The Fate of Heavy Metals in Pilot‐Plant Scale Upflow Sludge‐Blanket Clarifiers

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Abstract
SYNOPSIS: The Removal of heavy metals and aluminium residuals during upflow sludge‐blanket clarification was investigated using a pilot‐plant system. Pilot units treating predosed raw water on site were assessed for sludge‐blanket formation and stabilization, and were found to produce clarified water of a comparable quality to the main works units after the start‐up and stabilization periods were complete. The heavy metals Cr, Fe, Mn, and Ni were the only heavy metals detectable at the investigation site and these occurred at concentrations well below guidelines values for potable abstraction and supply.Mean removals of Fe, Mn, Cr, and Ni were about 98, 89, 51, and 30 per cent respectively, during steady‐state operation, and Fe, Mn, and Cr all displayed reduced removals during transient changes in surface load, whereas Ni removal appeared to be relatively independent. Aluminium residuals derived from the dosed alum coagulant also showed decreases in removal from about 83 per cent during steady‐state operation to between 40 and 50 per cent during transient experiments.