The retention rate of a filter membrane indicates the retention of defined particles or test organisms under defined conditions. In microfiltration, it is indicated in µm and can be nominal or absolute. For simplicity’s sake, it is often referred to as pore size.
- Nominal retention rates only indicate percentage retention rates of particles with a defined size. The details vary depending on the manufacturer, and can therefore not be used for a direct comparison of different filter membranes.
- Absolute retention rates indicate the smallest particles or test organisms still retained under defined conditions. Based on DIN 58356-1, the retention capacity of a membrane filter element for example for Brevundimonas diminuta is described as given if a sterile filtrate is obtained with a specified bacterial load of 1 x 107 pathogens per cm² of effective filter area. By definition, this membrane filter element then has an absolute retention rate of 0.2 µm. The following additional test organisms are used for the definition of other absolute retention rates:
||Absolute Retention Rate