If you’ve ever searched for the “cleanest” or “purest” drinking water, you’ve probably come across reverse osmosis (RO). It’s widely regarded as one of the most thorough filtration methods available.

And there’s a reason for that: RO systems remove a wide range of harmful contaminants, including heavy metals, fluoride, chlorine byproducts, and more.

But here’s what most people don’t realize:

That’s not a flaw in a particular brand or model. It’s a built-in limitation of how reverse osmosis works.

So before deciding if RO is right for your home, it’s worth understanding the full picture. 

Let’s dive into how it works, what it removes, what it doesn’t, and whether it’s still the best option today.

Reverse osmosis is a multi-stage filtration process designed to remove nearly everything from your tap water. 

Here’s how it works:

Water is typically pushed through several stages of filtration, including sediment and carbon filters, followed by a specialized semipermeable membrane. 

This membrane is what sets RO apart. 

It contains tiny pores (around 0.0001 microns) that catch a wide range of contaminants—including those that slip through standard filters like fluoride, arsenic, nitrates, and heavy metals.

But those pores don’t just block harmful contaminants, they’re also small enough to remove healthy minerals dissolved in water like calcium and magnesium.

The result is water that’s almost entirely free of Total Dissolved Solids (TDS). 

In other words, it’s pure. But is it too pure?

Picture a super vacuum that sucks up everything. 

All the dirt, dust, and stains? Gone. But with it, went your keys, your phone, and your wallet too.

That’s what happens with reverse osmosis.

So while it removes a range of harmful contaminants, it also strips out the naturally occurring minerals in your water, like calcium, magnesium, and potassium.

In other words, its biggest advantage could also be considered its biggest drawback.

A lack of healthy minerals—like potassium, calcium, and magnesium—can potentially be linked to conditions like brittle bones, dental issues, high blood pressure, and cardiovascular disease.

Research suggests your body needs these minerals to help:

• Regulate blood pressure + body temperature (to feel balanced)

• Manage risk of cardiovascular disease (to protect your heart)

• Repair & rebuild cells (to help you heal faster) 

• Support digestion (by absorbing more nutrients)

• And more

That’s why many RO users remineralize their water after filtration. 

Let’s talk more about that for a second:

Imagine this: Every time you pour a glass of water, you have to add back in the healthy minerals your filter just removed.

That’s common practice with many reverse osmosis (RO) systems—for two main reasons:

1. Health: As we mentioned earlier, naturally occurring minerals found in water support key functions in your body. Due to these health benefits, RO users often want them restored.

2. Taste: Water without minerals can taste . . . off. Some call it “flat,” “lifeless,” even “dead.” Others say it’s too pure.

For those reasons, many RO users choose to remineralize their water after filtration.

There’s a few ways to do it: Remineralization drops are popular, but they’re an added step and cost. The same thing goes for tablets and powder supplements. 

Because of that some users install an inline remineralization filter, mineral cartridge, or alkaline post-filter directly onto their system if it doesn’t already have one.

For every 1 gallon of filtered water they produce, most RO systems flush 3 to 4 gallons down the drain.

Why?

RO filters don’t trap contaminants inside the filter like many other systems do. Instead, they use that semipermeable membrane to separate clean water from everything else.

And only a fraction of the water makes it through the membrane.

The rest—along with all the contaminants—is flushed out typically through a separate drain line to prevent buildup and keep the system functioning, rather than clogging. 

In other words, reverse osmosis separates and flushes water. Other filtration systems trap and hold contaminants.

See the difference?

Over time, that separation adds up: Not just on your water bill, but environmentally, too. It’s an especially important consideration in areas facing drought or water shortages, where conservation matters.

After all, that “wastewater” simply exits the system and is discarded. 

If your household drinks just 1 gallon of filtered water per day, choosing an RO system could mean discarding more than 1,000 gallons of water every year.

Instead of rejecting water, they use filtration media that traps and holds contaminants inside the filter itself—kind of like a magnet.

Take Clearly Filtered’s 3-Stage Under-the-Sink Filtration System:

• It uses pressure from your tap to push water through multiple stages / cartridges of dense filtration media.

• Contaminants are captured inside the filters through a range of filtration methods powered by our advanced Affinity® Filtration Technology. 

• All the water that enters the system passes through and comes out the other side—clean and ready to drink.

• There is no drain line and no rejection line. Thus, there is no wastewater. 

• While the water flow slows slightly (because it's being forced through multiple dense filters), none of the water is “separated and flushed” the way many RO systems do it.

You can actually see this in action with our best-selling Filtered Water Pitcher:

The unfiltered water in the top chamber doesn’t just sit there. It moves down through the gravity-fed filter, then every drop comes out the other side, dripping into the lower-reservoir clean and filtered. 

When it’s time to replace the filter, you might even see the discoloration where those contaminants were trapped and held inside. 

That’s the system working exactly as designed.

While the flow may slow slightly due to the density of the filters, no excess water is lost, discarded, or wasted. 

It’s the difference between trapping contaminants and separating water.

Here are just a few more key things to keep in mind when considering an RO system: 

Professional & Permanent Installation 

• Under-sink RO systems typically require professional installation unless you're confident with plumbing tools and pipe fittings. 

• You may even need to drill into your countertop to add a dedicated RO faucet.

• If you’re renting—or simply not interested in modifying your kitchen—this setup may not be ideal.

Space For Storage Tank(s)s

• Many under-sink RO systems include a pressurized storage tank to collect filtered water—about the size of a basketball.

• Add in the pre-filter, RO membrane, post-filter, and any optional cartridges, and these setups can take up over half your under-sink cabinet. 

Convenience & Upkeep

• RO systems tend to be slower than other options, often filtering water at a rate of just 0.03 to 0.08 gallons per minute. That’s less than 1 cup every 30–60 seconds.

• Many RO systems have four to seven filters each on different replacement schedules ranging from every 6 months to every 2-3 years. As you can imagine, multiple schedules can be tricky to track.  

Many systems require tools or special filter wrenches for maintenance and upkeep, especially older models.

All of our best-selling filtration systems are powered by our advanced Affinity® Filtration Technology. It’s the only filtration technology that targets up to 365+ contaminants without removing healthy minerals.

While reverse osmosis systems are known for their broad contaminant removal, Clearly Filtered can target just as many contaminants. 

And in some cases, even more, including hard-to-remove chemicals like PFOA, PFOS, BPA, pesticides, and pharmaceutical residues. 

Plus, with Clearly Filtered there’s no excess wastewater, no remineralization, no long wait time, no second drain line, and no need for electricity, pumps, power tools, or pressure tanks—just clean water you can count on. 

Click here to shop our entire line of advanced filtration systems, including the most popular choice for those considering under-sink RO systems, our best-selling 3-Stage Under-the-Sink system.