The pressure that forces water through the membrane. Also, net driving pressure is the feed pressure, minus the permeate backpressure, minus the osmotic pressure
NDP = P feed – P perm – π ,where π is the osmotic pressure of the feed solution
- in systems open to the atmosphere, permeate back-pressure is negligible
- for most brackish water systems, osmotic pressure (π) is also negligible
- for seawater systems or other highly concentrated brines, π becomes an important factor