In this study, simulation methods of a system dynamics (SD) model for activated-sludge wastewater treatment
plants were developed to determine the extent of facility expansion due to changes in inflow rate based on the
model presented in Das et al. (1997). The system dynamics methodology is an effective simulation tool to model
the dynamic nature of a system. It deals with information and/or material feedback structure inherent in working
mechanism of a system and focuses on modeling the changes in behavior of a system, thereby facilitating analysis
of the effects of changes in operational policies of a system. In the activated-sludge wastewater treatment processes
‘return of activated-sludge’ works as material feedback. This feedback links water, biomass, and substrate in a
wastewater treatment plant. Therefore, the system dynamics methodology is appropriate in modeling activatedsludge
wastewater treatment processes.