Reverse osmosis for Phoenix drinking water
Reverse osmosis (RO) is the system people add for drinking water. It works on a different problem from a water softener, and in the Valley the two are often installed together.
How reverse osmosis works
Water is pushed under pressure through a semi-permeable membrane. Water molecules pass; most dissolved minerals, salts and many other contaminants are held back and flushed away. The University of Nebraska–Lincoln Extension lists sodium among the ions household RO units remove. Most systems also include carbon and sediment filters before and after the membrane.
Softener and RO side by side
| Water softener | Reverse osmosis | |
|---|---|---|
| Treats | The whole house | One tap, usually the kitchen |
| Removes | Calcium and magnesium (hardness) | Most dissolved solids, including sodium |
| Adds | A little sodium (or potassium) | Nothing |
| Waste water | Each recharge | Some with every gallon made |
| Main upkeep | Salt | Filter and membrane changes |
Why they’re often paired
A softener in front of an RO unit protects its membrane from hardness scale. And RO at the kitchen tap takes out the sodium the softener adds, so you get soft water everywhere and low-mineral drinking water.
What to look for
- Certification to NSF/ANSI 58, the standard for RO systems, for the exact model
- The WaterSense label, which also limits how much water the system wastes
- How often filters and the membrane need changing, and their price
- A tank size that suits how much drinking water you use
Common questions
- Do I need reverse osmosis if I have a water softener?
- Not for hardness. A softener handles that. RO is for drinking water with fewer dissolved minerals, including the sodium a softener adds.
- How much does reverse osmosis remove?
- It varies by system. The U.S. EPA's WaterSense specification for RO systems requires at least 75 percent reduction of total dissolved solids under the NSF/ANSI 58 test.
- Can I use reverse osmosis for the whole house?
- Rarely. RO is slow and sends part of the water to the drain, so it is normally used at one tap.