The study focused on the effects of inorganic coagulants and polyelectrolytes use and its both impact on sonicated digested sewage sludge. For sludge conditioning coagulant PIX 123 and polyelectrolyte Zetag 8160 were used. For untreated and prepared sludges the following tests were done: capillary suction time, vacuum filtration, structure observation and measurement of the suspension and COD in supernatant sludge. On the basis of the CST tests doses of PIX 123 and a dose of Zetag 8160 were chosen. The results have shown that the use of PIX 123 and Zetag 8160 and their combination causes a decrease in the value of CST, improves parameters of vacuum filtration. The usage of PIX 123 together with a fixed dose of Zetag 8160 reduces suspended solids and COD in the supernatant sludge.