As mentioned above in the exposed examples, when working with free enzymes in solution, size-exclusion membrane operations are often chosen to separate the soluble biocatalyst 42 (retained in the retentate in cross-flow operations, see Figure ) while isolating the product in the permeate . In such size-exclusion operations, the membranes porosity -which defines its molecular weight cut-off (MWCO, in kDA) -has to be carefully chosen based on the molecular size of the enzyme (if present in free form), substrate(s), and product(s). Given that enzyme molecular weights typically range from 10 to 150 kDa, ultrafiltration (UF) membranes are commonly used in membrane reactors designs. The UF membrane pore size must ensure complete enzyme retention while ensuring unobstructed product transport. Nanofiltration (NF) membranes can also be employed in size-exclusion membrane operations design, especially for biocatalysts displaying small molecular weights (typically 0.2 to 10 kDA, such as for example such as for example some cysteine proteases 83 ) . Additionally, NF membranes are effective in concentrating target compounds as a secondary separation step 86 . As product purification is known to be a major driver of the cost of drugs manufacturing, this is a crucial point.