Merits of Alternative MBR Systems

Two membrane bioreactor (MBR) systems were evaluated. Both MBR systems are able to meet Arizona Class A+ reuse requirements. The Zenon configuration has a total maximum air requirement of approximately 11,000 scfm. This assumes coarse bubblers in the membrane tank, and fine pore diffusers in the aeration tank. Since the MLSS will be approximately 10,000 mg/l, the alpha for fine pore aeration will be about 0.42 and for the coarse bubblers 0.62.The Kubota configuration has a maximum air requirement of approximately 23,000 scfm, using coarse bubble spargers. This is assumed to be on a constant basis to provide an average D.O. concentration of 1.5 mg/l. If the Kubota system is operated at the recommended MLSS concentration of not less than 12,500 mg/l, sufficient air to meet the process demand could not be introduced given the existing tankage. Part of the anoxic tankage would need to be aerated to make up the deficit.One possibility for the Kubota alternative is to adopt their two-tier approach whereby cassettes are stacked one on top of the other. By this means, approximately half of the membrane tank could be freed up for fine pore aeration. However, this alternative lacks experience at the scale of the Randolph Park WRF.There are still considerable unknowns regarding performance of MBR systems. Therefore, close monitoring of the operation, as well as off-gas testing of the aeration system operating at the higher MLSS concentrations is recommended.