Objective: to develop a method for determining the optimal hydrological characteristics of systems for treating contaminated surface and drainage waters. Methods: the pollutant balance equation was applied. For comparison, the method of the Vodgeo Research Institute was used. The costs of treatment and environmental damage from pollution of watercourses were studied for various water levels of the year and the parameters of treatment facilities. The main parameters chosen are the types of pollutants (suspended substances and petroleum products) and the separation coefficient of the runoff in the separation chambers. Results: a method has been developed for optimization calculations of the hydrological characteristics of surface and drainage water treatment systems, taking into account the likelihood of runoff during high water and floods, the costs of construction and operation of treatment facilities and damage from pollution of watercourses. As an example, optimization calculations were performed for several options for treating melt, rain and drainage runoff. Practical importance: the method of optimization calculations of environmental protection will allow a more reasonable and accurate determination of the required degree of wastewater treatment, productivity and parameters of treatment facilities, and in some cases — to reduce the degree of pollution of watercourses.