All Your Reverse Osmosis Questions Answered

When I decided to install a home water treatment system, I had to choose which type to buy. There were lots of options!

I wanted my family to have clean drinking water, but I also had to consider cost, maintenance requirements and ease of use. A friend of mine recommended reverse osmosis (RO) for the multi-layer filtration it offers. 

After doing some research, I chose an RO system for my home. I am glad I did.

What is Reverse Osmosis?

RO is a filtration system used to purify water. Before we go into where it came from and how it works, we need to discuss what osmosis is.

Osmosis is a natural process

We often joke that we wish we could learn a tough academic subject “by osmosis”. Have you ever thought about what that phrase really means?

Osmosis is a type of diffusion (spread of molecules) specific to water. It has to do with the natural movement of water molecules through semipermeable cell membranes. Semipermeable just means the membrane is picky about which molecules can pass through and which ones cannot.

This movement occurs from an area that has a higher concentration of water molecules to a lower concentration of them. It’s like they are passengers on a crowded subway train. As soon as the doors open, they want to pour out onto the platform that is less crowded.

Osmosis is important in our bodies because it helps us to maintain internal balance as water moves into and out of our cells.

Let’s take the example of what happens to our cells when they are surrounded by a high-sodium environment.

The water inside our cells is in an area with a high concentration of water molecules. The salty environment outside contains lots of sodium, and therefore a lower concentration of water. The water inside the cell will naturally flow through the membrane to the salty environment outside.

That is why we feel dehydrated and bloated when we consume salty foods.

History of RO Water Systems

The RO process is also referred to as hyperfiltration. It was first documented in a patent filed by the University of California in 1962.

The invention was eventually adapted by the US Navy. The April 1979 issue of the Naval Engineers Journal describes a process for making sea water potable (safe for humans to drink). The abstract reported that “[i]nvestigations of reverse-osmosis systems and components conducted at a seawater test site have demonstrated significant progress in extending membrane life and system performance through improved prefiltration and water pretreatment techniques.”

Research and inventions are ongoing to improve on the RO system.

Research published in the November 1992 issue of Desalination discusses the results of a prototype RO unit installed on a Canadian Navy ship. It “confirms the potential benefits of centrifugal reverse osmosis: reduced energy consumption, improved reliability, lower levels of noise and vibration, and reduced requirements for pretreatment.”

A patent for a pumpless, tankless residential RO system was filed in 2008. 

How It Works

Hyperfiltration technology throws the pattern of osmosis movement backward by the use of air pressure and a synthetic membrane.

The air pressure, which must be higher than natural osmotic pressure, pushes the water molecules in a direction contrary to their natural flow: from an area of lower water concentration to an area of higher water concentration. Another term for this movement is “up the water gradient”.

The water on the lower concentration side has a higher concentration of contaminants than water molecules. The opposite is true for the environment on the other side of the membrane.

Using the subway analogy from above, the air pressure is like a group of subway employees pushing the passengers from the open platform, through the doors and back into the very crowded train. Only the large outlaws (sediment and toxins) are not allowed back on. They are kicked out of the station altogether!

The RO membrane is actually the third step of a 4-step process in standard filtration units. The complete process of hyperfiltation is as follows:

  1. Water from your cold water supply line (feed liquid) flows through a sediment filter that holds back larger particles.
  2. The water then flows through an activated carbon filter to remove smaller particles and toxins.
  3. The resulting water is pushed through the RO membrane where it loses even more impurities. From here the stream of water is split into two streams: one of waste water (concentrate) that gets sent down the drain, and one of purified water that goes to the next step.
  4. The purified water passes through another carbon filter to remove any remaining odors before it is sent either to the faucet or a holding tank.

More steps are added as more components are incorporated into the system, like a re-mineralization unit. The hyperfiltration membrane has multiple layers made of materials that may include:

  • Polysulfone
  • Polyester
  • Polyamide

These are all synthetic materials chosen for their nanofiltration ability, durability and flexibility. Even with those qualities, though, the membranes age or “foul” and have to be replaced at least yearly.

The membrane is rolled up into a scroll around a porous collection tube. As the water encounters the membrane, it is pushed by the air pressure through the layers of the scroll in a spiral toward the center. As the water moves, water molecules are being pushed through the membrane itself.

The membrane only allows the tiny water molecules to be pushed through. It filters down to 1 micron. For comparison, the diameter of a person’s red blood cell is about 5 microns. A human hair is 75 microns in diameter.

Instead of semipermeable, it should be called barely permeable!

Larger molecules of salts, bacteria and viruses cannot be pushed through the membrane. These are collected in a separate series of hoses and flushed down the drain.

What About the Waste Water?

Opponents of RO treatment units argue that they create more waste water than drinkable water. Some of this waste can be minimized by:

  • reusing the waste water in a dual-membrane system
  • mixing with equal amounts of tap water and using the waste to water plants, wash the car, do laundry or mop the floor

Where Else Is this Technology Used?

RO technology is used for more than treating drinking water in your home. It is also used in:


What Are the Pros and Cons of Having a Treatment System at Home?

Like any other water purification system, there are advantages and disadvantages to home RO treatment.


The US Environmental Protection Agency (EPA) lists RO as a “…best available technology (BAT) and Small System Compliance Technology (SSCT) for uranium, radium, gross alpha, and beta particles and photon emitters.”

It also has several other benefits.

  • Clean, soft water (salts, bacteria, viruses and harmful toxins like lead, arsenic and mercury are filtered out)
  • Longevity (systems are built to last 15-20 years)
  • Low energy use
  • Low maintenance
  • Ability to hook into refrigerator water line for cold filtered water and clear ice cubes
  • Choice between filtered or nonfiltered water depending on the intended use
  • Added peace of mind for residents who use well water or tainted tap water

Potential drawbacks

  • Cost
  • Drilling required
  • Space required
  • Low faucet pressure
  • Acidic water due to mineral removal
  • Water waste

The last two items on the list can be adjusted by using a wider hose leading from the storage tank to the faucet (3/8 inches versus 1/4 inches in diameter), and by adding a unit to reintroduce desired minerals.

Can You Buy Bottled RO Water?

Bottled RO water is available, but at a higher price. A gallon of bottled water costs several dollars, whereas water from a home RO treatment system costs only a few cents per gallon.

Can You Tailor the Mineral Content?

Standard reverse osmosis water has had the minerals removed along with the pathogens and other heavy metals. You can customize some units by adding a mineralizing component. This will add in minerals before the water reaches the faucet.

How Should You Choose an RO System for Your Home?

There are a few things to consider prior to deciding which water treatment system to purchase for your home.

Parts and maintenance

RO treatment systems for residential use usually include an RO unit (4 housings containing filters and a membrane) and a tank. Tanks are required because “typical RO systems for home use have slow purified water output.”

Tank capacities range from about 2 gallons to 20 gallons, but another measure you need to be aware of is the amount of water per day a unit can filter. This number can range from 50 to 100 gallons per day. You will need to estimate how much water your family typically uses each day to have a good idea of which unit will best meet your needs.

The filters need to be replaced yearly. It is very important that you replace them promptly to avoid damage to the system.

Over time the filters fill up with contaminants and lose their ability to keep chlorine from passing through. Chlorine will damage the membrane, which in turn lets harmful substances like lead and chromium 6 pass through to the faucet. Without proper maintenance, you could be drinking contaminated water without realizing it.

Choose between brand name or generic

One important consideration to make when you are purchasing an RO treatment system is whether to go with a proprietary (restrictive brand) or nonproprietary (flexible brand) unit. 

Proprietary units will require that only brand name parts (and usually professional maintenance) are used. Nonproprietary systems allow for interchangeable and less expensive parts as well as easy, diy maintenance.

Look for certifications

Make sure that the system you choose is certified by either the NSF International (formerly National Sanitation Foundation) or the Water Quality Association.

Certification by these organizations is voluntary, but they have high standards for consumer products. 

Consider the cost

There is a variety of reverse osmosis water systems to choose from, each with a different price tag attached. Units usually start around $2,000. In general, the more components you add to your system, the more expensive it will be.

Do your research to find what works best for your home. It could be that having a system installed costs less over time when compared to purchasing bottled water year round.

Think about your available space

The smallest RO tank is a bit larger than a basketball. Paired with the RO unit itself, this could mean a significant loss of space under your kitchen sink. 

If your home has a basement, this problem could be avoided by having the RO system housed in the basement under your kitchen. Talk to your consultant about configuration options.

Plan for installation

Unless you choose a countertop or freestanding system, your kitchen will undergo some minor construction. 

  • A hole needs to be drilled through the countertop/sink for the extra faucet.
  • A valve will need to be attached to your cold water line.
  • Holes may need to be drilled through your kitchen floor into the basement.
  • Plumbing codes for your area will need to be followed.

Depending on your comfort level, you can do the installation yourself or you may have to spring for professional installation. This is especially true with proprietary systems.

Insist on a warranty

You will probably have your RO unit for many years. Whether a product comes with a warranty should be one of your main priorities. Lifetime warranties are the best option.

Seek out reviews

Because there are lots of reverse osmosis water systems to choose from, it is helpful to hear from real people who have used the filtration unit you are interested in. They may bring up issues or concerns you had not taken into account.

Having an RO system installed in your home is a big decision. The multi-layer filtration, low maintenance and system options make RO the right choice for my family.

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