This study was performed to identify suitable conditions for the in-situ reduction of excess sludge production by intercalated digesters in recycle-activated sludge (RAS) flow. The objective was to compare and model biological sludge mass reduction and the biodegradation of endogenous residues (X-P) by digestion under hypoxic, aerobic, anaerobic, and five intermittent-aeration conditions. A mathematical model based on the heterotrophic endogenous decay constant (b(H)) and including the biodegradation of XP was used to fit the long-term data from the digesters to identify and estimate the parameters. Both the bH constant (0.02-0.05 d (1)) and the endogenous residue biodegradation constant (b(P), 0.001-0.004 d (1)) were determined across the different mediums. The digesters with intermittent aeration cycles of 12 h-12 h and 5 min-3 h (ON/OFF) were the fastest, compared to the aerobic reactor. The study provides a basis for rating RAS-digester volumes to avoid the accumulation of XP in aeration tanks. (C) 2015 Elsevier Ltd. All rights reserved.