The impact of heavy metals on biocenotic population of wwtp’s activated sludge

Abstract

Heavy metals are toxic substances which, at certain concentrations have serious impact upon water treatment plants. This leads to impaired biological processes of activated sludge, changes in composition of biocenosis and, eventually, no biological activity. The purpose of lab experiments was to test the influence of some heavy metals (cadmium, copper, zinc and nickel) at different concentrations, in batch bioreactors, with a retention time of wastewater of 24 hours. The inoculum was biological sludge and the bioreactors were fed with municipal wastewater supplemented with heavy metals. The community of ciliates is an indicator of the activity of biological sludge, therefore these were predominantly monitored throughout the experiment. The experimental results indicated that of all tested metals, cadmium at a concentration of 0.5-1 mg/l was mainly toxic, while zinc and nickel were less toxic, even at concentrations of 1.5-2.5 mg/l. The most sensitive species of protozoans to heavy metals were Chilodonella uncinata and Oxytricha sp., while the most resistant were the attached ciliates, like Opercularia coarctata and Vorticella microstoma. The attached ciliates resistant to high metal concentrations underwent morphological changes, like legthening of stem or forming buoyant individual cells or colonies. The study emphasized the presence and survival of ciliates in activated sludge in the presence of heavy metals at concentrations exceeding the admitted limits in WWTP influents.