The study assesses the influence of the loads of some pollutants (BOD5, suspended solids, total nitrogen and total phosphorus), discharged to the Ner river, the right-bank tributary of the Warta river, before the construction of a municipal sewage treatment plant in Łódź, and during its start-up and further operation. Despite its low natural flow, Ner discharges annually a relatively high load of pollutants into Warta. In the research period (1995-2003), the annual average flow of Ner constituted 10% of the annual average flow of Warta below the mouth of the tributary, while the contaminant loads carried by Ner averaged 27% of total nitrogen, 37% of phosphorus, 39% of BOD5, and 28% of suspended solids loads in Warta. In the years 1995-2000, before the start-up of the biological and chemical sections of the treatment plant, the respective values were 29, 41, 44, and 37%, and then, after the plant reached its full capacity, they decreased to 23, 29, 25, and 16%.

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