Process Simulation: Assisting Engineers in Design and Operation of Membrane Bioreactors

Biological behaviour in nutrient removal activated sludge systems with membranes for solid-liquid separation essentially is the same as in conventional systems with settling tanks, and the process design principles are no different. However, design of MBR-based systems must account for differences in mixed liquor concentration between reactors due to recycle flows, changing concentrations under diurnal flows, and the impact of recycle arrangements on solids distribution and concentrations. These factors, which do not occur with conventional systems, should be recognized in design where a main objective is to operate with a target sludge mass distribution between anaerobic, anoxic and aerobic zones. Also, analysis of dynamic behaviour is important due to time-varying concentrations under diurnal flow patterns. Aeration efficiency is reduced at the higher MLSS concentrations that can be achieved in MBR systems. Evaluating the balance between aeration efficiency and process volume requirements for different recycle arrangements resulting in lower/higher MLSS upstream of the membrane tank is important. Process simulation provides an efficient tool for analysis of both the design and operational issues that are unique to MBR systems.