Evaluation of a Filter-Based Dental Wastewater Mercury Removal System

Objective:  This study evaluated the ability of chairside filters, of several pore sizes, to remove particulate-based mercury (Hg) from dental-unit wastewater.  Methods: Chairside filtration systems, with nominal filter pore sizes of 50μm, 15μm, 1μm, 0.5μm, or with no filter, were installed in five dental operatories. Daily chairside wastewater samples were collected on ten consecutive days from each room and brought to the laboratory for processing:  Wastewater samples were filtered at 0.45μm and these filters (with residual debris) were digested in HCL and HNO3 (3:1) at 95°C for 2 hours then allowed to cool to room temperature.  Hg concentrations were determined with USEPA method 7470A; total Hg in units of mg Hg per chair per day were calculated.  Filter systems were exchanged (after 10 samples were collected) so all five of the configurations were evaluated in each room (with assignment order balanced by a Latin Square).  The number of amalgam-surfaces-placed and removed per room per day was tracked.   Results:  The mean Hg per chair per day when no filter was installed was 1,087.38 mg (sd=993.92).  The means for the 50μm, 15μm, 1μm, 0.5 μm samples were 79.13 (sd=71.40), 23.55 (sd=23.25), 17.68 (sd=17.35) and 4.25 (sd=6.35) mg Hg per chair per day (n=50 for all groups).  Calculated removal efficiencies were 92.72%, 97.83%, 98.76%, and 99.61%, respectively.  ANCOVA on data from the four filter groups, with amalgam-surfaces-removed included as a significant covariate, was statistically significant (P<0.0001). Tukey post-hoc comparisons indicated that the 50μm filter removed less mercury than all other filters, and the 0.5μm removed more mercury than the 50μm and 15μm filters.   Chairside vacuum levels were not adversely impacted.  Conclusions: Chairside filtration systems substantially reduce Hg emissions into wastewater, at low cost and with minimal maintenance.  This study was supported by the U.S. Navy Bureau of Medicine and Surgery and the USEPA.