This paper discusses the effect Thiobacillus denitrificans DSM 807 has on the denitrification processes in the municipal sewage treatment plant MEANDER. The experiments consisted of three stages. At the first stage, the efficiency of denitrification process in MEANDER's chambers was analysed. The research was conducted in the spring-autumn period. The results proved the occurrence of weak denitrification during 24 h sedimentation periods. It must be noted that the strongest denitrification occurred in January. At the second stage, laboratory tests proved that in the conditions of moderate aeration Th. denitrificans shows the same denitrification path as in the conditions of zero aeration. On the other hand, slowing down the denitrification processes in the conditions of high concentration of tiosulfate was observed. At the third stage, attempts to inoculate activeted sludge in the chambers of MEANDER with Th. denitrificans culture showed that the intensity of nitrate reduction increased several times in comparison to the analogous process without inoculation.