Simulation of surface wastewater flow volume using the city of Kyiv as an example

Abstract

Although observations of the negative impact of surface runoff on water quality in water bodies have a long history, the development of national regulations governing surface water quality took place only in 2022. Accordingly, the Law of Ukraine "On Water Drainage and Wastewater Treatment" was adopted, which entered into force in August 2023 and is part of the process of adapting Ukrainian legislation in the field of water drainage in accordance with Council of Europe Directive 91/271/EEC "On Urban Wastewater Treatment". The paper analyzes the problem of determining the volume of annual surface runoff to be cleaned at treatment facilities. Using the data of the hydrometeorological service on the maximum daily amount of precipitation of various types for the city of Kyiv, the dependences of the distribution of the probability of daily layers of precipitation, in the form of the total depth of precipitation and the total duration of precipitation, were obtained. The conclusion that the predominant volume of runoff is determined by microrains that have a high frequency of recurrence is confirmed. The problem of finding the ratio of precipitation and runoff in the regulatory documents of Ukraine is covered very primitively, which is why there is a need to determine the depth of runoff on a more advanced basis, taking into account infiltration, which is determined by the parameters of the filtration properties of soils. The effect of imperviousness on the drain layer is determined through simulations using the SWMM distributed model. The F/P parameter is proposed, which characterizes the volume of losses from the total amount of precipitation depending on the properties of the surface, namely waterproofness and filtration coefficient. The modeling made it possible to obtain the dependence of the F/P parameter on the filtration coefficient and the imperviousness of the catchment area. Calculations confirmed that the runoff in most cases is formed as a result of precipitation, the value of which exceeds 2.5 mm. For the city of Kyiv, the volume of surface wastewater runoff for the warm period of the year from an area of 1 ha was calculated depending on the soil filtration coefficient at 60% imperviousness. As a result of these studies, it is recommended that sewage treatment plants capture and treat rainwater with a probability of 70 to 90% of the total depth of precipitation that falls on the catchment area. At the same time, the value of the flow volume of surface wastewater must be confirmed by calculation.