Ultrafiltration (UF) is a pressure-driven membrane process similar in some ways to reverse osmosis. However, in this case, as opposed to the situation encountered in an RO system, the flow of water through the membrane is generally through pores and not through the space between the lattices in the polymer, so osmotic pressure is not a factor.

Furthermore, there is little or no chemical interaction between the transported species and the membrane itself. UF membranes may be tailor-made to meet virtually any type of removal specification. Although they cannot reject any dissolved salts or other low-molecular-weight soluble matter, UF systems can remove very fine particulate material and high-molecular-weight organic matter from water streams.

To remove any low-molecular-weight soluble species with a UF membrane, a process must occur to convert the soluble matter to particulate form.As examples, soluble phosphorus may be precipitated with a metal salt, soluble organics may be adsorbed onto powdered activated carbon, and soluble iron may be oxidized to particulate form.

All of these processes and others will allow a UF membrane to remove even soluble matter. There are several different membrane module geometries on the market. Spiral-wound membrane modules are similar to RO membranes. Although they have a high membrane density, it is not possible to feed them with a high suspended solids concentration because of the narrow passages available through the module.

This geometry is suitable for the separation of high-molecular-weight organics in combination with low suspended solids. A second geometry is tubular, in which the feed flow passes through tubes at high velocity. The tube diameters range from about 0.5 to 1 in (12 to 25 mm). Because of the large tube diameter, tubular membranes are able to effectively separate liquid from biomass slurries of relatively high concentration, about 2 to 3 percent solids by weight.

The permeate flow in the tubes is from the inside to the outside. Pressure, typically on the order of 70 to 90 psig (483 to 621 kPa), is required to force the permeate through the membrane pores. A high feed-flow velocity [12 to 15 fps (3.7 to 4.6 m/s)] through the tubes is required to ensure sufficient shear at the membrane surface to keep the membrane clean and reduce concentration polarization.

A third variety of geometry is immersed hollow fiber, in which the membranes are submerged directly in the feed fluid without the need for a pressure vessel. Hundreds of small-diameter [0.07-inch (1.9-mm)] vertically oriented hollow fibers are supported at the top and bottom.The permeate flow is from the outside to the inside so that fibers can handle very high solids concentrations, as only clean, pure water flows inside the membranes.

The vacuum required to operate the hollow fiber design is very small, about −2 to −4 psig (−13.8 to 27.6 kPa). This vacuum is normally provided by a standard pump connected to the membranes via a piping manifold.To create the required shear on the membrane surface, low-pressure air is diffused intermittently under the fibers.The air rises through the fiber bundle, providing the necessary shear.

The nature of the solids–liquid separation requirement will dictate the kind of membrane module geometry that is best suited for a project. For small flows and very low suspended solids, the spiral-wound membrane can be used. Care must be taken to ensure that high solids concentrations do not develop within the membrane module by keeping the recirculation flow high.

For small flows and high suspended or emulsified solids concentrations, including tramp oils, tubular membranes should be selected. Free oils are very fouling to the membrane and must be prevented from coming in contact with the membrane. For high flows and high suspended solids concentrations, immersed hollow-fiber membranes are usually the most economical because of their lower pressure operation and hence lower energy requirements.

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