Treating Wet Weather Flows In a Membrane Bioreactor: Changes in Mixed Liquor Properties Cannot Be Neglected
Data from two unrelated pilot-scale submerged membrane bioreactors (SMBRs) studies are presented. Both SMBRs treated wastewater from combined sewer systems on the west coast of the United States: a flat sheet Kubota SMBR in Seattle, Washington (KC SMBR) and a hollow fiber Zenon SMBR in San Francisco, California (SF MBR). The SF SMBR was operated for a total of 834 d, through 3 rainy seasons, and the KC SMBR was operated for about 365 d through 1 rainy season. The SF SMBR was operated at a membrane flux of 30 LMH (18 gfd) and membrane-fouling rates were determined and compared. For the KC SMBR, membrane-fouling rates were compared for short-term peaking studies that operated at a flux of 54 LMH (32 gfd). The membrane-fouling rates were always higher following significant storm events for both facilities studied. Microscopic analyses showed that the mixed liquor deflocculated. The bioflocs became diffuse and the mixed liquor contained large amounts of unicells and colloidal material. Analyses of the mixed liquor properties showed that the colloidal content increased in the SF SMBR following large storm events, confirming what was observed under the microscope. It is important that engineers and researchers understand the significance of mixed liquor properties on membrane performance and what influences these properties. This work shows that designing an SMBR to treat peak wet weather flows on a combined sewer system, or a system with large I&I flows, requires diligent engineering. An additional process for addressing peak wet weather flows appears to be required.