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The study presents the research results on the rejected water generated in dewatering sludge stabilised in Autothermal Thermophilic Aerobic Digestion (ATAD) technology. The research was carried out in three municipal wastewater treatment plants (WWTPs), with a capacity of 1500 to 3260 m3 d−1 and a sludge node capacity of 835 to 2000 kg DM d−1. The mean content of Kjeldahl nitrogen (TKN) in the rejected water samples taken from each object ranged from 485 to 1573 mg N L−1, ammonium nitrogen 318 to 736 mg N L−1, and the average concentration of total phosphorus ranged from 96 to 281 mg P L−1. The average content of organic matter expressed as five-day biological oxygen demand (BOD5) ranged from 205 to 730 mg O2 L−1, while chemical oxygen demand (COD) ranged from 767 to 4884 mg O2 L−1. The study determined the kinetics of the biochemical decomposition of organic matter, assuming that it follows the first-order equation. The average reaction rate constant k in subsequent treatment plants was estimated at 0.424, 0.513 and 0.782 d−1. The R2 coefficient determining the model's adjustment to empirical values was not lower than 0.952. The organic matter biodegradability index average values ranged from 0.17 to 0.26. [ABSTRACT FROM AUTHOR]