Both too large and too small volume of sewage inflowing to wastewater treatment plant in relation to its volume determined during designing process, can contribute to the reduction of sewage treatment effectiveness. Taking into consideration, in this article, the analysis of the sewage quantity discharging from the Sanok agglomeration and treated in domestic wastewater treatment plant in 2013-2015 was performed. The analysis indicated that in each year of the analyzed period, to the Wastewater Treatment Plant in Trepcza, a similar volumes of sewage was inflowing and the maximum hydraulic capacity of the object was not exceeded, what is evidenced by the average daily sewage flow values. It is noted that in rainless periods, the quantities of inflowing sewage were smaller than during days with wet weather. Despite of periodically situations of the hydraulic overload and hydraulic underload, it can be stated that the effectiveness of the sewage treatment processes is not endangered by the excessive or insufficient quantities of inflowing sewage.