The physicochemical speciation of Cd, Pb, Cu, Fe and Mn in the final effluent of a sewage treatment works and its impact on speciation in the receiving river

A scheme for the speciation of metals in freshwaters has been applied to the analysis of the final effluent from a sewage treatment plant and to the receiving river upstream and downstream of the effluent outfall. The treatment plant was selected because of the high influent and effluent concentrations of Cd. The metal speciation patterns in the effluent are interpreted primarily in terms of organic interactions, which appear to be exerting a solubilizing effect on Cd and Cu, but not on the Pb and Fe which are principally associated with the particulate size fraction (> 12 μm). The influx of metals with the sewage effluent alters the speciation pattern in the river. A large part of the Cd is added to the smallest size fraction (< 0.015 μm). However, the major part of each metal, with the exception of Mn, is associated with the colloidal and particulate size fractions, thus minimising the immediate toxic significance to aquatic life.