Molecular weight cutoff (MWCO)
Molecular weight cutoff (MWCO) is an important term in membrane filtration. It describes the pore size or rejection capability of a membrane. MWCO is in the unit of Dalton, or Da in short, it equals to the molecular weight (MW) of a known compound e.g. dextran, polyethylene glycol, which is 90% retained by the membrane. For example, in Figure 1, the MWCO of this membrane is regarded as 10,000 Dalton.
Because the method of MWCO determination is not standardized, there could be rejection variations between membranes with the same claimed MWCO among different membrane suppliers.
Figure 1 – Determination of membrane MWCO using solutes with different MW: solute rejection vs. solute MW. (Source:Synder)
Membranes are divided into four classes according to their Membrane MWCO or pore size, as shown below in Table 1.
Table 1 – Membrane classification and their applications.
| Membrane type | MWCO | Pore size | Species rejected | Main applications |
| Microfiltration (MF) | > 100 kDa | 0.1 – 10 µm | Suspended solids, bacteria, colloids, fats etc. |
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| Ultrafiltration (UF) | 1 – 500 kDa | 0.01 – 0.1 µm | Suspended solids, bacteria, viruses, colloidal silica, polypeptides, proteins, antibiotics, etc. |
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| Nanofiltration (NF) | 0.2 – 10 kDa | 1 – 10 nm | Multivalent ions, organic matters, dyes, sugars, antibiotics, etc |
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| Reverse osmosis (RO) | < 200 Da | < 1 nm | Salts, sodium ions, micropollutants e.g. PFAS, pharmaceuticals residues, etc. |
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Lenntech water and filtration and separation specialists help clients to select the most appropriate membrane based on the particle size or molecular weight of the species need to be filtered out/retained. Please contact us for more information or quotation at [email protected], or call us on +31 152 610 900.
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