June 1, 2001
by Timothy Tuominen
Pretreatment of wastewater prior to discharge to the sewer system has been a common practice for many industrial wastewater dischargers for a number of years. Treatment of industrial discharges high in metals is commonly used to prevent upsets of biological treatment systems at municipal wastewater treatment plants or pass through of metals into the environment. It has been assumed that treatment at the source is less costly because of the low volume of water and the high concentration of the target chemical.
Treatment of the wastewater from dental office vacuum systems is especially attractive because the waste from a dental chair is less than a liter per day and has a concentration much higher than most wastewater. Mercury from dental offices comes from dental amalgam, which contains between 40-60% mercury. When a filling is placed or removed some portion of that amalgam is drawn into the vacuum suction device. Researchers have estimated the total mercury waste is about 2 grams per dentist per day. The portion of that amount which is discharged to the sewer varies greatly from day to day and from practice to practice.
Little is known about how much of the mercury from amalgam waste actually reaches the wastewater treatment plant. A portion is captured on traps in the dental office, and some portion accumulates in dental suction lines, as well as in sewer lines and wet wells. Researchers have shown that some mercury may become dissolved and reach the treatment plant. It is also likely that some fine particulate amalgam reach the treatment facility.
Two basic types of vacuum pumps are used in dentistry. A “wet vacuum pump” or liquid-ring pump (see Figure 1) uses water in order to maintain a vacuum seal in the pump and adds a large volume of water to the discharge. A “dry vacuum pump” or turbine type vacuum pump (see Figure 2) adds no water to the discharge. Each system has advantages and disadvantages, however wastewater treatment is simpler if the extra water from a wet vacuum pump does not need to be treated.
The first question that arises when looking at treating this source of mercury is: “What level of treatment are you trying to achieve?” Conventional wet vacuum systems capture large particles of amalgam by use of a chair-side trap (screen). The pump is protected by the use of a vacuum pump trap where some further capture of amalgam particles occur. The wastewater is then discharged along with the seal water from the wet type vacuum pump. These systems can capture 60-90 percent of the mercury suctioned from patient’s mouths.
Dry vacuum systems usually also use chair-side traps. The waste then passes into an air-water separator where the fine amalgam particle have an opportunity settle in the tank. However the tank usually drains completely when shut off, releasing both the water and the amalgam particulates. It has been suggested that use of a chair-side trap with a smaller pore size would be simple way to reduce amalgam discharge. There may also be opportunities to use air-water separators as sedimentation chambers if the plumbing design was changed.
CHAIR-SIDE TRAP IMPROVEMENTS
There have been a few ideas to improve the capture rate of the metal or plastic screens that capture large particles at chair-side. The most common chair-side traps are plastic screens with a 700-micron opening (0.0331 inch). One idea was to make a finer replaceable filter out of glass fiber that would fit into the standard chair-side trap. It was thought that the filter would need to be changed on a daily basis instead of the weekly change out of the standard trap. The result would be an increase in capture over the standard trap of about five grams of amalgam bound mercury per week per chair. This filter was never developed commercially.
A second advanced type of chair-side trap is designed to be used in conjunction with an advanced treatment system. This trap is a small sedimentation trap to allow sedimentation to occur chair-side and only needs to be replaced every three months.
IMPROVING AMALGAM CAPTURE
A number of different devices have been developed in order to improve the capture of amalgam particles. Some units even attempt to capture the small amount of mercury that becomes dissolved when acidic or basic vacuum line cleaning solutions are used.
The type of vacuum system (wet or dry) may determine what type of advanced treatment system is appropriate. The type of vacuum system will also determine where in the suction line an advanced capture device will be installed. In a system with a wet-ring pump the amalgam capture devices are installed just prior to the vacuum pump. This requires cutting into the suction line, which is under vacuum. This creates some difficulty because vacuum pressure must be maintained in the line in order for the vacuum pump to maintain suction. In a dry vacuum system, treatment systems can be installed after the air-water separator, because there is no added water (such as the seal water in a water ring vacuum pump).
Among the factors the dentist should consider when evaluating advanced treatment systems, reliability is most important. Dentists don’t want the suction device to quit during a crucial stage of placement of restoration material because a dry tooth is needed to place the restoration. Other factors dentists are concerned about include amount of noise, size, odor, ease of installation, maintenance, and disposal of collected materials.
Basic treatment systems fall into two basic types of technology: filtration and sedimentation. If a very high level of treatment is needed, absorption is another option as a final step. There are many variations on these basic technologies. Measuring the effectiveness of treatment systems can be quite difficult due to the variability of the waste stream. Because of the high density of amalgam, particles can settle in the low spots of the vacuum system. Mercury and amalgam can further amalgamate to metal piping and also become incorporated into biological growth in the vacuum piping. This material can slough out of the vacuum lines when cleaned, or at any time during use. Some researchers have suggested that 50 or 60 samples should be taken in order to accurately assess a wastewater as variable as that from a dental vacuum system.
In Europe, where treatment systems are required by some countries, the International Standards Organization (ISO) has developed a standard for the effectiveness of treatment systems. This standard measures the effectiveness of the treatment unit’s removal of a standard particulate amalgam solution. In order to pass this standard, the unit must remove 95% of the standard particles. Some have suggested that this standard may not be appropriate because the standard solution does not include any dissolved mercury.
Filter units are specifically designed to capture amalgam particulates in the dental vacuum system. Since biological growth can occur in the vacuum lines, a filter can plug when the bio-growth breaks loose and enters the filter. Thorough cleaning and maintenance of the vacuum lines can control the plugging problem. One unit has a quick-change design that allows office staff to change them in less than a minute.
Some of the simplest systems are also cost effective. One system was developed based on the research done in Seattle in the early 90’s. Amalgam is very dense and settles fast; staff at Metro King County developed a cylinder that was filled during the day during active hours of the practice. The fine amalgam particles are allowed to settle over night and the clear supernatant is then drained off in the morning. This simple sedimentation tank can easily achieve 90 percent removal rates. Some fill and drain sedimentation tanks have even achieved as high as 99 percent removal rates with some concentrated waste streams by allowing sedimentation to occur over the weekend. A few commercial units have been developed and marketed based on this simple technology.
When installed in a dry vacuum system after the air-water separator, a homemade unit can be as simple as be a pail with a valve 2/3 the way down. The pail is filled during the day and the supernatant is drained the following morning and in order to achieve the desired capture. In a wet vacuum system the sedimentation tank must be installed on the vacuum side of the vacuum pump. The treatment unit is under considerable vacuum so it must be strong enough to withstand collapse. The negative side of fill-and-drain sedimentation tanks is that an individual must drain them on a regular basis.
A number of commercial units have been designed with enhanced sedimentation that allows the water to flow through the unit without human involvement. Baffled sedimentation tanks allow the particles to settle with less short-circuiting. Commercially available centrifuges increase settling speed, which results in a system that requires human intervention only when the sludge is removed.
Some commercially available amalgam capture devices use multiple treatment technologies in series. These systems usually have preliminary treatment, such as sedimentation tank for large particles, followed by some type of absorbent material, which can capture ionic or dissolved mercury. These units can provide the best treatment available. However, size and cost could be an issue for a small practice.
An important question to ask when evaluating an advanced treatment device is what options are available for managing amalgam sludge managed after collection. Some manufacturers offer a full service option along with a treatment unit and include disposal service for a monthly or annual fee. Presently, most amalgam sludge is disposed via retorting, a form of distillation for metals (which then can be reused).
If a vendor of a treatment system does not provide recycling services for the sludge, the dentist must see that the sludge is properly managed. Many state dental associations have information available about amalgam waste management. Recycling Amalgam and Other Best Management Practices is a brochure, which includes a list of mercury recycling facilities that will accept amalgam sludge.
At large dental facilities, such as those serving the armed services, advanced treatment systems for amalgam capture are required by the local wastewater treatment facility. This seems very appropriate since the amount of amalgam discharged to the sewer from such facilities is very high compared to most municipalities. Other treatment plants may find that standard traps and filters and proper management of the waste from those traps is be sufficient to prevent water quality or sludge quality issues.
Tim Tuominen is a chemist for Western Lake Superior Sanitary District (WLSSD) in Duluth, Minnesota. He has spent the last 10 years helping commercial and industrial customers reduce their environmental impact. He has focused on reducing mercury and lead releases to wastewater and solid waste, which the WLSSD manages for the Duluth, MN, region.
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