems. Part 2 and the Appendices discuss the results of MWRA's analyses.

This report demonstrates that dental discharges are a significant source of mercury. Most of the samples exceeded MWRA's discharge standard of 1 part per billion (ppb), which were ranging as high as 41,400 ppb. MWRA estimates that dentists are contributing at least 10% of MWRA's mercury. 

This study is part of a broader effort by MWRA to reduce its mercury load. MWRA has been working cooperatively with a number of permitted facilities, particularly hospitals, who formed the Mercury Products Workgroup in June 1994. This Workgroup has developed extensive information on mercury-containing products, product substitution, infrastructure measures to remove mercury from pipes, traps and sinks, and mercury pretreatment systems. The Workgroup will be reporting on the results of its efforts in the course of 1997.

As described in Section 2.2, MWRA calculated a range of possible mercury contributions from dental facilities, based on differing assumptions about their volume of wastewater flow, and on an estimate of MWRA's total mercury load of 0.76 lbs/day. These calculations place dentists' contribution at anywhere from 13% to 79% of MWRA's mercury load, with the midpoint at 46%. Because of the uncertainties in the flow and load estimates, MWRA believes that its high-end estimate is unrealistic, but is confident that the low-end, rounded down to 10%, is a reasonable minimum estimate. This estimate is consistent with the findings of other studies: studies estimated dentists' contributions to the total mercury load at 8-13% in San Francisco, 14% in Seattle, and 26% in Duluth (see Section 1.3).

MWRA's sampling also indicated that the bulk of mercury (80-90%) discharged from dental offices is discharged as particles, while the remaining 10-20% is discharged as dissolved mercury. MWRA's study also showed that dental wastewater contains other regulated heavy metals in high concentrations, including copper, silver, zinc and lead.

Suction lines, with average concentrations of 4,781 ppb for those with air-cooled pumps, and 826 ppb for water-cooled pumps, had the highest mercury levels of any sources within dentists' offices, suggesting that segregation and removal of the mercury in these wastewater would be a significant step toward reducing the loading of mercury and other metals to the sewer. Operatory and laboratory sinks, and X-ray developers were smaller sources. The single ultrasonic cleaner that MWRA sampled had an average mercury level of 169 ppb, and an average zinc level in excess of MWRA's limit, suggesting that instrument cleaning and cleaning devices should be investigated further as potentially significant sources of mercury and other metals.

Note: For a full copy of this report, contact Charlie Bering at 788-2309. A copy is also located in the MWRA Library.