THE IMPACT OF MEMBRANE SOLID LIQUID SEPARATION ON THE DESIGN OF BIOLOGICAL NUTRIENT REMOVAL ACTIVATED SLUDGE SYSTEMS

Installing membranes for solid-liquid separation into BNR activated sludge (AS) systems makes a profound difference not only to the design of the BNR system itself but also to the design approach for the whole WWTP. The PWWF fixes the membrane surface area that is required in the aerobic zone and therefore fixes the aerobic zone volume. This in turn fixes the biological reactor volume because the aerobic mass fraction is generally fixed to ensure nitrification and sufficient anaerobic and/or anoxic mass fractions for N and/or P removal. Therefore usually with MBR BNR systems, the reactor volume is fixed by the PWWF. The sludge age is then calculated in order to achieve the required Xtaer for effective membrane operation (12 to 18 g/ℓ). For low aerobic mass fraction (<60%) MBR BNR systems without flow balancing treating settled wastewater, the sludge age to achieve the required Xtaer is extremely long and the membrane aeration system supplies significantly more oxygen than required for the peak total biological oxygen demand, resulting in uneconomical operation. Treating raw wastewater for this situation results in much shorter sludge ages and in much greater utilization of the oxygenation capacity of the membrane aeration system. Therefore for PWWF/ADWF ratios in the upper range (2.0-2.5), aerobi