HEAVY-METAL REMOVAL IN PRIMARY SEDIMENTATION .1. THE INFLUENCE OF METAL SOLUBILITY

Heavy metal removal in the primary sedimentation process is primarily influenced by metal solubility and the settleability of insoluble forms. The fraction of the total metal in soluble form in raw sewage was investigated over a range of suspended solids (SS) concentrations and chemical oxygen demand (COD) values, obtained by adding primary sludge and a soluble synthetic sewage, respectively, to raw sewage samples. The soluble fraction in raw sewage was influenced to a greater extent by suspended solids than by COD. Increasing the suspended solids concentration caused marked decreases in the soluble fraction of Cd, Cr, Cu and Tl. The addition of soluble heavy metals in order to achieve concentrations typical of mixed industrial/domestic sewage did not in general have a significant effect on solubility. Thus, the removal of heavy metals could be assessed at these elevated concentrations in a pilot scale primary sedimentation plant. The plant was operated at four different influent suspended solids concentrations at each of four different hydraulic loadings covering the range from 0. 5 to 5 dry weather flow (DWF) equivalents. Increases in influent suspended solids concentrations caused higher metal removals, whereas increasing the hydraulic loading had the opposite effect.