The Environmental Aspects of Mercury
in Dental-Unit Wastewater
In 1997 it was estimated that dental facilities in the United States used 40 metric tons of Hg (6). The Seattle Metro Study (7) and a later study by Barruci et al. (8) reported that about 8-to-14% of the Hg load to local sanitary districts originated from dental clinics. There have been few studies investigating the environmental aspects of the metals released from dental amalgam (9). Recent collaborative studies by Naleway et al. (10) and Cailas et al. (11) were the first to rigorously define the dental amalgam-wastewater stream. A later study (12) demonstrated the presence of significant levels of soluble (<0.45 µm) Hg (13) in the fluid portions of dental-unit wastewater.
Industrial wastewater-treatment technologies have been developed to address specific manufacturing applications (14). However the use, development, and research on waste-treatment technologies for dental-operatory wastewater are in their infancy (15, 16). Due to the relatively small quantity of dental-operatory wastewater generated and its heterogeneous nature, developing effective, nontoxic, and cost-effective treatments has been difficult (10, 11, 17, 18).
Under Section 402 of the Clean Water Act, all point source discharges of pollutants to waters of the United States (lakes, rivers, wetlands, etc.) must be authorized under a National Pollutant Discharge Elimination System (NPDES) permit. In addition to direct discharges to US waters, industrial discharges to sanitary sewer systems must also meet standards and other local limitations designed to protect the water treatment facilities of the publicly owned treatment works (POTWs). NPDES discharge permits are not required for these indirect discharges, but pollution control standards are implemented through locally issued permits under the Industrial Pre-treatment Program. Municipal NPDES permits regulate POTWs and are issued to these public bodies created by State or Federal law. Most POTWs were not designed to treat toxic pollutants. As a result, dischargers, including dental treatment facilities, may be required to pretreat their effluent prior to discharge. This is precisely what transpired at the Naval Dental Center, Norfolk (12). Waste pre-treatment consists of techniques or “management practices” used to reduce or eliminate contaminates that interfere with the microorganisms used by local POTWs to facilitate waste treatment.
Two agencies did not have local limits for Hg, one agency had a narrative pollution prevention standard for Hg, and one agency had a tiered Hg limit based on flow rates from facilities. This variability in local discharge limits can create difficulties for Dental Treatment Facilities trying to meet their POTW discharge limits. Naval Institute For Dental and Biomedical Research is currently assembling a database of national discharge limits that will be made available to dental treatment centers at a future date.
The 400-fold decrease in the detection limit for Hg achieved with standard method 1631 will necessitate the lowering of Hg discharge limits by POTWs.
In an effort to meet local POTW mandated discharge limits (0.1mg/liter average daily discharge and 0.05 mg/liter average monthly discharge) the clinic installed commercially available centrifugal amalgam separators. Six samples of the centrifuge-treated wastewater showed a mean total Hg concentration of 3.91 mg/liter (n=6, SD=0.274). Soluble Hg concentrations (< 0.45 mm) were found to be 0.37 mg/liter (n=6, SD=0.064). Mechanical separation was not sufficient to meet local POTW discharge limits and appeared to increase Hg levels when compared with sedimentation alone. Sedimentation alone can remove 95% of mercury. Further analyses determined that the concentrations of soluble Hg in the centrifuge-treated samples were higher than local POTW discharge limits.
The centrifuges were removed and the first Naval Institute For Dental and Biomedical Research designed system employing a combination of sedimentation, filtration, and ion exchange technologies was installed. A standard air-water separation tank was modified to enhance sedimentation. The clarified wastewater was then pumped through a graded series of filters and finally through cat-ion exchange columns prior to discharge into waste lines. This system reduced Hg levels sufficiently for the POTW to allow dental facility reconnection to the sewer system. The Naval Institute For Dental and Biomedical Research modified settling tank and method has been awarded a patent (22).
An important finding of this effort was the determination that a large amount of Hg is retained in wastewater lines. Some portion of amalgam waste never leaves the building, but is deposited in the wastewater lines. Five copper waste lines serving the dental-unit wastewater stream were collected and analyzed for total Hg using standard method 7471. An average of 1097 mg Hg per kg pipe was found, with a range of 606-to-1603 mg/kg (SD=399). This retained Hg can be solubilized through the action of oxidizing line cleaners (23) and may in itself cause excessive Hg releases.
Commercially available polymers provide an effective treatment option for some dental facilities. Naval Institute For Dental and Biomedical Research tested the ability of two such polymers, individually and in combination, to remove Hg from dental-operatory wastewater (24). The two polymers selected for use in this study were an aqueous 20-to-40% solution of aluminum hydroxychloride and polyquaternary amine (25) at pH 4.0 (Nalco polymer N8186, Nalco Chemical Company, Naperville, IL) and an aqueous solution of polymeric precipitant and salt at pH 11.5-13.0, with metal chelating molecules bound to a polymer backbone (26,27) (Nalco polymer N8702, NALMET, Nalco Chemical Company, Naperville, IL). The company recommends a pH range of 6.0-9.0 as optimal for the use of these polymers. Contact time between the polymers and mercury in the waste stream is an important factor in the effectiveness of this system.
The polymer based treatment system has been installed in a large 45-chair dental clinic and is currently on-line pre-treating the wastewater stream. A standard plate and frame filter press dewaters the sludge produced by the pre-treatment process. The dewatered sludge is sent for recycling at a licensed Hg retorting facility. Hg levels leaving the press have been as low as 9 mg/liter.
Two Hg binding materials are currently being tested as a possible technology to pre-treat dental-unit wastewater. The first, Keylex resin (Solmetex, Inc., Billerica, MA) is being tested at a 35-chair dental-treatment facility. The wastewater is pumped from four 50-gallon air/water separating tanks into a 125-gallon pre-treatment tank where chlorine is added to disinfect and oxidize the waste. Gross filtration is used to remove large particles prior to passing through 25mm and 1 mm filters. The filtered waste then flows through the Keylex resin containing cartridges. The flow rate is maintained at <250 ml/minute. Baseline Hg levels from the holding tank averaged 6.3 mg/liter (n=10, SD=1.2). The Hg levels of the Keylex treated samples were all at non-detectable levels when a method detection limit of 0.2 mg/liter was employed (28). Two earlier samples were found to have non-detectable levels of Hg using standard method 1631. Work continues on optimizing this system and determining the cartridge replacement intervals.
The second material being tested is a microbial biosorbent derived from genetically engineered bacteria. These bacteria express a metal binding motif on their cell surface. This biosorbent is capable of removing at least 94% of Hg from dental wastewater in bench top jar testing (29). Other devices are being constructed and will soon be installed at test clinics for evaluation and modification. Some of these may be suitable for use at clinics with 30 or more chairs and others for clinics with four or fewer chairs.
Best Management Practices
1. U.S. EPA. Mercury in the Midwest: Status and Future Directions. Conference fact sheets 1 and 4, 22-23 October 1996. Chicago, IL: U.S. Environmental Protection Agency, Region 5, 1997.
2. Bernier J, Brousseau P, Krzystniak K, Tryphonas H, and Fournie M. Immunotoxicity of heavy metals in relation to Great Lakes. Environ Health Perspect 103:23-34,1995.
3. Rice CD. Neurotoxicity of lead, methylmercury, and PCBs in relation to the Great Lakes. Environ Health Perspect 103:71-87, 1995.
4. Knobeloch LM, Ziarnik M, Anderson HA, and Dodson VN. Imported seabass as a source of mercury exposure: a Wisconsin case study. Environ Health Perspect 103:604-606, 1995.
5. U.S. EPA. Great Lakes Water Quality Initiative Criteria Documents for the Protection of Wildlife: DDT; Mercury; 3,3,7,8-TCDD; PCBs. EPA-820/B-95-008. Washington, DC: U.S. Environmental Protection Agency, 1995.
6. The Knight-Ridder CRB Commodity Year Book. New York: John Wiley and Sons, 1998.
8. Barruci M, Dana J, Moeller G. Interactive Qualifying Project Report: Problem Constituents Discharged from Dental Business in the City and County of San Francisco. Rpt no.052-069a-950. San Francisco, CA: City and County of San Francisco Department of Public Works, Bureau of Environmental Regulation and Management, 1992.
9. Arenholt-Bindslev D. Dental amalgam environmental aspects. Adv Dent Res 6:125-130, 1992.
10. Naleway C, Ovsey V, Mihailova C, Chou H, Fan PL, Whitlock R, Meyer D, Cailas M, Ralls S, and Cecil J. Characteristics of amalgam in dental wastewater. J Dent Res 73:105, 1994.
11. Cailas MD, Ovsey VG, Mihailova C, Naleway C, Batchu H, Fan PL, Chou HN, Stone ME, Meyer DM, Ralls SA. Physical-chemical properties of dental wastewater. In: Proceedings of WEFTEC’94, Water Environment Federation, 67th Annual Conference and Exposition, 15-19 October 1994, Chicago, IL. AlexaNaval Institute For Dental and Biomedical Researcha, VA. Water Environ Feder 76:317-327, 1994.
12. Stone ME, Deutsch W, Roddy W, Ralls S, Meyer D, Cailas M, Batchu H, Naleway C, Chou HN, Mihailova C. Mercury levels and particle size distribution in the dental unit wastewater stream at Naval Dental Center, Norfolk, Virginia, USA [abstract]. Presented at the Conference on Pharmaceutical Science and Technology (International Symposium on Separation Technologies for Dental and Other Health Care Facilities), 22-25 August 1995, Chicago, IL.
13. U.S. EPA. Ambient Water Quality Criteria for Mercury-1984. EPA-440/5-84-026. Washington, DC:U.S. EPA, 1985.
14. Patterson JW. Industrial Wastewater Treatment Technology. Boston:Butterworth-Heinemann, 1985.
15. U.S. EPA. Methods for the Determination of Metals in Environmental Samples, Supplement I. EPA-600/R-94/111. Cincinnati, OH:U.S. EPA, 1994.
16. Pederson ED, Stone ME, Ralls SA, Roddy WC, and Choo PL. Mercury removal from dental operatory wastewater using polymers. J Dent Res 75:37, 1996.
17. Ralls SA, RoddyWC, and Pederson ED. Mercury removal method and device for dental operatory wastewater. J Dent Res 75:169, 1996.
18. Ovsey VG. Recycling potential of the dental wastewater stream. [Master’s thesis]. University of Illinois at Chicago, Chicago, IL, 1995.
19. Title 40 Code of Federal Regulations, Protection of the Environment. Chap 1.
20. U.S. EPA Methods for the Determination of Metals in Environmental Samples, Supplement I. EPA-600/R-94/111.Cincinnati, OH: U.S. EPA, 1994.
21. Title 40 Code of Federal Regulations, Part 136.
22. U.S. Patent 5,885,079. Method and System for removing mercury from dental wastewater. Ralls, SA, Roddy WC, and Pederson ED. Mar. 23, 1999, Naval Dental Research Institute.
23. Stone ME, Pederson ED, Auxer RA, and Davis SL. Line cleanser/disinfectant effects on the soluble mercury content of dental wastewater. J Dent Res 78: 207, 1999.
24. Pederson ED, Stone ME, and Ovsey VG. Mercury removal from dental operatory wastewater by polymer treatment. Environ Health Perspect 107:3-8, 1999.
25. Nalco Chemical Company. A-ULTRION: Water Clarification/Pollution Control Chemicals, Cationic Coagulants. Naperville, IL:Nalco Chemical Company, 1995.
26. Choo PL, Siefert KS, and Sparapany JW. Novel chemical solutions to heavy metal removal from wastewater. Presented at the Water Environment Federation Conference, 20-24 October 1992, New Orleans, LA. Reprint 585. Naperville, IL:Nalco Chemical Company, 1992.
27. Nalco Chemical Company. Product Bulletin UN-8702: Technologies for Soluble Metal Control. Naperville, IL:Nalco Chemical Company, 1995.
28. Stone ME, Pederson ED, Jones GK, Karaway RS, Auxer RA, and Davis SL. Evaluation of an integrated system to remove mercury from dental-unit wastewater. J Dent Res 79:(In press), 2000.
29. Pazirandeh M, Stone ME, and Pederson ED. Mercury removal from dental wastewater by a microbial biosorbent derived from genetically engineered bacteria. J Dent Res 79:(In press), 2000.