Influence of cadmium on the performance of an activated SBR sludge treatment

High concentration of heavy metals is a toxic factor for most microorganisms. As a result, such metals give rise to severe disruption in wastewater treatment operations. It has been noticed, however, that biological systems may adapt to heavy metals when their concentration is limited. In fact, such systems can even produce some degree of metal elimination provided that an optimal exposure tempo is guaranteed. On the other hand, irreversible damage may occur when metal concentration is sufficiently high. It is the aim of the present study to report on the inhibiting effects due to Cadmium on the activated sludge treatment method. To that end, different Cd concentrations were tested (ranging from 0.25 to 14 mg l(-1)) in a Sequential Biological Reactor, using synthetic wastewater without recirculation. The feed water providing 325 mg l(-1) COD and 30 mg l(-1) NH4+ -N. The process was screened by checking elimination percentages for COD and ammonia, as well as the evolution of ciliate protozoa. In addition, research was carried out on Cd assimilation by the activated sludge system by means of measuring in the mixed liquor and in the effluent. Results showed a marked decrease in purification efficiency when Cd-concentration rose over 14 mg l(-1), in which case COD elimination dropped from 98% to 41% while ammonia elimination went down from 98% to 32%. The Sludge Biotic Index decreases to 0 at Cd concentrations above 10 mg l(-1).