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.  Following graphic shows 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 which is the only membrane expressed in pore opening.  The former ones are expressed in MW cut off.