A type of ultrafiltration membrane separation process involving a semi-permeable membrane to remove salts and microsolutes from a solution. These are small molecules that are separated from larger molecules, which are retained as the retentate. Unlike dialysis, which uses osmotic pressure to drive the solutes across the membrane, diafiltration uses an external force such as pressure. The process is typically used to reduce the level of salts in solutions containing proteins, peptides, nucleic acids, and other biomolecules from biochemical processes.