Conversion to sulfide precipitation to meet water quality based effluent limits

Many industrial wastewater treatment plants (IWTPs) are facing increasingly stringent treatment requirements resulting from the imposition of Water Quality Based Effluent Limits (WQBELs). While pollution prevention will be a preferred part of efforts to meet these requirements, it is likely that upgraded IWTP performance will ultimately be necessary in many cases. Letterkenny Army Depot (LEAD) faced stringent WQBELs for several metals in its revised NPDES permit. These new permit limits included mass-based effluent limits which did not previously exist for these metals. Operating experience indicated that the new permit limits for cadmium of 3 parts per billion (ppb) and lead of 14 ppb would be most problematic. While LEAD`s long range plans focus upon pollution prevention via source control and waste minimization, near term improvements in IWTP performance were considered essential to meeting WQBELs. Previous bench scale treatability testing of processes such as ion exchange and carbon adsorption had shown that these processes would not provide a satisfactory solution to the increased performance requirements. However, conversion to sulfide precipitation, with final effluent filtration, was considered potentially applicable. This option was developed through bench-scale and pilot-scale testing, leading to full-scale implementation.