Toxic Effect of Chemical Disinfection of Wastewater on Freshwater Ciliates

The toxic effect of chemical disinfection of sewage treatment plants was tested on freshwater ciliates inhabiting the receiving water bodies. For this purpose, the effluent from an activated-sludge plant was treated with sodium hypochlorite (NaClO), chlorine dioxide (ClO sub(2)) and peracetic acid (PAA) for 15 min and the treated effluent was inoculated for 24 h with the ciliates Dexiostoma campylum, Euplotes patella and Spirostomum teres. The effluent treated with ClO sub(2) was highly toxic to S. teres but resulted only slightly toxic to the other two ciliate species. The effluent treated with NaClO produced a moderate toxic effect only on E. patella. The effluent treated with peracetic acid caused mortality only to E. patella. Since the poly-isosaprobic ciliate D. campylum - the smallest of the ciliate species tested - showed the lowest sensitivity, this can lead to the hypothesis that toxicity of the treated effluents may be related to both saprobity and size of the ciliates.