The article considers the problem of rationing wastewater discharge in terms of environmental risk management, caused by the pollution of waterbodies. The cases of using a simple indicator of water quality in the form of concentration of a substance, as well as a complex indicator of water quality are considered. The relevance of using a complex indicator is caused by the similar effect of various substances on the ecosystem and man. For both cases algorithms for determining the permissible composition of wastewater, ensuring compliance with the established standards of natural water quality in the control points of the water body with a given probability have been developed. In the case of using complex indicators in order to unambiguously solve the problem of rationing wastewater composition, permissible concentrations of substances are finally determined by solving the optimization problem. The optimization criterion is the minimum cost of water treatment. The method of ecological standardization of wastewater composition suggested in the article is applicable for the case of normal probability distribution of a substance in wastewater according to data of field measurements.