While there is no information on how much amalgam in dental office waste-water actually reaches wastewater treatment plants, and even though no mercury was detected when amalgam particles were subjected to simulated wastewater treatment processes,⁴ there is a growing impetus across the United States to decrease the amount of amalgam in dental office wastewater that is discharged to sewers. This has resulted from national, state and local initiatives to decrease or virtually eliminate the discharge of mercury and mercury-containing items into the environment.^5,6 In addition, wastewater treatment plants are facing discharge permits that have lower mercury concentration limits than those in the recent past.

As a result, wastewater treatment agencies in several regions are looking to reduce mercury and mercury-containing waste at the sources of discharge. Dental offices are identifiable sources because of the waste amalgam particles that are discharged in wastewater. Amalgam separators were developed with the aim of reducing the amount of amalgam in dental office wastewater that is discharged into sewers.

Amalgam separators use one or multiple technologies to remove amalgam from dental office wastewater. These technologies include sedimentation, filtration, centrifugation and ion exchange. The performance of amalgam separators, especially in regard to their efficiency in removing amalgam from wastewater, is of interest to dentists, regulators and wastewater agencies. Recognizing the need to obtain additional information about amalgam separator performance, we conducted a laboratory evaluation to determine the amalgam removal efficiency of these devices and the total mercury concentration in their effluent.

Amalgam separators use one or multiple technologies to remove amalgam from dental office wastewater.

	MATERIALS AND METHODS

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION Amalgam in dental office... REFERENCES

 
For this laboratory evaluation, we obtained from manufacturers or distributors 12 amalgam separators that are commercially available in the United States. Table 1 presents information about each of these amalgam separators, including the technologies they use to remove amalgam, product information provided by the manufacturers and the manufacturers’ suggested retail prices as of January 2002 (also see sidebar on page 585).

– amalgam removal efficiency;

– total mercury concentration in the effluent from the laboratory testing;

– total dissolved mercury concentration in the amalgam separator effluent. (The U.S. Environmental Protection Agency, or EPA, considers all mercury that passes through 0.45-micrometer filters to be dissolved mercury.)

Amalgam removal efficiency. International Organization for Standardization, or ISO, Standard 11143 for Amalgam Separators was used to evaluate the amalgam removal efficiency of the 12 separators.⁷ The ISO standard requires that the amalgam separator remove at least 95 percent of the amalgam when the separator is subjected to the test method specified in the standard. Using the ISO standard, we evaluated one representative sample of each amalgam separator. The figure (page 580) shows a schematic of the ISO test arrangement.

View larger version (52K): [in this window] [in a new window]   Figure. Schematic of the International Organization for Standardization test arrangement.

We determined the sample weight and made a slurry composed of the amalgam particles and 1 liter of filtered (1-µm nominal pore size) water containing 1 g of sodium pyrophosphate. Within minutes of mixing, the slurry is stirred and then poured into the amalgam separator, along with filtered (1-µm nominal pore size) tap water to achieve the maximum flow rate specified by the manufacturer of the amalgam separator. The effluent water from the amalgam separator is collected in a vessel.

The collected effluent water, containing amalgam particles not retained by the amalgam separator, is filtered through a series of three preweighed filters with nominal pore sizes of 12 µm, 3 µm and 1.2 µm. The filters, with the amalgam particles collected on them, are dried to constant weight in a desiccator at room temperature. We weighed the filters using a balance (Mettler AE 163, Mettler Toledo, Columbus, Ohio), which is accurate to 0.0001 g. We used the amount of amalgam retained by the filters to calculate the efficiency of each separator according to this formula:

ISO 11143 requires that amalgam separators be tested both empty and full. The full condition means that the amalgam separator is filled to its stated capacity with a combination of glass beads (1-mm diameter) (70 percent of the capacity) and amalgam scrap (maximum particle size, 0.3 mm) (30 percent of the capacity). We tested the empty amalgam separators by first placing no amalgam in the amalgam separator and then conducting two more tests without removing the retained amalgam. For the full testing condition, we filled each amalgam separator as defined by ISO 11143 and tested it for three runs. We calculated the mean efficiency separately for the empty and full amalgam separators. The lower of the two mean values for each amalgam separator is considered to be the ISO test result.

We obtained amalgam samples from the Becker Company (bm becker messtechnik GmbH, Winnenden, Germany), which certified that the samples were in compliance with the particle-size distribution specified in ISO 11143. In addition, we confirmed each amalgam sample’s compliance with the ISO 11143 particle-size distribution requirement using the electrozoning (Coulter) method, and tested 11 separators empty and full. One separator (Hg10, SolmeteX Inc., Northborough, Mass.), designed to handle multiple dental units in large dental clinics, was tested empty only because it was impractical to fill it with more than 15 kilograms of amalgam scrap for the full test. For this amalgam separator, the empty test result was used as the ISO test result.

We subjected the amalgam removal efficiencies of the separators to statistical analysis using analysis of variance and multiple comparison (Tukey) tests. For each amalgam separator, the mean efficiencies of the empty device and the full device were compared using the Student t-test.

Total mercury concentration in effluent. Because regulators often use the total mercury concentration in wastewater in deciding on wastewater discharge limits and water quality limits, we decided to evaluate the total mercury concentration in the effluent from each amalgam separator test. To do this, we calculated the contributions of mercury from two sources: the amount of amalgam retained in the preweighed filters per volume of the effluent, and the mercury concentration in the filtrate (1.2-µm nominal pore-size filter). The total mercury concentration in the effluent is the sum of these two amounts.

We also measured the volume of effluent that passed through the preweighed filters. To determine the total mercury concentration in the effluent attributed to the amalgam retained in the preweighed filters, we used a mercury-alloy ratio of 1:1 and applied the following equation:

To determine the total mercury concentration in the effluent attributed to amalgam in the filtrate (1.2-µm nominal pore-size filter), we took two aliquots of the filtrate for each amalgam separator run and tested them for mercury using the U.S. EPA Method 245.1. EPA Method 245.1 involves acid digestion (sulfuric acid and nitric acid), oxidation (potassium permanganate, potassium persulfate) to mercury ion (Hg²⁺), reduction (stannous chloride) to elemental mercury (Hg⁰) and then measurement of mercury using atomic absorption spectrometry. We also tested samples of filtered tap water used in the ISO test.

Total dissolved mercury in effluent. We filtered two aliquots of each filtrate further (nominal pore size, 0.45-µm filter), and analyzed each resultant filtrate (0.45-µm filtrate) for total dissolved mercury using EPA Method 245.1. The detection limit for mercury was 0.2 parts per billion, or ppb.

	RESULTS

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION Amalgam in dental office... REFERENCES

 
Amalgam removal efficiency. The amalgam removal efficiencies of the 12 amalgam separators evaluated were 96.09 percent or greater, exceeding the ISO 11143 requirement of 95 percent. Table 2 presents the results for the amalgam separators tested. The results show statistical differences between amalgam separators. However, no differences were found between the separators in group 1 (that is, Hg10, MRU [DRNA Dental Recycling North America Inc., Hackensack, N.J.], Rasch 890-4000 [AB Dental Trends Inc., Lynden, Wash.], Amalgam Collector [R&D Services, Seattle], RME 2000 [Rebec, Edmonds, Wash.], Hg5 [SolmeteX Inc.] and Asdex [Avprox Inc., St. Petersburg, Fla.]), group 2 (that is, Rasch 890-4000, Amalgam Collector, RME 2000, Hg5, Asdex and MSS 2000 [Maximum Separation Systems Inc., Sean Heights, Sannichton, British Columbia, Canada]) group 3 (that is, Hg5, Asdex, MSS 2000 and BullfroHg [DRNA Dental Recycling North America Inc.]) and group 4 (that is, Durr 7800/7801 [Air Techniques Inc., Hicksville, N.Y.] and ECO II [Metasys, Miami]. The results for A1000 (Air Techniques Inc.) were statistically different from those for the other amalgam separators.

Total mercury concentration in effluent. Because of the large range of values for each separator, we did not calculate mean values or standard deviations for total mercury concentration in the effluent from the laboratory testing. Instead, Table 3 provides the range of total mercury concentrations in the effluent for empty amalgam separator tests, and Table 4 (page 583) provides the range of values for full separator tests.

	DISCUSSION

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In our laboratory evaluation of the amalgam removal efficiency of amalgam separators, we used the test method specified in ISO 11143. This test method and the particle-size distribution of the amalgam sample specified in the ISO standard were developed after considerable discussion among countries involved in the development of ISO standards. The particle-size distribution requirement was chosen to reflect amalgam particles generated during dental office procedures.^8,9

Our test results show that the amalgam separators removed 96.09 percent or more of the amalgam particles that simulate particle sizes generated during dental procedures. The ISO amalgam sample contains particles as large as 3.15 mm. Particles of that size, along with those as small as 0.7 mm, would be retained by chairside traps, since most of these traps have nominal pore sizes of 0.7 mm. If vacuum filter traps were used in the dental office suction line, amalgam particles as small as 0.4 mm would be removed from dental office wastewater. Thus, these amalgam particles would be removed before they ever reached an amalgam separator.

Nevertheless, our results show that amalgam separators can remove a considerable amount of amalgam particles. At present, very limited information is available in regard to the amalgam removal efficiency of separators in actual dental office settings. A Danish study reported that the mean amount of mercury from amalgam found in wastewater from dental offices without amalgam separators was about 6.9 times that from dental offices with amalgam separators.¹⁰ However, this study did not report the amalgam removal efficiency of the separators. An evaluation of point-of-discharge (that is, into-the-sewer) treatment technologies used by commercially available amalgam separators also did not address amalgam removal efficiency.¹¹

Amalgam removal technologies. Several types of removal technologies are used in the design of amalgam separators. Our results show little difference among the separators in regard to efficiency. Sedimentation technology is used in the majority of amalgam separators, sometimes in conjunction with filtration and ion exchange. Because of its high specific gravity (about 10), amalgam particles settle quite readily from suspension in water. A study of the settling of amalgam in dental office wastewater samples reported that, in most samples, more than 90 percent of amalgam particles in a water column settled from suspension within about two hours.¹² Thus, sedimentation may play an important role in enabling amalgam separators that use this technology to achieve high removal efficiencies.

Filtration also is used, either by itself or in conjunction with sedimentation, as another removal technology. Our results show that an amalgam separator based solely on filtration results in amalgam removal efficiency similar to that of amalgam separators that use sedimentation in conjunction with other removal technologies. Our results also show that the efficiency of the amalgam separator that used centrifugal technology was statistically different from that of all but two amalgam separators that used sedimentation technology. However, although our results show statistical differences between amalgam separators, the differences are not likely to be practically significant because each of the separators tested had an amalgam removal efficiency of 96.09 percent or greater.

Total mercury concentration. We evaluated the total mercury concentration in effluent from the amalgam separators because regulatory agencies almost always use this concentration to define wastewater discharge limits or water quality limits. Our results show that the total mercury concentrations in the effluent were in the parts-per-billion range and varied widely for each amalgam separator tested. This could be due to the variation in the number of small amalgam particles in each of the ISO samples used. We should note that although the results cover a wide range, they may suggest that the total mercury concentrations in the effluent from some amalgam separators are higher than those from other separators. However, our data in regard to total mercury concentration in the effluent are too limited to allow us to make such a conclusion.

Previous studies have reported that effluent from laboratory testing of amalgam separators contained amalgam particles 100 µm or smaller.¹³ Variations in the number of amalgam particles that were 100 µm or smaller in the ISO amalgam samples could have caused the wide ranges that we observed in the total mercury concentration values. Additional research to determine the total mercury concentration in effluent using amalgam samples containing only particles 100 µm and smaller would provide further information to address this issue.

Total dissolved mercury concentration. We also evaluated the total dissolved mercury concentration in effluent from the amalgam separators. The EPA considers all mercury species that pass through 0.45-µm filters to be dissolved mercury. Thus, colloidal amalgam particles smaller than 0.45 µm and ionic mercury in solution are both included in the total dissolved mercury determination.

As is the case for total mercury concentration in effluent, we observed wide ranges in values for total dissolved mercury for each amalgam separator. However, these values were higher than the value of mercury in filtered tap water. Variations in the number of amalgam particles smaller than 0.45 µm in the samples tested could explain the wide-ranging values, since the ISO standard does not specify the number of amalgam particles that can be smaller than 0.45 µm. To evaluate the efficiency of amalgam separators in removing dissolved mercury, samples containing defined numbers of colloidal amalgam particles and a defined amount of ionic mercury in solution would be needed.

The results of our laboratory evaluation show that amalgam separators removed more than 96 percent of the amalgam particles in ISO-defined test samples. However, the effluent from the amalgam separators, when tested according to the EPA method (which includes acid digestion to change amalgam to cationic mercury for chemical analysis), contained total mercury concentrations in the parts-per-billion range. Regulatory agencies typically use the EPA test method to measure total mercury in wastewater. Because the effluent limits for mercury in some areas of the country are lower than the parts-per-billion range and approach the parts-per-trillion range, our test results show that the effluent from amalgam separators would not meet these limits for mercury.

	CONCLUSION

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION Amalgam in dental office... REFERENCES

 
In this laboratory evaluation of 12 commercially available amalgam separators, we found that all had amalgam removal efficiencies of 96.09 percent or higher, which surpasses the 95-percent requirement specified in the ISO 11143 standard for amalgam separators. The statistical differences in efficiency between separators probably are not practically significant. The total mercury concentrations in the effluent from laboratory testing of the amalgam separators were in the parts-per-billion range and varied widely for each separator, as did the concentrations of total dissolved mercury in the effluent. Additional research is needed to develop test methods for evaluating the efficiency of amalgam separators in removing small amalgam particles, colloidal amalgam particles and ionic mercury in solution.

	Amalgam in dental office wastewater

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION Amalgam in dental office... REFERENCES

 
Addressing the issue In recent years, the U.S. Environmental Protection Agency, or EPA, has substantially lowered the maximum level of mercury allowed in rivers, streams and other bodies of water. As a result, municipal sewage treatment plants now are required to substantially decrease the amount of mercury in their treated waste-water discharge.

Source control, which is the elimination of mercury from the wastewater entering sewage treatment plants, is the method being promoted by the EPA and the sewage treatment agencies for the reduction of mercury discharged into the surrounding aquatic and natural environment. In certain parts of the country, most notably states in New England, in the Great Lakes region and on the West Coast, dental offices are being asked to practice source reduction to decrease the amount of amalgam discharged into wastewater. Source reduction can vary from best management practices, including the proper disposal of chairside traps and vacuum filter traps, to the voluntary installation of amalgam separators.

The installation of amalgam separators that meet the international standard of 95 percent amalgam removal efficiency may not satisfy the increasingly stringent regulatory requirements that have been established in some locations.

Recognizing the need for information about amalgam separators, the Association conducted an evaluation of amalgam separators in regard to their amalgam removal efficiency in a laboratory setting, according to International Organization for Standardization Standard 11143 for Amalgam Separators.¹ The evaluation also measured the total mercury concentrations in effluent from the amalgam separators during laboratory testing. Although each amalgam separator tested exceeded the international standard of 95 percent amalgam removal efficiency, the installation of amalgam separators that meet this standard still may not satisfy the increasingly stringent regulatory requirements that have been established in some locations.

In addition to the laboratory evaluation results presented in the preceding article, more detailed product information about commercially available amalgam separators is provided in the following table. When making a decision to install an amalgam separator, dentists should consider the following: one’s professional need to meet established regulatory requirements for mercury discharge; the dental office’s plumbing configuration; the physical space required for installation; the maintenance that will require attention by dental office personnel; and the proper disposal of collected amalgam waste.

The Association continues to actively address the issue of amalgam in dental office wastewater through the implementation of an action plan, which was approved by the ADA House of Delegates in 2001. This action plan includes further evaluation of amalgam reduction technologies and providing assistance to state and local dental societies in their response to amalgam wastewater issues in their localities. Dentists facing amalgam wastewater issues are urged to contact their state and local dental societies for additional information.

1 International Organization for Standardization. ISO 11143:1999. Dental equipment—amalgam separators. Geneva, Switzerland: International Organization for Standardization; 1999.