Achieving low mercury concentrations in chlor-alkah wastewaters

In the early 1980s, EPA established production-based effluent limitation guidelines for mercury in wastewater discharges from mercury cell chlor-alkali plants based on the application of best available technology economically achievable (BAT). The principal process for achieving the BAT guidelines has been chemical precipitation with sulfide compounds, followed by filtration to remove the mercury as insoluble mercury sulfide. This treatment process typically results in effluent mercury concentrations ranging from 10 to 50 mug/kg (ppb). In response to changes in environmental laws in Maine in 1997, HoltraChem Manufacturing Company in Orrington, Maine, began searching for new technologies to reduce mercury in its effluent. Working with suppliers, an onsite pilot plant test with sulfur-impregnated, activated carbon, MERSORB(R) LW mercury adsorbent, indicated it was feasible to achieve effluent mercury levels of <100 ng/kg (ppt). A full-scale treatment system was designed and installed after the existing primary sulfide treatment process to treat 100 gpm of wastewater. Mercury concentrations after initial start-up of the secondary adsorption process were one to two orders of magnitude higher than expected. After optimization of the sulfide treatment process, optimizing pH of the influent and the addition Of 0.5 micron filters prior to the adsorption trains, mercury concentrations averaging <100 ppt and as low as 20 ppt have been achieved.