Flux is the throughput per unit membrane area, and the rate at which a membrane can run without rapid fouling. Concentration polarisation and recovery set the practical ceiling on sustainable flux.
Flux (litres per square metre per hour) sizes the membrane area and sets the fouling rate; conservative flux extends life.
Operating below the critical flux keeps fouling slow and reversible.
Flux, with recovery, fixes the membrane area and the number of elements.
Rejected solutes accumulate in a thin boundary layer at the membrane, raising local concentration above the bulk.
The polarisation (beta) factor quantifies this build-up and raises local osmotic pressure and scaling risk.
Crossflow velocity sweeps the boundary layer, limiting polarisation and fouling.
At the membrane surface, rejected salts are carried in faster than they diffuse back, so the wall concentration exceeds the bulk by the concentration-polarisation (beta) factor, typically 1.1 to 1.3. This raises the local osmotic pressure, reducing net driving pressure, and lifts the surface saturation of sparingly soluble salts, which is why scaling appears at the membrane wall before the bulk reaches saturation. Adequate crossflow and conservative flux keep beta, and therefore fouling and scaling, in check.
Flux is set by feed quality, conservative for difficult waters to protect life.
Element and spacer selection manages polarisation and pressure drop.
Recovery is capped by scaling and osmotic limits along the array.
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