What Reverse Osmosis Takes Out of Valley Water, and What It Leaves

Phoenix tap water meets federal drinking-water standards, and a good share of the people who live here would rather not drink it. Both facts are true at once. The water is safe. It is also hard, faintly chlorinated, and in late summer it can taste like the shallow end of a pond. Safety was never the complaint.

The complaint has causes, and they are worth knowing before you buy anything to fix them.

Why the glass tastes the way it does

Most Valley water starts as snowmelt and rain in the Salt and Verde watersheds, or as Colorado River water carried across the desert by the CAP canal. Open channels under Arizona sun make comfortable habitat for algae, and in the warm months some of those algae release compounds the human nose can detect in astonishingly small amounts. That is the earthy, musty note that turns up most summers. Utilities treat for it. They cannot treat it to zero.

Next comes the disinfectant, added so the water stays safe across a very long network of pipe. You taste it as the faint swimming-pool finish.

Then the minerals. Valley water commonly carries 10 to 20 grains per gallon of hardness, mostly calcium and magnesium, along with sodium, sulfate, and chloride in amounts that shift with the source and the season. Hard water in the Valley covers what those minerals do to your appliances. At the kitchen tap they do something smaller. They make the water taste like water that has been somewhere.

Three machines, three different jobs

People shopping for better water tend to compare three things that don’t actually compete.

The pitcher filter. Activated carbon in a plastic cartridge. It grabs chlorine and many of the compounds behind off tastes and odors, and for the price it does that well. It does very little about dissolved minerals. Pour pitcher water into a kettle and the kettle still scales. Useful, cheap, and limited by a cartridge the size of a candle.

The water softener. Ion exchange at the point where water enters the house. It pulls out calcium and magnesium and swaps in a small amount of sodium. That protects the water heater, the fixtures, and the shower glass. It does nothing for chlorine or taste, and it nudges the sodium in your glass upward. A water softener is house protection, not drinking-water treatment.

Reverse osmosis. A multi-stage system, usually under the kitchen sink, that feeds its own small faucet. It is the only one of the three that removes most of what is dissolved in the water.

For a lot of Valley homes the honest answer is a softener for the house and RO for the glass. They were never rivals.

What the membrane removes, and what it doesn’t

An RO system works in stages. A sediment filter catches grit. A carbon filter removes chlorine, which would otherwise damage the membrane, along with most taste and odor compounds. Then household water pressure pushes water against a semipermeable membrane with pores so fine that most dissolved minerals, salts, and metals can’t follow. A final carbon polish sits between the storage tank and the faucet.

A healthy membrane rejects the large majority of total dissolved solids. In practice that means most of the:

• Calcium and magnesium behind hardness and kettle scale
• Sodium, including the sodium a softener adds
• Sulfate and chloride, two sources of the flat, faintly salty edge some people taste
• Nitrate, lead, and arsenic, the last of which occurs naturally in some Arizona groundwater and is worth a lab test on any private well
• Fluoride, which matters if you were counting on it, and is a fair question for your dentist

What it does not do matters as much. It treats one faucet, or two if you run a line to the refrigerator. The shower, the dishwasher, and the water heater still get whatever arrives from the street. RO is not a disinfection system for untreated water, so a well with a bacteria problem needs a different conversation. Dissolved gases and some small organic compounds slip through the membrane, which is why the carbon stages exist. And a spent carbon filter lets chlorine reach the membrane, which is how a system fails quietly while looking perfectly fine.

The water that goes down the drain

Here is the part of the brochure set in small type. To keep the membrane from clogging with what it rejects, an RO system rinses that concentrate to the drain as it works. Conventional under-sink units send several gallons down the drain for every gallon they deliver. Newer designs do better, some with a permeate pump (a small hydraulic pump that keeps the storage tank from pushing back against the membrane), and softened feed water helps too, because a membrane fighting less scale works more efficiently.

Keep the number in proportion. RO supplies drinking and cooking water only, a few gallons a day in most households, and the drain water behind that is a fraction of one load of laundry. In a desert it still deserves a sensible design. It is also why we check for a failed shutoff valve, the part that lets a system trickle to the drain all night after the tank is full.

The flat-taste objection

Stripped of minerals, RO water tastes clean to some people and flat to others. Coffee drinkers argue about it with unusual heat. The fix is a remineralization stage, a small cartridge after the membrane that adds back a measured amount of calcium and magnesium. It won’t restore fluoride, and it won’t bring back the pond. Try it both ways; the cartridge is easy to add or remove.

Maintenance, stated plainly

• Prefilters and the final carbon: about once a year, sooner on well water or on unsoftened Valley water.
• The membrane: every few years, depending on hardness and how much you draw.
• The storage tank: check its air charge when the RO faucet slows to a trickle.
• The proof: a pocket TDS meter costs little. Test the regular tap, then the RO faucet. When the second number starts creeping toward the first, the membrane is finished.

A system nobody has serviced since the previous owner’s housewarming isn’t dangerous. It is simply an elaborate way to run tap water through a used filter.

Where it goes, and what the install involves

Most units live in the cabinet under the kitchen sink. The install needs a cold-water feed, a drain connection placed above the trap so nothing can siphon back into the system, a hole in the sink deck or countertop for the RO faucet, and enough room left over to change filters without removing the trash can and half the cleaning supplies. Where the sink has a disposal, the drain line is placed so food waste can’t back up into it. A branch to the refrigerator’s ice maker is usually a short run and worth doing while the cabinet is open. The install often folds neatly into other kitchen plumbing work, like a new faucet or sink.

We start with the water, not the catalog. Every reverse osmosis installation begins with a TDS and hardness reading at your tap, and the system is sized and priced flat before anyone opens the cabinet. If the rest of the house needs a softener, you’ll hear that too, along with why. The hard water cost calculator estimates what untreated water is costing everything past the kitchen, and our free water hardness test puts your own number on the counter. When you’re ready, book a visit.

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