Membrane filtration is a physical separation process that uses a semipermeable membrane to remove suspended solids from a liquid stream. MF and UF, along with nanofiltration (NF) and reverse osmosis (RO), are all examples of pressure-driven membrane filtration.

MF/UF are flexible filtration methods, and both are used to remove a variety of particles, pathogens, and microorganisms from process liquids. MF/UF is typically incorporated into a larger system, occurring downstream from media filtration units that capture large particles, and upstream from processes that provide further separation, purification, or waste treatment.

MF and UF systems share much in common with one another, with the main difference being that UF is able to catch finer particles than MF due to the comparatively smaller pore sizes of UF membranes.

MF and UF physically separate solids from liquid streams based on the principle of size-exclusion. As a feed stream is passed through the MF/UF membrane, any solids that are too large to pass through the membrane’s pores are retained, while any liquid or small particles are permitted to flow through. In either MF or UF, the portion of the feed stream that has passed through the filter membrane is referred to as the filtrate or permeate, while the remainder is known as the retentate. Depending upon the industrial application at hand, the filtrate and/or retentate may each be directed to other systems, as appropriate, for waste treatment, or purification through RO.

MF/UF units support direct flow and crossflow filtration and in direct flow filtration, also known as dead-end filtration, a feed stream is forced through a porous membrane. Particles too large to fit through the membrane pores build up on the filter in a residue known as a filter cake. Dead-end filtration provides more comprehensive filtration of the feed stream, and is typically performed in batch or semi-continuous flows, allowing for the membrane to be replaced or cleaned regularly.