Aerobic stabilization of biological sludge characterized by an extremely low decay rate: Modeling, identifiability analysis and parameter estimation

[Display omitted] •The sludge produced had only 2 fractions: heterotrophs, XH and its residues, XP.•The endogenous decay rate was very low, bH=0.02, one-tenth of usual value, 0.2d−1.•Apparently, the digested biomass was made up of glycogen accumulating organisms.•The COD/VSS ratio of XP (icv=1.06) was shown to be different from that of XH (1.48).•With low bH, it could be misleading to use SOUR standards as stabilization criteria. Aerobic digestion batch tests were run on a sludge model that contained only two fractions, the heterotrophic biomass (XH) and its endogenous residue (XP). The objective was to describe the stabilization of the sludge and estimate the endogenous decay parameters. Modeling was performed with Aquasim, based on long-term data of volatile suspended solids and chemical oxygen demand (VSS, COD). Sensitivity analyses were carried out to determine the conditions for unique identifiability of the parameters. Importantly, it was found that the COD/VSS ratio of the endogenous residues (1.06) was significantly lower than for the active biomass fraction (1.48). The decay rate constant of the studied sludge (low bH, 0.025d−1) was one-tenth that usually observed (0.2d−1), which has two main practical significances. Digestion time required is much more long; also the oxygen uptake rate might be