By Harold Fravel, executive director, American Membrane Technology Association
By Harold G. Fravel Jr., Executive Director, American Membrane Technology Association (AMTA) and Karen Lindsey, Vice President, Avista Technologies, Inc.
We are programmed to believe that the word “fouled” is inherently bad, identifying something that is dirty, unclean, or contaminated. But for those of us in the membrane industry, the physical presence of “foulants” simply confirms that our treatment was necessary and that our process is successfully producing a more pristine effluent.
Ultrafiltration (UF) and microfiltration (MF) are processes that rely on pore size to exclude particulates in a feed source from passing through a membrane, resulting in a buildup of solids on the feed side of the membranes surface. These particulates include silt, sand, bacteria, coagulated organics, and other materials that are insoluble in the water being treated. Effective removal of suspended solids is particularly critical in direct potable applications such as a municipal plant sourced from the Great Lakes, which does not require further total dissolved solids (TDS) reduction. In the majority of potable applications though, the addition of reverse osmosis (RO) is necessary to further reduce the salt and ionic content of a feedwater. In the case of RO, the suspended solids must be removed with effective pretreatment to avoid plugging the narrow feed channel spacer within the membrane element and reduce the rate at which foulants accumulate on the membrane surface. As the foulant layer builds up, more feed pressure is required to pass water through the membrane and achieve the desired permeate flow. RO systems are typically cleaned two to four times per year (or more depending on the fouling rate) by taking the system off-line and initiating an in-situ chemical cleaning. In contrast, foulants in UF and MF are systematically and continually removed as part of the routine low pressure membrane process. Interestingly, fouling is detrimental in one membrane process but the objective of another.
Fouled UF membrane, dissected
Low pressure membranes are available in a number of configurations including hollow fiber and flat sheet. For hollow fiber, there are two feedstream methods. When the feed water is directed into the center channel of the fiber and the filtrate is recovered on the outside, we call this an “inside-out” process. When the feed water is outside the fiber and is driven through to its center so that the filtrate is recovered on the inside, we call this an “outside-in” process. The resulting flow path will dictate whether foulants build up on the inside or the outside of the hollow fiber. Solids that are larger in diameter than the membrane pores are unable to pass through that barrier and will concentrate on the surface to form a filter cake. As this layer accumulates, the transmembrane pressure increases until it reaches a level that becomes prohibitive to operate. A systematic backwash is initiated to loosen the foulants and remove them from the process. The interim between backwash cycles varies depending on the amount of material in the feed, the nature of the material being removed, and the desired operational flux rate.
Systems operating at higher flux rates may require shorter times between backwashes since the material being removed will likely accumulate faster. Some low pressure systems add an air scour process that helps dislodge more resistant filter cake materials and extends the time between backwashes. The frequency and duration of air scour will vary from system to system with the goal of reducing it as much as possible as these methods require additional energy and necessitate a temporary shutdown of the applied system.
Air scouring in a submerged membrane system
Several low pressure system manufactures have taken innovative approaches to minimize the need for backwash and air scour or help make these processes more effective. One supplier incorporates a pulsed air method to eliminate the need for a continuous delivery of air and others have added microbubbles to improve the scouring effect.
Membrane bioreactors (MBR) pose a unique challenge. Because these membranes are perpetually submerged in a highly concentrated solids bath, the likelihood of solids accumulation at the surface is subsequently magnified. In MBR systems, backwash is not applied to dislodge solids, but to use initiate a “relaxation mode” and impede the accumulated solids ability to inhibit flow. Most systems employ continuous air scour to constantly dislodge solids and allow the system to effectively operate while others incorporate pulsed bursts of air to accomplish the same result.
One manufacturer recently introduced a mechanism that gently and continually modulates the fiber bundles. This “no-aeration MBR” applies inertial force created by horizontal reciprocation of the submerged membranes to constantly dislodge particulates that would adhere to the membrane surface. The continuous fiber movement during the back and forth rocking of the cassette is also said to prevent sludge accumulation. Several pilot plants are currently in operation to qualify and document this method and verify the related energy savings, operational flux, and filtrate quality. The elimination of air scour is desirable as it minimizes the associated energy demand, making it a key advantage if it proves successful.
Another company has developed an MBR cassette in which the membranes at one end are not potted (adhered to the cassette) leaving the fiber tips to move freely within the biosolids liquor. Fiber stress on the unrestrained end is subsequently minimized, reducing the likelihood of breakage. Single header, submerged hollow fiber UF modules for MBR claim to offer improved performance, reduced fouling, and lower energy consumption. Central aeration is introduced, combining a high packing density with low energy demand for module air scouring.
One industry supplier recently introduced a hybrid membrane that combines hollow fiber and flat plate membrane technologies, claiming several advantages including improved filtration performance, greater strength, extended and sustainable flux rate, space efficiency, and optimized cleaning. The newly designed hybrid system is said to have a high membrane packing density, which results in a significantly reduced system footprint compared to conventional hollow fiber systems and even some flat plate designs.
Technical innovations in low pressure membrane systems focus primarily on minimizing energy usage and reducing the associated costs. Improvements have included the elimination of aeration to control solids accumulation, developing membranes with higher packing densities, and improving overall membrane properties to extend run times. Additional development currently underway includes improved porosity of the membranes, uniformity of pores, and improvements in membrane material composition.
Effective solids removal in low pressure MF, UF, and MBR membrane applications is the goal of these processes so that fouling is not necessarily undesirable, but actually a critical part of the process and a tangible indicator of its success. Exciting improvements in both system design and membrane composition will increase their efficiency and cost effectiveness and expand their market application, providing the resources necessary for continued innovation.
Harold Fravel accepted the position of Executive Director for the American Membrane Technology Association (AMTA) after working for Dow Chemical /FilmTec Corporation for 36 years. He has a PhD in Organic Chemistry from the University of North Carolina and a BS in Chemistry from Florida State University. He resides in Jupiter, FL.
Karen Lindsey is an Executive Member of the American Membrane Technology Association (AMTA) Board of Directors. She is the VP and co-founder of Avista Technologies and has 30 years’ experience in the water treatment industry, working with companies that cast cellulose acetate membrane, produced polyamide elements, and formulated specialty chemicals.