When thinking of osmosis, reverse or otherwise, one might be reminded of a bit of circular reasoning seen on a geeky T-Shirt: “In order to understand recursion, one must first understand recursion.” It’s a math joke. Osmosis is kind of like that because in order to understand reverse osmosis, one must first understand osmosis. Please review the definitions in the terminology sidebar before proceeding with this article.

Osmosis Basics

Osmosis is a fundamental chemical and biological concept taught in both middle and high school, but some people find the concept hard to grasp. Osmosis is a kind of diffusion. When we think of things diffusing, we don’t have much trouble with the notion that, if given the chance, the atoms or molecules of gases and solutes in a liquid solvent or in air tend to naturally spread out and randomize rather than come together.

Say you pour a bit of gasoline onto a plate in a room. Initially, you can only smell the gasoline right next to the plate, but not on the other side of the room. Eventually, the gasoline evaporates and diffuses throughout the room until the molecules are equally spaced from each other and are as far apart from each other as they can get.

This process is driven by entropy, a basic operating principle of the universe. The probability that the gasoline molecules will spontaneously all find themselves back together as a liquid on the plate at some point in the future is so small (although it isn’t zero!) that we confidently assume it to be an impossible event.

The same principle applies to solids such as salts when they are dissolved in water. To the extent that a substance is soluble in a solvent such as water, a solute will distribute itself randomly in the solvent until equilibrium is reached and the concentration of solute is the same everywhere in the solution. The universe likes things this way.

Let’s move onto osmosis. Here’s a simple situation to explain it: For osmosis to occur, a barrier must exist between two solutions that have different solute concentrations. This barrier must allow water to pass through it while blocking the passage of whatever solute molecules exist in solution (remember, this is a highly simplified example).

The water will move across the selectively permeable membrane in the direction that decreases the concentration of solute molecules. This makes the solute particles farther apart from each other and less organized (entropy in action). Osmosis continues until a new equilibrium state is reached. Water molecules continue crossing the membrane, but the rate of crossing in one direction is equal to the rate of crossing in the opposite direction.

The reason people have an easier time understanding “regular” diffusion is because it’s more intuitive to imagine the solute molecules moving until they are as randomized as possible. It is harder to picture the solvent molecules diffusing to randomize the solute particles. But that’s the way it is.

Here is one final example to solidify an understanding of osmosis: Say you place some red blood cells in a solution of pure water—a classic experiment. Cell membranes are selectively permeable. They allow water and many other things to pass, but there are even more things, such as sugars and large protein molecules, that cannot pass.

Since the solute concentration inside the cells is much greater than the solute concentration outside the cells, which is essentially zero, water flows into the cells by osmosis to try and make the two concentrations equal. In fact, the cells will eventually burst from so much water flowing in. On the other hand, if red blood cells are placed in a highly concentrated salt solution, water will flow out of the cells into the salt solution and the cells will shrivel.

Osmosis works the same with plant cells, but since plants have cell walls, they don’t burst when placed in pure water and their membranes get pulled away from the cell wall as they shrivel if placed in a highly concentrated solution. This is why plants wilt if the nutrient solution concentration is too high.

So, what about reverse osmosis? In reverse osmosis, we are interested in making water as pure as possible by removing solutes and keeping the purified water. The universe doesn’t really approve of this happening naturally, so we have to come up with a way of reversing osmosis so that water moves across a selectively permeable membrane toward a lower solute concentration rather than a higher one.

How a Reverse Osmosis Water Filtration System Works

Water flows across a membrane because of a difference in osmotic potential. This potential is a pressure difference that is a function of the different solute concentrations on either side of the membrane. If we apply an external pressure in a direction opposite to the natural osmotic pressure and force the water through a selectively permeable membrane, we have created a reverse osmosis (RO) system. A system must also be implemented for removing the solute as it tries to move across with the water. A selectively permeable membrane does just that.

For this next example, let’s assume the input water is city water and we want to remove dissolved ions. Typically, city water contains an ionized disinfectant (a chlorinated compound), dissolved metals from pipes and salts that naturally exist in the water supply. Here is the basic idea behind an RO system:

  • A pump pressurizes water to force it through a selectively permeable membrane. The membrane allows the water to pass, but blocks the passage of undesired molecules.
  • A secondary flow parallel to the membrane is applied to flush away the solute molecules that get trapped. In this system, the membrane doesn’t get loaded with solute molecules, allowing it to keep passing water without clogging. This differs from bulk filtration, which entrains particles that are filtered from the flow.
  • The higher the solute concentration in the feed water, the higher the pressure must be to overcome the osmotic potential.
  • Multiple stages can be used to perform repeated RO filtrations to increase the purity of the water to any desired level up to the maximum achievable. Water with an electrical resistance of 18.2 megohms per cm is the best you can get.

The first RO membranes were made of cellulose acetate, but virtually all of today’s RO membranes are the thin film composite (TFC) type. These membranes consist of a thin layer of polyamide over a thicker, highly porous, polysulfone layer. The water that passes through the membrane is called thepermeate. The recovery rate of an RO system is the ratio of permeate flow to input flow and in some systems can be higher than 97%.

With a basic understanding of osmosis, reverse osmosis is not difficult. We could go into a lot more technical detail of specific RO systems, how they work, how they are maintained, calculation of rejection rates, etc., but this is just a primer—all of that other stuff will have to wait for another day.


OSMOSIS VOCAB

Diffusion – The movement of particles from a region of high concentration to a region of low concentration.

Concentration – The amount of a substance per unit volume. There are a lot of ways to express concentrations. The most useful units are grams per liter (g/L) and milligrams per liter (mg/L). Mg/L is also known as parts per million (ppm). You might also see things like, “a 10% solution of x” but unless more information is known, you can’t always be certain what that means exactly (mass/volume, volume/volume, or…?).

Entropy – Entropy is one of the most difficult and most misunderstood thermodynamic state functions. For simplicity’s sake, just refer to it as the tendency of the universe to become less ordered over time. Without an energy input, things naturally get more random; they do not become more organized.

Osmosis – The diffusion of water across a selectively permeable membrane from a region of low-solute concentration to a region of high-solute concentration.

Selectively Permeable Membrane – A barrier that allows some things to cross it while blocking the passage of others. A thin sheet of cellophane or a cell membrane are typical examples.

Solvent – The part of a solution that exists in the greatest quantity. For example, when a bit of sugar is dissolved in water, water is the solvent.

Solute – The part of a solution that exists in the smallest quantity. In a solution of sugar and water, the sugar is the solute.

Solution A perfectly mixed mixture. The concentration of particles, no matter what they are, is the same everywhere in the mixture.