What Is Reverse Osmosis Water? A Plain-English Guide

Reverse osmosis water is tap water that has been pushed through a very fine semi-permeable membrane that lets water molecules through and holds back most dissolved salts, metals and other contaminants. The result is water with far fewer total dissolved solids than what comes out of the tap. Home RO systems usually pair the membrane with sediment and carbon filters, so the water that reaches your glass has been filtered several ways.

How reverse osmosis works

Osmosis is what happens naturally when water moves across a membrane from a less concentrated solution toward a more concentrated one. Reverse osmosis uses pressure to push water the other way, from the concentrated side (your tap water) to the clean side, leaving dissolved substances behind.

The membrane's pores are small enough to hold back most dissolved ions, not just particles. That is what sets RO apart from a pitcher or a basic faucet filter, which mainly rely on carbon.

A typical under-sink RO system has several stages:

1. Sediment filter to catch sand, rust and grit.
2. Carbon filters to remove chlorine, chloramines and many organic chemicals, and to protect the membrane.
3. RO membrane to remove dissolved solids.
4. Post-filter to polish taste before the faucet.

The Sanavia Hydra uses five stages in this order: sediment, carbon, carbon block, reverse osmosis membrane and post-filter. Its fifth stage adds some minerals back after the membrane.

What reverse osmosis removes

RO membranes are among the most effective home options for reducing a wide range of dissolved contaminants. In general, reverse osmosis is used to reduce:

• Total dissolved solids (TDS), the overall measure of what is dissolved in water. See what TDS in water means.
• PFAS, the so-called forever chemicals.
• Lead and other metals.
• Arsenic.
• Nitrate, a common concern in farm areas and well water.
• Fluoride.

The carbon stages handle a different job. They are what reduce chlorine, chloramine and disinfection byproducts such as trihalomethanes, which form when chlorine reacts with organic matter in source water. How well any system performs depends on its design, its filters' condition and your water, so real test results matter more than general claims.

What the lab found with the Hydra

We had an independent accredited lab test a real Arlington, VA home's tap water before and after the Hydra. Total trihalomethanes fell from 58 ppb to 2.79 ppb, a 95% reduction. The full before-and-after results are published on our proof page.

Does reverse osmosis remove minerals?

Yes, reverse osmosis removes most minerals. Calcium, magnesium, potassium and other minerals are dissolved solids, and the membrane does not distinguish between a mineral you might want and a contaminant you do not. That is also why RO water often tastes softer or flatter than tap water.

Is reverse osmosis water good for you?

For most people, reverse osmosis water is a perfectly good everyday drinking water. The mainstream view is that most of the minerals we need come from food, not water, and drinking water is usually a minor source of them for people who eat a varied diet.

Some researchers have raised questions about drinking very low-mineral water over the long term, especially for people whose diets are low in minerals. The evidence on that is not settled. If it is a concern for you, remineralizing is simple, and anyone with a specific medical condition should ask their doctor.

What RO water clearly does well is reduce contaminants that many people would rather not drink. That trade-off is the main reason people choose it.

How to remineralize RO water

If you want minerals back in your water, for taste or peace of mind, there are a few common ways to remineralize RO water:

• Remineralization cartridge. An inline filter added after the RO stage that slowly dissolves minerals such as calcium and magnesium into the water.
• Mineral drops. Liquid concentrates you add to a glass or pitcher. They are inexpensive and let you control the amount.
• Mineral pitchers. Pitchers with a mineral cartridge that you fill with RO water.

For more on this, including pH and alkaline water, see how to make alkaline water at home and remineralize RO.

RO waste water: why some water goes to the drain

Every reverse osmosis system sends some water to the drain. The membrane holds back contaminants, and a stream of water carries them away so they do not build up and clog the membrane. That stream is often called RO waste water, reject water or concentrate.

How much goes to the drain compared with how much you drink varies by system design and by your water pressure, temperature and TDS. Older tank systems are generally known for sending more water to the drain than newer designs. If water use matters to you, compare manufacturers' published figures and ask how they were measured.

Reverse osmosis vs distilled water

Both are low in dissolved solids, but they are made differently. Distillation boils water and condenses the steam, while RO pushes water through a membrane. RO is far more practical for everyday home use. Read the full comparison in reverse osmosis vs distilled water.

Should you get reverse osmosis?

Start with what is in your water: look up what your utility reports on our water quality page, or get a free water test (a mailed kit anywhere in the U.S., or a free in-home TDS test in Northern Virginia). For a specific contaminant such as lead, use a certified lab test.

If RO makes sense for you, the Sanavia Hydra is tankless, filters on demand at up to 800 gallons per day, and shows your water's TDS in real time. It is $999 to self-install or $1,250 professionally installed in the DC, Maryland and Virginia area, with free U.S. shipping, 60-day returns and a 3-year warranty.