Sanitizing a reverse osmosis system is a routine job done at the annual cartridge change, and there is one step that must not be skipped: the membrane comes out before any sanitizing solution goes in. Chlorine destroys thin film composite material, so sanitizing with the membrane in place ruins the most expensive component in the system.
Before you start
Close the feed valve to the system and the storage tank valve, then open the dedicated faucet and let the tank drain completely, which takes longer than expected and is worth waiting out. Depressurize fully before opening anything. Have the replacement cartridges ready, along with a fresh set of O rings, food grade silicone lubricant, a housing wrench and a bucket. Photograph the arrangement of tubing before disconnecting anything, since it is easy to misremember which line went where.
The sequence
Remove all the cartridges and take the membrane out of its housing and set it somewhere clean, or discard it if it is being replaced. Wash the housings in warm soapy water and rinse thoroughly. Add the sanitizing solution, commonly unscented household bleach at the dilution the manufacturer specifies, into the first stage housing, reassemble the housings empty, and open the feed to draw the solution through the system and into the tank. Let it stand for the specified contact time.
Rinsing out and reassembling
Flush thoroughly: fill and drain the tank several times, running the faucet until there is no chlorine smell at all, since residual bleach reaching a new membrane damages it immediately. Only then fit the new cartridges and refit the membrane, noting its orientation. Fit fresh O rings with silicone lubricant, never petroleum grease. Then discard the first two or three tankfuls, which carry carbon fines and taste of the new cartridges, before drinking any of it.
How often, and what else to check
Annually is the usual interval, aligned with the cartridge change, and sooner if the system has stood unused for a long period or if the water has developed an off taste. While it is apart, check the storage tank's air charge with the tank empty, inspect the flow restrictor, the automatic shutoff valve and the permeate check valve, and take a fresh feed and permeate reading afterwards as a baseline. Our whole house reverse osmosis guide covers the annual service.
Questions people ask about how to sanitize a reverse osmosis system
Do I have to remove the membrane first?
Yes. Chlorine destroys thin film composite membrane material, so sanitizing with it in place ruins the most expensive component.
What solution should be used?
Commonly unscented household bleach at the dilution the manufacturer specifies, drawn through the empty housings and into the tank.
How thoroughly must it be rinsed?
Completely. Fill and drain the tank several times until there is no chlorine smell, since residual bleach damages a new membrane.
How often should this be done?
Annually with the cartridge change, and sooner after any long period of disuse or if the water develops an off taste.