Ultra-filtration (UF) in water treatment typically uses hollow fibers of membrane material The feed water flows either inside the fibers, or around the outside of the fibers. Suspended solids and solutes of high molecular weight are retained, while water and low molecular weight solutes pass through the membrane. This separation process is used in industry and research for purifying and concentrating macromolecular compounds. When combined with other purification technologies in a complete water system, UF is ideal for the removal of colloids, bacteria, pyrogens and macromolecules larger than the membrane pore size from water. The primary removal mechanism is size exclusion. UF can be used as pretreatment for reverse osmosis systems or as a final filtration stage for purified water. Periodic flushing and cleaning are required.