How Does a Tankless Reverse Osmosis System Work?

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When reverse osmosis filtration systems first hit the market, they came with a storage tank. Its purpose was to store filtered water from the slow filtration process, so you always had a clean supply.

Nowadays, newer tankless models are overtaking the old, thanks to their compact design.

But how can you keep up with a household’s water demand without a water tank? And how does a tankless reverse osmosis system work anyway? Find out here!

Key Takeaways

  • Most tankless reverse osmosis systems work by connecting directly to the cold water supply above or under the kitchen sink.
  • The water enters the system where it flows through the pre-treatment stages. Here, sediment filters remove larger particles of dirt. Carbon pre-filters eliminate chlorine and other chemicals. Both protects the RO membrane from damage and improves water taste and odor.
  • Next comes the RO membrane which rejects almost all leftover water impurities. Any salts, heavy metals, and other stuff are flushed down the drain.
  • A system with post-filtration may use a carbon filter to remove residual taste. Or remineralization may add beneficial minerals back into the water. If UV disinfection is used, any waterborne pathogens are eliminated.
  • The purified water flows out of the dispenser when opened.
  • Tankless reverse osmosis systems often use a pump that pushes the water through the entire unit and out of your faucet in no time. This gives a consistent flow of water on demand and reduces the amount of reject water produced by a great deal.

How Does a Tankless Reverse Osmosis System Work?

So, how does a tankless reverse osmosis system work?

Most tankless RO systems directly connect to the cold water line under the kitchen sink (some also attach to the existing kitchen faucet or are standalone units).

  • The cold feed water enters the RO system and passes through the first pre-filtering stage. Here, one or more sediment filters remove larger dirt particles like dust, sand, rust chips, etc. This protects all following filter stages, the reverse osmosis membrane in particular, from clogging.
  • Next come one or more carbon pre-filters. These can be granular carbon or carbon blocks. Either way, activated carbon is highly effective at removing chemicals, including chlorine, as well as bad taste and odor. It’s important to get rid of any chlorine at this point, as most RO membranes are rather sensitive to it.
  • This stage makes the heart of any reverse osmosis system: The semipermeable RO membrane rejects up to 99% of leftover contaminants. Water molecules pass through the tiny membrane pores; everything else remains on the other side and is flushed down the drain.
  • Lastly, we often have post-filtration. Here, another carbon filter may remove residual tastes. Or a remineralization stage adds healthy minerals lost before back into the water which is ideal for drinking. Some tankless RO systems even use UV disinfection.
  • Now the filtered water is ready to flow out of the RO faucet when turned on.

blue reverse osmosis membrane

Besides, tankless RO water systems usually have pumps in them. These create pressure that pushes the water through the entire system and out of your faucet in a very short amount of time.

This gives a consistent flow of water on demand and reduces the amount of wastewater that you would find in an RO system without pump significantly.

Some countertop models do not have a pump, so the water pressure will be considerably slower in those.

Is Tankless Better Than Tank-Based?

Tankless and tank RO models more or less filter contaminants just as well as each other. Each type has advantages and disadvantages, but in terms of purifying water, they are pretty much on par.

It really comes down to a personal choice in other areas, such as size, cost, and functionality. Knowing their differences may help determine which is suitable for you and your home.

The Differences Between a Tank and a Tankless RO System

The main difference between the two types is the size. Tankless RO units are at least 50-75% smaller and easier to move around and install.

Under the sink tank units have the filtration system, plus a 3-gallon (sometimes smaller/larger) storage tank. A tank-based reverse osmosis system needs this as it usually doesn’t have a pump to push the water through the various filter stages and up to the sink faucet; instead, it uses air pressure within the tank to output the water.

Tank RO systems are triggered to filter water and top up the supply when the tank water level lowers and are also triggered to switch off production when the backpressure reaches a certain level, indicating the tank is full.

The other notable difference is that tank RO systems are often standard-sized, meaning they can be modified to include post-filtration steps, such as UV disinfection and remineralization filters, whereas tankless systems often use proprietary sizes and cannot be altered.

Advantages of Tankless RO Systems

We compiled the following list of tankless reverse osmosis system advantages assuming that there is a built-in pressure pump.

More Free Space

Tiny kitchen? No problem. A tankless system won’t take up much space. While the tank models are also highly effective, they really do not do well in small spaces.

Less Waste Water Production

RO systems produce waste water and quite a bit of it. There isn’t any getting around it, but you can choose a system that will waste less, and those are often the tankless models.

While tank systems can waste up to 5 gallons of water for each gallon produced, tankless systems can significantly reduce that down to as much as only 0.25 gallons wasted.

The pressure pump in the tankless system is what we have to thank for that. The force they use to push the water through the RO membrane ensures less water is left behind.

No Secondary Pollution

Unfortunately, with tank systems, you can’t get in and clean the storage tank. This means there is a chance microorganisms could build up in your tank, re-infecting the purified water. Also, if you let the water sit for too long, even a clean storage tank can leach plastic taste.

Tankless systems eliminate this risk.

Faster Filtration

The internal pump in a tankless reverse osmosis system means you can filter enough water on demand, unlike with a tank system. You can also filter a higher amount of GPD (gallons per day) than with a tank model.

Easy Installation and Maintenance

Tankless systems are easy to install and maintain. Some countertop models, for example, simply need to be plugged in, and you are good to go. Tank systems are more fiddly and harder to maneuver as they are much larger.

They are also harder to maintain as you have to monitor tank pressure and additional valves.

Different Water Filter Cartridges and Membranes

More Effective Contaminant Removal

Because of the increased pressure in a tankless RO filter system, it helps to make contaminant reduction slightly more effective, meaning that traces of contaminants left in your water may actually be lower.

Drawbacks of Tankless RO Water Filters

Again, we will base the following section on tankless reverse osmosis water filters using an internal pump.

Often More Costly

Tank systems sound great, but they may only be suitable for some budgets, as they are usually more costly to purchase.

Depend on Electricity

Tankless systems require electricity to operate, unlike the tank system, which works off plumbing pressure and pressurized air.

No Purified Water During Blackout

Your RO system won’t function if there is a power cut, which leaves you with no access to purified water unless you have some already stored in a container somewhere (we recommend this!)

Pump Noise

If your tankless system comes with a pump, or if you have had a pump added, it is much noisier than the tank models.

Less Options for Customization

The design of the system doesn’t allow for customization most of the time, unfortunately.

When to Buy a Tankless RO System and What to Consider?

When trying to determine if an RO system without tank is right for you, here are some questions to ponder;

  • Do I use a lot of water and need water on demand?
  • Is the pressure in my feed water usually low?
  • Am I better off with a system that is easier to maintain?
  • Do I want to create less wastewater?
  • How much space do I have? Is it too small for a tank model?

If you answered yes to all these questions, congratulations, you and the tankless RO system may be a match. Other things to consider when purchasing a unit include the following:

Feed Water Quality and Filtration Process

Check your feed water. If you have a lot of sediment or high amounts of a particular contaminant, you may need specialized pre-filters to protect the RO system from damage.

And you want to make sure that, whichever RO system you choose, it’s capable of reducing the specific contaminants found in your water supply.

NSF Standards

NSF tests and certificates mean you can rest easy knowing your machine has been vetted and will do everything it says it will do to the appropriate standard. It also indicates the unit is made from high-quality materials.


RO removes all the good minerals from the water as well as the bad stuff. If water without minerals concerns you, consider looking at ways to remineralize your water supply.

young woman drinking water

Reject Water

Check the expected wastewater output of the unit you want to purchase to ensure it fits your needs. It will be different depending on the manufacturer.

Filter Capacity

Filters in all reverse osmosis water systems need to be replaced, or the unit just won’t work, and you will have contaminated water. There are various types and sizes of filters, some last longer than others. Also, if you have a lot of contaminants in your water, you will need to replace your filters more regularly.

So this is something to keep in mind as it will also affect operating cost.

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

About the Author Alexandra Uta

Alex is a content writer with an affinity for research and a methodical attention to detail. Since 2020, she has fully immersed herself into the home water treatment industry only to become an expert herself. Alex has been using water filters and similar products for years which has gained her lots of hands-on experience.
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