Explanation

In reverse osmosis, water is forced through a semipermeable membrane against its natural osmotic tendency, from the concentrated (saline) side to the pure-water side. This reversal is possible only when the applied pressure exceeds the solution's osmotic pressure. Pressure equal to or below the osmotic pressure would merely allow normal osmosis, or nothing at all, rather than desalination. The applied pressure must therefore be greater than the osmotic pressure.