WRRCTR No.82 Fate of Heavy Metals in the Sewage Treatment Process

Abstract

Heavy metals have long been known to exert toxic effects upon plants and animals at all trophic levels. Through the use of water as a cleansing and transporting agent, various types and forms of metals find their way into the sewerage system. To determine the fate of heavy metals sewage treatment processes, a study covering a period of five months was conducted at the Wahiawa Sewage Treatment Plant. The study involved determining the distribution of metals in the various fractions of the wastewater and sludges in addition to the overall removed in the system. The step-aeration activated sludge treatment process, employed at Wahiawa, removed most of the heavy metals, Al, Cd, Cu, Fe, Hg, Pb, and Zn, that were detected at the plant in concentrations ranging from a few parts per billion of mercury to a few parts per million of iron. Hexavalent chromium was removed somewhat less efficiently and nickel was reduced in concentration only slightly compared to the other metals monitored. Results of this investigation showed that most of these metals were removed by precipitation with the sludges in primary treatment and further removal occurred through biological uptake in the secondary phase of treatment. The residual concentrations of metals in the final effluent discharge were usually below toxic levels, with the exception of nickel, for freshwater aquatic organisms and plants.