Heavy metals effect on the heterotrophic activity of activated sludge

Respirometric techniques were applied to estimate the toxic effect of several heavy metals (chromium(III), lead, nickel, mercury and zinc) on the activity of the heterotrophic bacteria present in activated sludge and to ascertain how the presence of these pollutants can affect biological treatment. For the assays, activated sludge was collected from the biological reactor (oxidation ditch) of a sewage treatment plant that also receives industrial wastewater. The tests were performed in a batch respirometer operating in dynamic mode, and the corresponding respirograms were obtained. The values obtained (between 0.47 and 0.71) for the specific growth yield coefficient in the different activated sludge samples pointed to the adequate performance of the biological treatment applied. The experimental data obtained showed that inhibition percentage is a logarithmic function of metal concentration, the toxicity of the heavy metals following the order: Hg ≫ Zn > Cr > Pb > Ni.