Ultrafiltration is a pressure-driven process which uses semi-permeable synthetic membranes to separate certain chemicals and materials from water. Various membrane configurations are available. Some are fibers or tubes with the active membrane being found on the inside, while others are flat sheets which can be stacked in frames or rolled into a spiral configuration. They all function by a similar mechanism.
When a cross-section is viewed under a high magnification microscope, it is possible to see that the membrane has an asymmetric structure. The pores appear as inverted conical-shaped holes, with a narrow diameter at one end of the membrane (in the range of 0.01 micron to 0.1 micron) and a wider one at the other end. The narrow end is best described as a skin, usually of the same material as the rest of the membrane. That particular surface has been chemically, thermally or mechanically modified. In total, the asymmetric (or anisotropic) membrane is only up to 50 micron thick, so this is cast onto a supporting medium to give the membrane additional strength.
The pores at the surface are so small that they will allow only water and small dissolved chemicals to pass through, while stopping bigger molecules and particles. Due to the small pore size, the water will flow through the membrane only by applying pressure. Typically, a pressure differential of at least 15 psi across the membrane is required.
A build-up of contaminants at the surface of the membrane is prevented by operating the system in a cross-flow mode. Here the flow is pumped parallel to the membrane surface which results in a constant scouring action, sweeping particles away from the membrane and back into the feed flow. Without this cross-flow, a cake layer would build up, thus restricting the flow of water through the membrane, as is the problem with conventional filtration.
Cross-flow filtration results in the division of one stream (feed) into two others-a dirty water stream (retentate) and a clean water stream (permeate).
A phenomenon of ultrafiltration is known as concentration polarization. This is a buildup of chemical contaminants at the membrane surface as the water filters through to the other side. The condition is not permanent and can be removed by flushing the membrane with either more dilute feed or with clean water, or permeate.
Despite the asymmetric structure and cross-flow operation, ultrafiltration membranes can still become clogged as particles of a similar size are pushed into the pores, causing them to plug. Some membrane configurations allow for backwashing. This is a periodic reverse flow through the membranes from the clean water side, back through to the feed water side. As the water is forced through the pores under high pressure, it pushes out the contaminating particles back into the feed stream and unplugs the pores.
Ultrafiltration membranes can be chemically cleaned to restore high fluxes (clean water flow rates) and maintain separation performance. With a well-designed system and the use of approved cleaning chemicals, longer membrane lifetime can be expected.
If the application is one where relatively clean water is being filtered, it is possible to operate ultrafilters with relatively large pores, bordering on microfiltration, using a conventional filtration mode (full flow). This requires only low feed pressures and flow rates which make membranes economically competitive with conventional technology. However, most applications for ultrafiltration use cross-flow which requires larger pumps, capable of delivering higher flow rates and pressures.
System designs vary depending upon the application, and how they are to be interfaced with a particular process. Some systems operate as a batch concentration where a set volume of liquid is filtered down to leave a small, highly-concentrated feed volume. Other processes require the ultrafiltration to be done as a continuous on-line (feed and bleed) process. Typically the systems include a holding tank where any necessary pretreatment can be done.
Pretreatment may include solids settling and removal, oil removal, in-line screen to remove small particles, or cartridge filters. A transfer pump passes the feed to a working tank from where it is pumped around the membrane by the feed (recirculation) pump. The systems can be designed to be fully automatic with on-line analysis or backwashing.
One of the benefits of using ultrafiltration is the quality of permeate produced. Its small pore size provides an absolute barrier to particles, bacteria, high molecular weight organic molecules, emulsified oils and colloids. Water supplies can often contain chlorine-resistant pathogens and their spores, such as Cryptosporidium and Giardia. Conventional systems can be ineffective against the removal of these spores. Ultrafiltration will remove them, providing a form of non-chemical disinfection.
Ultrafiltration can be used independently or in association with other treatments such as coagulation or precipitation. It also can be used effectively as a method of pretreatment for osmosis. Its benefits range from producing a high quality water stream for use or recycling, to reducing waste volumes and recovering valuable materials from wastestreams.
