Ultrafiltration (UF) utilizes a semi-permeable membrane to physically remove suspended particles from water based on particle size and the pore size rating of the UF membrane. Among membrane technologies commonly used, UF is typically one step “tighter” (meaning it has smaller pore size) than microfiltration. Membranes tighter than UF are nanofilters and reverse osmosis. Nanofiltration and reverse osmosis membranes are capable of removing dissolved ions in addition to suspended solids while UF membranes cannot remove dissolved ions. UF membranes are available in a range of “tightness” or pore sizes and are typically specified based on the molecular weight cutoff, expressed in Daltons, to describe size of molecules they will allow to pass through the membrane. Because of the large pore size, much lower pressure is used to force fluid through the UF membrane as compared to nano, reverse osmosis or microfilters.
Types of Membranes
Similar to reverse osmosis and nanofiltration, ultrafiltration may be carried out in spiral wound membranes utilizing cross-flow separation, where a feed stream is introduced into the membrane element under pressure and passed over the membrane surface in a controlled flow path. A portion of the feed passes through the membrane and is called permeate. The rejected materials are flushed away in a stream called the concentrate. This arrangement allows high membrane surface area in a small footprint but is more subject to fouling since it cannot be backwashed.
Another common arrangement is use of hollow fiber membranes. The membrane is formed into long, very thin tubes or fibers (typically 0.6 to 2 mm in diameter) which are sealed into connectors at both end. Hundreds of these fibers with one inlet and outlet connector are called a “bundle” or “cartridge” and may be grouped together to form a “module”. The feed solution typically flows through one end of the fibers while the opposite end is completely or partially closed off, thus forcing the fluid through the membrane where it is collected in the cartridge area surrounding the fibers and leaving the suspended materials on the inside of the membrane. Membrane cleaning may be achieved by allowing a very small portion of the flow to exit the opposite end of the membrane while the membrane is “on-line”, followed by a cleaning step in which clean water or water with a cleaner is caused to flow “backward” in the membrane (i.e., from outside the membrane to the inside).
Filtration can also be carried out from the “outside-in” and some systems on the market utilize a vacuum to draw fluid from the outside to inside the membrane. This design resists fouling since the collected materials on the outside of the membrane surface can be more easily removed from the membrane by reversing the flow of water and oftentimes using dissolved air with the water which are forced “backwards” into the center of the fibers. These designs are capable of being immersed into clarifiers in water or wastewater treatment plants and can produce very clean water, thus eliminating conventional slow sand filtration.
Due to difficulty in consistently removing very small particles using conventional water treatment, and increased concerns about chlorine-resistant organisms (e.g., Giardia, Crypto), ultrafiltration is increasingly becoming the method of choice for municipal water treatment plants. UF membranes have demonstrated greater than 6-Log (99.9999%) removal of Cryptosporidium and Giardia lamblia. Ultrafiltration eliminates the extreme sensitivity of conventional plant treatment steps of coagulation, sedimentation and filtration to variable influent turbidities and particle charges (i.e., zeta potential).
Ultrafiltration is very effective at removing tiny particles which can quickly foul reverse osmosis membranes, thus reducing the silt density index of the water. For this reason, it frequently serves as pretreatment for surface water, seawater and biologically treated municipal water upstream of RO.
UF is also used in industry to separate suspended solids from solution. Industrial applications include power generation, food and beverage processing, pharmaceutical production, biotechnology, and semiconductor manufacturing.