What is ultrafiltration (UF) in terms of membrane filter technology?

Ultrafiltration (UF) is a type of membrane filtration in which hydrostatic pressure forces a liquid against a semipermeable membrane. A semipermeable membrane is a thin layer of material capable of separating substances when a driving force is applied across the membrane. 

Once considered a viable technology only for desalination, membrane processes are increasingly employed for removal of bacteria and other microorganisms, particulate material, and natural organic material, which can impart color, tastes, and odors to the water and react with disinfectants to form disinfection byproducts (DBP). As advancements are made in membrane production and module design, capital and operating costs continue to decline.

Ultrafiltration uses hollow fibers of membrane material and the feed water flows either inside the shell, or in the lumen 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 (103 - 106 Da) solutions, especially protein solutions. Ultrafiltration is not fundamentally different from reverse osmosis, microfiltration or nanofiltration, except in terms of the size of the molecules it retains. 

When strategically combined with other purification technologies in a complete water system, UF is ideal for the removal of colloids, proteins, bacteria, pyrogens, proteins, and macromolecules larger than the membrane pore size from water. The primary removal mechanism is size exclusion, though surface chemistry of the particles or the membrane may affect the purification efficiency. UF can be used as pretreatment for reverse osmosis systems or as a final filtration stage for deionized water.

The primary advantages of low-pressure UF membrane processes compared with conventional clarification and disinfection (post chlorination) processes are:

• No need for chemicals (coagulants, flocculates, disinfectants, pH adjustment);
• Size-exclusion filtration as opposed to media depth filtration;
• Good and constant quality of the treated water in terms of particle and microbial removal;
• Process and plant compactness; and
• Simple automation.

Ultrafiltration (UF) is used to remove essentially all colloidal particles (0.001 to 1.0 microns) from water and some of the largest dissolved contaminants. The pore size in a UF membrane is mainly responsible for determining the type and size of contaminants removed. In general, membrane pores range in size from 0.005 to 0.1 micron. 

UF membrane manufacturers classify each UF product as having a specific molecular weight cutoff (MWC), which is a rough measurement of the size of contaminants removed by a given UF membrane. A 100,000 MWC UF membrane means that when water containing a given standard compound with a molecular weight of around 100,000 daltons is fed to the UF unit, nearly all of the compound will not pass through the membrane.

Ultrafiltration is used in:

• Laboratory grade water purification
• Wastewater treatment
• Drinking water treatment
• Paint Recovery in the automotive industry
• Desalting and solvent-exchange of proteins
• Dialysis and other blood treatments

Substances with a molecular weight of 100,000 daltons have a size of about 0.05 microns to about 0.08 microns in diameter. UF membranes are used where essentially all colloidal particles (including most pathogenic organisms) must be removed, but most of the dissolved solids may pass through the membrane without causing problems downstream or in the finished water. UF will remove most turbidity from water.