Nutrient Removal MBR Systems: Factors in Design and Operation

The kinetics and stoichiometry 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. A number of case studies demonstrate these factors in design where a main objective is to operate with a target sludge mass distribution between anaerobic, anoxic and aerobic zones. Analysis of dynamic behaviour is important due to time-varying concentrations under diurnal flow patterns; this can impact nutrient removal performance. Aeration efficiency is reduced at the higher MLSS concentrations applied in MBR systems. Different recycle arrangements and recycle rates from the membrane tank to upstream reactors result in lower/higher MLSS upstream of the membrane tank. It is important to evaluate the balance between aeration efficiency, process volume requirements, and effluent quality.