Research conducted in winter and summer aimed to establish the level of nitrogen elimination from fishponds fertilized with municipal sewage. Biologically treated sewage in the quantity of 30 000 m3 per day was obtained from the sewage treatment plant for Bielsko-Biała municipality and supplied to ponds of a total area of 41 ha. Research concentrated on five ponds. Two of them operated in the bead system, while the remaining ones had independent inflows and outflows. In winter one pond was stocked, and in the season the carp was stocked in three variants in polyculture with bigheaded carp and grass carp. In winter nitrogen elimination was very limited; only in the pond stocked with carp did it reach 20% for ammonia nitrogen and 13% for nitrate nitrogen. In the season, mean ammonia nitrogen reduction amounting to 23% and nitrate nitrogen reduction amounting to 61% was observed, which corresponds to 80 kg N deposited daily in the ponds provided such a level of nitrogen elimination. The use of municipal sewage for ponds fertilization is beneficial from the environmental (sewage utilization) and economic (reduced spending on artificial fertilizers) point of view.

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