With increasing concern over drinking water quality, more people are turning to home water purification technology to either enhance or protect their own tap water. Reverse Osmosis (RO) technology is a popular technology choice for homeowners. From a quality standpoint, these machines are great. They offer a rejection of 98 – 99% of total dissolved solids (TDS) such as lead, iron, chromium, and other metals. RO also offers a virtual 100% removal of bacteria and viruses. Designed properly, RO can provide a user with very safe, high quality drinking water.

Unfortunately, when it comes to home RO systems, the technology is extremely wasteful and does not contribute to sustainability. Let’s take a closer look.

A single RO membrane is roughly 17% efficient. That means for every 100 gallons of water fed into the unit, 17 gallons of good water will be produced while 83 gallons will be sent down the drain. This drain water is called reject water.

Industry makes RO much more efficient by applying the engineering process known as membrane staging. Staging is a design in which multiple membranes are used and the reject from the first membrane is used to feed the second membrane. Then the reject from the second membrane is used to feed the third and subsequent membranes, or sets of membranes.

So what happens is, a one membrane system is 17% efficient; a two membrane system can be made 34% efficient; a three membrane system can be therefore made to be 51% efficient. The limit on efficiency however is governed by feed water quality and system design. In general, RO system efficiency tops out at about 75%, although some manufacturers claim higher inefficiencies.

The reason that an RO machine can never be 100% efficient is because the contaminants that are removed from the feed water must somehow be carried to the drain. This is the purpose of the reject water; it transports the waste products away from the membrane. Each system, based on feed water quality has a scientifically determinable maximum efficiency. If operated above that efficiency the rejected contaminants begin to scale and plug up the RO membranes thereby shutting down the system.

Due to very low flow rates of home RO systems, in general they are designed with only one membrane. This makes most home RO systems 83% wasteful, and not very compliant with today’s conservation and sustainability goals.