Systems in which a selective membrane allows only species below a given size or molar mass to pass through. Under a pressure gradient, the permeate crosses the membrane; the concentrate is rejected. The separation scale is molecular or submicron — well beyond conventional Filters.
Usual types
Classified by separation cut-off size:
- Microfiltration (MF, 0.1–10 μm) — removal of bacteria, colloids.
- Ultrafiltration (UF, 1–100 nm) — removal of proteins, viruses.
- Nanofiltration (NF, 0.5–5 nm) — partial removal of monovalent salts.
- Reverse osmosis (RO, < 1 nm) — desalination; produces ultra-pure water.
Reliability: fouling (biofouling, salt scaling, colloid deposition — the dominant mode), chemical degradation by chlorine/oxidation, membrane compaction under high pressure. Central strategy: adequate pre-treatment and chemical cleaning (CIP) before fouling becomes irreversible.