TCLP Analysis of Chairside Dental-Unit Wastewater Samples

Objective: Concern has been raised that concentrations of mercury (Hg) and silver (Ag) in dental wastewater may be sufficient to make it qualify as hazardous waste.  To determine if a waste is hazardous, the Environmental Protection Agency (USEPA) designed a laboratory test entitled Toxicity Characteristic Leaching Procedure (TCLP; USEPA method 1311).  This study analyzed chairside dental wastewater samples to determine if they would exceed Resource Conservation and Recovery Act (RCRA) limits for Hg and Ag, two RCRA regulated metals found in amalgam.  Methods: Daily chairside wastewater samples (n=24) were collected from dental operatories over two weeks.  The number of patients treated, volume of wastewater collected, and numbers of amalgam surfaces placed and removed were recorded.  Wastewater samples were analyzed for percent solids, TCLP [Hg] and TCLP [Ag].  Hg levels in extracts were determined using EPA method 7470A and Ag levels were determined using EPA method 6010B. Results: Method 1311 procedures for percent solids (<0.5%) were appropriate.  Mean values for number of patients treated, volume of wastewater, amalgam surfaces placed and removed, and Hg and Ag concentrations in TCLP extracts are presented below. Statistical analysis showed none of the variables tracked in this study to be predictive of TCLP Hg levels.  None of the twenty-four samples exceeded the 0.2 mg/L RCRA limit for Hg (range: 0.006-0.154, 95% CI: 0.048-0.077). Conclusions: The twenty-four dental wastewater samples analyzed had Hg and Ag levels less than RCRA limits and therefore didn't meet the criteria of a hazardous waste.  The mean Hg level in TCLP extracts was 31% of the RCRA limit. This study was supported by the Navy's Bureau of Medicine and Surgery.

Patients Seen	Volume Collected	Amlgm. Surf. Placed	Amlgm. Surf. Removed	TCLP [Hg] Levels	TCLP [Ag] Levels
4.1	498 ml	14.9	2.9	0.062 mg/L	not detected
SD=1.5	SD=212	SD=6.9	SD=3.5	SD=0.0348	----

Detection limits for Hg and Ag in this study are 0.004 and 1.25 mg/L, respectively.  RCRA limits for Hg and Ag in TCLP extracts are 0.2 and 5 mg/L, respectively.