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Basic Principles of Ultrafiltration 1 Overview Ultrafiltration is a filtration process driven by tangential flow of fluid and pressure, which separates particles in water based on their molecular weight. The pore size of ultrafiltration membranes is in the range of 0.002–0.1 micrometers (with an MWCO of approximately 1,000–500,000). Substances that are soluble and smaller than the membrane pore size can pass through the membrane as the permeate, while those that cannot pass through the membrane will be retained and concentrated in the effluent. Therefore, the produced water (permeate) will contain water, ions, and small molecules, while colloidal substances, particles, bacteria, viruses, and protozoa will be removed by the membrane. A hollow fiber ultrafiltration membrane is a very thin polymeric material made of polysulfone PS, polyethersulfone PES, PVDF, or polyacrylonitrile (PAN), and features an asymmetric microporous structure. Asymmetric ultrafiltration membranes have an extremely smooth and thin inner surface (0.1 micrometers thick) with pore sizes ranging from 0.002 to 0.1 micrometers; this inner surface is supported by a asymmetric mesoporous structure whose pore sizes can reach up to 15 micrometers. The combination of this smooth membrane surface with a small pore size and a supporting material with a larger pore size results in very low flow resistance when filtering tiny particles, making clogging less likely. 2 Basic Principles Ultrafiltration is a process that combines cross-flow and tangential flow, with the liquid to be filtered flowing along the surface of the membrane. This creates conditions of fluid shear on the inner wall of the hollow fiber, making it more difficult for contaminants to form on the membrane surface. The water to be filtered is pressurized by an ultrafiltration feed pump and then fed into the membrane module. Due to the pressure difference between the inside and outside of the membrane, some water penetrates through the membrane, while the impurities in the water remain in the remaining water and are filtered out. If too much of the impurity to be separated deposits on the membrane, depending on the type of membrane, this can lead to the deposition of insoluble salts or the formation of a partial coating layer. To avoid this, part of the water will become a concentrated stream and be removed; depending on the type of membrane and the application, this process either continues or takes place during reflux.