The work is devoted to the task of simplifying the assessment of the effect of effluents from treatment facilities on the river hydrobiocenosis. The studies were carried out on the mountain river Uzh (Uzhgorod, Ukraine). Our approach to assessing the impact of waste treatment facilities on the river receiver is based on the estimate of the similarity of species composition and quantitative characteristics of populations of organisms from the aerotank and from the river. It is shown that the quantitative development of populations of species of ciliates from the aeration tank is a good indicator for assessing the degradation of organic matter coming with wastewater. The use of qualitative and quantitative characteristics of the protozoa from the wastewater treatment plant as a criterion for assessing the quality of the environment in the area of wastewater discharge showed their representativeness and effectiveness. The use of a limited number of species makes it possible to conduct an express assessment of the effect of effluents on receiving reservoirs for specialists working with activated sludge in the laboratories of treatment facilities.