Heavy metal shock load in activated sludge uptake and toxic effects

Waste sludge from laboratory units at four different steady-state conditions (5–20 days sludge ages) was employed for studying heavy metal shock load by the jar test system. Metal uptake (Hg, Cd, Ni) and oxygen depletion rates were simultaneously followed. The potential adsorption constants ( k am) measured reveal a metal affinity sequence Hg > Cd > Ni, in partial contrast to the metal toxicity (MT) one, Hg > Ni > Cd. This suggests different sludge adsorption sites. Hg is preferentially adsorbed on the cell, while Cd is adsorbed on extracellular polymer slimes and Ni on capsular polymers and the cellular wall. Therefore sludge age influences Cd and Ni toxicity, while no effect is observed for Hg.