Summary:

The paper presents changes of the San river water quality (from km 28+800 to 23+200) in result of treated municipal and industrial sewage discharge at km 26+440. Along the analyzed section, the San is under protection in the framework of Natura 2000 habitat areas programme (The Lower San River Valley). Analyses of pollutants in treated sewage originating from the Municipal and Central Treatment Plant were conducted twice a month. Municipal Sewage Treatment Plant, a maximum throughput of 17500 m3•24 hours-1, receives municipal sewage from Stalowa Wola city, was modernized in 2008 and is capable of receiving the pollutant load to 98500 RLM. The neighbouring Central Sewage Treatment Plant treats sewage from the Stalowa Wola Iron Works area and from the Tarnobrzeg EURO-PARK WISŁOSAN Special Economic Zone in Stalowa Wola. Its maximum throughput is 100000 m3•24 hours-1, but during the rainless weather the amount of sewage does not exceed 20000 m3•24 hours-1. The assessment of sewage effect on the San river water quality was carried out in two measurement-control points (2360 m above and 3240 m below the sewage discharge point) on the basis on four-year (2009-2012) analyses of 19 pollution indices, including 3 physical, 2 oxygen, 2 biogenic, 2 salinity, 7 metals and 3 industrial ones.In measurement-control points quality classes of the San river water were determined according to the Regulation of the Minister of the Environment on classification of ecological status, ecological potential and chemical status of homogeneous parts of surface water [Journal of Laws of 2011, No.259, item 1549].In order to determine the effect of treated sewage on surface water quality in the receiving water using U Mann-Whitney non-parametric test, the significance of difference in pollutant concentrations was checked between the points situated before and below the sewage discharge.The San river water both upstream and downstream from the sewage treatment plant was classified to ecological status below good due to high value to five-day biological oxygen demand, however the value was statistically insignificant on the level α = 0,05.