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Environmental Health

Reverse Osmosis (RO) Devices Home Water Treatment Devices:

How It Works:

Reverse osmosis is used to remove salts and other impurities from water in order to improve the color, taste or properties. A reverse osmosis unit substantially reduces most suspended and dissolved matter from water. RO systems are NOT appropriate for treating water supplies that are contaminated by coliform bacteriaImage of Reverse Osmosis Devices

Reverse osmosis uses a membrane that is semi-permeable, allowing water to pass through it, while rejecting the contaminants that remain. The process requires a driving force or pressure to push the water through the membrane. Contaminants are removed by forcing this water through a membrane having microscopic holes that allow water molecules, but not larger compounds, to pass through.

Reverse osmosis units remove many inorganic contaminants from household drinking water supplies. The removal effectiveness depends on the contaminant and its concentration, the membrane selected, the water pressure and proper installation. Membranes are made of a variety of materials that differ in effectiveness for different chemicals. Be sure to study water test data and identify the chemicals to be removed.

Limitations and Maintenance:

A Typical Home RO System Includes: (1) particle filter, (2) reverse osmosis
membrane unit, (3) pressurized treated-water storage container,
(4) carbon adsorption post-filter, and (5) separate treated-water tap.

Image of Reverse Osmosis System

Although reverse osmosis removes many organic chemicals, it does not remove all. And it does not remove 100% of most chemicals. RO systems are not appropriate for treating water supplies that are contaminated by coliform bacteria.

These units waste large amounts of water. Household RO units typically deliver small amounts (2 to 10 gallons per day) of treated water and waste 3 to 20 times the amount of water treated. Waste water is typically connected to the house drains and will add to the load on the household septic system.

The membrane can develop problems from precipitate buildup and scaling. A softener must be installed ahead of the reverse osmosis unit if hard water is used. Otherwise, the minerals will quickly plug the membrane filter.

Frequently, mechanical and/or activated carbon filters are installed before the reverse osmosis unit to remove turbidity and improve taste and odor. This can result in improved overall water quality and extend the life of the reverse osmosis membrane.

The reverse osmosis membrane will need periodic replacement according to the manufacturer's recommended schedule. Occasional cleaning and flushing of the whole reverse osmosis unit may be required.

Additional Information:

Certification of Residential Water Treatment Devices

When a manufacturer claims that a drinking water treatment device will reduce contaminants [such as lead, Cryptosporidium (protozoan cysts), pesticides, herbicides, solvents, heavy metals, bacteria and viruses] or makes other health related performance claims, the device must be certified by the DHS, pursuant to Health & Safety Code Section 116830.

DHS does not regulate devices that make aesthetic claims. Aesthetic claims include improvement in taste, odor and appearance.

Also, DHS does not regulate backpacking or camping filters, sports bottles, shower filters or water softeners.