Understanding Reverse Osmosis

Here's an e-mail I received about the common confusion regarding reverse osmosis:

"In your explanation of reverse osmosis you state 'Reverse osmosis occurs when the water is moved across the membrane against the concentration gradient, from lower concentration to higher concentration'. But then in the same paragraph you state 'In reverse osmosis, pressure is exerted on the side with the concentrated solution to force the water molecules across the membrane to the fresh water side'. I believe the first statement is incorrect, please confirm."

My response:

Both statements are true. Think of it like this: if you are using reverse osmosis to desalinate seawater, you will get the first bit of fresh water across the membrane from simple diffusion. Once the concentration of the water is the same on both sides, it will require energy or force to get more fresh water from the salty side to the freshwater side. Normal osmosis is basically diffusion, with water (from higher to lower concentration) and across a membrane. Reverse osmosis is forcing water to cross a membrane against the concentration gradient. What's confusing is, there is more than one concentration gradient involved. There is the water, which becomes less concentrated in the seawater side, and there are the other molecules. As water is forced out of the seawater, it becomes 'more concentrated' with respect to these other molecules. Its solvent is being removed.

Reverse Osmosis | Hard & Soft Water