A complete different world of filtration is accomplished by membranes.  As described earlier they operate on sub-micron particles and dissolved molecules like proteins and salts. 

The graphics on the right explain the difference between Dead-End and Cross-Flow filtration and show the membrane filtration range, starting with RO for removing all dissolved solids, through Nano Filtration (NF),  for partial dissolved solids removal for small organic molecules,   Ultra Filtration (UF),  for bigger dissolved solids, like larger Molecular Weight (MW) proteins and finally Micro Filtration. MF is usually defined by pore opening while RO, NF, UF are usually defined in Molecular Weight (MW) cut off.

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In the bioreactors of Waste Water Treatment plants submerged membranes of around 50 nm are used to increase the bacteria concentration and to obtain clear filtrate.