Directflow operation is now the membrane process of choice for water treatment applications. In most water sources, feed solids loadings are normally very much lower than for process or waste stream feeds used in crossflow applications. This has allowed membrane companies to develop an alternative to crossflow. At first, crossflow velocities were reduced to a bleed flow, and then stopped altogether, relying simply on an intermittent backwash with Liquid Solid Separator .
With the elimination of crossflow, it has become possible to use much finer feed channels, since filtration cycle pressure drop ceased to be the limiting factor. This increased the packing density of the module greatly, and the combination of low capital and operational expenditure has opened up the market for large scale water treatment.
UF membranes are defined as having a pore size in the range 0.001-0.02 μm and they retain fine particles, colloidal material, bacteria, viruses and some other pathogens and pyrogens such as endotoxins. In contrast, microfiltration (MF) membranes typically have a pore size of greater than 0.1 μm, and hence will not remove colloids or all viruses but they are efficient at removing bacteria and fine inorganic particles. Reverse osmosis (RO) separates dissolved material and operates in the range below 0.001 μm.
In a pressurised UF system trans-membrane pressure (TMP) is generated in the feed by a pump, while the permeate stays at atmospheric pressure. Membrane modules are generally standardised, allowing the design of membrane systems to incorporate membranes selected for the application.
UF systems can also be provided in an immersed configuration. Here membranes are suspended in basins containing the feed and are open to atmospheric pressure. Pressure on the influent side is limited to the static head above the membranes with TMP generated by a pump that develops suction on the permeate side of the membrane Liquid Solid Separator. UF, like other filtration methods, can be operated as a continuous or batch process.
