The aim of the study was to define the scope of the spring snowmelt impact on the increase of inflow to the separated sewer system sewage treatment plant. The research was carried out from 1^st January 2010 to 31^st March 2010 in four selected sewer systems located in the Lesser Poland voivodeship. Hourly air temperature and daily sewage flow measurements were performed. Intensive snowmelts in 2010 began on 19^th February and ended on 4^th March. The average inflow percentage in the sewage discharge to each treatment plant during the snowmelt season reached from 36.6% to 58.1%. The highest inflows occurred on 1^st March (average daily air temperature – 6.6°C), and their percentage compared to the average daily flow at the rainless weather reached respectively: 56.8% in the sewer system A, 74.3% in the sewer system B, 54.7% in the sewer system C and 76.5% in the sewer system D. These numbers indicate that inflow caused by the snowmelts constituted over 50% of the daily flow in all analysed sewer systems. The following amount of inflow was discharged into the analysed sewer systems during the greatest snowmelts, i.e. from 19^th February to 3^rd March 2010: 3641 m³ into the sewer system A, 6926 m³ into the sewer system B, 1854 m³ into the sewer system C and 5846 m³ into the sewer system D. Openings in the inspection chamber manholes as well as illegal connections of gutters and yard inlets to the sanitary sewers were considered as the cause of melt-water discharge into the sewer systems.

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