The sewage treatment plant was monitored to determine the efficiency of technologies employed to reduce microbiological threats. Every four weeks, for a year, sewage was sampled to investigate the effects of post-mechanical and post-biological treatment. Changes in Escherichia coli, D-group streptococci and Salmonella spp. number were analyzed. The reduction of the bacteria in sewage treated mechanically was not significant and did not exceed 1 log. However, a significant reduction in an average number was found. In ad-dition, seasonal variation was found to correlate with the number of each of the selected bacteria groups.