This type of polymer membrane has an asymmetric microporous structure, consisting of two layers: an upper functional layer with dense micropores and the ability to intercept large molecules, with a pore size of 1–20 nm; and a lower support layer with a large open-pore structure, which increases membrane strength.
The functional layer is relatively thin, resulting in a high water flux. Generally, various types of modules, such as tubular, plate, spiral wound, and capillary types, are first fabricated, and then multiple modules are assembled together for application to increase the filtration area. The ultrafiltration process is essentially a mechanical sieving process, and the size of the membrane surface pores is the most important controlling factor. Ultrafiltration membranes can separate solutes (polymers or solutions) with a size of 1–30 nm. The substances it repels are related to the membrane's properties, the shape, size, flexibility of the molecules, and operating conditions. Early ultrafiltration membranes used cellophane and nitrocellulose membranes.