Reverse-Osmosis Water Filters

A reverse-osmosis (R/O) water treatment device can often remove a wide range of particulate contaminants. However, the processing of your water is relatively slow, and the cost can sometimes be relatively high. While technically a “filter,” because water must pass through a membrane, most people think of this as a unique technology.

How Reverse-Osmosis Units Work

To understand how a reverse-osmosis unit works, it’s best to understand osmosis and reverse osmosis. Osmosis is the natural process by which water spontaneously passes through a semipermeable membrane (a very fine material that allows only very small molecules to pass through it). Actually, water goes through such a membrane in only one direction: it passes from a solution containing a low concentration of dissolved substances, through the membrane, to a solution containing a higher concentration. Eventually, this results in the same concentration of water on both sides of the membrane. By the way, osmosis is the process by which water passes from the stems and leaves of plants (sites of low concentrations of dissolved substances) through the cell walls (which are semipermeable membranes) into the interiors of the cells (sites of high concentrations of dissolved substances).

Reverse osmosis, on the other hand, is not a naturally occurring process. Instead, it requires the addition of pressure to create the opposite effect from that of osmosis. In other words, in reverse osmosis, water is forcibly pushed through a semipermeable membrane so that the water goes from a solution containing a high concentration of dissolved substances to a solution with a much lower concentration. The result is a different water concentration on each side of the membrane. The intent of reverse osmosis is to separate water from dissolved substances.

Through this process, reverse-osmosis (R/O) units are capable of effectively treating home-water supplies. In practice, your home’s water pressure (the pressure in your water pipes), and sometimes additional pressure, drives a stream of water through a special membrane. If your water supply is chlorinated, the membrane might be made of cellulose acetate or cellulose triacetate. On the other hand, polymid resin film might be used with unchlorinated water (other membrane types are also available).

Interestingly, reverse-osmosis membranes aren’t all capable of removing the same-size particulates. In fact, the pores (tiny openings) in various R/O membranes are rated as to the largest matter that can pass through them. For example, a typical membrane used in residential units might have a rating of 0.01 microns. Although water molecules under pressure are able to pass through these minute openings, the holes are just too small to let solids over 0.01 microns pass through. (In reality, a membrane will not have all pores of the same size. The pores in a membrane with...