Through 1994 and 1995, several wastewater treatment plants sought variances from mercury permit limits. During the process, the facilities discovered, after using grab sampling and more sensitive analytical techniques, that mercury concentrations were at least an order of magnitude lower than originally believed. These experiences, combined with our research efforts, led permit staff to the conclusion that mercury detections from traditional sampling and analysis methods were unreliable. That strategy recognizes that although we are able to measure mercury at less than 1 nanogram per liter, the practical aspects of controlling contamination during sampling may compromise our ability to consistently obtain reliable results at these low levels. By placing the emphasis on source identification and control, the focus for mercury analyses is on the influent to the treatment plant rather than the effluent.
All municipal wastewater contains some level of mercury.
The concentration of mercury in influents to municipal wastewater treatment plants can vary widely depending on the size and types of discharges to the sewer system. The largest quantities of mercury are associated with the large plants simply because of their flow volumes. Since that influent data was collected, a number of communities have voluntarily initiated pollution minimization programs. Due to its behavior, an extremely high percentage of mercury contained in wastewater becomes concentrated in treatment plant bio-solids or sludge. Thus, bio-solids concentration data are good predictors of what plants have elevated mercury loadings.