Reverse Osmosis RO Membrane
Removal of dissolved salts, minerals and organic compounds with high-pressure membranes, typically at 95-99%.
What Is Reverse Osmosis (RO)?
Reverse osmosis (RO) means applying a pressure above the osmotic pressure to the feed side of a semi-permeable membrane so that water is driven in the opposite direction to natural osmosis, and dissolved salts, minerals and organic compounds are held back behind the membrane. Able to separate even the ions that UF and NF let through, RO is the tightest rung of the separation ladder: its separation limit is expressed in the order of ~0.0001 µm, and typically 95-99% of dissolved salts are removed.
RO is a critical technology in industrial water reuse, ZLD (Zero Liquid Discharge), process and boiler water production, and in brackish and sea water treatment. It is the heart of the reuse chain (biological treatment → UF/MBR → RO): dissolved salt and conductivity can only be lowered at this stage.
The purpose of this article is not to sell but to explain: why osmotic pressure drives the energy bill, why cross-flow is what makes RO possible, why recovery cannot be pushed to 100%, and which parameters an RO unit is sized with. The numerical ranges given are typical orders of magnitude found in the literature; the exact values for any plant are determined by water analysis, membrane projection and preferably a pilot test.
Looking for the bigger picture? This page explains the RO technology itself. For the biological stage that produces the ideal RO feed, see What Is MBR? → · Technology summary page: Reverse Osmosis (RO) · Water reuse scope: Water Reuse Solutions
Typical RO System Flow
High Pressure
Operating pressure of 10-70 bar (fresh water: 10-15, sea water: 55-70 bar). Energy recovery units.
Membrane Configuration
Spiral wound membranes, 4-inch and 8-inch elements. Single or double pass design.
Membrane Protection
Antiscalant dosing, CIP (chemical cleaning) system, SDI and turbidity monitoring.
The Physics of RO: Osmotic Pressure, Cross-Flow, Recovery
Understanding RO means understanding three concepts. This trio sits behind every design decision — from pump pressure to membrane life, from the energy bill to the concentrate volume.
Osmotic pressure — nature's resistance
If there are different salt concentrations on the two sides of a semi-permeable membrane, water moves spontaneously from the dilute side to the concentrated side (osmosis). The pressure that must be applied to the concentrated side to stop that flow is the osmotic pressure, and it rises with the dissolved ion concentration. RO reverses the flow by applying a pressure above the osmotic pressure to the feed. The force that actually drives the membrane is the net driving pressure: applied pressure − osmotic pressure − line/element losses. As salinity increases, so does the osmotic pressure and therefore the pump pressure and the energy demand — this is why brackish water is typically operated at ~10-20 bar and sea water at ~55-70 bar.
Cross-flow — the membrane sweeping itself
In a classic filter the water flows perpendicular to the surface and everything retained accumulates there (dead-end). In RO the feed flows parallel to the membrane surface inside the spiral-wound element: part of it passes through the membrane as permeate, and the rest sweeps the salts away and leaves as concentrate. That sweeping limits the accumulation of salt at the membrane surface (concentration polarisation). If the cross-flow velocity drops, the concentration at the surface becomes far higher than in the bulk stream; both salt passage and the risk of crystalline precipitation (scaling) increase. This is the reason for the recovery limits per element.
Recovery — the single knob for both gain and risk
Recovery is the ratio of permeate flow to feed flow. The higher it goes, the more water is recovered and the smaller the concentrate becomes; but the salt concentration of that shrinking concentrate multiplies by the concentration factor ≈ 1/(1−recovery): about 2-fold at 50% recovery, about 4-fold at 75% and about 10-fold at 90%. At some point the sparingly soluble salts (CaCO₃, CaSO₄, silica) pass saturation and crystallise on the membrane. This balance is why single-stage industrial RO typically targets 50-75% and multi-stage designs typically target 85-95%.
The physics in one line: RO is the balance of a net driving pressure that overcomes the osmotic pressure, a cross-flow that limits polarisation, and a recovery that does not trigger scaling. Most field problems begin when one of these three moves outside its limits.
RO Design and Operating Parameters
An RO unit is sized by solving these interdependent parameters together. The values are typical ranges quoted in the literature; the exact design is made with a water analysis and membrane projection software.
| Parameter | What It Represents | Typical Range (literature) | What It Affects |
|---|---|---|---|
| Operating pressure | Pump pressure applied to the feed | Typically ~10-20 bar (brackish water / reuse), ~55-70 bar (sea water) | Net driving pressure, energy consumption |
| Permeate flux | Water passing through unit membrane area (L/m²·h) | Typically ~10-25 L/m²·h (depending on the source)* | Fouling rate; high flux = rapid fouling |
| Recovery | Permeate flow / feed flow | Typically 50-75% (single stage), 85-95% (multi-stage) | Concentrate volume, scaling risk, osmotic pressure |
| Salt rejection | Share of dissolved salt retained by the membrane | Typically 95-99%+ | Permeate conductivity / quality |
| Feed SDI | Colloidal plugging index (Silt Density Index) | Typical target SDI15 <5, preferably <3 | Fouling rate, CIP frequency, membrane life |
| Free chlorine | Oxidant reaching the polyamide membrane | Typically <0.1 mg/L (in practice ~0) | Irreversible membrane damage (rising salt passage) |
| Concentration factor | Concentration coefficient of the concentrate ≈ 1/(1−recovery) | ~4-fold at 75% recovery | Saturation indices, antiscalant requirement |
| Feed temperature | Affects flux through water viscosity and diffusion rate | Typically ~15-35 °C | Flux (rises with temperature) and salt passage (which also rises) |
*A higher flux is selected for clean surface or well water feeds, and a more conservative (lower) flux for treated wastewater reuse. All values in the table are typical literature magnitudes, not commitments; the final sizing is done with a water analysis, a membrane projection and preferably a pilot test.
In Which Sectors / Applications Is It Used?
RO comes into play wherever dissolved salt has to be removed. The chains below are typical orientations — the exact flow scheme is built according to the water analysis.
| Sector / Application | The Role of RO | Typical Chain | Use of the Permeate |
|---|---|---|---|
| Food & Beverage | Upgrading treated wastewater to process quality | DAF → MBR → RO | Process, washing/CIP, boiler and cooling make-up |
| Textile & Dyehouse | Removing salt and residual colour and returning the water to the process | Biological → UF → RO (2 stages) | Dyeing and rinsing water |
| Metal & Plating | Recovery of rinse waters, separation of the salt load | Chemical → UF → RO → (ZLD) | Rinse water |
| Energy / Steam Boiler | The main stage of demineralised water production | Softening/UF → RO → (EDI / mixed bed) | Boiler feed water |
| Paper & Pulp | Salt control of process water going into the closed loop | DAF → Biological → UF → RO | Process water |
| Industrial Zone / Municipal | Upgrading treated wastewater for secondary use | MBR → UF → RO | Irrigation, parks and gardens, industrial make-up |
| Sea / Brackish Water | Water supply (desalination) | Pre-treatment → SWRO/BWRO | Drinking/process water (with remineralisation) |
Note: the chains in the table are typical configurations, not commitments. For each project the flow scheme is determined by laboratory analysis and a feasibility study.
Technical Highlights
The values are typical magnitudes in the industry; actual performance varies with the feed water analysis, the membrane selection and the design recovery.
Operating Tips
In RO the membrane set is the most critical consumable item; what protects it is not equipment but disciplined operation. Five fundamental habits:
Monitor the data in normalised form
Track permeate flow, salt passage and pressure differential as trends cleaned of temperature and feed variations (normalised). While the raw data still says "everything is fine", the normalised trend shows that fouling has started weeks earlier — the CIP decision is made from that trend.
Treat pre-treatment as the membrane's insurance
Measure SDI regularly, monitor the cartridge filter pressure differential and verify the integrity of the UF/MBR barrier. Most membrane failures begin not in the RO itself but in the stage before it.
Verify the antiscalant dose and stay alert to chlorine
The antiscalant is selected according to the saturation indices at the target recovery, and its dose is verified by pump calibration. Polyamide membranes are defenceless against free chlorine: if disinfection is in use, make sure it is removed upstream of the membranes (e.g. bisulphite / activated carbon).
Do not force recovery — optimise it
Pushing recovery above the design value looks like "free water" in the short term; in the medium term it comes back as scaling, more frequent CIP and membrane replacement. An operating point that balances water, chemical and energy consumption performs better than the maximum on paper.
Flush during shutdowns, keep records
Flush the membranes with permeate or low-salinity water at every shutdown; use a preservation solution for long stops. Records of pressure, conductivity, flow and chemical consumption are the only way to find the root cause of a problem.
Common Operating Problems and Their Solutions
The five RO problems most frequently met in the field, their symptoms and their likely root causes.
1. Permeate flow is falling — fouling
Symptom: normalised flux is decreasing, the first-stage pressure differential is rising. Possible cause: colloidal/organic fouling or biofilm; usually inadequate pre-treatment or an increase in SDI. Approach: verify SDI and the pre-treatment; apply a CIP weighted towards alkaline/detergent chemistry; where biofouling is suspected, review the cleaning programme and the feed disinfection scheme.
2. Performance loss in the last stage — scaling
Symptom: flux loss and rising salt passage, especially in the last elements; crystal traces in the concentrate line. Possible cause: recovery exceeding the saturation limit, a wrong antiscalant dose or type, or a dosing pump failure. Approach: lower the recovery, verify the antiscalant dose, apply an acidic CIP; for a permanent solution, redo the saturation projection.
3. Permeate conductivity is rising — salt passage
Symptom: permeate quality deteriorates while the flux is normal or even higher. Possible cause: chlorine/oxidant damage (irreversible), an O-ring or seal leak, or surface damage to an element. Approach: locate the leak by conductivity probing on a pressure-vessel basis; replace the O-ring if that is the cause, and if it is oxidant damage, cut off the source and replace the affected elements.
4. Pressure differential (ΔP) is rising rapidly
Symptom: the inlet-outlet pressure differential of a stage climbs within a short time. Possible cause: plugging of the feed channels (spacers) with solids or biofilm; at an advanced stage, the risk of mechanical deformation of the elements (telescoping). Approach: carry out the CIP before the ΔP alarm limits are exceeded; if coarse solids are getting through, correct the cartridge filtration and the pre-treatment.
5. Performance does not return after CIP
Symptom: despite cleaning, the normalised flux does not return to its former level. Possible cause: a cleaning that was left too late (hardened/layered fouling), the wrong chemical selection, or a CIP at the wrong temperature/pH. Approach: identify the fouling type (autopsy/analysis) and build the chemical recipe accordingly; where the loss is permanent, plan element replacement and tighten the CIP trigger thresholds.
Related Solutions & Detail Pages
MBR (Membrane Bioreactor)
The biological + membrane combination that produces the ideal feed for RO.
Ultrafiltration (UF)
The standard protective barrier ahead of RO: retention of suspended solids and colloids.
RO Technology Page
The summary technology page for RO within the advanced membrane family.
Water Reuse Solutions
GESU's scope and approach in industrial and municipal water reuse.
DAF Systems
Oil, grease and TSS removal at the head of the reuse chain.
Ask for an RO / Water Reuse Proposal
Apply with your water analysis; we will answer with a feasibility study and a membrane projection.
Frequently Asked Questions — Technical
The questions below focus on the engineering of RO. If you cannot find the answer you are looking for, get in touch with our engineers.
What is osmotic pressure and why does it matter so much in RO?
When two solutions of different salt concentration are separated by a semi-permeable membrane, water naturally moves from the dilute side to the concentrated side (osmosis); the pressure that has to be applied to the concentrated side to stop that movement is called the osmotic pressure. In RO, pressure above the osmotic pressure is applied to the feed so that the flow is reversed. The saltier the water, the higher the osmotic pressure; the net driving pressure is found by subtracting the osmotic pressure and the line losses from the applied pressure. That is why saline waters demand a higher pump pressure and therefore more energy.
Why does RO operate in cross-flow?
In classic filtration the water flows perpendicular to the filter surface and everything retained accumulates on that surface (dead-end). In RO the feed flows parallel to the membrane surface: part of the water passes through the membrane and becomes permeate, while the remaining stream sweeps the salts and contaminants away and leaves the system as concentrate. This continuous sweeping limits the build-up of salt at the membrane surface (concentration polarisation) and is what makes uninterrupted RO operation possible. If the cross-flow velocity drops, polarisation increases and both salt passage and the risk of scaling rise.
What is recovery, and why can it not be 100%?
Recovery is the ratio of permeate flow to feed flow. As recovery increases the concentrate volume shrinks, but the salt concentration inside it rises exponentially (concentration factor). Beyond a certain point the sparingly soluble salts (calcium carbonate, calcium sulphate, silica) exceed their solubility limit and crystallise on the membrane (scaling); in addition, the rising osmotic pressure pushes up the pump pressure. This is why single-stage industrial RO typically targets 50-75%, while multi-stage designs typically target 85-95%; the exact value is determined by the water analysis and projection software.
What is the difference between fouling and scaling?
Fouling is the covering of the membrane surface by suspended solids, colloids, organic matter or biofilm; it is generally seen in the first elements near the feed and increases the pressure differential. Scaling is the crystalline precipitation of dissolved salts once the solubility limit is exceeded; it starts in the last elements, where the concentration is highest. They are also managed differently: fouling is controlled by pre-treatment and alkaline/detergent cleaning, scaling by antiscalant dosing, limiting recovery and acidic cleaning.
What is SDI and why is it critical for RO feed water?
SDI (Silt Density Index) is a standard test that measures the tendency of water to plug a 0.45 µm membrane, and it indicates the colloidal fouling potential of the feed. In the literature, the typical target for RO feed is SDI15 < 5, with < 3 preferred. Water with a high SDI fouls membranes rapidly; this is why a UF or MBR barrier is placed upstream of the RO. A low and stable SDI is one of the most important guarantees of membrane life.
When should CIP (chemical cleaning) be carried out?
Common engineering practice is to carry out a CIP when the normalised permeate flow drops by roughly 10-15%, when the normalised salt passage increases noticeably, or when the stage pressure differential rises by roughly 10-15% (typical thresholds — the membrane manufacturer's instructions govern). The critical point is to base the decision on normalised rather than raw data; temperature and feed variations make raw values misleading. A delayed cleaning hardens the fouling layer and can lead to irreversible flux loss.
Why is UF or MBR needed upstream of RO?
The feed channels (spacers) of spiral-wound RO elements are below a millimetre; suspended solids and biological fouling plug those channels quickly. UF or MBR retains suspended solids and bacteria behind a physical barrier and thus delivers a stable feed with a low SDI. In industrial water reuse, biological treatment plus UF — or MBR directly — is the standard protective layer ahead of RO; a cartridge filter on its own is not pre-treatment but a final safeguard.
Is RO permeate directly suitable for every use?
No. Because permeate contains very few minerals it has an aggressive character, and it should not be fed into any line without a corrosion assessment. For boiler feed it usually requires additional conditioning (degasser, chemical conditioning, and where very high purity is needed, further polishing with EDI or a mixed bed), while for process or drinking water use it may require remineralisation or blending with raw water. The correct approach is to design the final treatment step according to the quality specification of the point where the permeate will be used.
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