The influence of flow conditions on the removal of heavy metals in the primary sedimentation process

A pilot scale primary sedimentation pilot plant, treating raw sewage, was used to assess the efficiency of removal of Cd, Cr, Cu, Ni, Pb, and Zn. Heavy metal removals were determined during conditions designed to simulate normal dry weather flow, three times dry weather flow and diurnal variations in flow. During these experiments, the flow of sewage to the works where the pilot plant was sited also varied. It was found that short term variations in heavy metal concentrations in the raw sewage were related to the flow, although percentage removals appeared to be relatively unaffected by heavy metal concentrations. Removals during normal dry weather flow conditions were generally in the range of 25 to 50%, with Cu and Pb removals highest and Ni removal lowest. Lower removal efficiencies were observed when the flow to the pilot plant was increased three-fold, and the poorest removals were observed under variable flow conditions.