Comparison of TMT-15^® and Nalmet^® in the Removal of Mercury (Hg) from Dental Wastewater

Objective: This study compared two different chemicals for removing Hg from dental-unit wastewater. Methods: Wastewater from a 107-chair dental clinic was collected in a 250-gallon tank and pH was adjusted.  Nalmet®, a proprietary Hg specific polymeric precipitant (Nalco, Inc.) or TMT-15®, (15% 2,4,6-Trimercapto-s-triazine, Degussa, Inc.) were added (0.5% and 1.0%, respectively) and mixed for 20-minutes.  A coagulant, Ultrion® (Nalco, Inc.) was added (0.5%) and mixed for another 20-minutes.  Treated wastewater was pumped through a Netzsch® filter press.  Two samples were collected (and averaged) at baseline, 5, 30, and 120-minutes from 8 batches of wastewater treated with Nalmet® and 6 batches treated with TMT-15®.  Data were analyzed for total Hg (method 7470A).  TCLP studies (method 1311) for Hg and Ag were completed on the sludge.  Results: Statistical evaluation showed the baseline Hg level to be positively correlated with observed treatment levels (r=0.244, 0.613, 0.423; P=0.400, 0.020, 0.132; for baseline versus 5, 30, and 120 minute data, respectively).  Because of these associations, the effect of treatment group was evaluated using a repeated measures analysis of covariance, with baseline as the covariate.  Raw and covariate adjusted means are: 

Conclusions: Because of sample size limitations and large variances, this data set did not yield categorical evidence of significant effects.  Time was not significant (P=0.082 though a quadratic effect (unadjusted for time scale) was (P=0.025).  Neither the effect of Group (P=0.096) nor the Time by Group interaction (P= 0.721) were significant.  Sludge samples from both Nalmet® and TMT-15® passed TCLP testing for Hg and Ag.  Data collection continues.  This study was supported by the USEPA Great Lakes National Program Office: IAG# DW17947929-01-0.