Ultrafiltration and reverse osmosis address different water treatment tasks. Both technologies use membranes, but they differ in pore size, separation principles, and the range of substances they retain. Therefore, the choice between UF and RO should be made according to the requirements of the specific technological process after analyzing the source water and determining the target purification parameters. Water treatment for business must take into account the quality of the permeate, productivity, water recovery, energy consumption, and further handling of the concentrate.
The working principle and pore size of ultrafiltration membranes
What is ultrafiltration from a technical point of view? It is a membrane process in which water passes through a porous membrane under the influence of pressure difference. Separation occurs mainly based on particle size. In industrial systems, ultrafiltration membranes for water treatment can have pore sizes in the range of 0.02–0.05 microns.
Such a structure retains suspended particles, colloids, bacteria, and some viruses while allowing water and low-molecular dissolved substances to pass through. This is why UF belongs to barrier purification technologies but not to desalination.
Reverse osmosis operates on a fundamentally different level of separation. Its dense semi-permeable membrane retains dissolved salts and other low-molecular impurities, and higher pressure than osmotic pressure is required for water to pass.
What ultrafiltration removes and what remains in water
The main difference between the technologies is the composition of the permeate after membrane separation. The advantages of ultrafiltration lie in its effective removal of mechanical and colloidal contaminants. However, it is not designed to remove most dissolved salts.
| Component or contamination | Ultrafiltration | Reverse Osmosis |
| Suspended particles | Removes | Removes |
| Colloids | Removes | Removes |
| Bacteria | High removal efficiency | High efficiency |
| Viruses | Significant removal depending on the membrane and mode | High efficiency |
| Dissolved salts | Passes | Retains |
| Mineral composition | Remains | Significantly reduced |
| Desalination | No | Yes |
If it is necessary to reduce mineralization or obtain water with controlled low salt content, UF alone will not be sufficient.
Preserving mineral composition while fully removing bacteria
One of the important advantages of ultrafiltration is the ability to obtain clearer and microbiologically safer water without significantly removing dissolved mineral components.
This can be crucial for processes where the mineral composition of water should not be changed, but it is necessary to remove colloidal particles and microorganisms. UF can also act as a barrier before subsequent stages of deep cleaning.
Key differences between ultrafiltration and reverse osmosis
| Parameter | Ultrafiltration | Reverse Osmosis |
| Main Task | Removal of particles and colloids | Desalination and deep cleaning |
| Working Pressure | Usually no more than 3 bar | Significantly higher |
| Dissolved Salts | Predominantly pass | Predominantly retained |
| Mineral Composition | Remains | Significantly changes |
| Concentrate | Forms backwash/concentrate flow | Forms concentrate with increased salt content |
| Energy Costs | Generally lower | Higher due to the need for increased pressure |
Working pressure, discharge to drainage, and energy costs
Ultrafiltration requires significantly less pressure. For example, for pressurized UF systems, the typical feed pressure is less than 30 psi (~2.1 bar).
For reverse osmosis, the pressure is significantly higher. In industrial systems, some low-selectivity RO membranes are estimated to operate at approximately 8.6–10.3 bar under standard test conditions, while for high-selectivity membranes, the working pressure is usually between 10-11 bars to 15.
The difference in pressure directly affects the energy consumption of pumping equipment. Therefore, at the same productivity, UF usually requires less electricity than RO, although actual consumption is determined by the installation scheme, pumps, temperature, quality of the source water, and operating mode.
Water handling also differs. In UF, part of the flow can be used for periodic flushing of membranes, and the operational recovery of the system can be high.
In RO, part of the water is discharged into the concentrate to carry away retained salts. Therefore, when designing, it is necessary to consider not only the productivity for permeate but also the volume of concentrate and the possibilities for its disposal.
The fields of application of both technologies in industry
Ultrafiltration is advisable to apply where the main problem is suspended substances, colloids, bacteria, and other high-molecular contaminants. The technology is used for preparing surface and technological waters, treating wastewater, and obtaining stable water before subsequent membrane processes.
Reverse osmosis is needed when water requirements include reducing the concentration of dissolved salts. This is relevant for industries where mineralization affects product quality, the technological process, or equipment resources.
It is the analysis of the source water and the technology requirements that determine the optimal membrane scheme. Invent Group implements comprehensive solutions in the field of water treatment, including preliminary filtration, partial desalination, and membrane technologies.
Ultrafiltration as a stage of preliminary purification before osmosis
In many industrial systems, UF and RO do not compete but work sequentially. Ultrafiltration removes particles and colloids that can cause fouling of RO membranes. This reduces the load on osmosis and can decrease the frequency of chemical cleans and downtimes.
This scheme is particularly justified in cases of complex source water. UF creates a more stable feed stream for RO, while reverse osmosis has another task — reducing the content of dissolved salts.
For the enterprise, this means that water ultrafiltration is not a 'cheap version' of reverse osmosis. It is a different technology with a different function. If desalination is not needed, UF can be a more rational solution. However, if it is necessary to reduce mineralization, UF will not replace RO.
