Membrane Filters Explained: What They Are and How They Work

Membrane filters act as barriers designed to remove contaminants from water by physically rejecting the particles responsible for impairment. Processes like reverse osmosis (RO), ultrafiltration (UF), and nanofiltration (NF) leverage these membranes, each adapting the basic principle of membrane technology to suit specific contaminants. To clarify, our Master Water Specialist, John Woodard, provides a concise overview of membrane filters and their in-action dynamics in on-site water treatment systems.

What is membrane filtration?

Membrane filtration includes reverse osmosis membranes, ultrafiltration membranes, and nanofiltration membranes. Each type offers a unique configuration and operates on similarly innovative but distinct underlying mechanisms tailored to the removal challenge faced.

How does a membrane filter work?

In reverse osmosis, the unit exerts pressure across a semipermeable membrane, compelling only water molecules to diffuse while sodium, magnesium, and other dissolved ions are routed through a concentrate stream to the drain. This results in a divided water stream destined either for production or for waste, as only the desired, purified water continues across the membrane.

As per GMI Research, the Membranes Market was touched at USD 7.8 billion in 2024

Ultrafiltration functions without separating water by dissolved solids weight; instead, it inhibits the passage of particles via simple mechanical criteria. The membrane precisely measures openings as small as 0.025 microns across, so even fine colloids and organic impurities—substantially smaller than human hair—are physically retained and sequentially flushed from the system.

What’s the difference between reverse osmosis and ultrafiltration membranes?

The two membranes work at different filtration levels. RO targets dissolved solids, pulling out dissolved minerals right down to the ionic level, while UF screens out larger particulate matter, leaving dissolved salts and most minerals in the water when it passes. Simply put, UF is great at turbidity and larger microbes, but it lets dissolved minerals through, while RO catches almost everything smaller, including dissolved inorganics.

What are membrane filters made of?

That depends on the technology. The most common RO membranes are classified as thin-film composites: a layered material blended on a porous support to create a super-thin, selective barrier. Earlier designs used cellulose triacetate, but they’re now rare in the market. CTA membranes were sensitive to low pH and produced low flow rates, limiting system size and membrane area. Moving to thin-film technology, manufacturers created membranes that generate greater permeate volumes in smaller footprints, allowing larger membrane sizes without the need for oversized housings. UF versions use a similar polymer system but tweak the formulation to optimize rejection of larger solutes and yet keep flow rates high, so the two technologies end up on almost indistinguishable surface materials, but with distinct selective characteristics.

How often do you swap out your membrane filter?

For a reverse osmosis (RO) membrane, think in two- to three-year chunks based on your feed water’s grit. Over time, the membrane holds back more dissolved stuff, and some begin to crystallize and stick to the surface. Running the RO on softened water buys you a healthy five-year stretch, as long as you keep the pre-filters swapped on schedule.

Cleaning a membrane filter in point-of-use (POU) setups isn’t the norm. Manufacturers aimed these units to run and eventually replace the RO module. Industrial systems? Sure, you might chemically clean. But home-size filters would need you to use proprietary cleaning solutions to dislodge calcifications. The math often still favors a new cartridge, simpler than the labor plus chemicals. One tip, though: run a short out-of-service flush after a long standby. It primes the membrane and verifies that the whole unit fires up in good standing.

What’s the proper way to rinse a filtration membrane?

An ultrafiltration (UF) membrane is specifically designed to operate as a flow-through device, not as a discrete separation unit within the overall process. At start-up, the membrane only needs to purge the residual air binary in place since it has been settled internally during the packaging phase. For reverse osmosis systems, moving water through the element at rated flux will resaturate the membrane; these units are usually manufactured in a dry state, so rehydration is beneficial. In a point-of-use unit, prime the system by filling two or three product-water tanks and allowing the full volume to flow to drain; this will effectively rinse the element and create a steady-state hydraulic condition before the final potable product is used.