The ecolog-microbiological structure of activated sludge in nitrification-denitrification biological excess phosphorus removal (NDBEPR) systems in classic version with real wastewaters and in the presence of inhibitor o-nitrophenol (oNP) has been investigated. The amount of the essential physiological groups microorganisms (aerobic-heterotrophic, nitrifying, denitrifying, phosphorus removing bacteria (Poly P, Poly P denitrifying), nitrophenols-degrading and heterotrophic ammonia releasing bacteria) has been defined in the presence of low and high oNP-concentrations. The dynamics of activated sludge microbial structure has been discussed in effect on treatment efficiency towards different parameters – COD, PO43− elimination, ammonia-oxidation, oNP elimination.

The results show that oNP decreases efficiency of biotreatment from 26% to 80% with the increasing of oNP concentration from 9.35 to 85.44 mg/l. The large flexibility and compensative possibilities of AS structure in NDBEPR systems have been established. The amount of poly P and oNP degrading bacteria is increased at high oNP concentration (85.44 mg/l). These microbial groups play the important buffer role for stabilization of BEPR in the presence of inhibitor, as well as for the biodetoxication of oNP.