The issue of reject water has to be considered in almost every biological municipal or industrial wastewater treatment plant (WWTP) that applies aerobic or anaerobic digestion of sewage sludge. Reject water is usually returned to the beginning of the treatment process, which results in periodical disturbances in stable and efficient sewage treatment. Due to planned modernization of one of the biggest dairy WWTPs in Poland, a laboratory scale research has been carried out to determine quality characteristics of reject water. Aerobic and anaerobic digestion was applied to a mixture of two kinds of sludge: excessive and flotation. According to research performed by the author results (range value) of reject water were: 7.3 to 12.9 mg N-NH4/L after aerobic and 460.0 to 574.0 mg N-NH4/L after anaerobic digestion. The study has confirmed a higher value of organic substances in reject water after anaerobic digestion in comparison with aerobic. Due to high concentration of ammonia nitrogen in reject water obtained during co-digestion of excessive and flotation sludge, a separated system for its treatment should be applied. The results of research work presented in this paper provided a base for the project of the pilot installation with constructed wetland.

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