The inhibition effect of the disinfectants was investigated under laboratory conditions. COD removal, nitrification process and oxygen uptake rate were the observed processes. Disinfectants can be divided into a few groups depending on the present biocides. The results of the experiments showed a significant influence of the disinfectants containing sodium hypochlorite on the activated sludge. Domestos and Savo caused the highest inhibition on the respiration, 99% and 100%, respectively; while Asanox and Clorox had the highest effect on COD removal, 97% and 100%, respectively. Bref duo active, which also contains sodium hypochlorite, caused the lowest inhibition for all observed processes. Disinfectants based on other biocides did not cause significant inhibitions. (c) 2007 Elsevier Ltd. All rights reserved.