Dentistry and Wastewater FAQ
Why has dental amalgam become a wastewater issue?
Dental amalgam contains mercury, and mercury is identified as one of 12 “persistent, bioaccumulative, and toxic” chemicals targeted by local, state, and federal environmental regulators. With the enactment of various environmental and occupational safety regulations, the use of mercury by U.S. industries began a steep decrease in the mid-1980s. As industries eliminate mercury from some products, such as paints and pesticides, or reduce its use in batteries, lamps, and electrical switches, environmental regulators have turned their attention to mercury’s use in healthcare. Thermometers, blood pressure cuffs, medicines, and dental amalgam are among the healthcare products that regulators have identified as possible sources of mercury that can be released into the environment. It should be noted that dentistry’s use of amalgam has decreased significantly in recent years with decreased caries and with the popularity and improved properties of tooth-colored restorative materials.
Regulatory interest is highest in regions where fish are found with high concentrations of methylmercury. Some forms of mercury, under certain conditions, can be transformed by a biological process to become the more highly toxic methylmercury and bioaccumulate in fish. Mercury is a naturally-occurring element that cannot be destroyed but can change forms. U.S. EPA and state and local environmental agencies measure and regulate for “total mercury.” Other recent developments also drive regulators’ interest in dental amalgam: (a) new tests that detect very low levels of mercury previously undetectable; (b) lower regulatory standards for mercury in water and mercury in fish; (c) exceeding the new standards; and (d) international agreements to reduce mercury release and uses.
What is the fate of dental amalgam once it enters a dental office’s plumbing lines and the community’s sewer system?
During placement or removal of amalgam restorations, most amalgam debris is caught by the chairside filter and by a secondary filter at the vacuum. Amalgam particulate is heavy, and research indicates that most of the particulate in a wastewater stream will be entrained in amalgam-capturing devices or settle in pipes over time. If amalgam particulate makes it to a wastewater treatment plant, it is likely to be captured in one or more sediment, or grit, chambers at the plant. The resulting sludge (also called “biosolids”) may be disposed at a landfill, used as a soil amendment, or incinerated. Sludge is regulated for mercury content.
How much mercury is released from dental amalgam into wastewater is unknown, and the process is likely dependent on a variety of factors. Research indicates that some line cleaners, primarily those with oxidizers, may release mercury from amalgam. Therefore, use of oxidizing agents for line cleaning is discouraged. Nevertheless, ADA-sponsored research conservatively estimates that the amount of mercury from dental offices that theoretically may be released into waterways and become available for conversion to methylmercury is less than one percent of all sources of mercury.
Why is wastewater regulation not the same across the state?
Water quality standards vary from community to community. The federal Clean Water Act allows states and communities to establish regulations specific to their own water quality criteria. For example, the elevated level of mercury in San Francisco Bay has led to greater regulation of all potential sources of mercury, including dental office wastewater. In contrast, mercury is not a significant concern in some Central Valley watersheds.
What is dentistry being asked to do?
Individual sanitation agencies (publicly-owned treatment works or “POTWs”) have contacted dental offices, component dental societies, and CDA for a variety of reasons. Some POTWs are conducting surveys and/or educational outreach programs. A few POTWs have implemented regulatory programs which include permits, fees, and monitoring.
CDA recommends all dental offices implement “Best Management Practices” (BMPs) for dental office waste. Because wastewater regulations differ from community to community, CDA’s approach is to address each situation on a case-by-case basis. CDA maintains open communication with local sanitation agencies and strongly encourages local dental societies to meet with local agencies when contacted.
GLOSSARY
Best Management Practices: In dental offices, a combination or series of practices determined to be effective and practical means of preventing or reducing the amount of amalgam waste disposed into the environment. BMPs promoted by some environmental agencies also may target photochemicals and disinfectants.
Clean Water Act: Growing public awareness and concern for controlling water pollution led to enactment of the Federal Water Pollution Control Act Amendments of 1972. Commonly known as the Clean Water Act (CWA), it established the basic structure for regulating discharges of pollutants into the nation’s waters. The statute employs a variety of regulatory and nonregulatory tools to sharply reduce direct pollutant discharges into waterways, finance municipal wastewater treatment facilities, and manage polluted runoff. These tools are employed to achieve the broader goal of restoring and maintaining the chemical, physical, and biological integrity of the nation's waters so that they can support "the protection and propagation of fish, shellfish, and wildlife and recreation in and on the water."
Local Discharge limit: POTWs may regulate businesses discharging wastewater to their sewer systems. One method of regulation is to require businesses to meet local discharge limits for specific pollutants. Local discharge limits in California for mercury range from 0 to 0.1 ppm (or mg/l).
NPDES: National Pollutant Discharge Elimination System. This is the permit system established by the Clean Water Act (CWA) to regulate direct wastewater discharges from wastewater treatment plants and industry. Wastewater dischargers may be required to comply with a specific mercury discharge limit (concentration and/or mass limit) or may only be required to monitor their discharges for mercury.
POTW: A Publicly Owned Treatment Works as defined by Section 212 of the CWA, which is owned by the state or municipality (as defined by Section 502(4) of the CWA). This definition includes any devices or systems used in the storage, treatment, recycling, and reclamation of municipal sewage or industrial wastes of a liquid nature. It also includes sewers, pipes or other conveyances only if they convey wastewater to a POTW treatment plant. The term also means the municipality that has jurisdiction over indirect discharges to and the discharges from such a treatment works. POTWs may require businesses discharging wastewater to their systems to meet local discharge limits for specific pollutants and/or comply with other permit conditions, such as BMPs.
TMDL: The Total Maximum Daily Load is a calculation of the maximum amount of a pollutant that a waterbody can receive and still meet water quality standards. TMDLs determine what level of pollutant load would be consistent with meeting water quality standards. The TMDL regulatory process also allocates acceptable loads among sources of the relevant pollutant.
Water Quality Criteria: Levels of individual pollutants or water quality characteristics, or descriptions of conditions of a waterbody that, if met, will generally protect the designated use of the water.
Water Quality Standards: Includes three major components: designated uses, water quality criteria, and antidegradation provisions.