Phoenix Drinking Water and Scale: Planning Two Systems From the Entry Line to the Kitchen

Scale and drinking water in a Phoenix home are usually planned as two separate tasks, with a softener treating the lines chosen for scale control, from the hot-water supply alone to most of the house, and a reverse osmosis (RO) system under the kitchen sink handling the water people drink and cook with. The softener protects the water heater and fixtures from the hard water the city's report describes, while the RO system serves one faucet and anything branched from it. For that kitchen half, the Viomi M1 pairs RO filtration with a mineral stage that adds selected naturally sourced minerals back for a smoother taste.

Some homes need only one of the two, and the equipment already in the house often decides which. The earlier roundup of water filter brands for Phoenix tap water compares the brands for each half.

What the City's 2025 Numbers Say About Hardness

The City of Phoenix 2025 Water Quality Report gives a range of 172 to 302 ppm for total hardness in its distribution system, or 10 to 17.6 grains per gallon, and 464 to 716 ppm for total dissolved solids (TDS). On the USGS hardness scale, 121 to 180 mg/L counts as hard and anything above 180 mg/L as very hard, so the city's reported range runs from hard to very hard.

The city treats hardness as a comfort and maintenance matter. Its report calls it one of the "harmless, aesthetic aspects that some consumers may notice", with scaling in pipes and water heaters and on faucets and showerheads, and states that tap water delivered to about 1.7 million residents in 2025 "met or surpassed all federal and state drinking water standards". The city's Water Services FAQ describes filters and softeners as a personal choice, which makes the plan a question of which jobs a household wants done.

Those figures describe the City of Phoenix system.

Which System Serves Each Tap and Appliance

University of Arizona Cooperative Extension's guide Arizona: Know Your Water (AZ1578) gives the division used in the table. For very hard water, which it defines as 10.5 grains per gallon or more, point-of-entry (POE) treatment covering most of the water entering the home may be needed; otherwise, AZ1578 suggests treating only the hot water supply to limit scale in the water heater and pipes. RO, meanwhile, "is placed beneath the kitchen sink to treat water used for cooking and drinking". Phoenix's reported range of 10 to 17.6 grains spans that 10.5-grain line, so both arrangements on the entry side are realistic for different addresses.

Sodium is part of why the drinking tap sits in the RO column. A sodium-based softener exchanges hardness minerals for sodium, and Cooperative Extension's AZ1799 converts grains per gallon of hardness to added sodium by multiplying by 7.9. Applied to Phoenix's reported range, that formula works out to roughly 79 to 139 mg/L of added sodium. Household RO units list sodium among the ions they reduce, according to University of Nebraska-Lincoln Extension's reverse osmosis guide, which is one more reason drinking water is planned at the RO faucet.

Starting From the Equipment Already in the House

An installed softener keeps serving the lines it was plumbed to serve, and its installation records show which ones those are. The extension guide notes that new homes are often plumbed to send soft water to the kitchen and laundry room, so those records are worth reading before anything is added at the sink. What remains is the kitchen itself, where an RO system covers drinking water, cooking water and a refrigerator line.

A kitchen pitcher or faucet-mounted filter, on the other hand, points the other way. If the complaints are crusted showerheads and spotted glassware, the missing half is on the entry side, and the decision there is whether the whole house or only the hot-water supply needs softening, guided by the extension guide's 10.5-grain threshold.

In a house with neither, the table above works as the planning sheet. Taste complaints lead to the kitchen column and scale complaints to the entry column, and a household that wants both can begin with whichever matters more.

The Kitchen Half: RO at the Sink

For the kitchen system, taste is a good place to begin. RO lowers the total dissolved solids in Phoenix water, and the Viomi M1 then puts selected minerals back so the glass keeps a rounded taste. The guide to what TDS means for water quality explains how that dissolved-solids figure is read.

Volume matters on cooking days. The Viomi M1 and the Viomi V8 PRO are both tankless, so a stock pot fills straight from the faucet without waiting for a tank to refill, and the Viomi V8 PRO's LED faucet shows the TDS reading while it fills.

Knowing when to change a cartridge takes no record keeping on either system, because each faucet shows remaining filter life and the Viomi M1's app also sends a reminder when a change is due.

Where the Two Systems Share Space, Drains and Upkeep

A softener sits where the lines it serves are plumbed, whether that is the main line or the hot-water supply, with enough room around its brine tank for bags of salt to be poured in. The kitchen system sits under the sink, and a short note of what already occupies each spot (an existing softener or filter, the cold-water valve, the sink drain and the outlet under the sink) gives whoever installs either system a clear picture of the house.

The extension guide points out that most of the added sodium or potassium used in softeners, along with the brackish water from resin regeneration, is discharged into sewage, septic and gray water systems. A home with a septic system or a gray water setup for landscaping therefore has a reason to plan the softener's discharge with care.

Softener upkeep "is largely confined to restocking the salt supply", in AZ1578's words, using salt made specifically for ion exchange units. On the kitchen side, the faucet's filter-life display takes care of the timing.

FAQ

Can an older softener stay in place when only a kitchen RO system is added?

An existing softener keeps serving the lines it was installed on, and the RO system is added at the kitchen sink with its own faucet for drinking and cooking water.

Which hardness figure belongs in the plan?

Two figures are worth writing down. The City of Phoenix's 2025 report gives a range of 10 to 17.6 grains per gallon across its system, and a hardness reading taken from water before it reaches the softener, noted with the date and the tap used, gives the figure for the house itself.

Do the two systems share a drain?

Each one is planned on its own terms. The plan notes where the softener's regeneration water will discharge and where the kitchen RO system will connect to the drain pipe below the sink, so both connection points are on record before work starts.

The kitchen already has a faucet-mounted filter. Does it stay?

An under-sink RO system brings its own dedicated faucet, so the existing filter can keep serving the regular kitchen faucet while drinking and cooking water come from the RO faucet.

Why does the water heater sit at the top of the softener's list?

According to the USGS, solid calcium carbonate deposits can form when hard water is heated in a home water heater, and that scale can shorten equipment life, raise water heating costs, lower the efficiency of electric water heaters and clog pipes. AZ1578 accordingly suggests that, below the very-hard threshold, treating only the hot water supply can be enough.

Does softened water taste different from Phoenix tap water?

AZ1578 notes that the taste of soft water may not be as pleasant as that of hard water, which is one more reason the drinking faucet in a softened Phoenix home is planned around the kitchen RO system and its own taste profile.

Sizing the Softener's Reach and the Kitchen's Role

With hardness of 10 to 17.6 grains per gallon in the city's 2025 report, the softener decision in Phoenix is mostly about reach: the whole house or the hot-water supply alone, depending on where scale is a problem. The kitchen decision is about the glass, and it can be made on its own timeline, whether or not a softener is already in place. The city's FAQ treats filters and softeners alike as personal choices, so the order of the two projects can follow whichever outcome the household values more.