RO Water Systems 101

What Is Reverse Osmosis Water and How Does an RO Filter System Work


    On this page you will learn:

    What is Reverse Osmosis Water?

    Reverse osmosis water, or RO water, is water that was purified by the use of reverse osmosis filtration technology. Reverse osmosis is a water treatment method that removes almost all impurities. Therefore, RO water mainly consists of H2O molecules and contains only tiny amounts of other particles.

    Pureness is also the reason why RO water is used in medical applications (e.g. dialysis and injections) and other industry processes that require (almost) contaminant-free water, and why it’s suited for drinking and cooking.

    How ‘pure’ reverse osmosis water really is depends on a variety of factors, such as the filtration system itself and feed or input water quality. As a general rule, feed water that contained more impurities before the filtration, will also contain more impurities afterwards, compared to feed water that was cleaner in the first place.

    It is also relevant what types of contaminants are present in water. Large bacteria and viruses can be removed much easier and more effectively than small ions such as fluoride.

    But what about an actual number representing purity levels? Well, manufacturers and sellers usually claim that their home use RO units remove somewhere between 95 and 99 percent of all contaminants, if the systems are well maintained. These numbers are oftentimes tested and certified by independent third party organizations like NSF International.

    Is Reverse Osmosis Water Good For You or Is It Bad?

    We’ve already mentioned that RO water can be used for drinking and cooking. Therefore, one would expect it to be safe for our health. But people tend to be skeptical of new technologies, and rightly so.

    Even though reverse osmosis is not exactly new, there are still many people who doubt its ability to produce healthy water. The main basis of this claim is that RO systems strip away minerals that are considered healthy for the human body and thereby produce what is often referred to as ‘dead water’. Are these people right in saying that RO water is bad?

    Safety and Health Benefits

    safety and health

    The average tap water supplied by your municipal is far from pure. Various studies have discovered that it contains contaminants that can cause skin rashes, tooth damage and allergic reactions. Even harmful substances like arsenic, lead and barium were found.

    In case you are getting your water from a ground source, there might be even worse stuff lurking in it. That’s why it’s important to install a water filter in your home that can remove all these substances, and an RO system is one of your best options.

    Reverse osmosis is considered one of the safest water filtration methods. Most of the bottled water you see on supermarket shelves was in fact ‘Purified by Reverse Osmosis’ – next time you go grocery shopping, take 5 minutes of your time and check out some of the labels of bottled water and other beverages.

    As we mentioned before, RO water may not be 100 percent pure, but that doesn’t mean that it’s not safe. The remaining solutes are unlikely to cause any health problems.

    Are You Looking For a Filter System That Will Provide 100% Pure & Safe Water to Your Family? Click to View Our Reverse Osmosis System Reviews.

    Mineral Lack

    Unfortunately, a water filter cannot differentiate between ‘good’ and ‘bad’ impurities. RO systems will remove up to 99% of all contaminants including calcium, magnesium and other healthy minerals.

    But simply because most minerals have been removed from the water you drink from day to day, does not necessarily mean that your body is starving for these nutrients. Water is not the only mineral source for our body. In fact, only about 20 percent of our daily mineral intake comes from drinking water.

    A balanced diet with plenty of organic fruits, vegetables, and nuts that are not deprived of nutrients should more than adequately cover your body’s mineral requirements. Those who are still concerned about losing out on mineral intake can supplement their diet with a mineral blend or enrich their water with a pinch of mineral-rich sea salt.


    The pH-value or level of a fluid shows how acidic or alkaline the solution is. Perfectly balanced water is what we call ‘neutral’ and has a pH-value of 7. A pH-value above 7 means that water is alkaline, a value below indicates that water is acidic.


    In general, it’s healthier for us, our teeth, our cardiovascular, etc., if what we drink does neither contain too many acidic ions nor too many alkaline ions. In other words: As often as we can, we should stick to beverages with a pH-value close to 7.

    It’s possible that reverse osmosis filtration reduces the pH-level of water to a small extent, which mostly depends on feed water composition. Tests conducted in California showed that RO filtration lowered the pH-value of a test water sample from 8.12 to 8.06, proving that the impact is minimal.

    For comparison purposes: The pH-value of orange juice is around 3.3-4.0 and lemonade around 2.5, which is a lot more acidic than filtered water can be. So, before you start worrying about RO water, make sure to eliminate these other drinks first.

    Where to Buy RO Water and How Much Does It Cost?

    If you want to have a never-ending supply of RO water, the wisest thing you can do is buy an RO system for your home or business. Even people on a tight budget can get something small to place on their countertop – just follow this link:

    However, if you don’t want to deal with the costs and hustles of a system in your home, you can simply go and buy RO water in a store (although we warn you that in the long run this is more expensive).

    Like we said, most bottled water – unless it’s spring water – is RO purified tap water. So check your local WalMart first. Chances are that they also sell RO water by the gallon. You should also try to stick to glass bottles, as plastic containers add new contaminants into the water.

    Look out for labels that state ‘Purified by Reverse Osmosis’ or something similar. The price per gallon can range between $0.30 and $0.50. Most HEB stores also sell RO water by the gallon. Check other stores in your area, too.

    Another great source for RO purified water are delivery service companies or water service stores/clubs.

    How Does Reverse Osmosis Work?

    Reverse osmosis is most commonly used to desalinate sea water in areas where fresh drinking water is hard to access. It is also used to recycle commercial wastewater. There are RO systems for homes, for businesses, and large plants for industrial applications. RO filtration makes your water taste great, so you might even want to drink more of it. But how exactly does reverse osmosis work?

    Osmosis Process Diagram

    Reverse Osmosis Process Diagram

    Reverse osmosis is the opposite of the natural osmosis process. If you remember your chemistry, osmosis occurs when a solvent moves from a low concentration solution to a high concentration solution through a semi-permeable membrane; semi-permeable meaning that only some, but not all substances can pass through it.

    This process continues until solute concentrations on both sides of the membrane are equal. The movement of a solvent to equalize solute concentrations creates osmotic pressure.

    In reverse osmosis the exact opposite happens. External hydraulic pressure is used to overcome the osmotic pressure and the flow of the solvent, usually water, is reversed. The solvent molecules now move from the high concentration solution (feed water) to the low concentration solution (filtered water). The aim is to separate the solvent from the dissolved solids resulting in purified water.

    Looking for more visual explanation? Try this 90-second video:

    How Does an RO Membrane Work?

    An RO membrane rejects contaminants based on their size and ionic charge – the higher, the better. Compared to micro and ultrafiltration membranes, a reverse osmosis membrane has much smaller pores that water molecules can diffuse through, however, the majority of impurities can’t and are flushed down the drain. Thin Film Composite (TFC) RO membranes reject up to 99% of all Total Dissolved Solids down to 0.0001 microns.

    But there is a key difference between reverse osmosis and ‘regular’ filtration and that is the predominant removal mechanism. Regular filtration works by size exclusion. This simply means that if a solvent is too large to fit through the pores, it will get filtered out. Theoretically, you can achieve perfect exclusion here, if membrane pores are only small enough.

    But not so with reverse osmosis, because it involves a diffusive mechanism so that filtration efficiency also depends on solute concentration, water pressure and temperature, and water flux rate. Higher water pressure means higher filtration efficiency, as lower water temperature results in lower efficiency.

    The Ideal Pressure For Operating an RO System Is Around 60 PSI. If Pressure Falls Below 30 PSI, It’s Generally Considered Insufficient and Should Be Increased Using a Pressure Pump.


    The majority of commercially manufactured RO membranes are made from TFC, cellulose acetate (CA), or cellulose triacetate (CTA). Each membrane has 2 layers, the active barrier skin about 0.25 microns in thickness and a support layer about 100 microns in thickness.

    TFC membranes are more durable than CA and CTA membranes and also have higher rejection rates, 98% for standard contaminants on average. CA and CTA membranes are better at tolerating chlorine, but more susceptible to fouling from bacteria.

    Reverse Osmosis vs. Carbon Filter

    RO membranes have much higher contaminant rejection rates than carbon filters, which are best suited for water filtration pre-treatment. Using a carbon filter, you won’t be able to remove the majority of ions like heavy metals from your water, as well as waterborne pathogens such as viruses.

    However, carbon and sediment pre-filters are key to protect an RO membrane from scaling, fouling and damaging in general. Therefore, if you are looking for a drinking water filter, the question should not be ‘reverse osmosis or carbon filter?’. Rather you should look out for the best combination of both.

    Reverse Osmosis vs. Water Softener

    A water softener is primarily used to remove hardness minerals from water in an ion exchange process. The main goal is to protect your plumbing system and household appliances from scaling. It’s not about making water safe for consumption.

    Does Reverse Osmosis Waste Water?

    One of the common charges against reverse osmosis filtration is water wastage, and it’s true, every time RO water is produced, a certain amount also goes down the drain. The reason for this is that an RO system needs water to rinse itself and flush out the impurities. However, strictly speaking, this water is not being ‘wasted’.

    You have to consider that many mechanical or chemical processes with a specific end product require an input of resources. It could be energy, it could be chemicals, it could be human labor, or as in our case water. We also use water to wash the dishes and our clothes, or to flush the toilet.

    Yes, an RO system uses more water in its operation than it later actually provides, but you won’t notice it on your water bill. What’s more, water is only being used while the tank is filling. Afterwards the whole system shuts down and no more goes down the drain.

    By the way, water for cooking and drinking makes up only a fraction of the total amount of water you use every day…

    Water Use Per Capita

    The USGS estimates that each person uses about 80-100 gallons of water per day for the following purposes:

    Bath36 gallons on average
    Shower2.5 gallons per minute
    Washing hands/face1 gallon
    Face/leg shaving1 gallon
    Dishwasher6-16 gallons
    Dish washing by hand8-27 gallons
    Washing machine25-40 gallons per load
    Toilet flush1.6-4 gallons
    Drinking water0.06 gallons (8 oz.)
    Cooking1-2 gallons

    How Much Water Does Reverse Osmosis Waste?

    Ratios of 20:1, meaning 20 gallons of water wasted for 1 gallon of filtered water, are misleading as they refer to industrial applications that desalinate sea or recycle brackish water. Here, much more water and much higher pressure is needed to achieve the desired levels of purity.

    Most residential RO systems will drain away 3 to 6 gallons of water for every gallon of purified water they produce. This may seem like a lot, but with advancing technology, manufacturers are coming up with new systems that have significantly less water draining away. One great example is the Brondell H2O+ Circle – click here to read our review – which features a 2.1:1 ratio.

    Additionally, as the amount of water that’s needed highly depends on feed water pressure, what you can do is add a pump to your filter system, which can reduce water wastage by up to 80%.

    Our Favorite RO System That Uses a Permeate Pump to Increase Water Pressure Is the Home Master TMAFC-ERP, Which We Reviewed Here.

    Last but not least, why not reuse the wastewater to irrigate your garden or for many other general household purposes, like cleaning?

    What Is an RO Water Filtration System?

    RO Water Treatment Process – How Does a Reverse Osmosis Water Filter Work?

    We had a brief glance at the scientific principle behind osmosis and reverse osmosis. A modern filter system combines RO membrane technology with carbon and/or mechanical filtration to produce highly purified drinking water. Thereby, it uses the pressure present in your home’s water system to push the water through the different filter stages. The removed impurities are then flushed down the drain and filtered water is provided for a designated faucet.

    But what exactly are the stages water goes through in an RO filter system?

    Fortunately and apart from a few exceptions, most systems are very similar to each other. There are some basic components you will find in almost all of them and they work in roughly the same way. Here is a detailed look at the inner workings:

    • Cold water inlet – This is where the unfiltered tap water is fed into the system. A feed water valve either connects to the cold water line above the cold water shut-off valve, or it connects to the cold water line below an existing faucet with the help of a faucet adapter.
    • Pre-filter(s) – A pre-filter, of which there could be several depending on the system, helps to remove large particles such as sand and silt, but also other organic contaminants that could clog or damage the sensitive RO membrane, like chlorine. Pre-filters thereby extend the life of an RO membrane significantly allowing it to tackle smaller impurities more effectively. Common pre-filter examples are carbon block filters and polypropylene sediment filters.
      • Sediment pre-filter – Melt blown polypropylene filters remove sediment, dirt, and rust particles with a size of 5 microns and more.
      • Carbon pre-filter – Activated charcoal filters remove chlorine, tastes, and odors with a filtration capability of 0.5 to 10 microns.
    • Shut-off valve – We always recommend our readers to look out for systems with an automatic shut-off valve for maximum efficiency. When the storage tank is full, the valve closes to prevent more water from entering the membrane and also blocks the drain line. This way, no water is wasted unnecessarily. When filtered water is drawn from the faucet and pressure in the storage tank drops, the valve opens up again so the filtration process can continue.
    • RO membrane – The heart and center of every RO system. It’s where the majority of impurities are removed, if the membrane functions properly.
    • Flow restrictor – Water flow through the RO membrane is regulated by the flow restrictor to maintain the right amount of pressure on the inlet side of the membrane. Without the restrictor almost all incoming water would simply go down the drain line.
    • Drain valve/saddle – The valve connects to the drain line to remove wastewater from the system.
    • Storage tank – The storage tank is used to store the purified water. Standard-sized tanks can hold anywhere between 2-5 gallons.
    • Post filter – When you draw water from the storage tank, it goes through a final filtration stage to remove any remnant odors and tastes; a sort of a final touch. The inline filter typically clips onto the top of the system housing.
    • Faucet – This is where you can draw the filtered water from. The additional faucet usually gets installed on your kitchen countertop.

    Schema RO Systems


    • Remineralization stage – Some systems come with an additional remineralization stage, which enriches the water with calcium, magnesium, or potassium. This solves the mineral-lack problem.

    Reverse Osmosis System Cost

    How much does a reverse osmosis system cost? That’s a tough one to answer.

    RO systems vary in price depending on their size and complexity. For a few hundred dollars, you can get an under sink system that connects to a single faucet in your kitchen. For up to several thousand dollars, you can get a whole house system that cleans all the water coming into your home.

    Some systems will have a complicated 5-step filtration process, even using UV light to purify your water, others come with only 3 different filter stages. Your needs, therefore, will dictate how much you will have to pay. So, evaluate what your water purification needs are and what cost range is within your financial comfort.

    Generally, reverse osmosis systems cost between $100 to $300 on the lower and $1,000 and above on the higher end. And don’t forget to take into account installation and maintenance costs!

    Cheap Systems, Middle-Class Systems, High-End Systems

    When evaluating the costs of RO systems, there are three general categories you need to be aware of; cheap systems, mid-class systems, and high-end systems.

    • Cheap systems – Cheap systems will cost anywhere between $50 and $250. These are small home-based systems that usually connect to a single faucet only and can be placed on your countertop or go under the sink. These systems are affordable because they use the lowest quality parts available. While they will purify your water just as well as other higher priced systems, individual components will wear out more quickly. After about six months to a year, you will have to replace various parts such as filters and membrane.
    • Middle-class systems – They provide the best value for homeowners. They range in price between $250 and $600 depending on size and complexity. They are also much better in build and quality and are likely to last for many years. Some sellers even provide lifetime guarantees. If you are planning to buy a high-quality RO water filtration system without spending thousands of dollars, look for a middle-class system.
    • High-end systems – On the pricey end of the scale we have high-end units. These are usually whole house systems that provide filtered water for every tap in your home. The price range starts at approx. $600 and goes into the thousands. However, be careful when buying a supposedly high-end product. Some come with useless bells and whistles that will bring you no benefit. Others compromise on important features just to maintain their form and style. Make sure that when you pay for a high-end RO system, it actually delivers on function and performance.

    Does Reverse Osmosis Remove …

    The pre-filters of an RO system remove large particles like sand and silt. Carbon pre-filters also remove chlorine, which would otherwise cause damage to the RO membrane.

    When the water reaches the membrane itself, theoretically any molecule larger than water will be filtered out. Most minerals will be stripped off, dangerous substances like lead and asbestos will be removed, and compounds like fluoride and calcium will likewise be filtered out of the water. Even bacteria and many viruses are not spared from the filtration process.

    By way of example, listed below are some of the contaminants that an APEC Water filter system is certified to remove:

    ContaminantReduction RateContaminantReduction Rate
    Chromium-694-99%Radium 226/28893-98%


    calcium stone

    Salt (Sodium)

    Yes, reverse osmosis removes sodium from water. Remember that anything bigger than a water molecule cannot pass through the membrane? Dissolved salt will certainly not make it through there. This is why reverse osmosis is used in desalination, where salty seawater is made potable. The removal of salt also improves the taste and smell of water.


    Yes, reverse osmosis also removes calcium, which can be quite problematic in water because it causes hardness. It can clog up your plumbing, causes pipes to rust and leaves a white film on everything it touches.

    But although reverse osmosis has proven to be are very effective method for removing calcium from water, clogging is likely to occur and damage the membrane, especially if you have very high levels of water hardness. In that case there are better ways to reduce the amount of calcium in your water, e.g. by installing a softener.


    Yes, an RO system removes chlorine. Although, it is important to note that chlorine is removed by the carbon pre-filters rather than the semi-permeable RO membrane at the heart of the system.


    Countless tests and experiments have shown that a reverse osmosis filter system is also very well capable of removing lead.


    Getting rid of fluoride with the help of reverse osmosis? No problem at all!


    A reverse osmosis membrane will separate iron compounds from water with ease. However, the iron will plug the membrane over time, which is why we recommend different treatment methods.


    Yes. In the microscopic world, bacteria and viruses are literally giants (especially the former). An RO membrane is designed to filter out molecules that are a thousand times smaller than the smallest bacteria. So yes, reverse osmosis will remove bacteria and many viruses from water with ease.

    Reverse Osmosis Water vs. …

    Tap Water

    Do you remember the Flint water crisis, where highly-elevated blood lead levels were found in children? And that’s only the tip of the iceberg. No matter where you live in the country, there is a lot more than just H2O in your tap water!

    One big problem is that impurities can enter the water supply system at many different points in the distribution channel. Possible causes include groundwater contamination due to agriculture, corroded pipes, and water breaks.

    If you want to learn more about tap water contamination, you should definitely check out one of our infographics about the topic – ‘The Current State Of U.S. Public Tap Water‘.

    Bottled Water

    plastic bottle

    The major disadvantage of bottled water is that it’s expensive. Apart from that, plastic bottles create a lot of waste that all too often ends up in a landfill – unless you stick to glass containers, of course. On top of that, bottled water is not as pure as you might think. In fact, most of the time it’s simply filtered public tap water, and to be honest they don’t even do a very good job.

    In contrast, an RO system will pay itself off in no time, because you only pay pennies for each gallon you use. RO water is always fresh unlike bottled water that has been sitting in a plastic bottle for weeks or even months. And you always have the water at the ready right there in your kitchen, so you don’t even have to do any heavy lifting.

    Distilled Water

    Is RO water the same as distilled water? No, it isn’t. Though, both are suited for drinking and cooking, because they contain very low amounts of contaminants.

    Like a teakettle, a water distiller heats water until it reaches the boiling point and vaporizes leaving all other solutes behind. Then the steam condenses in a cooling coil and gets collected in a pitcher. Distillers reduce the amount of Total Dissolved Solids in water even more effectively than RO systems providing almost pure H2O.

    Unfortunately, they aren’t great when it comes to volatile chemicals that have a low boiling point, like chloramine which remains in the water. This is why it’s a great idea to use carbon filters for pre-treatment.

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    If you have any questions or thoughts about reverse osmosis in general or a specific RO filter system in particular, please don’t hesitate to leave a comment below!

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