Ultrafiltration vs. Reverse Osmosis – When to Use Which?

Ultrafiltration (UF) and reverse osmosis (RO) are the two key membrane processes in modern drinking and domestic water treatment. Both work with fine-pored membranes but pursue different goals: one retains particles, turbidity and microorganisms, the other additionally removes dissolved salts and the finest contaminants. For installers and planners, the right choice determines water quality, operating costs and maintenance effort. This guide classifies both processes and shows when each makes sense.

How the two membrane processes work

The key difference lies in the separation limit of the membrane – that is, in the size of the particles that are still retained.

Ultrafiltration: a fine particle and germ barrier

An ultrafiltration membrane typically has a pore size in the range of about 0.1 micrometre. It reliably retains suspended matter, turbidity, particles as well as bacteria and a large proportion of viruses, but allows dissolved minerals to pass. The water thus remains mineral-rich and natural in taste. UF systems work solely with the mains pressure, so they require no additional pressure boosting and produce virtually no wastewater. In the GWS range, the UltraWave Max series is available for this as a system for the point-of-entry (POE), whose UF membrane manages without backwashing thanks to an internal automatic flushing device.

Reverse osmosis: removal of dissolved substances

Reverse osmosis goes one step further. Its semi-permeable membrane is so dense that even dissolved salts, heavy metals, nitrate, pesticides and a high proportion of further micro-contaminants are separated out. For this, the water must be pressed through the membrane under increased pressure; part of the water is discharged in the process as concentrate (wastewater). The result is very pure, low-salt water. GWS offers several Reverse Osmosis solutions for this – from the compact under-sink systems AquaWave Classic and AquaWave Premium to the direct-flow system AquaWave Pro G500 for higher demand.

The main differences at a glance

• Separation limit: UF removes particles and microorganisms and leaves minerals in the water; RO additionally removes dissolved salts and the finest substances.
• Mineral content: UF water remains mineral-rich; RO water is low in salt and can be remineralised if needed.
• Water yield: UF works almost without wastewater; RO inherently produces a concentrate.
• Pressure and energy: UF uses the mains pressure; RO requires higher pressure, depending on the system with a pressure-boosting pump.
• Place of use: UF is suitable for central treatment at the building entrance (POE), RO mostly for the drinking water draw-off point (point-of-use, POU).

When ultrafiltration is the right choice

Ultrafiltration plays to its strengths when it comes to a safe, hygienic water barrier for the entire building without changing the natural mineral content. Typical use cases:

• Protection of the entire domestic water network against turbidity, particles and germs with well, spring or unsafe mains water.
• Applications in which mineral-rich water is expressly desired (taste, neutral water hardness balance).
• Locations without a wastewater connection at the installation site or with a desire for minimal water loss.
• Pre-treatment to protect downstream treatment stages from particle load.

When reverse osmosis is a better fit

Reverse osmosis is the first choice when dissolved substances are the problem and the highest purity at the tap is required:

• High salt or TDS content, nitrate, heavy metal or pesticide contamination in the drinking water.
• Premium drinking water at the kitchen tap as an alternative to bottled water.
• Commercial applications with a demand for consistently pure water, for example in catering, cafés or medical practices.
• Processes that require low-salt water, for example high-quality coffee and beverage preparation.

Because RO water is very low in salt, it can be remineralised in a targeted way if needed – for example with a mineral filter such as the GWS MineralWave pH+, which adds calcium and magnesium and rounds off the taste. For storing the treated water, suitable RO pressure storage tanks are available.

Practical selection checklist for planning

These points help to determine the appropriate process before sizing:

• Water analysis first: is the problem with particles and germs (UF) or with dissolved substances such as salts and nitrate (RO)?
• Clarify the place of use: should the entire house be protected (POE → UF) or only one drinking water tap be supplied (POU → RO)?
• Mineral content desired? should the water remain mineral-rich (UF) or be maximally purified and, if necessary, remineralised (RO)?
• Check pressure and electricity: is the mains pressure sufficient (UF) or is a pressure-boosting pump necessary (RO)?
• Water consumption and wastewater: is low-wastewater operation important (UF) or is the concentrate acceptable (RO)?
• Plan maintenance: take pre-filters, membrane replacement and flushing intervals into account in the operating costs.

The two processes often complement each other: a central ultrafiltration secures the entire domestic water, while a reverse osmosis system at the kitchen tap delivers premium drinking water. A sound water analysis remains in any case the basis for the right decision.

Practical guide: Which size for which application?

Whole-house ultrafiltration systems (Point-of-Entry, POE) are not sized by the number of people alone, but by the simultaneous peak flow of your draw-off points. The following table provides a rough orientation based on the performance data of the GWS UltraWave Max UF20J, which delivers around 33 l/min at 1 bar up to about 58 l/min at 5 bar, depending on the applied pressure. These figures should be understood as orientation and do not replace an individual system design.

Typical application / household size	Recommended scale	Note
Single draw-off point or small apartment (few simultaneous draw-offs)	One UF unit of the UF20J class is amply dimensioned here	With only one active draw-off point, the flow potential is generally not fully utilised.
Single-family house (typically 1 to 2 simultaneously used draw-off points)	One UF unit of the UF20J class covers the usual peak demand	The usable flow rate increases with the supply pressure (approx. 33 l/min at 1 bar up to approx. 58 l/min at 5 bar).
Small multi-family building (about 2 to 4 units)	One UF unit of the UF20J class depending on simultaneity, possibly several units in parallel	What matters is how many draw-off points are realistically used at the same time, not the number of units alone.
Larger multi-family building or small business (higher simultaneity demand)	Several UF units of the UF20J class connected in parallel	The total flow rate results approximately from the sum of the units operated in parallel.
Commercial / agricultural use with high or continuous demand	Several parallel UF units; design as an individual project specification	For continuous operation, peak load, pre-filtration and maintenance intervals must be considered separately.
Industry / high-rise building (very high peak flow)	System design with several parallel units or a larger system	A professional project design is required here; a single POE unit is not sufficient.

Important note: The scales given here are purely orientation values. The actual system size required depends on several factors and, in case of doubt, should be designed by a specialist company or directly by GWS.

Factors influencing the design

• Simultaneous peak flow: Number of draw-off points realistically used at the same time.
• Supply pressure: Higher inlet pressure increases the usable flow rate of the UF unit.
• Usage profile: Short peaks (household) versus continuous demand (commercial/agricultural).
• Number and type of consumers: Shower, washing machine, outdoor taps, etc.
• Water quality and pre-filtration: Affect operation, service life and maintenance effort.
• Reserve and expandability: Possibility of connecting further units in parallel as demand grows.

Frequently Asked Questions

What is the main difference between ultrafiltration and reverse osmosis?

Ultrafiltration uses a membrane of around 0.1 micrometre to remove particles, suspended matter as well as bacteria and a large proportion of viruses, but leaves minerals in the water. Reverse osmosis uses a much denser membrane and additionally separates out dissolved salts, heavy metals, nitrate and the finest contaminants. The result is very pure, low-salt water.

Are the minerals retained in the water with ultrafiltration?

Yes. The pores of an ultrafiltration membrane are too large to retain dissolved minerals such as calcium and magnesium. The water therefore remains mineral-rich and natural in taste, while particles and microorganisms are reliably separated out.

Does reverse osmosis consume more water than ultrafiltration?

Reverse osmosis inherently produces a concentrate (wastewater), since part of the water serves to discharge the dissolved substances. Ultrafiltration, on the other hand, works almost without wastewater and solely with the mains pressure. However, modern RO systems such as direct-flow systems are significantly more efficient than older designs.

Should reverse osmosis water be remineralised?

Since reverse osmosis water is very low in salt, it can be remineralised in a targeted way if needed. A mineral filter such as the GWS MineralWave pH+ adds calcium and magnesium, raises the pH value and improves the taste – this is a matter of taste and can be sensible depending on the application.

Can ultrafiltration and reverse osmosis be combined?

Yes, the two processes complement each other well. A central ultrafiltration at the building entrance (point-of-entry) protects the entire domestic water network against particles and germs, while a reverse osmosis system at the kitchen tap (point-of-use) delivers high-purity premium drinking water.