Redundancy and Reliability – The 5th Generation of Membrane Bioreactor Design
Membrane bioreactor (MBR) facility designs have progressed through four generations of development (Crawford et al, 2001). The four generations are generally described as evolving from long Solids Retention Time (SRT) package plants, to the inclusion of nutrient removal (total nitrogen and/or phosphorus), to equipment and facility design optimization by reducing both the SRT and the Mixed Liquor Suspended Solids (MLSS) concentrations, and finally to the fourth generation characterized by larger plant designs with the Owner/Engineer assuming increased risk and design responsibility. The small MBR installations of the first three generations, as well as most fourth generation plants, were not required to operate with full reliability and redundancy – most MBR plants to date have included very large flow equalization or flow diversion basin capabilities, or withdraw their influent wastewater from a collection sewer that continues to a larger downstream plant. At these plants, reliability and redundancy is desirable but not essential – system shutdowns can occur without causing a raw wastewater bypass. Some recent MBR designs however are of the 5th generation of design; characterized by the inclusion of fully reliable and fully redundant design features that are essential for end-ofpipe treatment facility design. Detailed design information from operating MBR installations showing the unique reliability and redundancy requirements of end-of-pipe MBR system designs will be presented.