What Does Reverse Osmosis Do?

Bottled water was supposed to be the safe option. Reverse osmosis is the smarter one. Most people who make the switch to reverse osmosis say the same thing: why didn't I do this sooner? So, digging deeper, you must be thinking, what does reverse osmosis do? 

Reverse osmosis is a water purification process that pushes tap water through a semipermeable membrane to remove up to 99% of dissolved contaminants, ing lead, PFAS, arsenic, chlorine, and microplastics, producing cleaner, safer drinking water at home.

In this guide, we cover how it works, what it removes, and how to choose the right RO system. By the end, you'll have everything you need to make the switch with confidence.

How Does Reverse Osmosis Work?

At its core, reverse osmosis (RO) works by forcing tap water through an incredibly fine (0.0001 microns) membrane under pressure. That membrane has pores small enough to block dissolved contaminants, heavy metals, chemicals, and microplastics, while allowing clean water molecules to pass through. 

The contaminants that don't make it through are flushed away down the drain, and what reaches your glass has been stripped of up to 99% of its original content. 

It sounds simple, but there's a precise sequence of filtration stages that makes the process work, and understanding each one helps you see exactly why RO outperforms every other home filtration method.

What Is Osmosis (and How Does RO Reverse It)?

To understand reverse osmosis, it helps to understand regular osmosis first, because RO is literally the opposite.

Osmosis is a natural process where water moves through a membrane from a less concentrated solution towards a more concentrated one. It's a system trying to find balance, equalizing both sides of the membrane. You see it in nature constantly: it's how plant roots absorb water from soil and how your body cells regulate fluid.

The problem is that natural osmosis would move water toward the contamination, not away from it.

Reverse osmosis flips this by applying pressure to the contaminated side, forcing water to move in the opposite direction, through the membrane, and away from the contaminants. The membrane's pores are small enough to block dissolved solids, chemicals, and heavy metals, while water molecules pass through freely.

Think of it like a screen door that only lets the breeze through, not the bugs. The screen doesn't stop the air; it stops everything you don't want coming in with it.

The 5 Stages Inside an RO System

Most home RO systems don't rely on a single filter; they run water through a sequence of stages, each targeting a different layer of contamination. Here's what happens at each step.

Stage 1: Sediment Pre-Filter:

Water enters the system and passes through a sediment filter first. This stage catches the physical stuff, dirt, rust, sand, and larger particles that would otherwise clog or damage the stages downstream. It's essentially a protective first filter that extends the life of the entire system.

Stage 2: Carbon Pre-Filter:

Next, water moves through an activated carbon filter. This is where chlorine, chloramines, and organic compounds are removed, the things responsible for the taste and smell most people associate with tap water. Stripping chlorine here also protects the RO membrane, which can degrade with prolonged chlorine exposure.

Stage 3: RO Membrane: 

This is the core of the entire system. The semipermeable membrane filters down to 0.0001 microns, fine enough to block dissolved solids, heavy metals like lead and arsenic, PFAS, microplastics, fluoride, and hundreds of other contaminants. Anything that can't pass through the membrane is flushed away. What continues forward is clean water at its most fundamental level.

Stage 4: Post-carbon polishing filter:

Before the water reaches your tap, it passes through a final carbon filter. This stage refines taste and removes any residual odors picked up during storage or the final run through the system. It's the difference between water that's technically clean and water that actually tastes that way.

Stage 5: UV Sterilization And Alkaline Remineralization

The most advanced systems add two final stages that address what basic RO leaves out. UV sterilization uses ultraviolet light to neutralize bacteria and viruses that a membrane alone may not catch. 

Alkaline remineralization reintroduces calcium and magnesium, the beneficial minerals that the RO membrane removes along with contaminants, restoring a natural taste and balanced mineral content. Advanced, high-end systems, like the illi Water Tankless RO, include both of these as built-in stages, not optional add-on cartridges.

Where Does the Wastewater Go?

Water waste is the most common concern people raise about RO, and it's a fair one. Here's what's actually going on and why the concern is less of a trade-off than it appears. .

Every RO system produces some wastewater. As water is pushed through the membrane, the contaminants it leaves behind are flushed down the drain in a concentrated stream:

• Older/cheaper systems waste as much as 3–4 gallons per gallon of clean water, a 1:4 ratio that's the source of most criticism you'll see about traditional RO systems.

• Modern high-efficiency systems have closed that gap significantly, achieving 2:1 or better, some close to 1:1.

• The wastewater isn't lost; it drains into the sewer system, gets treated, and returns to the local watershed.

• Bottled water companies use RO, too, which means that wastewater exists either way. Add plastic production and transportation on top, and home RO is the more resource-responsible choice regardless of its ratio.

That's where the illi Water Tankless RO stands apart. Running at a 2:1 pure-to-drain ratio, it's built for efficiency from the ground up, and the impact is quite impressively measurable. 

A family of four switching to the illi Water’s Tankless RO can save up to 19,000 liters of water per year compared to a traditional system. That's not a minor upgrade. That's a system designed to make water waste the last reason you'd hesitate to go RO.

What Does Reverse Osmosis Remove?

The short answer is RO removes most of the substances that you don't want in your water. It removes 99% of impurities, including bacteria, viruses, heavy metals, unwanted chlorine, PFAS, microplastics, and total dissolved solids (TDS). 

RO membranes are very fine, with pore sizes as small as 0.0001 microns. That’s why they catch contaminants that standard filters easily miss. That includes:

• Heavy metals like lead, arsenic, and mercury
• Chemicals, including PFAS (forever chemicals), pesticides, VOCs, and pharmaceuticals
• Biological contaminants such as bacteria, viruses, and cysts
• Microplastics and chlorine
• Fluoride and total dissolved solids (TDS)

PFAS and microplastics are the most harmful contaminants of the lot and the main reason for people to install RO even when their municipal water technically passes standard testing in Canada. The reality is that most Canadian cities don't test for either of these chemicals. Passing a water quality report doesn't mean your water is free of them.

That being said, effectiveness varies by system and certification. Not all RO filters perform equally, and the contaminants a system is certified to remove should always be verified before you buy.

For a full breakdown of every contaminant reverse osmosis removes, along with the mechanism of different filters, check our guide on how different water filters work.  

Does Reverse Osmosis Remove Minerals from Water?

Yes, RO does remove minerals from water, and this is the biggest concern people have about RO. The same RO membrane that removes lead, PFAS, and microplastics also removes calcium and magnesium. Structurally, it can’t differentiate between impurities and the good mineral. That's just how an RO membrane works. 

There's some debate about whether this is actually a problem. Some experts say it doesn't matter much, as you get most of your minerals from food anyway, so losing a little from water is no big deal. But the WHO says water is still an important source of minerals, especially for people who don't always eat a balanced diet, like those in developing countries. 

What most people do agree on is how the water tastes. Without remineralization, RO water can taste flat and empty, almost like distilled water. It's clean, but something feels missing. A lot of people notice it right away. The good news? This problem has already been solved.

Modern RO systems include an alkaline remineralization stage that puts calcium and magnesium back into the water after it's been filtered. You get clean water that also tastes the way water is supposed to taste. 

The illi Water Tankless RO includes this as a built-in stage, not an add-on you have to buy separately or a feature you have to request. It's just part of how the system works, so you never have to choose between purity and taste.

How Is Reverse Osmosis Different from a Regular Water Filter?

If you're looking into RO, you've probably wondered whether a simpler option, a pitcher filter, a faucet attachment, or a carbon system would do the job just as well for less money. It's a fair question. There are many types of water filters out there, and they're not all built to do the same thing.

RO is the most thorough home filtration option you can buy. But it's not always the only solution. If your water tastes fine and your only concern is the chlorine smell, a carbon filter might be all you need. 

The difference is that RO is the only method that handles heavy metals, PFAS, microplastics, fluoride, and biological contaminants all at once. Read our detailed guide about all the types of filters to better understand which option fits your needs. 

What Are the Benefits of Reverse Osmosis?

RO doesn't just filter water; it changes how you experience it every day, from what you taste in your morning coffee to how much you spend at the grocery store while buying bottled water. 

Here's what you'll actually notice once you make the switch.

Reduces Contaminants That Standard Filters Miss

Your tap water might be passing every government test and still contain elements you wouldn't want to drink. Municipal treatment is designed to meet federal minimums, not to catch everything. PFAS, microplastics, and pharmaceuticals often aren't regulated at all, which means water treatment plants aren't required to remove them. 

Your water can be fully "compliant" and still carry contaminants that nobody is testing for. RO closes that gap. It doesn't filter to a legal standard; it filters to a physical one. If a particle can't fit through a 0.0001 micron membrane, it doesn't get through. That's the difference between water that's legally acceptable and water that's actually clean.

Improves the Taste and Smell of Your Water

The difference isn't subtle; most people notice it on day one. Chlorine and dissolved solids are the main culprits behind that flat, slightly chemical taste that tap water often has. Once they're gone, everything made with water tastes better. 

Your morning coffee tastes cleaner and brighter. Tea brings out more of its natural flavor. Soup and rice cooked with RO water taste noticeably different. Ice cubes come out clear instead of cloudy. 

And if you're making baby formula, you're starting with water you can actually feel good about. It's one of those changes that sounds small until you experience it, and then switching back to bottled water becomes impossible.

Saves Money Compared to Bottled Water

A gallon of tap water costs about $0.002. Run it through an RO system, including the cost of wastewater, and that goes up to around $0.006 per gallon. A gallon of bottled water costs $1–$2. 

A family of four using 3 gallons of RO water per day spends roughly $35–$50 per year on water and filter replacements combined. The same family buying bottled water spends $1,000–$1,500 per year. 

Most RO systems pay for themselves within a few months, not years. The math isn't close, which is why people often say they wish they’d switched to RO filters much earlier. 

Reduce Plastic Waste at Home

Switching from bottled water to RO eliminates 2,000–3,000+ single-use plastic bottles per household every year. But the environmental concerns go deeper than that. Bottled water companies use RO to produce their product; the same wastewater is generated either way. 

The difference is that when you buy bottled water, you're also paying for the plastic bottle, the transportation, the refrigeration, and the retail markup. None of that adds anything to the water quality. You're essentially buying RO water in a plastic wrapper at a 200x markup. Switching to a home system keeps the clean water and cuts everything else out.

Works with Both City Water and Well Water

RO works well regardless of where your water comes from, but for different reasons. City water is treated before it reaches you, but it still picks up contaminants along the way. 

Aging pipes add lead and sediment to the water. Disinfection byproducts form inside the water during the municipal treatment. And as discussed before, emerging contaminants like PFAS aren't part of the standard treatment process. 

Well water presents a different challenge; it's completely untreated and unregulated. No municipality is monitoring it. Whatever is in the ground around your well is potentially in your water. 

RO provides the treatment layer that simply doesn't exist otherwise. For well water users with high iron, hardness, or sediment levels, a pre-filter or softener before the RO system is worth considering to protect the membrane.

Low Maintenance and Long Filter Life

RO systems are easier to maintain than most people expect. Pre- and post-filters last 6–12 months. The RO membrane lasts 2–3 years. Total annual filter cost typically runs $40–$80, less than a month of bottled water. Modern systems use twist-and-pull cartridges that swap out in seconds with no tools required. 

The best systems go further with smart TDS monitoring and filter life alerts that tell you exactly when something needs replacing—no guesswork, no checking a calendar. You can also connect most RO systems directly to your fridge water dispenser and ice maker, which eliminates the need for separate fridge filters entirely. One less thing to buy, one less thing to remember.

How Do You Choose the Right Reverse Osmosis System?

Now that you know what RO does and why it matters, the next step is finding the right system for your home. Here's what to look at before you buy.

Test Your Water First

Before you spend money on any filtration system, it's worth knowing what's actually in your water. You might have contaminants you can't see, taste, or smell, and your results will tell you whether you need RO specifically or whether a simpler carbon filter would do the job.

Well-water users especially need to test their water before choosing any filtration type. No municipality is checking your supply, so you're the only one monitoring what comes out of your tap. 

But city water households need testing too, as the aging pipes, local infrastructure, and seasonal changes all affect the quality of water that flows between the treatment plant and your faucet.

At-home test kits are an affordable and easy starting point. For more detailed results, a lab test gives you a full picture. Either way, testing first is the smart move; it takes the guesswork out of choosing the right system.

What Certifications Should You Look For?

Not all RO systems perform the same way, and marketing claims aren't enough to go on. Certifications tell you what a system has actually been tested and proven to remove.

Here's what the key NSF/ANSI standards mean in plain terms:

• Standard 42: taste and odour reduction
• Standard 53: health contaminants like lead
• Standard 58: the RO-specific standard, and the most important one to look for
• Standard 401: emerging contaminants like pharmaceuticals
• P473: PFAS removal

If a system isn't certified to NSF/ANSI 58 at a minimum, you're trusting marketing claims instead of lab results. WQA and IAPMO are two reputable third-party organizations that validate these certifications independently. Look for their certified filters before you buy any system.

Should You Get a Tank or Tankless System?

This is one of the most practical choices you'll make, and it comes down to how you use water and how much space you have.

Tank systems store filtered water in a pressurized reservoir under your sink. They work well and have been around for decades, but they take up significant cabinet space, and if you use a large amount of water at once, you may have to wait for the tank to refill.

Tankless systems filter water on demand, so there's no reservoir and no waiting. They're generally newer technology, tend to have better waste ratios, and free up a lot of under-sink storage space.

The illi Water Tankless RO filters 800 gallons per day and fills a 250ml glass in 8 seconds, freeing up around 4 square feet of cabinet space in the process. If space and flow rate are your concerns, tankless is a preferred upgrade. If you want reliable performance at a lower price point, the illi Water Standard RO is a solid option.

How Easy Is It to Install and Maintain?

Installing an RO system is quite easy with hassle-free maintenance. Most under-sink RO systems are designed for DIY installation in 30–60 minutes using basic tools and the kit that comes in the box. You need access to a cold water line and a drain connection under your sink; that's it, no plumber required.

Ongoing system maintenance is just as simple as the installation. Pre- and post-filters swap out every 6–12 months using a one-click replacement system, no tools, no mess. The membrane lasts 2–3 years. Total annual filter cost runs $40–$80. It's about as low-effort as a home appliance gets.

The illi Water Tankless RO comes with an all-in-one installation kit built for a 30-minute setup. Yes, that means you can easily install the system on your own. Additionally, the unit features a smart digital stainless-steel faucet that shows real-time TDS readings and filter life alerts right on the tap. It eliminates the guessing when something needs replacing.

Where Else Is Reverse Osmosis Used?

RO isn't just an under-sink gadget; it's one of the most widely used water treatment technologies in the world. Here's where you'll find it working at scale.

• Desalination: Dubai uses RO to filter roughly 416 million gallons of seawater into drinking water every single day. It's the backbone of clean water supply across water-scarce regions worldwide.
• Restaurants and coffee shops. High-end cafés and kitchens invest in RO specifically because water quality directly affects how food and beverages taste, the same reason home cooks notice the difference.
• Medical facilities. Dialysis machines require RO-grade water. Tap water, even treated municipal water, isn't pure enough for direct medical use.
• Breweries and dairy: Consistent water chemistry means consistent product. RO gives manufacturers a clean, neutral baseline to work from every time.
• Maple syrup production: RO is used to separate water from sap before boiling, concentrating the sugar content and cutting production time significantly.
• Wastewater recycling: Several water-scarce cities use RO as the final stage in recycling treated wastewater back into the drinking supply, which is sometimes called "toilet to tap."
• Hydroponics and greenhouses: Growers use RO to control exactly what their plants absorb; some operations run through 100 gallons a day or more.

Your Next Step: Cleaner Water Starts Here

Reverse osmosis is the most thorough home water purification method available, and modern systems have taken care of the drawbacks that used to hold people back. Water waste is down. Minerals are restored. Installation takes 30 minutes. The systems are smarter, more compact, and more efficient than anything that existed a decade ago.

If you're ready to stop buying bottled water and start drinking cleaner water straight from your tap, illi Water provides a range of advanced RO systems for every need.  We even provide consultation to help you find the right solution for your water challenges. Contact us today.

Frequently Asked Questions:

How much does a reverse osmosis system cost?

Entry-level under-sink systems run $150–$300. Mid-range systems with advanced features like smart monitoring fall in the $300–$500 range. Premium tankless systems start at $500+. Ongoing filter costs are typically $40–$80 per year. Most systems pay for themselves within 2–4 months simply by replacing bottled water purchases, making the upfront cost much easier to justify.

Is reverse osmosis water safe to drink every day?

Yes. The concern that RO water is "dead" or nutritionally empty isn't supported by evidence. Most of your dietary minerals come from food, not water; you won't develop a deficiency from drinking RO water daily. Systems with an alkaline remineralization stage restore beneficial minerals and natural taste. RO water has also been used safely in medical dialysis settings for decades.

How long do reverse osmosis filters last?

Pre and post filters last 6–12 months, depending on usage and water quality. The RO membrane lasts 2–3 years, sometimes up to 5. With proper maintenance, a full system can last 10–15 years. Smart systems with filter life alerts take the guesswork out entirely. Using a TDS meter (around $20) lets you monitor membrane performance. When the output TDS starts creeping up, it's time to replace it.

Can you install a reverse osmosis system yourself?

Yes, most under-sink RO systems are designed for DIY installation in 30–60 minutes with basic tools and zero plumbing experience. You need access to a cold water line and a drain connection under your sink. Most systems include a complete installation kit with everything you need. Some setups require drilling a small hole in the countertop for a dedicated faucet, which is simpler than it sounds.

Does reverse osmosis remove fluoride?

Yes, RO is one of the few home filtration methods that effectively reduces fluoride. While fluoride was originally added to municipal water for dental health, most people now get adequate fluoride through toothpaste. Growing research suggests excess fluoride may have negative health effects, particularly for children. RO gives you direct control over what stays in your water rather than leaving that decision to your municipality.

Can you use reverse osmosis on well water?

Yes, and well water is actually one of the strongest use cases for RO, since wells aren't regulated or treated by any municipal system. Depending on your iron, hardness, and sediment levels, your well water may benefit from pre-treatment with a sediment filter or water softener before the RO stage. Testing your well water first is essential; your results will show whether additional pre-filtration is needed to protect the membrane and extend its lifespan.