Stage 4: ST-RO50 50GPD QC RO Membrane Cartridge (Filmtec) – Replace every 2 years – NSF 58
The RO membrane is the "engine room" of your R/O. All the crucial work is done here, so all the filters before the membrane are primarily there to protect the membrane.
This membrane is a 0.00001-micron rating. It is classed by the manufacturer to be a U.S 50 gallon per day membrane (190 Litres).