Reverse osmosis or ultrafiltration? Choosing the right membrane
Ultrafiltration, with a 0.01–0.1 micron pore size, retains suspended solids, bacteria and viruses but passes dissolved salts; reverse osmosis uses a semi-permeable membrane to also separate dissolved ions, producing demineralised water. The right choice depends on the contaminant type in the raw-water analysis and the target quality — and often both are used together as sequential stages.
When people say “membrane” in water treatment, two technologies come to mind: ultrafiltration and reverse osmosis. Both treat water by passing it through a barrier, but the class of contaminant they retain — and therefore their purpose — is fundamentally different.
The difference: pore size
The simplest distinction is pore size.
- Ultrafiltration (UF): 0.01–0.1 micron pore size. It physically retains suspended solids, turbidity, bacteria and viruses. It passes dissolved salts and ions.
- Reverse osmosis (RO): Not a pore but a semi-permeable membrane. It reaches down to dissolved ions, separating dissolved contaminants such as salt, nitrate and heavy metals.
Simply put: UF clarifies water and makes it microbiologically safe; RO demineralises water.
Which contaminant, which membrane?
| Contaminant | Ultrafiltration | Reverse osmosis |
|---|---|---|
| Turbidity, suspended solids | Retains | Retains |
| Bacteria, viruses | Retains | Retains |
| Dissolved salt (salinity) | Passes | Retains |
| Nitrate, sulphate | Passes | Retains |
| Heavy-metal ions | Partly | Retains |
| Hardness (Ca, Mg) | Passes | Retains |
The table shows why the choice depends on the raw-water analysis: if the problem is turbidity and microbiological load, UF is enough; if the problem is salinity, hardness or dissolved contaminants, RO is required.
Why often together?
In practice the two technologies are frequently installed together as sequential stages. Ultrafiltration acts as pre-treatment ahead of reverse osmosis: by retaining suspended solids and microorganisms it prevents the RO membrane from fouling and wearing prematurely. This markedly extends RO membrane life and lowers the system’s total operating cost.
Energy and recovery
Because RO operates under pressure, it consumes more energy than UF and discharges some water as concentrate (waste). The recovery rate — how much of the feed water is converted to product water — is typically in the 65–75% range; with energy-recovery and concentrate-recycle options it can be raised higher for certain waters.
UF, by contrast, operates at low pressure, consumes little energy and has a high recovery rate. That is why, in applications requiring only clarification and disinfection, UF alone is both sufficient and economical.
How is the decision made?
The right membrane combination is not selected from a catalogue; it emerges from the raw-water analysis. The water’s turbidity, conductivity, hardness, ion profile and microbiological load are measured, compared with the target quality, and the minimum set of stages required is determined. Too many stages mean unnecessary cost; too few mean missing the target — which is why the analysis is the starting point of every project.