How Does Reverse Osmosis Work

Posted by Donavan Jones on

You may be thinking, "How Does Reverse Osmosis Work"?

Basically, reverse osmosis works as water is forced across a semi-permeable membrane, leaving the contaminants behind to be flushed down the drain, and the clean water is collected in a storage tank for later use.


illustration of reverse osmosis

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Table Of Contents:

What Is Osmosis?

Osmosis is a natural process that balances two bodies of water separated by a semipermeable membrane. Water flows through the semipermeable membrane from the less concentrates solute (fewer contaminants) to the highly concentrated (more contaminants) to restore equilibrium between the two sides.

The semipermeable membrane has small holes to trap contaminants but it allows the water to flow right through it. This flow of water may be reversed if pressure is applied to the solute with a higher concentration.

What's Reverse Osmosis?

Reverse osmosis occurs when pressure is applied to the solute with a high concentration of contaminants. For example, when pressure is applied to a volume of saltwater, the salt is left behind when the water passes to the other side of the membrane. 

Now, the saltwater is drinkable. The freshwater that was produced is called permeate and the concentrate that is left over is waste.

How Does A RO System Work

The feed water flows to the more concentrated side of the RO membrane when pressure is applied. The pressure forces the water through the semipermeable membrane. The atoms of the contaminants and the membrane contain ions that have the same charge. Since the charge of the atoms of the contaminants and the membrane are the same, they repel each other. This is what stops the contaminants from reaching the other side of the membrane.

Stages Of An RO System

Reverse osmosis is only one of the stages of an RO system. RO systems are made up of 3 to 5 stages of filtration. Water is always treated in an RO system with reverse osmosis in conjunction with pre and post-filters. Typically, an RO system contains one or more of the following filters.

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  • Sediment Filter - This filter reduces particles like dirt, dust, and rust.

  • Carbon Filter - This filter reduces volatile organic compounds (VOCs), chlorine, and other contaminants that give the water a bad taste or odor.

  • RO Membrane - This is the heart of the system. It removes up to 98% of total dissolved solids (TDS)


point of use RO system

When water enters an RO system, it goes through the prefilters first. The prefilters catch sediment, chlorine, and particles that cause bad taste and odors. The prefilters stop these contaminants from reaching and clogging the RO membrane. After the water flows through the prefilters it finally goes through the RO process.

Next, the water flows into the storage tank, where it is held until it's needed. RO systems produce water slowly. If you were to turn your RO faucet on to fill a glass of water at the RO membrane production rate, you would wait about 5 minutes before your glass was filled. A few ounces of water, about 2 or 3 ounces, are produced every minute. This is why a storage tank is necessary. The RO system will continue to process water until the storage tank is full and when you turn on your RO faucet, water comes out of the storage tank and goes through another postfilter to polish the water before it goes to the RO faucet.

What Happens To The Filtered Contaminants

When the water source flows through the system, the water gets divided into 2 streams. One stream carries the filtered water to your RO faucet. The second stream carries the rejected impurities like salts, dissolved pollutants, and minerals to the drain.

Since the contaminants are carried away with the brine water, they cannot clog the RO membrane.

Does A Reverse Osmosis System Waste Water

A small amount of water goes to the drain when carrying rejected impurities. The wastewater is needed to carry the rejected impurities to the drain. If you want to waste the least amount of water as possible then you need to add a permeate pump to your system. Permeate pumps reduce the rejected water by 75%. Most RO systems come with a shut of device that stops the wastewater from going down the drain when the storage tank is full.

Many people with RO systems use the water that goes to the drain for landscaping. The drain water will have a higher TDS reading but it is safe for your lawn and garden.

Reverse Osmosis Wastes Less Water Than Bottled Water

bottling water

Think about it! Bottled water wastes more water than an RO system.


First of all, the water in the bottle was most likely produced by using reverse osmosis. In addition, the production of the bottle requires water and petroleum. Lastly, the truck gets washed at the end of the day to prepare for the next day.

Does Reverse Osmosis Demineralize The Water

Yes, but is that a bad thing? 


There are some materials in your drink water that are fine to drink, but the EPA recommended that the number of TDS not to exceed 500 parts per million. This means a large portion of the USA should be removing some total dissolved solids. Removing minerals from the water will not hurt anyone who has enough food to eat. Besides, food is the primary source of essential nutrients, not water.

Our bodies are around 70-80 percent of water and that water-primarily hydrates, lubricates joints, and aids in organ function. Minerals don't help with this. You would need to drink water in excessive amounts for your body to absorb enough minerals. If you are deficient in minerals, then you need to eat more fruits and veggies.

Will A Reverse Osmosis System Soften My Water

Yes. However, using an RO system to treat hard water can shorten the life of the RO membrane and lead to more frequent replacements. If you have hard water, use a water softener to treat it instead.

A water softener does not treat water for contaminants. It only removes calcium and magnesium to treat hard water. An RO system is safe to use with a water softener. It removes the sodium added by the water softener and extends the life of the RO membrane.

A water softener can remove iron from the water before it goes into your RO system. Iron can stain your toilet, shower, hair, clothes, and clog the RO membrane.

What Does A Reverse Osmosis System Remove

Your RO system includes the RO membrane, pre, and post-filters for filtering out a broad range of contaminants.

Does A Reverse Osmosis System Remove...

  • Fluoride? Yes.

  • Salt? Yes.

  • Sediment? Yes.

  • Chlorine? Yes.

  • Arsenic? Yes.

  • VOCs? Yes.

  • Herbicides and pesticides? Yes.

  • Other contaminants? Yes. The contaminants listed are some of the most popular ones treated with an RO system, but the system also removes many other contaminants.

  • Bacteria and Viruses? No. Water entering your RO system should already be microbiologically safe. Reverse osmosis may remove some bacteria, but bacteria could grow on the RO membrane and potentially enter your water supply. To remove viruses, we recommend UV disinfection.

What Can A Reverse Osmosis System Be Used For

Is an RO System right for your needs?

Under Sink Reverse Osmosis? Yes.

Reverse osmosis is typically applied at a point of use in a home in places like under the kitchen sink, bathroom sinks, but a point-of-use reverse osmosis system could also be mounted in a cabinet or your garage or basement.

Refrigerator? Yes.

Connecting an under the sink RO system to a refrigerator is easy and worthwhile. The reverse osmosis process removes minerals from ice and water, making this setup a no brainer. Ice cubes produced with RO water last longer.



Whole house? Rarely.

Reverse osmosis is only used to treat water for your entire home if the home's water supply has saltwater intrusion, high levels of silica in the water, or some other problem that only reverse osmosis can treat.

Shower? No.

shower head

You need a huge tank if you want reverse osmosis water for your shower. Instead, all you need is a carbon filter or a water softener.

Pools? No.

The only time you may need an RO system for a pool is when the water contains a contaminant that no other water filtration system can remove. You will send about 10,000 gallons of water down the drain so you can filter about 20,000 gallons of water to fill your pool.

When swimming in your pool, the dissolved solids in a pool really don't matter, so other systems do a better job at cleaning pool water.

Agriculture? Depends.

hydroponic farming

Reverse osmosis works well for hydroponic farming, but not all plants survive or thrive with RO water.

Businesses? Yes.

Commercial or industrial RO systems are used because commercial units allow the drain water to be redirected back into the feed supply. Reverse osmosis removes paints, dyes, and other industrial contaminants. 

Aquarium? Yes.

fish tank

If you’re a fish enthusiast, then an RO system is perfect for you. An RO system will remove all minerals from the water.

RVs? No.

RO systems require proper draining, and draining hookups aren't available at campsites. Storage tanks are also difficult to attach to an RV.

How Long Do Reverse Osmosis Systems Last

RO systems last a long time, but the filters should be changed every 6 months to a year. The RO membrane should be replaced every 2-4 years, this depends on your feed water conditions.

filter change schedule


There are many different reasons why you should use an RO system. The above article should be able to help you understand,  reverse osmosis systems, and how they can help you with your water filtration needs. If you have any questions reverse osmosis or any of our water filtration systems, then please contact us so we can help you further.

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