RO water system

A semipermeable membrane has small pores that block contaminants but allow water molecules to flow through. 

In osmosis, water becomes more concentrated as it passes through the membrane to obtain equilibrium on both sides.

Reverse osmosis, however, blocks contaminants from entering the less concentrated side of the membrane.

For example, when pressure is applied to a volume of saltwater during reverse osmosis, the salt is left behind and only clean water flows through.