Pushing the Limits of Technology to Achieve Low Nutrient Levels Using Membrane Bioreactors

Operating results from full-scale wastewater treatment facilities which incorporate membrane bioreactors (MBRs) practicing biological and chemical phosphorus and biological nitrogen removal to meet stringent effluent nutrient limits are analyzed. A series of MBR design guidelines were developed to achieve stringent effluent nutrient concentrations, as low as 0.05 mg-P/L total phosphorus (TP) and 3 mg-N/L total nitrogen (TN). The guidelines were developed as a result of full-scale and special studies conducted at these facilities, and include directing the membrane recirculation flow to the aerobic zone, providing intense mixing at the inlets of the anaerobic and anoxic zones, maintaining internal recirculation flow rates to maintain the desired mixed liquor suspended solids distribution, and controlling the supplemental metal salt addition in proportion to the phosphorus remaining after biological removal. Staging the various process zones and providing effective dissolved oxygen control also enhances nutrient removal performance. The results also demonstrated that process performance can be characterized by the International Water Association (IWA) activated sludge model number 2d (ASM2d) and the Water Environment Federation chemical phosphorus removal model. These models were used to develop unique process configurations which are currently under design and/or construction for several full-scale nutrient removal MBRs.