Both heterotrophs and autotrophic nitrifiers are present in aerobic sludge digestion systems. The endogenous heterotrophic metabolism releases ammonium that, via oxidation to nitrite and nitrate, serves as the energy substrate for growth and survival of nitrifiers. The population interaction was used to explain the two-phase decay behavior observed in this study for the biosolids collected from rotating biological contactors (RBC) as well as the mixed primary and secondary sludge. Separated by an intermediate transition, the two phases followed different first-order decay kinetics, with the first phase being much faster. The two-phase phenomenon was supported by the on-line fluorescence profile monitored by an NAD(P)H fluorometer. The fluorescence showed an initial rise to a high plateau, a sharp decline after staying at the plateau for 30-50 h, and a trailing very slow decrease. The initial plateau corresponded to the first-phase of rapid sludge digestion; the trailing fluorescence decline corresponded to the second phase of slow digestion. The transition between the two phases occurred during the sharp decline from the plateau. The fluorescence profile was consistent with our earlier observation of negligible NAD(P)H fluorescence from nitrifiers compared to that from heterotrophs. The VSS reduction was, therefore, modeled by considering the decay and interaction of heterotrophs and nitrifiers. The model described the experimental results very well. The best-fit average decay constants were 0.067 d(-1) for nitrifiers and 0.95 d(-1) for heterotrophs in the RBC biosolids. The apparent heterotrophic decay constants (in the presence of nonbiomass VSS) were found to be 0.48-0.55 d(-1), consistent among different runs of mixed sludge digestion.