Combined sewer systems in cities are increasingly equipped with additional storage facilities or other installations necessary for keeping the wastewater treatment plants from overloading during wet weather and reducing combined sewer overflows into receiving waters. Effective methods for reducing such negative phenomena include the temporary storage of wet weather flow in an end-of-pipe separate tank or in a sewer system. In this paper, four scenarios of wastewater storage for the Group Wastewater Treatment Plant (GWWTP) in Lodz (Poland) have been analysed: a storage in a separate single tank located in GWWTP, a storage in the bypass channel in GWWTP, in-sewer storage, and a combination of the aforementioned variants, also with real time control (RTC) system introduced. The basic calculations were performed using the EPA’s SWMM software for the period of 5 years (2004-2008). The chosen solution - storage in a separate storage tank - has been verified based on the inflow dataset from the years 2009-2013. The specific volume of the separate storage tank should be at least 22 m3 per hectare of impervious catchment area, but it could be reduced if additional in-sewer storage with RTC were introduced. Both options allow the effective protection of receiving waters against discharge of untreated sewage during wet weather.
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