What is known about the toxicity of silver-bearing waste?
Silver in wastes derived from the photoprocessing industry, because of the high rate of physical and chemical binding, is hundreds to thousands of times less toxic than “free” silver. This means that silver is not toxic to aquatic organisms in environmentally realistic concentrations, even close to POTW discharges. Silver thiosulfate, the dominant silver compound in photoprocessing effluents, has been demonstrated to have a toxicity in the range of 160,000 ug/L, while silver nitrate, which dissociates into ionic silver, has a toxicity range of 1-10 ug/L. Silver nitrate is not expected to be found in the environment. The high concentration of organic and sulfur-based materials present in municipal wastewater treatment systems ensures that any active silver materials are quickly combined and sequestered into the sludge. Typically, 90% or more of the incoming silver load is removed in this fashion.
