In the performed research, the variability of sewage inflow to the Wastewater Treatment Plant in Bielany during dry and wet weather in different years and months in the period of 2008–2012 was analyzed. Particular attention was paid to the problem of accidental water which inflows to the sewage system in Bielany. Considering the period of observation, the hydraulic capacity of wastewater treatment plant was most frequently exceeded in 2010 (172 cases), least frequently in 2012 (54 cases). The probability of the appearance of daily sewage inflows, which exceed the hydraulic capacity of wastewater treatment plant in the period between 2008 and 2012 was 42%. The performed research has shown that, after eliminating the inflow of accidental water to the sewage system, the average-daily hydraulic load would not exceed 78.1% of the designed hydraulic capacity.