Maintenance, cell death and predation are endogenous processes of microorganisms and play an important role in governing the overall performance of biological wastewater treatment systems. However, in the previous studies on the activated sludge system, a widely used biological wastewater treatment process, the endogenous processes are summarized as a single process and the kinetics of all related processes are lumped into a single parameter set. In order to better understand the endogenous processes, efforts are made to quantify the maintenance, cell death and predation processes separately with both experimental and mathematical approaches in this work. A model-based analysis on endogenous processes of activated sludge is performed. Both oxygen uptake and biomass concentration gradually decrease with the increasing length of aerobic starvation. Model predictions are in accordance with experimental data. The active bacteria content is reduced to about 23% of the initial value after 10-day starvation, and the maintenance energy consumption rate under non-growth conditions is found to be about four times of that when the microbial growth is of its maximum level. The active microorganisms form the prey for the predator growth. The oxygen consumption related to the predation of active bacteria significantly contributes to the total oxygen consumption.