How Does an Under Sink Reverse Osmosis System Work?

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Reverse osmosis is unparalleled in its ability to remove a wide spectrum of water impurities.

Most RO systems are installed under the kitchen sink – leaving you with clean, great-tasting water right out of the sink faucet.

But how does an under sink reverse osmosis system work exactly? Let’s find out!

Key Takeaways

  • Under sink reverse osmosis systems are point of use filters, making them efficient and cost-effective.
  • They directly connect to the cold water supply under a kitchen sink and purify water using several pre-filters, a semipermeable reverse osmosis membrane with tiny pores, and one or more post-filters.
  • Other key components of under sink RO systems are the storage tank holding the filtered water, the ASO and check valve controlling water flow, the flow restrictor which prevents too much wastewater going down the drain, the drain line flushing out any rejected contaminants, and the filtered water dispenser.

What Is Reverse Osmosis Water Filtration?

In simple terms, reverse osmosis water filtration works by forcing unfiltered water through a semipermeable membrane to capture even the tiniest contaminants. An RO membrane is an ultrafine filter that can capture organic and inorganic particles down to .0001 microns – which is significantly finer than other filtration methods.

By the way, RO contaminant removal is a function of membrane pore size, solute concentration, and feed water pressure and temperature.

Reverse osmosis systems typically feature several other filtration stages before and after the main membrane, which work to remove other contaminants and those that could damage the RO membrane, improve the water’s taste, etc.

What Is an Under Sink RO System?

An under sink RO system is a home reverse osmosis filter installed underneath the kitchen sink. These are the most popular home RO systems, as they filter only the water supply at said sink.

Under Sink Reverse Osmosis System

It’s sometimes called a point-of-use system, as it only filters water at the point where you drink it.

This makes under sink reverse osmosis systems significantly less expensive and lower maintenance than whole house units. The latter filters the entire water supply to a house – which sounds good in theory – but tends to be prohibitively expensive and unnecessary for most people.

How Does an Under Sink Reverse Osmosis System Work?

So, how does an under sink reverse osmosis system work?

Under sink reverse osmosis systems work the same as any other RO system but are installed in the water supply to the kitchen faucet.

These systems usually feature between 3 and 5 filtration stages, depending on the filter quality and price point. In addition to the primary membrane filter, RO systems will typically have a sediment filter to capture large particles and two carbon filters to remove chlorine/chloramine and improve the water’s taste.

Basic Components of Under Sink RO Systems

Feed Water Valve

The feed water valve connects the RO system to your sink’s cold water supply line. The unfiltered water flows through this valve before it reaches any of the filtration stages.

Pre-Filter Stages

Before unfiltered water can be filtered through the semipermeable reverse osmosis membrane, it passes through two or more pre-filter stages. These stages remove suspended particulate like sediment and silt (sediment pre-filter) as well as chlorine and other chemicals (carbon pre-filter), which can damage the RO membrane.

RO Membrane

This is the primary filtering step and rejects a wide variety of impurities from your water. This membrane is ultra-fine and will remove contaminants down to .0001 microns in size!

Any rejected water contaminants leave the under sink reverse osmosis system via the drain line.

Storage Tank

After passing through the RO membrane, the filtered water flows into a pressurized water storage tank. RO filtration is a slow process, so a storage tank is necessary to keep a sufficient amount of water ready.

An RO system will continuously filter water until its storage tank is full – so you’ll have filtered water on tap whenever you need it.


Before the water flows out of your kitchen tap, it passes through one final filtration stage. This takes place after the water accumulates in the storage tank. The purpose of this final filtration stage is to refine the water’s taste and remove any remaining impurities.

ASO (Automatic Shut-Off Valve)

The auto shut-off valve prevents the system from continuously filtering water once the storage tank is full. When the tank is full, the valve is activated – which prevents any more water from passing through the RO membrane.

When you turn on your tap and use RO water from the storage tank, the pressure drops, which deactivates the valve causing water to flow through the membrane again.

Check Valve

The check valve is a valve that blocks the flow of water back from the storage tank toward the RO membrane.

ASO and check valves only work when used in conjunction.

Flow Restrictor

The flow restrictor, as the name suggests, restricts the flow of water, wastewater to be more specific, in an RO system.

This maintains a high enough pressure level for the reverse osmosis membrane filter to work correctly and prevents excessive water waste.

Drain Line

The drain line disposes of the wastewater generated during the filtration process. It runs from the membrane to your sink’s drain.

Filtered Water Dispenser

The filtered water dispenser is simply the faucet that dispenses the filtered water at your kitchen sink. The RO faucet is separate from your main faucet, so you can still use unfiltered water for cleaning and washing dishes.

What Does a Reverse Osmosis System Remove?

RO systems are effective at removing a wide spectrum of contaminants from drinking water, particularly the non-organic contaminants that municipal water suppliers are less adept at filtering.

  • Heavy metals including lead, cadmium, and mercury
  • VOCs (volatile organic compounds)
  • Microbes like bacteria, cysts, and protozoa
  • Water disinfectants (chlorine and chloramine)
  • Disinfection byproducts
  • Pesticides and herbicides
  • Minerals and salts (sodium, calcium, magnesium, potassium, fluoride)
  • Sediment and silt
  • Nitrates/Nitrite
  • Overall Total Dissolved Solids (TDS)
  • Transition metals (iron, chromium, nickel)

bacteria in water

Why Does Reverse Osmosis Waste Water?

Reverse osmosis systems produce so-called “wastewater” as a part of the filtration process. As water passes through the membrane filter it is separated into two streams – one being the filtered water and the other being the contaminant-filled wastewater.

Most standard RO systems without pumps will produce about 4 parts wastewater to 1 part clean filtered water. This is a natural part of the filtration process, so calling it a waste is a bit of a stretch.

What Are the Benefits of Installing an Under Sink RO Water Filter?

Advanced Filtration

Without question, reverse osmosis systems offer one of the most thorough filtration methods out of any home treatment systems.

They remove up to 99.9% of contaminants like lead, nitrates, arsenic, fluoride, chlorine, chloramines – which are by far the most common contaminants in our water supplies.

Healthy Drinking Water

Advanced filtration means you’re left with clean, healthy drinking water on tap whenever you want it. And unlike filter pitchers – there’s no waiting for your water to filter. The storage tank automatically refills itself whenever you draw water from the faucet.

Better Tasting Water

RO filtration results in incredibly neutral, clean-tasting water without any distinctive taste. It’s a huge improvement over tap water – as the filtration process removes the overwhelming majority of sodium and other minerals that can give water an “off” taste.

Easy to Install and Maintain

Reverse osmosis systems have the advantage of being rather easy to maintain, and if you’re handy with tools, they’re not too hard to install.

The sediment and carbon filters only need to be changed about once every six two twelve months, while the membrane will last between two and three years.

As far as installation goes, if you’re comfortable with a little basic DIY and can follow instructions, you should be able to install one in less than two hours.


If tinkering is your thing, then you’ll be happy to know you can customize your own RO system using standard-sized parts available from water filter suppliers. This will allow you to tailor the filter design to your exact specifications.

  • Want to remineralize your RO water before use? Add a remineralization filter to your system!
  • You are facing elevated levels of iron? Install an additional iron pre-filter!



As mentioned previously, the filtration process produces wastewater at a ratio of about 4 parts waste to 1 part filtered water. This means you’ll be using a fair bit of wastewater to create clean filtered water.

Side note: Adding a simple permeate pump to your setup can reduce waste by as much as 80%. Booster pumps are even better.

Loss of Essential Minerals

RO systems remove somewhere between 85% and 98% of a given mineral from the water. This is simply due to the membrane’s design, so you’ll be removing some good with the bad.

The good news is that despite common belief, water is not a significant source of minerals. In fact, drinking water provides less than 5% of our daily mineral intake.


Generally speaking, RO systems are not slow, but neglecting to change filters on schedule or a clogged membrane can lead to the system functioning slowly. The solution to this is simply changing the filter(s) or replacing the aged membrane.

Space Needs

A typical under sink RO system requires about 2 feet x 2 feet of space for the entire unit including the storage tank. Smaller sinks may not have the necessary space, but you can also install the system in a nearby cabinet if it won’t fit directly under the sink.

Extra Faucet

As the filtered water flows out of the system through a separate faucet, you’ll need to install a second faucet on the sink. If your sink has a hole for a soap dispenser or a spray hose, use one of these to install the faucet. Otherwise, you’ll need to drill a new hole to fit the faucet.

Drain Connection Required

Because RO systems create wastewater that needs draining, they must be hooked up to the drain pipe beneath your sink. This is not a major drawback, but it does mean you’ll need to connect a wastewater saddle to your sink’s drain during installation.

What to Consider When Buying an Under Sink RO Filtration System

An under sink reverse osmosis system is a significant purchase, and there is a lot to consider when choosing a system that’s suited to your needs.

young woman drinking water

Water Source Quality

RO water systems are not the ideal solution for softening hard water. Running particularly hard water through an RO system will cause your filters and membrane to age prematurely, so consider adding a pre-treatment water softener if you have hard water in your area. The same goes for high iron levels etc.

So basically, you need to have the right pre-treatment in place if required.

NSF Testing/Certification

NSF standards evaluate RO systems based on several key criteria including efficiency, materials safety, and contaminant reduction.

So when you purchase an under sink filter system that’s been NSF verified you can be confident you’re getting the real deal.

Also, you want to choose a unit that’s tested/certified to remove exactly those contaminants found in your home water.

Your Water Demand

Typical under sink RO systems can make between 30 and 75 gallons of drinking water per day, which is well above what the average consumer will ever need.

Keep in mind that while RO system production is rated in gallons per day (GPD), the actual production will be between 50% and 75% of the rated GPD.

Water Pressure

Another consideration when shopping for an under sink RO water filter system is the feed water pressure needed for it to work correctly. Most systems operate best at 60 psi, and below 40 psi they cease to function correctly.

Low water pressure will cause your system to create excess wastewater, which means a slow-functioning filter. Luckily, this issue can be fixed by installing a pump.


A solid under sink RO system doesn’t need to cost an arm and a leg. You can purchase a quality system for just several hundred dollars. This is far less than the thousands of dollars a whole-house RO system will run you.

Another thing to consider when purchasing are the maintenance costs of filter and membrane replacement. A decent system will cost somewhere between $50 and $200 annually to maintain.

Pure to Wastewater Ratio

Again, a typical RO system creates a ratio of 4:1 (wastewater to filtered water), which represents a significant increase in water demand over other filtration methods. Some RO systems use a permeate pump to improve this ratio to about 1:1. Booster pumps can reduce wastewater to as little as .2 gallon per gallon filtered.


The nature of reverse osmosis systems means that minerals will be stripped out of the final filtered water. This is not a cause for concern as water is not a significant mineral source in our diet, but some people prefer the taste of mineralized water over RO water.

If this is an issue for you, there are remineralization filters that can be added to RO systems to add healthy minerals like calcium, magnesium, and potassium back into the water.

Installation and Maintenance

Most under sink RO systems can be installed using basic tools and little DIY knowledge. Additionally, manufacturers provide detailed instructions on how to complete the installation yourself. Of course, if you feel this is beyond your abilities, you can always hire a plumber to complete the installation for you.

As far as maintenance is concerned, the only major things that need regular replacement are the sediment and carbon filters. The manufacturer will provide a schedule for replacing these – which is generally every 6-12 months.

The RO membrane will also need replacement, but they generally last for 2 to 5 years before needing to be changed.

Lastly, the storage tank will need cleaning and sanitizing every 12 months or so to prevent bacteria and biofilm from accumulating.

If you have any thoughts about the question, how does under sink reverse osmosis work, please don’t hesitate to leave a comment below!

About the Author Gene Fitzgerald

Gene Fitzgerald is one of the founders of BOS and currently head of content creation. She has 8+ years of experience as a water treatment specialist under her belt making her our senior scientist. Outside of BOS, Gene loves reading books on philosophy & social issues, making music, and hiking.
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