Mercury: From the Dentist’s Chair to Public Treatment Works
“Congress declared it to be the national policy of the United States that pollution should be prevented or reduced at the source whenever feasible; pollution that cannot be prevented should be recycled in an environmentally safe manner, whenever feasible; pollution that cannot be prevented or recycled should be treated in an environmentally safe manner whenever feasible; and disposal or other release into the environment should be employed only as a last resort and should be conducted in an environmentally safe manner.” —Pollution Prevention Act of 1990 (Public Law 101-508)
As National Pollution Discharge Elimination System (NPDES) permits limit discharges for mercury and other contaminants closer to the zero level, states and publicly owned treatment works (POTWs) are looking at ways to reduce those metals at the source and meet stricter limits using cost-effective pollution prevention.
Dentists and mercury
The most common use of mercury in dental operations is in dental amalgam, a direct filling material used in restoring teeth. It is made up of approximately 40 to 50% mercury, 25% silver and 25 to 35% of a mixture of copper, zinc and tin. The total amount of mercury per amalgam can range from 327 to 982 mg, depending on the size of the amalgam.
Amalgam particles enter wastewater from dental offices when dentists remove old amalgam fillings or place new fillings. Estimates of the contribution of mercury in wastewater from dentists to POTWs range from 11% (San Francisco), to 14% (King County, Wash.), up to 80% (Minneapolis) of the total mercury load.
In a recent survey of seven major wastewater treatment plants in California, Minnesota, Ohio and Maine, dentists were identified as the largest contributors of mercury. The Central Contra Costa Sanitary District estimates that approximately 50% of the mercury in its wastewater comes from dental offices.3 A report released in 2002 by the National Association of Clean Water Agencies (NACWA) indicated that dental clinics are the largest single source of mercury in wastewater.4 There are more than 20 studies from Europe, Canada and the U.S. that identify the dental industry as the leading source of mercury to sewer systems.
According to the American Dental Association (ADA), the dental industry uses approximately 35 tons of amalgam annually. It is estimated that 29.7 tons of mercury, in the form of amalgam, are discharged to the internal wastewater systems of dental facilities annually. Traditionally, dental offices have captured this amalgam waste with chair-side traps and vacuum filters, but the ADA estimates that each year 6.5 tons of mercury bypass these filters and discharge to POWTs.
The NACWA, which represents wastewater treatment operators, released a “white paper” in January 2006 to highlight three options that its members can take to limit the amount of mercury effluent from dental offices. The options are listed in order of degree of intervention and effectiveness. Options include educating dentists on the ADA’s best management practices (BMPs), limiting the release into the sewer systems, implementing numeric wastewater limits for dental clinics and establishing potential requirements or recommendations for amalgam separators.
The white paper points to BMPs outlined last year by the ADA on how to handle the dental amalgam, such as stocking and storing it before it is shipped to a certified recycling operation. According to the NACWA, the dental amalgam is sometimes collected in chair-side traps or filters and washed out in sinks, directly discharging the contents into the sewer systems. The ADA itself notes that “although chair-side traps and vacuum filters remove some particles from the wastewater stream, particles that remain in the wastewater can settle along the waste pipe or be discharged into the sewer.”
The next option is to apply a local limit for mercury, either at the end of pipe or on a dental facility’s total discharge. The study indicates that this approach is complicated, expensive and can be technologically and economically infeasible for dental clinics.
The third recommendation is to advise or require the installation of amalgam separators that capture mercury and other wastes before they enter the sewer system. The white paper notes that a study is underway to “provide additional insight into whether the installation of amalgam separators at the dental clinics in a particular POTW’s service area may enable the POTWs to meet NPDES limits. Still, amalgam separators may be the best option some communities have to control mercury discharge.”
Amalgam separators are solids collectors installed on the vacuum lines of dental offices. They need to be certified under ISO 11143 to remove more than 95% of solids by weight. Most amalgam separators on the market in the U.S. are certified to greater than 98% ISO, providing dentists with a range of certified treatment options to purchase. Because 95% or more of amalgam is captured by the separators before it reaches the sewers, POTW reductions have been dramatic. Toronto recorded a 58% decrease in the amount of mercury in its sludge six months after the required installation of amalgam separators in dental offices.8 Wichita recorded a 74% reduction8, Seattle a 50% reduction9 and Victoria, B.C., Canada, a 70% reduction.9 Reports from Europe demonstrate up to 95% reductions in mercury loadings10 over the course of the last 10 years since amalgam separator regulations have been in place.
The three leading amalgam separators on the U.S. market represent approximately 90% of the devices sold. They have an average capital purchase cost of $846. Installation costs are approximately $250 and, in some cases, are rolled into the cost of the separator. Operational costs of these three separators average $528 per year. In comparison to BMPs where vacuum pump filters are changed regularly as recommended by the ADA, the annual operational costs of amalgam separators are actually less than the purchase and recycling costs of the vacuum pump filters.
Another benefit of amalgam separators is the recycling of mercury. In 2006, SolmeteX, a leading separator manufacturer, recycled approximately 5,300 canisters from its base of dental clients. This resulted in 1,325 lb of recycled mercury.
An undeniable trend
The effectiveness of amalgam separators has been recognized and regulations have been implemented in eight states: Connecticut, Maine, Vermont, Massachusetts, New Hampshire, New York, Rhode Island and Washington. These states have requirements for separators in dental offices. Montana, New Jersey and Pennsylvania are also evaluating statewide requirements. In addition to state regulations, POTWs from Seattle, San Francisco, Milwaukee, Wichita, Kan., and many other communities and counties have ruled that dentists must install amalgam separators.
The pollution prevention amalgam separator has proved a cost-effective solution for the removal of 50% or greater of the mercury loads to POTWs. Separation usage has been facilitated by statewide regulation, at the county or municipal level and by local POTWs individually. Amalgam separator usage also facilitates the recycling of mercury that is fulfilling the intent of the Pollution Prevention Act of 1990.