Effect of EDTA on Cadmium in Activated Sludge Systems

In the absence of a strong chelating agent, ethylenediamine tetraacetate (EDTA), the accumulation of cadmium (as Cd(II)), in activated sludge proceeded rapidly during the first 5 days and the first 2 to 6 hours, respectively, during long- and short-term experiments. The uptake rate then decreased to a slower level for both modes of experiment. The rate of Cd(II) uptake followed that of a simple first-order expression. Of the total amount of Cd(II) accumulated, 20% was an acid-digestible fraction that reached an equilibrium condition in 2 days during the long-term experimental runs. A plot of the amounts of Cd(II) uptake versus Cd(II) remaining in the supernatant yielded an S-shaped curve indicating strong competition between water molecules and Cd(II) species and lateral interaction among "adsorbed" Cd(II) ions. When EDTA was present, the extent of Cd(II) uptake was markedly reduced, whereas the rate of Cd(II) uptake remained relatively unchanged. The accumulation of Cd(II) tended to decrease the COD removal efficiency and enhance the ${\rm O}_{2}$ uptake rate. The contents of amino acids and proteins also increased with the amounts of Cd(II) accumulated.