The aim of the study was to analyse the impact of hydrodynamic disintegra-tion of thickened excess activated sludge, performed at different levels of energy density (70, 140, 210 280 and 350 kJ/l), on the respiratory activity of microorganisms. For this purpose the oxygen uptake rate test (OUR) was used. The experiment also included determination of the sludge disintegra-tion degree (DD). Studies have shown, that the value of the energy density and sludge disintegration degree affected the respiratory activity of micro-organisms. With a relatively low DD, below approx. 9 % (corresponding to an energy density of approx. 140 kJ/l), increase of microbial activity was observed (6,2 − 30,2 %), as compared with untreated sludge. However, above this value of DD, inactivation of microorganisms occurred. Deactiva-tion of microorganisms higher than 85 % was observed when the sludge disintegration degree was greater than approx. 24 % (corresponding to an energy density of approx. 210 kJ/l).