In the absence of Jo's link, I hope the following helps:
Osmosis is the diffusion of a solvent or a dilute liquid through a skin( permeable in only one direction) into a more concentrated solution.
Reverse Osmosis is therefore the passage of a concentrated solution through a skin (permeable in only one direction) into a more dilute one.
It is a process widely used in water treatement, and pressures can be applied to improved process.
Please let us know what other info you require in the absence of a working weblink. Best wishes Pauline RedR
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