In the article the analysis of the quantity variability of the sewage inflowing to the selected wastewater treatment plants of Sanok district: Zagórz and Besko was performed. The study was conducted based on the records of the daily sewage volume inflowing to the analyzed treatment plants in the period of 2007–2014 (Zagórz) and 2011–2014 (Besko). Based on the performed analysis it was concluded that analyzed wastewater treatment plants have worked under hydraulic underload conditions. Moreover, a significant irregularity of the volume of the sewage inflowing to the both wastewater treatment plants was noted. Performed analysis of trend has shown a significant growing trend of the sewage volume inflowing to the Wastewater Treatment Plant in Zagórz and the constant sewage inflow in the case of Besko treatment plant. The study of the seasonality has not shown a significant dependence between the quantity of the inflowing sewage and a particular months. In addition, it was concluded that there is a significant dependence between the precipitation amounts and the volume of the sewage inflowing both to the Wastewater Treatment Plant in Zagórz and in Besko.