Ultrafiltration (UF) and reverse osmosis (RO) are both membrane processes, and both produce clear water. They are nevertheless used for different jobs. Ultrafiltration is a very fine sieve that removes particles and germs. Reverse osmosis removes what is dissolved in the water: salts, hardness and most organic molecules. This guide compares the two processes and shows when one of them is enough and when they belong together.
What each process removes
The difference is the size of what the membrane holds back.
- Ultrafiltration has pores of 0.01–0.04 µm. It removes suspended solids, turbidity, bacteria, protozoa, most viruses and colloids. Dissolved salts, hardness, nitrate and small organic molecules pass through unchanged, so the conductivity of the water stays the same.
- Reverse osmosis has no pores in the usual sense. Water diffuses through a dense polyamide layer, while 97–99.5 % of the dissolved salts are held back, together with hardness, silica, nitrate, heavy metals and most organic substances.
Between the two lies nanofiltration, which removes hardness and larger organic molecules but lets most single-charged salts pass.
How the two processes work
Ultrafiltration: filtering at low pressure
Water is pressed through hollow-fibre membranes at 0.2–1.5 bar. Almost all the water passes the membrane; the solids stay on the membrane surface and are flushed out by an automatic backwash every 20 to 60 minutes. A chemically enhanced backwash with chlorine, caustic or acid follows daily to weekly. Details are on our ultrafiltration systems page.
Reverse osmosis: working against the osmotic pressure
A high-pressure pump raises the feed to 8–25 bar for brackish water and 55–70 bar for seawater. Only part of the feed becomes product water (permeate); the rest leaves as concentrate and carries the rejected salts away. The membranes are not backwashed. They are protected by pre-treatment and cleaned in place a few times a year. See our industrial reverse osmosis systems.
Ultrafiltration and reverse osmosis at a glance
| Criterion | Ultrafiltration | Reverse osmosis |
|---|---|---|
| Separation | Sieve, pores of 0.01–0.04 µm | Diffusion through a dense membrane |
| Removes | Particles, turbidity, bacteria, most viruses, colloids | Dissolved salts, hardness, nitrate, silica, most organics |
| Does not remove | Salts, hardness, dissolved organics | Dissolved gases such as CO₂ |
| Operating pressure | 0.2–1.5 bar | 8–25 bar (brackish water), 55–70 bar (seawater) |
| Recovery | 90–97 % | 50–85 % (brackish water), 35–50 % (seawater) |
| Energy | Below 0.1 kWh/m³ | 0.5–1.5 kWh/m³ (brackish water), 2.5–4.0 kWh/m³ (seawater with energy recovery) |
| Waste stream | Backwash water with the solids | Concentrate with the salts |
| Cleaning | Automatic backwash, chemically enhanced backwash | Cleaning in place a few times a year |
| Typical use | Clarification, disinfection barrier, pre-treatment | Desalination, demineralisation, process water |
When ultrafiltration is the right choice
- Drinking water from surface water or springs where turbidity and germs are the problem, not the salt content.
- Polishing of treated waste water for irrigation, washing or cooling water make-up.
- Replacing sand filtration where a constant filtrate quality is needed although the raw water changes.
- Pre-treatment ahead of reverse osmosis.
When reverse osmosis is needed
- Brackish water and seawater desalination.
- Boiler feed water, cooling water and process water with a limit on conductivity, hardness or silica.
- Removal of nitrate, sulphate, fluoride or heavy metals.
- High-purity water for pharmaceutical, electronics and laboratory use, followed by a second pass or EDI.
Why the two are often combined
Reverse osmosis membranes foul quickly when the feed contains particles and colloids. The usual measure for this is the silt density index (SDI), which should be below 3 at the RO inlet. Ultrafiltration delivers this value reliably, whatever the turbidity of the raw water. Surface water, seawater and waste water reuse plants therefore use ultrafiltration as pre-treatment and reverse osmosis for the desalination step. With clean well water, a multimedia filter and a 5 µm cartridge filter are often sufficient instead.
The reverse osmosis calculator estimates feed flow, concentrate flow and energy cost for a given recovery.
Common misunderstandings
- “Ultrafiltration softens water.” It does not. Hardness is dissolved and passes the membrane. Softening needs ion exchange, nanofiltration or reverse osmosis.
- “Reverse osmosis needs no pre-treatment.” It needs more than any other process, because the membranes cannot be backwashed.
- “RO water is sterile.” The membrane holds back germs, but the permeate side, the tank and the pipework can be recontaminated. Where the water must be germ-free, a disinfection step follows.
- “Ultrafiltration wastes no water.” The backwash uses 3–10 % of the filtered water, which is usually returned to the inlet of the treatment plant or to drain.
Data we need for a proposal
- Water source and a recent analysis: turbidity, suspended solids, conductivity or TDS, hardness, iron, manganese, organic carbon and temperature range.
- Required flow in m³/h and operating hours per day.
- Required water quality and the use of the water.
- Where the backwash water and the concentrate can be discharged.
Frequently asked questions
Does ultrafiltration remove salt or hardness?
No. Dissolved salts and hardness pass an ultrafiltration membrane. The conductivity before and after the membrane is the same.
Does reverse osmosis remove bacteria and viruses?
Yes, an intact RO membrane holds them back. It is nevertheless not used as the only disinfection barrier, because seals and the permeate side can let germs through.
Which process is cheaper to run?
Ultrafiltration, by a wide margin. It works at low pressure and uses a tenth or less of the energy of brackish water reverse osmosis. The two are not alternatives, however: if salts must be removed, only reverse osmosis or nanofiltration will do.
Can ultrafiltration replace a multimedia filter ahead of reverse osmosis?
Yes. For surface water and waste water it is the more reliable pre-treatment. For clean well water a multimedia filter with a cartridge filter costs less and is usually enough.
How long do the membranes last?
Ultrafiltration membranes typically last 5 to 10 years, reverse osmosis membranes 3 to 7 years. In both cases the service life depends mainly on the pre-treatment and on timely cleaning.
Not sure which process your water needs? Send us your water analysis and the required quality via our contact page. Our engineers will propose a suitable treatment line, with ultrafiltration, reverse osmosis or both.