HEAVY-METAL BEHAVIOR DURING THE ACTIVATED-SLUDGE PROCESS .2. INSOLUBLE METAL REMOVAL MECHANISMS

The mechanism of the removal of Ag, Bi, Cd, Co, Cr, Cu, Mn, Mo, Ni, Pb, Tl and Zn has been studied in batch tests using mixed liquor from an activated sludge pilot plant operated at sludge ages of 3, 6, 9 and 12 days. The solubilities of Co, Mn, Mo and Tl were high, being greater than 50% at all sludge ages; those of Cd and Ni were moderate at between 12 and 57% and those of the other six metals were insignificant at less than 6%. The percentage associated with the insoluble settleable fraction (percentage removal) was in general greater than 60% for the other six metals. Soluble fractions of Ag, Bi, Cr, Co, Pb and Zn were less than 12% in biomass-free mixed liquor matrix. Only a small proportion of all 12 metals that was insoluble settled in the absence of mixed liquor solids, except for Co and Ni, which were almost completely soluble at all sludge ages. The majority of metals that were well removed in the presence of biomass were associated with the insoluble non-settleable fraction to the greatest extent in the absence of biomass. Interaction of solids-associated metal with the mixed liquor was concluded to be a major metal removal mechanism in the activated sludge process.