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The contribution of the major technologically important microbial groups (ammonium and nitrite oxidizing, phosphate accumulating, foam inducing, and anammox bacteria, as well as plancto mycetes and methanogenic archaea) was characterized for the aeration tanks of the Moscow wastewater treatment facilities. FISH investigation revealed that aerobic sludge were eubacterial communi ties; the metabolically active archaea contributed insignificantly. Stage II nitrifying microorganisms and planctomycetes were significant constituents of the bacterial component of activated sludges, with Nitro bacter spp. being the dominant nitrifiers. No metabolically active anammox bacteria were revealed in the sludge from aeration tanks. The sludge from the aeration tanks using different wastewater treatment technol ogies were found to have differing characteristics. Abundance of the nitrifying and phosphate accumulating bacteria in the sludge generally correlated with microbial activity in microcosms and with efficiency of nitro gen and phosphorus removal from wastewater. The highest microbial numbers and activity were found in the sludge of the tanks operating according to the technologies developed in the universities of Hannover and Cape Town. The activated sludge from the Novokur'yanovo facilities, where abundant growth of filamentous bacteria resulted in foam formation, exhibited the lowest activity. The group of foaming bacteria included Gordonia spp. and Acinetobacter spp utilizing petroleum and motor oils, Sphaerotilus spp. utilizing unsatur ated fatty acids, and Candidatus 'Microthrix parvicella'. Thus, the data on abundance and composition of metabolically active microorganisms obtained by FISH may be used for the technological control of waste water treatment.Keywords: wastewater treatment plants, activated sludge, nitrifying bacteria, phosphate accumulating organ isms, foaming bacteria, fluorescent in situ hybridization (FISH)

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