The Effect of Treated Sewage Outflow From a Modernized Sewage Treatment Plant on Water Quality of the Breń River

The paper aimed at determining the effect of treated sewage discharged from a sewage treatment plant, modernized in 2007–2008, on the quality of the receiving water – the Breń river. The mechanical-biological sewage treatment plant is situated in the northern part of Dąbrowa Tarnowska town (Malopolskie voivodship). The treated sewage is disposed of by means of a collecting pipe to the Breń river, which abiotically is a lowland sandy stream, type 17. The hydrochemical analyses were conducted monthly, from May 2014 to April 2015 in three measurement points. The first and the third were situated on the Breń river, 30 m above and 400 m below the treated sewage outlet respectively, whereas the second one was on the outflow collecting pipe from the sewage treatment plant. In the analysed water, 17 physicochemical water quality indices were determined by means of reference methods. On the basis of the data analysis it was found that pollutant concentrations in the treated sewage discharged into the Bren river did not exceed the values stated in the water permit and only sporadically did not meet the requirements stated in the Regulation of the Minister of Environment regarding ammonium nitrogen concentrations in the winter-spring period. Slight dynamics of seasonal changes of physicochemical indices values in the treated sewage evidences a high proficiency of the sewage treatment plant operation, irrespective of the air or water temperature. The treated sewage affected the increase in 12 from among 17 analysed physicochemical indices in the Breń river, of which the dependencies were statistically significant in 8 cases. BOD₅ and ammonium nitrate caused a change of water quality class from I to II, and in case of phosphates a decline of ecological state from very good to below good. Despite a negative effect of the sewage treatment plant, the water quality state in the Breń river would be much worse if untreated municipal waste flowed into the receiving water.