How do I remineralize RO water for better taste?

An RO membrane takes out most of the dissolved minerals. The Pure Water Gazette, an independent water treatment newsletter, says the process "removes some 95% of water's mineral content" and, as a result, gives water that is "temporarily low in pH." The World Health Organization adds that "dissolved minerals contribute to the taste of drinking-water to varying degrees" and that "especially high or low mineral content may be considered unpalatable by consumers." Taste is personal, so some people never notice the change and others find RO water flat. The low pH and the missing minerals both play a part, and a remineralizing stage tackles both at once.

That matters in Phoenix because the starting point is high. The city's 2025 report lists total dissolved solids of 464 to 716 ppm at the tap. A membrane takes most of that out, so the jump from tap water to RO water is bigger here than in a soft-water city. Some households love the clean taste, and others miss the body of the tap water.

This is what most homeowners choose, and it is a straightforward add-on for a plumber.

• What it is. A cartridge that sits in the last stage of the RO system, after the membrane. The Pure Water Gazette says these cartridges are "comprised mainly of two common water treatment minerals, Calcite and Corosex," and that calcite, "a pure form of crushed marble or limestone," is the main ingredient in RO remineralizing filters. As the water passes through, calcium carbonate dissolves into it and the pH rises.
• How much they do. The same article says a cartridge with a "small bed of Calcite" is enough to "meet the requirement" of adding minerals, and that a small amount of the second medium gives the pH "an extra upward bump." It also cautions that "too much Corosex overcorrects and can produce alkaline, strong-tasting water." On tourmaline, one of the more exotic media, the article says "there is no indication what this might have to do with adding minerals to water."
• How long it lasts. The cartridge wears out. Replace it on the schedule in the manufacturer's instructions. Our page on RO filter replacement and maintenance covers the yearly routine.
• What it does not do. Calcite adds calcium, not a full spectrum of minerals. The Pure Water Gazette's own view is that RO water does not need mineral supplementation to be healthful, and that people choose the cartridge mainly because it makes "exceptionally good tasting water."

Good candidates for a cartridge: anyone with an existing under-sink RO who finds the water flat, and anyone installing a new system who wants the taste to be close to good tap water. Our guide on what to expect from an RO installation shows where the post-filter fits, and our page on whether RO is worth it covers the broader decision.

### How to add a cartridge

The swap is a short job on most systems. Here is the order to follow.

1. 1Check your RO system. Look at the last filter housing. Many systems have a standard post-filter cartridge that can be swapped for a remineralizing one, and some have a spare port.
2. 2Choose the cartridge. Look for a calcite-based cartridge made for your system's tubing size and housing. The Pure Water Gazette notes products range widely in price and claims, so compare the ingredients, not the marketing.
3. 3Install it after the membrane. Shut off the RO's water supply and the storage tank valve, relieve pressure at the faucet, fit the cartridge in line with the arrow pointing toward the faucet, then restore water and flush it as the maker directs.
4. 4Taste the water after a week. The new cartridge often needs a few days of use before the taste settles. If it tastes chalky or strong, the media may be too aggressive for you, and a different cartridge helps.
5. 5Put the replacement date on a calendar. A worn cartridge gives you plain RO water again, so set a reminder when you install it and write the date on the housing.
6. 6Do not stack cartridges to chase flavor. One remineralizing stage is enough. Adding a second calcite bed or a strongly alkaline blend risks water that tastes strong or chalky, which is the overcorrection the Pure Water Gazette warns about.

The WHO's guidance on point-of-use RO devices suggests a different approach: "the manufacturers of these devices may provide a suitable bypass of a portion of this water to maintain some level of these minerals in the water actually consumed (e.g. to a kitchen tap) or develop and add an appropriate remineralizing unit in the water line prior to the point of consumption."

The trade-off is that RO is a barrier. The WHO describes these devices as "functioning as a final barrier against potential trace-level contaminants" and notes they also remove nutrients. Blending tap water back in gives up some of the protection you paid for. If you want the minerals and the extra filtration, a cartridge is a cleaner choice than a tap blend.

The World Health Organization says point-of-use RO devices "remove virtually all the minerals from the input water" and that their use "may result in the reduction of the overall intake of nutrient minerals." Most of a person's calcium and magnesium comes from food, and our page on whether reverse osmosis removes healthy minerals goes through what health authorities say. The same WHO report notes that demineralization "will reduce levels of fluoride," so if fluoride matters to your family, ask your dentist. Everything on this page is about a single drinking tap. A whole-house RO system treats every fixture, and very low-mineral water at every pipe is a different question from one glass at the sink, so our guide to under-sink versus whole-house RO is the place to start there. If a doctor has you on a special diet, ask what your water should contribute, because that is a medical question, not a plumbing one.

When to call a plumber. Call one if your RO system has no spare filter position, if the tubing needs a tee for a blend line, or if you are not sure where the post-filter goes. HQ Plumbing & Air is licensed and insured, and we offer free estimates and upfront pricing on RO work in the Valley.