If a choice had to be made, the Reverse Osmosis process would be elected as state-of-the-art in water treatment technology today. Reverse Osmosis (RO for short) was developed in the late 1950's under U.S. government funding, as an economical method of desalinating sea water - a dream long sought after by mankind.
Reverse Osmosis is revolutionary because it uses a completely new mechanism for processing water - the semi-permeable membrane. Surprisingly enough, it looks a lot like common household sandwich wrap and is composed of very similar polymers (plastics). That's where the similarity ends because this near miracle material is now considered one of the greatest technological achievements of our century.
It's best to look at the semi-permeable RO membrane as providing two distinct water treatment processes. First, it is the ultimate mechanical filter, or ultra-filter, straining out virtually all particulate matter, turbidity, bacteria, microorganisms, asbestos - even single molecules of the heavier organics. To appreciate the fineness of this ultra-filter, as it is referred to in the industry, its pores are on the order of .0005 microns or .0000002 (two ten-millionths) of an inch! That's smaller than can be seen by the best optical microscopes.
Second, it removes dissolved impurities (e.g. mineral salts, toxic metals) - those even smaller than the water molecules themselves - by a remarkable phenomenon known as Reverse Osmosis. With RO, the membrane is said to reject these impurities by repelling them from its surface. It is however permeable to the water molecules so that they diffuse through in a pure state and collect on the opposite side to make the product water.
RO's Claim to Fame
The real claim to fame for Reverse Osmosis membranes is their ability to remove and reject such a wide spectrum of impurities from water and they do it with very minimal energy usage. In fact, it just requires water pressure. With the exception of distillation, RO is the only known process which can effectively remove the following types of impurities:
- Particulate matter, turbidity, sediment, etc.
- Colloidal matter
- Total Dissolved Solids
- Toxic Metals
- Radioactive elements
- Pesticides and Herbicides
- Heavier organic molecules (MW>300)
How RO Works: In Practice
Long sheets of semi-permeable membrane are ingeniously sandwiched together and rolled up around a hollow central tube in a spiral fashion. This rolled-up configuration is commonly referred to as a spiral wound membrane or module. They are available in a large selection of sizes for processing different quantities of water. Typically, a module for home water treatment is as small as 2" diameter and 10" long, while one for industrial use may be 8" diameter and 48" long.
Now that the membrane is in a usable form it must be put in some type of container (called a pressure vessel) so pressure can be maintained on its surface. It is this pressure that supplies the energy to force the water through the membrane, separating it from the impurities. The most amazing aspect of RO is that the contaminants left behind are automatically diverted to a waste drain so they don't build up in the system as with conventional filters and purification devices. This is accomplished by using a part of the unprocessed water (feed water) to carry away the rejected impurities to the drain, thus keeping the membrane clean. The flow of impurities to the drain is often referred to as reject water. This is the secret to why RO membranes can last so long and perform like new with minimum maintenance even after years of operation. It is also the reason behind the low cost of producing RO water.