January, 2010
Inspection News and Views from the American Society of Home Inspectors

What Clients Might Want to Know About Reverse Osmosis Water Filtration Systems


On occasion, we encounter Reverse Osmosis (RO) water filtration systems and the inevitable client who wants to know how the system works. Although the process is somewhat complex, I offer my clients a simplified explanation, starting with a brief example of the process of osmosis.

The basis of osmosis is that a less concentrated liquid solution always seeks to dilute a more concentrated solution. For example, if a semi-permeable membrane, which is membrane that allows some particles to pass but not others, depending on size (think filter), separates a saltwater solution and an equal volume of pure water, osmotic pressure will be created as the pure water passes through the membrane in an attempt to dilute the saltwater.

The naturally occurring phenomenon of osmosis is the reason people cannot drink ocean water. When saltwater is ingested, osmotic pressure begins drawing water out of the body in an effort to dilute the saltwater in the stomach. If a person were to continue to drink saltwater, he or she would eventually dehydrate and die.


In an RO water treatment system, we reverse normal osmotic pressure by forcing water through a semi-permeable membrane using the pressure of the feed water (the water pressure of a home’s water supply) to move the water on the more concentrated side of the membrane (feed water side) to the less concentrated side (purified water side). The pores of the semi-permeable membrane are large enough to allow water molecules through, but small enough to block most of the larger contaminant (salt, minerals, etc.) molecules. The “purified” water is stored in a tank beyond the membrane, to be used on demand through the sink-top glass filler. Typically, one or more pre-filters are used. These include a carbon or charcoal filter to remove organic contaminants that are small enough to penetrate the RO membrane (usually affecting taste and odor) and a particulate filter, usually removing (actually trapping) particles down to 5 or 10 microns.

RO systems require a considerable amount of water. The purification of a gallon of water, for example, may require four or more gallons of input water, depending on the filter (membrane) used and the quality of the domestic water supply. On the input side of the membrane, there is an inlet and an outlet for the water. Since only about 15-30 percent of the “purified” water makes it beyond the membrane, there is a lot of reject water. This water contains rejected contaminants and is usually evacuated to the house’s drain, waste and vent (DWV) system. The connection to the DWV system should be made on the sink side of the trap or, better yet, to an indirect drain.

Modern RO systems have a built-in air gap in the glass filler on the sink top for this drain to pass through to prevent a direct connection between potable water and the sewer system. These are identifiable by three tubes connecting to the glass filler base under the sink. Cross-connection of older RO systems is common and a topic for another article.

RO systems generally require maintenance a couple times a year. Of course, this depends on intensity of use and the number and types of contaminants in the feed water. Clients should be notified of the presence of the system and advised to contact the manufacturer and/or inquire with the seller regarding maintenance requirements and operation costs.

It also is worthy to note that many RO systems are rented from a water treatment company and it is a good idea to have your client check on this as well.