Dental Offices Wastewater
A concern over wastewater discharged from dental offices is its potential contamination with mercury, which is found in dental amalgam. Amalgam is made of two nearly equal parts: liquid mercury and a powder containing silver, tin, copper, zinc and other metals. Mercury is of special interest because it is a persistent bioaccumulative toxic element.
Most dental offices currently use some type of basic filtration system to reduce the amount of mercury solids passing into the sewer system (e.g., chairside traps, vacuum pump filters). However, the installation of amalgam separators and the use of best management practices have been shown to reduce discharges even further.
Over the past 10 years, some states and local governments have enacted regulations that require dental offices to install amalgam separators or equivalent equipment. More recently in 2016, EPA finalized national regulations referred to as effluent guidelines. The rule requires dental offices to install amalgam separators or equivalent technology to prevent the discharge of mercury to city sewers. The rule goes into effect December 2019. This federal rule effects virtually all dental offices. The only significant exception is dental offices that do not place or remove amalgam. These offices need only submit a onetime certification. For more information, see EPA fact sheet: Effluent Limitations Guidelines and Standards for Dental Offices.
Overview of the Mercury Problem
Mercury from dental offices contributes significantly to the overall mercury contamination in wastewater. In 2008 EPA estimated that there were approximately 122,000 dental offices (approximately 160,000 dentists) that used or removed dental amalgam in the U.S., and that those offices discharged approximately 3.7 tons of mercury each year to POTWs. Dental offices were found in 2003 to have been the source of 50 percent of all mercury pollution entering POTWs.
Unless it is captured and removed at the dental office, mercury-containing amalgam waste will find its way into the environment when new fillings are placed or old mercury-containing fillings are drilled out and waste amalgam materials are flushed into chair-side drains. Some of the waste amalgam particles that reach the sewer system settle out in the sewers, and some are carried to POTWs. The physical processes used in POTWs remove about 90% of the mercury received in wastewater. The mercury removed from wastewater then resides in the biosolids or sewage sludge generated during primary and secondary treatment processes. The sludge generated by the water treatment process is sometimes incinerated, sometimes disposed of in solid waste landfills, and sometimes applied to agricultural land as fertilizer. In any case, mercury enters the environment. Also, a portion of the dissolved amalgam/mercury passes through the POTW and is discharged to surface waters where it can bioaccumulate in fish and shellfish.
Amalgam separators are devices designed to remove amalgam particles from dental office wastewater through sedimentation, filtration, centrifugation, chemical removal by ion exchange or a combination of these mechanisms. An amalgam separator typically consists of a canister that is located either in the dental operatory or in the mechanical room adjacent to the vacuum pump. Amalgam separators meeting the International Organization for Standardization (ISO) standard (ISO 11143:2008) can capture over 95% of the amalgam discharged by dental offices into sewer systems.
Amalgam separators require regular monitoring and maintenance to ensure they are functioning properly. This varies depending upon the amalgam separator you chose and your practice volume. For example, the collected amalgam may have to be removed weekly with some units or in high-volume practices, while other units may only have to be serviced periodically (e.g., every three to 18 months). It is also important to plan how you will dispose of the amalgam waste and used parts of the amalgam separator (e.g., used cartridges and filters). Some amalgam separator companies offer disposal/recycling of the amalgam waste as part of a package when you purchase their product, while other companies leave it to the user to obtain disposal/recycling services.
Damon Highsmith (202-566-2504). EPA Effluent Guideline dental amalgam expert.
Mercury in Dental Amalgam. U.S. EPA.
Dental Amalgam Effluent Guideline. U.S. EPA.