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By Harold G. Fravel, Jr. and Karen Lindsey T he increasing desirability of microfiltration (MF) and ultrafiltration (UF) membrane systems over conventional water treatment plants is a direct result of their continued success in effectively reducing turbidity, rejecting pathogens and coagulated materials, and, in general, separating particulates from a sourced feedwater. Increasingly, MF and UF are the processes of choice for customers installing new water treatment systems and retrofitting or upgrading older, conventional plants. Both MF and UF membranes rely on distinct pore sizes to physically reject specific particles. Pore sizes vary deliberately, with MF membranes having relatively larger pores in the range of 0.1 to 0.2 microns and UF having smaller pores in the range of 0.005 to 0.01 microns. If the membranes have not been compromised, they will effectively reject any particle larger than the pore dimension. When the larger particulates are unable to pass through the membranes, they build up and can potentially become pinned to the membrane surface, developing into what is known in the industry as a "filter cake." If left unchecked, this cake will become thicker and thicker as more particulates bind to it. The resulting layer may become so dense that flow to the membranes is inhibited and an increased feed pressure is required to overcome the barrier and effectively pass water through the membranes. When that occurs, the surface of the membranes must be cleaned or treated in some manner to restore the flow of water through the membrane. There are several methods of cleaning MF/UF membrane surfaces. In most applications, the cleaning process includes a backwash — a method of applying short bursts of filtrate water from the opposite direction of the feed flow to help dislodge the filter cake and direct particulates away from the membranes and out of the system. An air scour may also be applied to increase the efficacy in dislodging particulates from the membrane surface. Enhanced chemical backwash and clean-in-place (CIP) cycles take these steps further by introducing some type of chemical to help disperse, dissolve, and remove accumulated solids and restore system performance. Any maintenance process that extends the time between backwash or chemical cleaning is a commercial advantage in reducing operating costs and increasing system efficiency. Getting Particulate: Membrane Bioreactors The numerous benefits of membrane bioreactors (MBRs) for wastewater treatment have resulted in a growing number of new installations. MBR systems apply MF or UF membranes depending on the system's design and application. This process treats biosolids and provides a higher-quality filtrate with a system that typically has a much smaller footprint than a conventional plant. The membranes are submerged in a mixed biosolids liquor, and the filtrate is drawn through the membrane. Biosolids, bacteria, and some viruses are unable to pass through the membrane's pores and remain in the mixed liquor. Because the feed flow is directed toward the MBR membranes while water is drawn from the mixed liquor to the membrane surface, particulates and solids inevitably accumulate. Combine this process flow with the extremely high solids content common to most MBR feedwaters, and the resulting issues related to filter cakes can be especially significant. Backwashing is not typically applied to MBR systems, as the particulate loading in the mixed liquors is simply too high to be effective. Instead, system operators apply intense agitation and circulation of the liquor to help minimize solids accumulation. There is also a procedure referred to as the "relaxation" step in which the draw from the mixed liquor is halted for a designated time while the agitation continues. Ideally, this reduces the depth of the filter cake on the membrane's surface. 24 wateronline.com n Water Innovations How To Control Filter Cake And Membrane Surface Fouling Who wants cake? Certainly not treatment plant operators who employ microfiltration and ultrafiltration membranes, and here's what they're doing about it. Air is introduced for scouring the membrane surface.