Dentists have long relied on amalgams for use in restorative dental work. Dental amalgam, more commonly known as a silver filling, is typically a mixture of mercury, and an alloy of silver, tin and copper. Mercury is used to bind the metals together, adds to the durability of the filing and makes up about 45–50 percent of the compounds used as dental amalgam.
Despite concerns over health risks associated with use of mercury and its presence in the environment, dental amalgam continues to be frequently used in dentistry both in North America and worldwide. Dental offices therefore generate a significant amount of mercury waste in the form of dental effluents from working on patients with amalgam filings, or from the application of new amalgam in dental work.
Increasingly, environmental regulations control and limit the disposal of mercury-laden wastes. Dentists are significant producers of mercury wastes and accordingly are turning to amalgam separators in order to comply with the regulations and significantly reduce their output of mercury in dental effluents into the public sewers system. Environmental standards in many cities now demand that 95% or greater of all mercury be removed from effluents before release into the public sewers, and many places are instituting more stringent standards. The International Standards Organization (ISO) 11143 standard Dental Equipment—Amalgam Separators for the removal of mercury by amalgam separators calls for at least a 95% reduction in the effluent level of mercury. While adherence to this standard will result in a significant reduction in mercury emissions from dental offices, these minimum standards are likely to become more stringent over time to better protect the environment. Amalgam separators suitable for use in dental offices should therefore offer a very high level of mercury recovery in order to meet both the present standards and the anticipated future increasingly strict environmental guidelines. Moreover, the space constraints present in many dental offices requires that the equipment and apparatus used for the removal of mercury wastes from dental effluent be of a reasonable size. The apparatus should also not require substantial ongoing maintenance or downtime, and should be able to be operated in a self-sufficient manner for extended periods of time.
Any device for the removal and storage of hazardous waste products from dental effluent in a clinical setting should also avoid the use of dangerous chemicals, and not produce gases or other byproducts that could be harmful upon exposure or release into the environment.
A number of devices have been described that are designed to remove waste products from dental effluents. These devices include one or more aspects such as gravity sedimentation of particles (e.g. US 6,592,752, 5,885,076, 5,795,159, US 200100479561A1, US 5,795,159 and US 4,732,739), centrifuges designed to separate the heavier toxic metallic particles such as mercury from the effluent, ion-exchange systems for binding and removing charged molecules (e.g. 5,885,076), and the use of chemical agents such as precipitants (e.g. 5,885,076, US 200100479561A1), chelating agents, flocculants, or adsorbants (e.g. US 6,592,752, US 6,592,754, US 200100479561A1). Acid-base adjustment, for example in mix tanks, has also been used in order to adjust the pH conditions to promote mercury precipitation in some systems (JP60197285A2, US 6,592,752). Other sedimentation methods or pump filtration systems rely on mechanical filtration for the removal of mercury and other hazardous materials (e.g. US 6,592,752, 5,885,076, 5,795,159, US 6,592,754, US 5,795,159. Such filters optionally incorporate methods based on ion-exchange, pH adjustment and absorbent columns (U.S. patent application Ser. No. 2001/0047956). There are problems with each of these separators. Some of them have very limited surface area for mercury sedimentation. This means that it is less likely that all the influent mercury will sediment before water is passed to downstream chambers or an extremely slow rate of flow to allow adequate time for the amalgam to precipitate before exiting the system would be required. Some systems attempt to overcome this problem by significantly increasing the size of the separator to increase the surface area available for settling resulting in devices of a size that would not be practical for most dental offices. Other systems attempt to overcome the lack of available settling area by specifying an extremely low flow rate through the system. The low flow rates specified by these systems become a significant problem when a dentist flushes vacuum lines with disinfectant and is restricted to adding disinfectant at the rate specified by the amalgam separator. Other systems that use absorbent substances and resins tend to get clogged due to the heavy demands placed on them to remove mercury as seen with some of the filter based amalgam separators currently in the marketplace. Systems that use a combination of sedimentation and absorbents or filters also tend to clog because of the increased proportion of amalgam removal by these systems caused by the inefficient settling system. In addition, the systems that rely on filtration or chemical adsorption are also susceptible to biofouling resulting in more frequent servicing and associated costs for the dental office. In some instances these systems recommend the use of disinfectants to control the growth of microorganisms in the filter or adsorption medium. This practice is not desirable as many of the disinfectants can solubilize and release the mercury previously trapped by the filter or adsorption medium. Some amalgam separators also require extensive down time to clean. Other separators give off gas and noxious odors such as hydrogen sulphides and organic amines. There remains a need to remove as close to 100% of mercury as possible without the problems of clogging, excessive down-time and gas emissions.
U.S. Pat. No. 6,592,754 teaches the use of chambers for removal of solid particles from the suction effluent by way of a sedimentary deposit tank with a series of baffle chambers connected in series through which the effluent flows in sequence as the chambers fill one by one. In each chamber sediment is deposited for later removal. Chemical injection of precipitants is optionally used to improve sedimentation. Positive air pressure or auxiliary pumps drive the effluent. Modular filters or adsorbants are installed downstream of the deposit tank.
However such a device suffers from a number of flaws stemming from the use of baffles chambers, including potential short-circuiting around the internal baffle system or overloading of one of the baffle chambers leading to the clogging of the system. Moreover, as the surface of the baffles is preferably parallel to the flow of the effluent through the system, such baffles are not efficient for inducing the sedimentation of waste materials. This inefficiency can be seen in the recommended flow rate of the device of 20 mL/min. At this flow, the rate at which a dental office could disinfect a line would be reduced to a steady trickle of drops.
Fan et al. evaluated the amalgam removal efficiency of 12 separators according to ISO standards (Fan et al. Laboratory evaluation of amalgam separators. Journal of the American Dental Association, Vol. 133 May 2002. p. 577.). Although they reported that all 12 devices removed greater than 95% of amalgam and therefore met the ISO guidelines, the range was from 96.09% to 99.99% efficiency such that the use of some amalgam separators may not assure compliance with the 0.01 mg/L limit for mercury in sewer effluent as found in various city bylaws such as Toronto Municipal Code c. 681. Similarly, Adegbembo et al. evaluated the performance of an ISO certified amalgam separator but found that only 99.4% of mercury waste was removed from the dental effluent such that the concentration of mercury in the effluent was only reduced to 0.1800 mg/L from 31.2973 mg/L (Adegbembo et al. The weight of wastes generated by removal of dental amalgam restorations and the concentration of mercury in dental wastewater. J. Can. Dent. Assoc. 2002 October; 68(9):553–8.).
There remains a need for an apparatus that allows for a very high level of mercury removal (greater than 99.99% recovery) in a system that is both convenient and suitable for use in dental offices.