Reverse Osmosis Explained: How It Works and Why Rius Water Uses It

Last updated: October 10, 2026

Reverse osmosis is simpler than it looks

A reverse osmosis (RO) system can look intimidating. There are housings, tubing, gauges, and a row of filters most people have never seen before. What it does is not complicated. Water is propelled by pressure through a membrane with openings so small that what is dissolved in the water cannot follow. Clean water comes out one side. Everything the membrane turned away is rinsed down the drain. Nothing is added to the water and nothing is heated. Pressure does the work.

Reverse osmosis is the foundation of every Rius Water setup, and it is the filtration we use and prefer. Water is a daily input. You drink it, cook with it, and pour it for your family and your pets every day, which makes it one of the easiest things a person can improve. Our contaminants guide covers what is in the water. This page answers the next question: what to do about it, and what living with a reverse osmosis system is like.

Ready to look at the equipment? See our reverse osmosis systems, in 75 and 150 gallons per day (GPD) sizes.

Not ready for a system? Drink Rius bottled water. Taste where this leads before you install anything. It ships in glass.

How does reverse osmosis actually work?

Osmosis is water's natural habit of moving through a thin barrier toward the side with more dissolved in it, the way a raisin swells in a glass of water. Reverse osmosis uses pressure to push water the other way, so the water crosses the barrier and what was dissolved in it stays behind.

The practical version comes down to three lines of tubing. One line brings tap water in under pressure. One line carries clean water out. One line carries the wastewater, also called concentrate, to the drain. The concentrate is the water that rinses the membrane and carries away everything it turned back. That steady rinse is why a membrane keeps working instead of clogging the way a strainer would.

The openings in the membrane are about 0.0001 microns across. A human hair is about 50 microns wide, roughly 500,000 times larger. That is the whole idea: pressure in, clean water out one line, wastewater out the other. A standard system has no motor and no moving parts doing the filtering.

What does each stage of the system do?

Reverse osmosis is a system, not a single filter. In our 4-stage systems, water passes through four stages in order, and each stage protects the one behind it.

Stage 1: Sediment filter

What it removes: dirt, sand, rust, and the fine particles that make water cloudy.

Sediment filters are rated in microns, which tells you the smallest particle the filter will catch. A micron is one millionth of a meter, and the lower the number, the finer the filter. Many systems work at 5 microns, which stops particles about one tenth the width of a human hair. Rius prefers 1 micron for our filters, because a finer pre-filter gives the membrane cleaner water to work with.

The advantage: it keeps grit from clogging the carbon filter and from scratching or plugging the membrane. It is the least expensive part of the system, and it protects everything downstream.

Stage 2: Carbon filter

What it removes: chlorine and chloramine (chlorine combined with ammonia, which many cities now use), the tastes and smells that come with them, and many chemicals the membrane is weaker on, including some pesticides, solvents, and disinfection byproducts.

Carbon works like a magnet. It has an enormous internal surface, and chemicals stick to it as the water passes through.

The advantage is twofold. Your water tastes and smells better, and the membrane is protected, because chlorine damages the material most membranes are made of. Chloramine is slower to remove than chlorine, so if your city uses it, changing the carbon filter on time matters even more.

Stage 3: Reverse osmosis membrane

This is the heart of the system. What it removes: the things that are dissolved in water and pass straight through ordinary filters. That includes total dissolved solids (TDS), lead, copper, arsenic, fluoride, nitrates, chromium-6, PFAS "forever chemicals," and more.

Membranes are sized in gallons per day (GPD), which is how much water the membrane can make in 24 hours under ideal conditions. Rius Water offers systems rated at 75 gallons per day (GPD) and 150 gallons per day (GPD).

The advantage: this is the stage that reaches what carbon cannot. A carbon filter working alone, which is what most pitcher and refrigerator filters are, does not remove fluoride, nitrates, or dissolved minerals.

Stage 4: Deionizing (DI) resin, the final polish

What it removes: most of the dissolved minerals and salts that slip past the membrane. The membrane takes out the large majority of what is dissolved in the water. Deionizing (DI) resin is a cartridge of small beads that grab most of what is left.

Resin comes in a few types. Cation resin grabs positively charged minerals such as calcium and magnesium. Anion resin grabs negatively charged ones such as nitrate and sulfate. Mixed-bed resin blends the two in one cartridge. Most resin is also color-changing: it shifts color as it is used up, so you can see how much life is left.

The advantage: it is the final polish that takes the water as close to pure as a home system gets. It does not remove particles, which is why it comes last and never first.

At this point the water is very clean and also very empty, because minerals leave along with everything else. Remineralizing and structuring come after filtration, in the Crystal Charging Chamber, which connects to the clean water line. That is its own step, and our frequently asked questions page covers it.

Why are the pressure gauges important?

The gauge on a reverse osmosis system answers one question: does your home or business have enough incoming water pressure to push water through the membrane effectively?

Pressure is the engine. A reverse osmosis system has no motor of its own, so the pressure in your pipes is the only thing moving water across the membrane. When there is enough of it, the membrane does its best work. It makes water faster, it turns back more of what is dissolved in the water, and it sends less water down the drain. When there is not enough, all three go the wrong way: production slows, more gets through, and more water is wasted.

We want people drinking plenty of good water, and we want that water made without waste. Healthy water and water conservation belong together, and pressure is how a reverse osmosis system delivers both.

Low water pressure is the most common issue we see with reverse osmosis systems. It is rarely the system. It is the building: an older home, an upper floor, a well pump set low, or a pressure regulator turned down. The gauge lets you see it instead of guessing.

What should the gauge read?

Pressure is measured in pounds per square inch (PSI). Read the gauge while the system is making water. The target is 70 to 95 pounds per square inch (PSI). In that range the system produces water at its fastest and most efficient, with the least water sent to the drain.

If your gauge reads below 70 PSI, the system is using more water than it should for every gallon it makes. A booster pump is how you bring it into the target range.

What is a booster pump, and when do I need one?

A booster pump is a small electric pump installed on the cold water line that feeds the system. It raises the incoming pressure into the 70 to 95 PSI target range. If your gauge reads below the target, a booster pump is the fix.

Rius Water offers the Aquatec 8800 Booster Pump Kit. It comes with the pump, a 24 volt transformer that plugs into a wall outlet, a pressure switch and wiring, and quarter-inch push-connect fittings. It is an add-on, not a rebuild. It connects inline with the same style of fittings as the rest of the system, and the result is faster production, better filtration, and less wastewater.

What else does the gauge tell me?

Watch it over time. If the reading drops gradually over weeks or months while the pressure in the rest of the building has not changed, the filters ahead of the gauge are filling up and are due for a change.

Why does Rius like this style of reverse osmosis system?

There are sleeker looking systems on the market, with everything sealed inside a plastic shell. We prefer the open style, with separate housings and a gauge, for four reasons.

Robust and built to last

It filters effectively for long periods. With the sediment and carbon filters changed on time, the membrane keeps doing its job, and our 4-stage reverse osmosis systems carry a Limited Lifetime Warranty.

Visual

You can see what is happening. Some reverse osmosis housings are not clear. We prefer clear housings, so you can see each filter and watch it change color with use. The gauge shows your pressure, and the dual total dissolved solids (TDS) meter shows the water going in and the water coming out. The system tells you what it needs.

Serviceable

The filters are standard and made to be changed by the owner. The fittings are push-connect, so there is no soldering and there are no special tools.

Effective

Up to 99 percent reduction of the contaminants reverse osmosis is suited for, which covers most of the list in our contaminants guide. The exceptions are named further down this page.

Where does the water go? Storage options

Reverse osmosis makes water slowly and steadily. A 75 gallons per day (GPD) system makes roughly 3 gallons an hour at its rating. So the practical question is where the clean water waits until you want it. There are three paths.

Under-sink storage tank with an auto top-off (ATO)

This is the most common setup in a home. A small pressurized tank sits under the sink and feeds a dedicated drinking water faucet.

Auto top-off (ATO) means the system refills the tank without you. Pour a few glasses and the tank level drops, so the system starts making water. When the tank is full, an automatic shut-off valve stops it. There is nothing to switch on or off, and there is water on demand at the tap.

The automatic shut-off valve is included with our systems. The Add-On Faucet Kit adds the pressurized storage tank and the dedicated faucet.

No tank: batch production

This is for renters, apartments, and anyone without room under the sink. Instead of storing water in a tank, you produce it in batches on a schedule, for example every week or two, and keep it in containers.

One rule matters more than any other. Always produce at a spot with a drain, where the water cannot overflow onto the floor if someone walks away and forgets. A sink is usually best. A laundry room sink or tub works, and so does a bathtub. Never a countertop, and never an open floor.

Connecting does not mean changing the plumbing. You can usually remove the screen on the tip of the sink faucet and connect to the faucet with a garden hose adapter. If your faucet will not take one, the other route is to buy a new faucet and connect to it with the garden hose adapter.

Set the container in the basin, start the system, and set a timer on your phone. If the timer gets ignored, the extra water goes down the drain and not across the floor or into the apartment below.

For planning: at its rating, a 75 gallons per day (GPD) system fills a five-gallon container in a little over an hour and a half, and a 150 gallons per day (GPD) system does it in under an hour. Cold water and low pressure both slow that down.

Proper containers make batching easy

• Five-gallon totes. This is the most useful size when producing: large enough to make a batch worthwhile, and at about 42 pounds full, still possible to carry.
• BPA-free quality totes. BPA (bisphenol A) is a chemical used in some plastics. Choose food-grade containers made without it, with a lid that seals.
• Glass totes. Glass adds nothing to the water and is easy to see into and clean.
• Glass gallon jugs. They pour well, fit in a refrigerator, and are the right size for daily use.

Most plastic totes can also be cured before you use them for drinking water. Fill the tote with water and leave it outside, exposed to the elements, then pour that water out. Curing helps the plasticizers in the plastic leach into the curing water instead of the water you drink.

Glass is what we trust for our own water. Rius Water sells and ships in glass, in 32 ounce bottles and one-gallon jugs. Wherever you store your water, keep it covered, cool, and out of direct sunlight.

Add structure after filtration: the Crystal Charging Chamber. Filtration gives you clean water. The Chamber is where you feel the difference.

Not sure which setup fits your home or business? Ask us. Email structure@riuswater.com or call (303) 219-0623 and we will walk through it with you.

How do I keep it running? Filter changes and the tag system

Filtration lives out of sight and out of mind. It sits under a sink or behind a door and works quietly for months. The filters are not hard to change. The hard part is remembering when you last changed them.

We recommend a tag system, and it takes two minutes:

• Tag the unit. Put a sticky note or a label on the system with the date each filter was changed.
• Set a calendar reminder for the next change at the same moment, while you are standing there.

This matters most for the membrane. It lasts a year or two, which is long enough for anyone to forget when it was last changed.

How often should each filter be changed?

These are typical intervals. Check your water, because heavy use, well water, and old pipes all shorten them.

• Sediment filter: every 6 to 12 months. Sooner if it looks dark through the clear housing or your pressure reading is drifting down.
• Carbon filter: every 6 to 12 months. Lean toward 6 if your city uses chloramine, or if a chlorine taste or smell comes back.
• Reverse osmosis membrane: every 1 to 2 years. The signal is a rising total dissolved solids (TDS) reading on the water coming out.
• Deionizing (DI) resin: every 6 to 12 months. Most resin changes color as it is used up. Refresh it when only about 25 percent of the resin is still its original color. For resin that does not change color, go by the total dissolved solids (TDS) reading on the finished water.

After any filter change, run the system for a few minutes before you drink from it.

What does reverse osmosis remove?

Working together, the four stages reduce up to 99 percent of what they are built to catch. Here is the short list, with the stage that does the work. Each one has its own section in our contaminants guide.

• Chlorine and chloramine: the carbon filter.
• Fluoride: the membrane, with the deionizing (DI) resin catching more of what is left.
• Lead and copper: the membrane.
• Arsenic: the membrane, for one of its two forms. The guide explains the difference.
• Nitrates: the membrane removes most. If nitrates are your concern, test your water after installing.
• PFAS "forever chemicals": the membrane, with help from the carbon filter.
• Microplastics: the sediment filter and the membrane. This is likely, though not yet proven for home systems.
• Dissolved solids: the membrane, then the deionizing (DI) resin.
• Pharmaceuticals: the membrane and the carbon filter. This is likely, though few home systems are certified for it.

What does reverse osmosis not do well alone?

There are limits to any filtration, and we would rather you hear them from us.

• Dissolved gases, such as radon, pass through the membrane.
• Some volatile organic compounds (VOCs), such as certain solvents, and some disinfection byproducts get past a membrane working alone. The carbon filter is in the system to catch them.
• Chlorine is not a job for the membrane. Chlorine damages it, which is why the carbon filter comes first.
• Disinfection. The membrane holds bacteria back, but it is a filter and not a sterilizer. The next section covers the add-on built for that job.
• The rest of the house. An under-sink system treats the tap it is connected to.
• Minerals. Reverse osmosis takes out calcium and magnesium along with everything else, which is one reason Rius Water does not stop at filtration.

What about bacteria and viruses? Ultraviolet (UV) sterilizers

The openings in a reverse osmosis membrane are far smaller than bacteria, and the membrane holds them back. Where bacteria are a concern, such as a private well, the dedicated tool is an ultraviolet (UV) sterilizer. It is a chamber with an ultraviolet (UV) lamp that the water flows past. The light inactivates bacteria, viruses, and other microorganisms so they cannot reproduce, and it adds nothing to the water.

A UV sterilizer is installed after the filters and plugs into a wall outlet. Rius Water can supply one for your system. Ask us and we will help you choose the right one. If your utility issues a boil-water notice, follow the notice.

Common questions about reverse osmosis

Is reverse osmosis water too pure, or "dead"?

It is very clean and very empty. Minerals leave with everything else, which is why many people describe plain reverse osmosis water as flat. We agree with them, and it is why Rius Water remineralizes and structures water after it is filtered. Filter first, then structure. Our frequently asked questions page goes deeper on structured water.

How much water does reverse osmosis waste?

Every reverse osmosis system sends some water to the drain. That water is doing a job: it carries away what the membrane turned back. A standard reverse osmosis system sends 3 to 4 gallons to the drain for every gallon it makes. Pressure is how you keep that number as low as it can be. At 70 to 95 pounds per square inch (PSI), the system makes each gallon with the least waste, which is why we point every owner to the gauge and, where it is needed, a booster pump.

Do I need a plumber to install a reverse osmosis system?

Most people do not. Our systems connect with push-connect fittings, with no soldering and no special tools, and most installations take under an hour. If you would rather not do it yourself, it is a short job for a handyman or a plumber, and we are glad to talk you through it by phone.

How long does a reverse osmosis system last?

The system itself lasts for many years. The housings, bracket, and fittings are the permanent parts, and our 4-stage reverse osmosis systems carry a Limited Lifetime Warranty. The filters are the parts that wear out and get replaced.

Can I use reverse osmosis in an apartment?

Yes. Batch production was made for this. You do not need a storage tank or room under the sink, and the system goes with you when you move. Check with your landlord before making any permanent change, such as adding a dedicated faucet.

What is TDS, and should I test it?

TDS stands for total dissolved solids. It is the amount of minerals, salts, and metals dissolved in water, measured in parts per million (PPM). Yes, test it, because it is the quickest scoreboard you have. Our systems include a dual total dissolved solids (TDS) meter that reads the water going in and the water coming out. Write down the readings when the system is new. When the outgoing number starts to climb, the membrane or the deionizing (DI) resin is due. One caution: the reading tells you how much is dissolved in the water, not what it is, so it is not a safety test.

Does a reverse osmosis system need electricity?

No. The system runs on water pressure. The exceptions are two add-ons, a booster pump and an ultraviolet (UV) sterilizer, which each plug into a wall outlet.

Is a reverse osmosis system noisy?

No. While it is making water you may hear a faint trickle going to the drain, and when it stops, it is silent. A booster pump adds a low hum while it runs.

Consider it, and ask us anything

Thank you for taking the time to learn about your water. Reverse osmosis is not mysterious. It is pressure, a very fine membrane, and a few filters that are changed on a schedule. Once you see how the pieces work together, it becomes one of the more approachable upgrades you can make at home or at work.

If it sounds right for you, consider it. If you are not sure, ask us anything. We would rather answer ten questions than have you guess.

Ready to look at the equipment? See our reverse osmosis systems, in 75 and 150 gallons per day (GPD) sizes.

Add structure after filtration: the Crystal Charging Chamber. Filtration gives you clean water. The Chamber is where you feel the difference.

Not ready for a system? Drink Rius bottled water. Taste where this leads before you install anything. It ships in glass.

Not sure which setup fits your home or business? Ask us. Email structure@riuswater.com or call (303) 219-0623 and we will walk through it with you.