The aim of the study was to analyze impact of treated wastewater discharged from the MPP on the receiver water quality, which constituted a drainage ditch. In addition, carried out studies were related to verification of water self-purification degree at the 15-km ditch section. The direct receiver of discharged treated domestic and industrial wastewater, rinsing waters form water treatment plant, precipitation and snowmelt waters discharged from facility area is specific drainage ditch that flows into the channel, which is the primary drainage ditch.The final receiver of discharged sewage and water, along the route from outlet and further drainage ditches system, is the river Szkwa. In order to determine fluctuations in water composition and its physico-chemical parameters as well as self-purification degree in drainage ditch and in drainage channel following indications were made: indicators of oxygen (COD, dissolved oxygen), nutrients (total nitrogen, total phosphorus). Samples were collected 3 times in a month, for 10 months period from November 2013 to August 2014. The monitoring of water from the receivers was implemented in six measuring points: one before inflow from sewage treatment facility, four points were deployed at ditch and canal, while the last point was located before the discharge to river. The obtained results showed that wastewater discharged from MPP increased the values of COD, nitrogen, phosphorus and decreased dissolved oxygen concentration in water from the outlet of facility specific drainage to ditch inlet drainage canal. On the basis of these indicators, self- purification of water was observed in watercourse on the canal mouth to river Szkwa. Water at the mouth of the River, was qualified to the first class of purity on the basis of dissolved oxygen, total nitrogen, pH and temperature value. Water quality was deteriorated by COD and phosphorus concentration. The concentration of this nutrient comparing to winter and spring seasons was the lowest in summer. Self-purification process was probably impacted by water temperature and correlated with larger absorption of phosphorus by microorganisms present in water as well as by plants in increased vegetation period. Plants growing at the edges were accumulating large amount of nitrogen and phosphorus compounds which resulted in water quality improvement in summer season.