The paper presents microbiological characteristics of the sewage sludge from the sewage treatment plant, in controlled conditions with biowastes (straw, sawdust, bark, hemp). In experiment the material was mixed in the appropriate weight proportions and located in bioreactor chambers with the constant air flow. The performed composting process aimed at the determination of the developement dynamics and survival of pathogenic microorganism in sewage sludge, composed with different additions in a cybernetic bioreactor. Compost samples were taken for laboratory analysis with reference to the same temperature value. Microbiological analyses were carried out on selective medium using the plate method, and the number of pathogenic bacteria from the Salmonella genus, the Clostridium perfringens and the Enterobacteriaceae family were determined. In additions, egg of Ascaris ssp., Trichuris ssp., Toxocara ssp., parasites were isolated using the floatation method. The results of both experiments have shown, that sewage sludge did not contain bacteria from Salmonella genus and ATT parasite eggs. The composting process entirely reduced the number of the Enterobacteriaceae bacteria, however it didn't contribute to the total elimination of the Clostridium perfringens bacteria. On the basis of presented analysis it was shown that reduction of the studies groups microorganisms was connected with temperature increase in all composts and it was shifted in time. The composts met the sanitary standards, according to Commision regulations (EC) No 185/2007, from 20 February 2007, amending Regulations (EC) No 809/2003 and (EC) No 110/2003 as regards extension of the validity of the transitional measures for composting and biogas plants under Regulation (EC) No 1774/2002 of the European Parliament and Council.