The purpose of the article is to clarify and supplement the methodology given in the current regulatory and methodological framework for the purpose of engineering calculation of the storm flow management scheme for sewage treatment plants. The flow control scheme, including an intermediate tank, a pumping station and a separation chamber, is analyzed by mathematical modeling using a computer algebra system. Changes are proposed in the calculation formula for determining the volume of storage tank for the scheme of emptying the tank by pumps, taking into account all types of separation chambers. The dependence is established and graphs are shown showing the effect on the volume of the tank of the coefficient, which takes into account the variability of the flow diverted by the separation chamber into the collector.