The aim of the study was to determine the extent to which heavy rainfall, that occurred in 2010, affected the infiltration into the selected sewage treatment plants in the Małopolskie voivodeship. The research was conducted in four separate sewer systems, located in poviats adjacent to the city of Kraków, dis-charging sewage to mechanical-biological treatment plants with a capacity below 1000 m3•d-1. The amount of sewage and extraneous water in the average wet year (2008) were used as control.
As a result of heavy precipitation in 2010 the sewer system A received 18 539 m3 more extraneous water than in 2008 (increase by 343%), the sewer sys-tem B - 22 822 m3 (increase by 163%), the sewer system C - 109 715 m3
(increase by 248%) and the sewer system D - 30 796 m3 (increase by 303%). Heavy precipitation in 2010 caused the increase of infiltration and inflow by 264% on average in all studied sewer systems compared to the average wet year. As the result of precipitation, whose annual total in 2010 was higher by 65% than the normal value in 2008, there was an increase in the annual share of extraneous water from 5.3 to 19.7% depending on the facility. The volume of extraneous water, which was discharged into the studied treatment plants in 2010 (the period of heavy rainfall), constituted the following share of the annual value: in the sewer system A - 41.3%, in the sewer system B - 21.2%, in the sewer system C - 14.4%, in the sewer system D - 22.9%. Assuming the average gross amount of 3 PLN paid for treatment of 1 m3 of sewage, cost of extraneous water disposal in 2010 amounted from 78 533 PLN to 552 165 PLN, depending on the facility. The results obtained in the present study suggest that eliminating or reducing infiltration and inflow into the analyzed sewer systems would allow for large financial savings associated with reducing costs both for their transport and treatment as well as for modernization of facilities to enhance their hydraulic ca-pacity.