What Is Ultrafiltration – How Do UF Membranes Work Exactly?

Ultrafiltration is the latest technology in home water filters. Once reserved for industrial use, it’s now available in pint-sized systems that fit under your kitchen sink.

But what exactly is ultrafiltration, how does it work, and is it the best solution for your unique water quality issues?

Let’s find out.

What Is Ultrafiltration?

Ultrafiltration (UF) walks the line between mechanical filtration and a membrane filtration process similar to reverse osmosis. Hydrostatic pressure forces water through a hollow fiber membrane that removes particulate material down to the pore size in the membrane material.

Like a sediment filter, it has the qualities of a mechanical filter but with the superior capabilities of membrane filtration. Used in hemodialysis, it removes low-molecular-weight solutes from blood and is equally effective at filtering water.

Which Contaminants Do Ultrafiltration Systems Remove?

Like a mechanical filter, an ultrafiltration membrane removes suspended solids — undissolved free-floating particulate material such as:

• Dirt
• Rust
• Bacteria
• Parasites
• Parasitic cysts
• Select viruses

The pore size of the membrane material determines which contaminants it removes. Unlike conventional mechanical filters, the pore size is much smaller — 0.01 to 0.1 microns versus 5-10 microns — for more complete filtration.

A UF membrane alone does not remove dissolved substances, such as volatile organic compounds, minerals and salts. Combined with carbon and other filters, however, it can be an important part of a complete water system.

Ultrafiltration Vs Nanofiltration

Nanofiltration is another membrane filtration method similar to ultrafiltration, but NF membranes have a smaller pore size. More efficient than ultrafiltration but less than reverse osmosis, no contaminant greater than 0.01 microns can pass through the membrane.

Nanofiltration removes even minute bits of sediment, microorganisms, some dissolved minerals and other natural organic material. Still, it doesn’t remove sodium, chloride, metal ions and many chemical pesticides.

Ultrafiltration Systems Vs Reverse Osmosis Systems

Reverse osmosis systems use incoming water pressure to force water through a semipermeable membrane. The same technology used in water treatment facilities, the membrane pore size eliminates most contaminants down to 0.0001 microns.

Among water filtration methods, reverse osmosis is the gold standard, eliminating up to 99.9 percent of dissolved solids including dangerous heavy metals, like lead.

RO membranes, however, struggle to remove chemicals from pesticides to volatile organic compounds. But as part of a multi-stage filtration system that includes a high-quality carbon or KDF filter, they’re the best protection money can buy.

Which Filtration System is Right for Me — Ultrafiltration or Reverse Osmosis?

Both UF and RO systems have benefits and drawbacks. So, let’s compare and contrast these two common water purification technologies.

Contaminant Removal

Ultrafiltration membrane pore size is larger than RO membrane pore size, so the filtration isn’t as comprehensive. Still, both UF and RO systems produce potable water, so if you need microbial removal, either is a good choice.

The biggest difference between UF and RO filters is the type of contaminants they reduce. UF membranes eliminate suspended solids while RO membranes remove particles dissolved in water, including beneficial minerals. It’s like drinking softened water but cleaner and without added sodium.

Neither RO nor ultrafiltration membranes remove all substances, so they’re generally combined with filters that broaden the spectrum of contaminants removal for more complete filtration. You can, however, buy a UF membrane-only system specifically to remove bacteria.

Taste

Minerals like calcium carbonate give water a familiar flavor and a refreshing quality. UF systems preserve those minerals — RO filters don’t, so water can taste flat at first.

Most people say they get used to it after a few weeks, while others prefer it to their untreated tap water from day one. But if you like the taste of minerals, consider an RO system with a remineralization filter that adds a touch of calcium post-purification.

Capacity

UF filter produces water on demand — RO systems don’t. The RO membrane processes water slowly, so 3-5 gallons are reserved in a storage tank to guarantee a steady supply of treated water. As the tank is drained, more is made — drinking water production never stops.

But the quantity you can use at any given time is limited to what’s in the tank, and the system can only produce a certain number of gallons per 24 hours. Running out of water, however, while possible, is rare.

Flow Rate

Any barrier placed between feed water and your tap, including filters, will reduce water flow. Both RO and UF filters have the potential to decrease water pressure, but RO systems have a greater impact.

UF membrane pores are larger and more forgiving. The hollow fibers range from least dense on the outside to most dense toward the center of the filters so pressure-reducing clogs are rare.

Footprint

RO storage tanks can be a challenge to fit under your sink, especially if you have a small cabinet or garbage disposal. An ultrafiltration system fits virtually anywhere.

Energy Consumption

Both RO and UF filters rely on incoming water pressure for separating substances. But RO membranes require higher incoming pressure to overcome the pressure difference that drives the RO process. If you have insufficient city water pressure or a low-flow well, you may need an electric booster pump.

RO filters also produce 2-3 gallons of wastewater for every gallon of purified water they make, so if you have city water, expect your bill to rise. UF filters need occasional backwashing, but waste is minimal.

Maintenance Needs

Ultrafiltration membrane replacement is the only maintenance a UF system needs. But RO systems have more parts. Some manufacturers recommend sanitizing the storage tank periodically.

Cost

You can purchase a reverse osmosis system for as little as $200, but plan to spend $300-$400 for sought-after features. A good quality ultrafiltration system can be had for under $100.

The long-term cost of ownership is low for both types of filter, but a UF membrane lasts longer than the semi-permeable membrane in an RO system — 5-7 years versus 1-5 years.

Ease of Installation

Because of the storage tank, RO systems are somewhat more complex to install than UF systems, but both are usually do-it-yourself jobs.

RO filters require a dedicated dispensing faucet, however, while many UF systems connect directly to your existing faucet. If your sink doesn’t have an extra hole pre-drilled, you may need a pro to help you prepare the sink.

Environmental Impact

The wastewater RO systems create is difficult to repurpose. Mineral-laden, it poses a problem for treatment facilities. But effluent water from an ultrafiltration system is clean and better for the environment. RO filters may be restricted in areas prone to frequent drought because of their heavy water consumption.

The Bottom Line

The choice of an RO or UF system depends primarily on the contaminants you want to remove — dissolved or suspended solids.

If your water looks, smells and tastes good, but you have bacteria in your well or get frequent boil orders, no other purification technologies remove enough microorganisms to make water potable short of UV light.

For broad-spectrum protection, nothing beats an RO filter. But if bacteria are the only concern, a single-stage UF filter is a more economical, environmentally friendly and space-saving choice.

If your drinking water tastes bad or contains chemicals from chlorine to disinfection byproducts, a multistage UF or RO system with a carbon filter is your best protection.

Opt for an RO filter if you want to remove dissolved salts, like fluoride, or heavy metals, like lead. A UF membrane will remove particulate but not dissolved metals.

Final Thoughts

Ultrafiltration systems fill an important niche between RO filters and the garden-variety carbon filters used to manage aesthetic impurities. Either is an excellent choice, and both produce very high purity water you’ll feel good about drinking.