Aim of the study The aim of the work is to assess the impact of pollution discharge limitation after the modernization of the ŁAM sewage treatment plant on pollutant loads in the middle section and downriver of the Ner river. In order to do this, pollutant concentrations and loads were compared in two periods: 1995–2003 and 2004–2011. Material and methods The sources of data included information obtained from wastewater treatment plant regarding the basic indicators of pollutants discharged to the river, as well as flows and concentrations of pollutants in the cross-sections of Ner and Warta rivers. As an indicator, we adopted the share of pollutants introduced into the Warta river by the Ner river in the pollution load of the Warta river in the first measurement cross-section on the Warta below the confluence with the Ner river. Results and conclusions In the period 1995–2003, the average annual flow of the Ner river was about 10% of the average annual flow of the Warta river below the confluence with the Ner river (Koło cross-section), and in the period 2004–2011, it remained at a similar level i.e. 13.8%, whereas the load of pollution brought in by Ner amounted to, on average: 27% nitrogen, 37% total phosphorus, 39% BOD₅ and 28% total suspended solids in the water of Warta river; whereas in the second period, the figures were 27.9%, 42.6%, 19.8% and 19.6% respectively. As in the first period, the lack of adequate impact was found of the decrease in the phosphorus discharged by the wastewater treatment plant on its content in the water of Ner river; on the other hand, the decrease of the BOD₅ index and total suspension ratio was recorded. In conditions of low phosphorus concentration in the discharged sewage (below 4g ∙ m⁻³) in 2007 it reached a maximum of 1.23 g ∙ m⁻³ discharged from the sewage treatment plant, and at the confluence with the Ner River, no improvement in water quality in the Chełmno profile was observed (average annual concentration 0.78 g ∙ m⁻³).
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