Concentration polarisation — the build-up of rejected solute at the membrane wall that raises osmotic pressure and lowers flux.
Membrane Flux & Recovery — in depth
Concentration polarisation is an inherent membrane effect: rejected solutes accumulate in a thin boundary layer at the membrane surface, raising the local concentration and osmotic pressure and reducing net driving force. Cross-flow and turbulence sweep the layer away, limiting its impact and protecting flux.
What matters in practice
Rejected solute concentrates at the wall.
Cuts the net driving force.
Turbulence thins the layer.
Reduces achievable permeate flux.
| Factor | Effect | Mitigation |
|---|---|---|
| Wall conc. | Higher osmotic P | Cross-flow |
| Flux | Reduced | Lower flux |
| Velocity | Thins layer | Higher |
| Spacers | Turbulence | Promoters |
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Fundamentals, design drivers and practical guidance
Concentration polarisation — the build-up of rejected solute at the membrane wall that raises osmotic pressure and lowers flux.
Concentration polarisation is the reversible build-up of rejected solute in the boundary layer at the membrane surface; it raises local osmotic pressure, depresses flux and can precipitate scale. Crossflow velocity sweeps this layer away, which is why velocity, spacer geometry and the resulting critical flux — the flux below which fouling is negligible — are central design parameters rather than afterthoughts.
Sustained operation depends on pre-treatment and recovery. Feed is conditioned to a target Silt Density Index to protect the membranes; system recovery is set to balance water yield against the scaling risk of an ever-more-concentrated reject; and clean-in-place chemistry — alkaline/oxidant for organics and biofilm, acid for scale — restores flux on a schedule driven by normalised performance, not the calendar. Array design (stages and the tapered pressure-vessel arrangement) keeps crossflow adequate as permeate is removed.
Reynolds & Bauhm designs membrane plant around critical flux, realistic recovery, robust pre-treatment and a normalised-data CIP regime, with array and energy design that holds rejection and flux over the membrane life — not just at start-up.
What our engineers assess on every scope of this type
| Parameter | Typical basis | Why it matters |
|---|---|---|
| Array | Staged, tapered | Holds crossflow as permeate leaves |
| SDI | Pre-treat to target | Protects membranes from fouling |
| Critical flux | Operate below it | Keeps fouling rate low |
| Crossflow | Velocity set by design | Sweeps polarisation layer |
| Recovery | Balanced vs scaling | Maximises yield safely |
| CIP | Alkali/oxidant + acid | Restores flux by foulant |
Common questions on membrane process engineering
Cleaning is driven by normalised data — when flux, differential pressure or salt passage drift past thresholds — not by the calendar. Alkaline/oxidant cleans lift organics and biofilm; acid cleans dissolve scale.
It is the reversible accumulation of rejected solute in the thin boundary layer at the membrane surface, which raises local osmotic pressure and depresses flux. Concentration Polarisation is managed largely by maintaining adequate crossflow velocity to sweep that layer away.
Critical flux is the flux below which fouling is negligible. Operating below it dramatically slows fouling, extends time between cleans and protects membrane life, so it is a primary design target rather than an afterthought.
Feed silt and colloids foul membranes irreversibly if uncontrolled. Conditioning the feed to a target Silt Density Index protects the elements and is fundamental to sustaining the performance that Concentration Polarisation relies on.
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