Ultrafiltration membranes are polymeric semi-permeable membranes used in the ultrafiltration process to separate polymeric colloids or suspended particles of a certain size from the solution. Driven by pressure, the membrane pore size is 1 to 100nm and is an asymmetric membrane type. The pore density is about 10/cm and the operating pressure difference is 100-1000 kPa. It is suitable for removing colloidal level particles and macromolecules and can separate solutions with a concentration of less than 10%.
This polymeric membrane has an asymmetric microporous structure and is divided into two layers: the upper layer is a functional layer with dense microporous and intercepting macromolecules with a pore size of 1-20nm; the lower layer has a support layer with a large through-hole structure, which increases the strength of the membrane.
The functional layer is thin and has a large water permeability flux. It is generally made into various types of components such as tubular, plate, rolled, capillary, etc., and then assembled into multiple components for application together to increase the filtration area. The ultrafiltration process of membranes is essentially a mechanical sieving process where the size of the pores on the membrane surface is the primary controlling factor. The solutes (macromolecules or solutes) that can be separated by an ultrafiltration membrane are molecules of 1 to 30 nm in size, and the substances it rejects are related to the shape, size, flexibility and operating conditions of the material molecules in addition to the characteristics of the membrane. Early ultrafiltration membranes used cellophane, nitrocellulose membranes, etc.
Usually made from various polymer materials such as cellulose acetate, cellulose acetate esters, polyethylene, polysulfone, polyamides and aromatic polymers.
Ultrafiltration membranes have been widely used for the deep treatment of industrial wastewater and process water, such as the concentration, purification and separation of macromolecules in the chemical, food and pharmaceutical industries, the decontamination of biological solutions, the separation of dyes in printing and dyeing wastewater, the recovery of glycerol from petrochemical wastewater, the recovery of silver from photochemical wastewater and the preparation of ultrapure water. In addition, it can also be used for sludge concentration and dewatering.
Performance is expressed in terms of pure water permeability in square metres per hour and retained molecular weight and retention percentage. The greater the pure water permeability the better, and the retention rate is generally required to be >99%. High quality ultrafiltration membranes have a high pore density and a narrow pore size distribution.
There are two types of membranes, asymmetric and symmetric, according to their structure. The former has a dense surface layer, with a thickness of 0.1 micron or less, and an orderly arrangement of micropores, and a bottom layer thickness of 200-250 microns, belonging to the surface filtration; the latter is homogeneous, without a skin layer, the pores are the same, belonging to the deep filtration. Asymmetric membranes are used in industry, with components in the form of hollow fibre, plate, plate and frame, tube and so on. Membrane forming materials include cellulose, cellulose acetate, polycarbonate, polyvinyl chloride, polysulfone, polyvinylidene chloride, polyacrylonitrile, modified acrylic polymers, cross-linked polyvinyl alcohol, sulphonated polysulfone and polysulfone amides. It is a new type of chemical unit operation, widely used in the separation, concentration and purification of various biological preparations, pharmaceutical and food industries, blood treatment, wastewater treatment and as a terminal treatment device in the preparation of ultrapure water.