Figure 1
Issue: 2/05

Many people have heard of reverse osmosis and know that it can be used to treat water to a high degree of purity, but their understanding of the technology ends there.

To understand reverse osmosis (called "RO"

Figure 2

Reversing Osmosis

Osmosis can be reversed by simply overcoming the osmotic pressure. If we apply enough pressure to the more concentrated salt reservoir, we can force water in the opposite direction, leaving a high percentage of the impurities behind.

Sometimes you will hear RO referred to as "membrane filtration."

How RO Works

Anytime one talks about "filtration,"


Even though it acts as a barrier to atom-sized particles in water, RO is an inefficient sediment filter. Used as such, RO systems would clog very quickly and stop producing water. Every RO membrane should be accompanied by proper pretreatment. In household applications, the pretreatment is typically built in. Pretreatment consists of sediment removal before the membrane by a string or fiber-type filter cartridge. Pretreatment also includes an activated carbon cartridge to protect the thin membrane polymer layer from substances that would oxidize it (such as chlorine in municipal water) and breach the membrane, allowing passage of untreated water. The water should also be free of high levels of precipitating ions such as iron or manganese.

There are two primary terms that describe RO operation:

  • "Rejection"


    To find out more about a particular RO product, it is advisable to seek out a manufacturer's Performance Data Sheet detailing rejection percentages for a range of contaminants and the unit's expected daily water production while operating at a specified temperature and pressure.

    An RO operated at a higher pressure will produce more permeate, as well as have a higher percentage of rejection of contaminants. An increase in temperature will do the same. Increases in dissolved solids, suspended solids or percentage recovery for a system will have the opposite effects.

    Also, for residential RO products, look for some form of third party certification to NSF/ANSI Standard 58 and/or WQA S-300. Third-party certification checks material safety (ensuring that wetted parts do not leach chemicals into the water) and structural integrity of the device under simulated operating conditions. The certifying body also verifies capacity numbers and recovery ratings. WQA's Gold Seal Program is one such certification.

    Mark Rowzee is the education director for the Water Quality Association, where he coordinates education activities for the association and the WQA Aquatech USA trade show event. For more information on this article or the WQA Aquatech USA 2005 trade show March 30-April 1, 2005, in Las Vegas, contact him at (630) 505-0160 x 513, or visit WQA's Web site at