Optimising mixing and nutrient removal in membrane bioreactors: CFD modelling and experimental validation

Membrane Bioreactors (MBRs) have been used successfully in biological wastewater treatment to solve the perennial problem of effective solids-liquid separation. The optimisation of MBRs requires knowledge of the membrane fouling, mixing and biokinetics. MBRs are designed mainly based on the biokinetic and membrane fouling considerations even though the hydrodynamics within an MBR system is of critical importance to the performance of the system. Current methods of design for a desired flow regime within the MBR are largely based on empirical techniques (e.g. specific mixing energy). However, it is difficult to predict how vessel design in large scale installations (e.g. size and position of inlets, baffles or membrane orientation) affects hydrodynamics, hence overall performance. Computational Fluid Dynamics (CFD) provides a method for prediction of how vessel features and mixing energy usage affect the hydrodynamics and pollutant removal and subsequently allowing optimisation of MBR design and performance. In this study, a CFD model was developed which accounts for aeration and biological nutrient removal. The modelling results are compared against experimental results of two full scale MBRs for the hydrodynamics and against a modelling benchmark for the biological nutrient removal component of the model.