The aim of this study was to determine of influence of atmospheric precipitations in flow to sewer system on temperature and the amount of raw sewage flowing into the sewage treatment plant. Studies carried out in the sewer system with a length of 15 km and a diameter of from 200 to 315 mm. Were chosen two characteristics winter study periods. The first period was from 01 to 31 January 2011 year and the second period was from 01 to 29 February 2012 year. The analysis found that as a result of accidental water inflow (rain water) occurred lowering the temperature of the influent wastewater to the treatment plant from 2.1 to 2.2 °C. Water flowing random (rainy water) is defined as the share of accidental waters in the total volume of waste water treatment plants result in an increased volume of sewage from 37.5 to 40.0%. To prevent negative phenomena which are reducing the temperature of plants and their increased volume as a result of the entry of rain water drains should be taken to detect and elimination of illegal connections residential gutters and sealed sewer system.