Full-scale assessment of the nutrient removal capabilities of membrane bioreactors

Operating results from two full-scale membrane bioreactors (MBRs) practicing biological and chemical phosphorus and biological nitrogen removal to meet stringent effluent nutrient limits are analyzed. Full-scale results and special studies conducted at these facilities resulted in the development of guidelines for the design of MBRs to achieve stringent effluent nutrient concentrations--as low as 0.05 mg/L total phosphorus and 3 mg/L total nitrogen. These guidelines include the following: (1) direct the membrane recirculation flow to the aerobic zone, (2) provide intense mixing at the inlets of the anaerobic and anoxic zones, (3) maintain internal recirculation flowrates to maintain the desired mixed liquor suspended solids distribution, and (4) carefully control supplemental metal salt addition in proportion to the phosphorus remaining after biological removal is complete. Staging the various process zones and providing effective dissolved oxygen control also enhances nutrient removal performance. The results demonstrated that process performance can be characterized by the International Water Association (London, United Kingdom) (IWA) activated sludge model number 2d (ASM2d) and the Water Environment Federation (Alexandria, Virginia) chemical phosphorus removal model. These models subsequently were used to develop unique process configurations that are currently under design and/or construction for several full-scale nutrient removal MBRs.