Many of us encounter Reverse Osmosis (RO) water treatment systems during inspections. Often a client will ask “how does that work?” This Kaplan Inspection Tip will explain the process.

First we need to understand the process of osmosis. The basic fundamental of osmosis is: the less concentrated solution will seek to dilute the more concentrated solution. If we place a semipermeable membrane between, say, more salty water and pure water, osmotic pressure will occur and cause the pure water to seek to dilute the salty water. This is why drinking salty (ocean) water will kill a person. When you put salty water in your stomach, osmotic pressure begins drawing water out of your body to try to dilute the salt in your stomach. Eventually, you dehydrate and die.

In an RO water treatment system we reverse normal osmotic pressure by forcing water through a semipermeable membrane with higher pressure on the more concentrated side of the membrane than the less concentrated side. The semipermeable membrane has pores 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 the tank beyond the membrane to be used on demand through the sink top tap. To help prevent the membrane from clogging quickly typically one or more pre-filters are installed to catch the larger particles.

RO systems need a lot of water. Obtaining 5-gallons of purified water requires about 45 to 90 gallons of input water. On the input side of the membrane there are an inlet and outlet for the water. Since only about 5-15 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 DWV system. 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.

RO systems generally require maintenance a couple times a year. Of course this all depends upon use and/or the amount of contaminants in the feed water. Clients should be notified of the presence of the system and to contact the manufacturer and inquire with the seller regarding the recommended maintenance and costs. Additionally, many of these systems are “rented” from a water treatment company. Clients should inquire with the seller regarding whether the system is owned or leased and costs.

Mike Casey
Kaplan Professional Schools
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