Reverse Osmosis

Diffusion is the movement of molecules from a region of higher concentration to a region of lower concentration. Osmosis is a special case of diffusion in which the molecules are water and the concentration gradient occurs across a semipermeable membrane.

The semipermeable membrane allows the passage of water, but not ions (e.g., Na⁺, Ca²⁺, Cl^-) or larger molecules (e.g., glucose, urea, bacteria). Diffusion and osmosis are thermodynamically favorable and will continue until equilibrium is reached. Osmosis can be slowed, stopped, or even reversed if sufficient pressure is applied to the membrane from the 'concentrated' side of the membrane. 

Reverse osmosis occurs when the water is moved across the membrane against the concentration gradient, from lower concentration to higher concentration. To illustrate, imagine a semipermeable membrane with fresh water on one side and a concentrated aqueous solution on the other side. If normal osmosis takes place, the fresh water will cross the membrane to dilute the concentrated solution. 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. 

Reverse osmosis is often used in commercial and residential water filtration. It is also one of the methods used to desalinate seawater. Sometimes reverse osmosis is used to purify liquids in which water is an undesirable impurity (e.g., ethanol).

"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."

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.

The Bottom Line

Ultralife Crystal Clear R.O. 25 GPD Unit is cost effective and removes up to 98% of mineral and chemical contaminants from tap water through 3 stages filtration.
 

• 3 stage filtration.
• Removes up to 98% of contaminants from tap water.

Cons

• None known.

Description

• 3 stages: 1 micron prefilter, 5 micron solid carbon block and Thin Film Composite (T.F.C.) membrane.
• Removes up to 98% of mineral and chemical contaminants from tap water.
• Fully assembled and comes with a standard 3/4" female adapter.

Guide Review - Ultralife Crystal Clear R.O. Unit

The Crystal Clear R.O. 25 GPD Unit, manufactured by Ultralife is one of the most inexpensive 3 stage, complete R.O. units on the market. It removes up to 98% of mineral and chemical contaminants from tap water and easily connects to any 3/4" faucet for quick set up. Setting aside the convenience factor of being able to make as much of your own R.O. water as you want, whenever you want, this unit will pay for itself in a short period of time if you are purchasing commercial R.O. water for top offs and water changes.