What Is Reverse Osmosis And How Does It Work?

Reverse osmosis is an extremely effective way to filter water by removing contaminants. While it’s best known as a filtration system for houses, this process has a range of other uses, including desalination (taking out salt from sea water) and wastewater treatment.

Despite their widespread use, reverse osmosis filters are still confusing for a lot of people. That’s understandable given how complicated the process of revere osmosis can be. To help shed some light over it, here’s our quick breakdown of reverse osmosis and the filters that utilize it.

What Is Reverse Osmosis?

Reverse osmosis is simply a filtration method where water is pressured to pass through a semipermeable membrane. This lets water molecules through but stops various contaminants such as salt, sediment, fluoride and chlorine.

Unlike other filtration systems, however, the contaminants aren’t retained inside the filter but are instead left in the water that hasn’t yet passed through the filters. This causes an overabundance of contaminants in the pre-filtered water which is why this water is periodically thrown out.

This “wastewater” is also called brine and for every 3 or 4 gallons of clean water that goes through a reverse osmosis (RO) system, at least one gallon of brine needs to be thrown out. While this is undoubtedly a waste of water, the benefit of this is that the filters don’t need to be changed, or at the very least – not so often as other filters.

Reverse osmosis is so called because it’s quite literally the reverse process of osmosis. During osmosis, the same filters are used but the goal here is to pass the water in the opposite direction and to “even out” the contamination on both sides of the filter. This isn’t useful for homeowners and people who just want clean water to drink. This is why reverse osmosis achieves that goal.

Simply put, reverse osmosis does the work of your kidneys before the water enters your body. It removes a variety of contaminants from the water that could be odorless, tasteless and invisible but have the potential to cause disease and harm.

What Do Reverse Osmosis Systems Remove From Water?

Reverse osmosis excels at removing contaminants that a lot of filtration systems struggle to get rid of. Tap and well water tend to have a lot of common pollutants that can make the water unpleasant and unsafe to drink. Sometimes, the water may appear to look, taste and smell fine, but could be containing various impurities. RO systems are very efficient in getting rid of such pollutants. Here are some:

  • Salt
  • Sediment
  • Fluoride
  • Chlorine
  • VOCs (Volatile organic compounds)
  • Heavy metals like arsenic, lead and nickel
  • Pesticides and herbicides
  • Pharmaceuticals
  • Rust
  • Iron
  • Natural minerals  like calcium, magnesium and manganese

Some contaminants that RO systems do NOT remove are viruses and bacteria. That’s because a lot of them can actually pass through the filters of RO systems and can even multiply in them. Fortunately, city water treatment plants take care of any and all viruses and bacteria in drinkable water by disinfecting the water. So it’s not necessary for residential RO filtration systems to remove microorganisms.

However, if this is something you’re worried about, you could purchase an RO system that comes with UV light which is highly effective in destroying pathogens in the water. There are also many ways to safely remove microorganisms from water which you can do with a little research.

How Does A Reverse Osmosis System Work?

RO filtration systems can actually differ from one another based on what and how many filters they have, as well as on some auxiliary components. That’s why it’s important to know all parts of standard RO systems so that you can decide which ones you need in your future RO system.

Here are the main parts of an RO system:

  • An RO membrane: This is the main filter for reverse osmosis and any RO system will have it.
  • Sediment filters: These filters remove all sediment particles such as dirt, rust, and dust from the water.
  • Semi-permeable membranes: These filters remove around 98% of total dissolved solids (TDS) from the water.
  • Carbon filters: Used in other filtration systems as well, carbon filters excel at removing VOCs, as well as chlorine and other contaminants that usually leave unpleasant odors and taste in the water.

A good RO system should have all these filters at the very least. The way water passes through them is first 1) through the carbon and sediment filters for “prefiltration”, then 2) through the RO membrane for the actual reverse osmosis process, after that 3) through the semi-permeable membranes, and last – 4) through “post-filters” for a final filtration.

Because RO systems tend to be slow on demand, they typically come with a tank to store purified water in. After the filtration, the clean water is stored in a clean water tank and dispensed through a faucet. If your RO filter is a whole house system, the purified water will be dispensed frome very tap in your house, whereas if it’s just in the kitchen, you’ll have clean water to drink and/or cook with from a single outlet.

Pros and Cons of Reverse Osmosis

If you’re considering purchasing an RO filter for your home, here’s a look at the main advantages and disadvantages of this type of system.

Pros and Cons of Reverse Osmosis

Reverse Osmosis Pros:

  • Removes most contaminants: RO systems remove most contaminants in water, including heavy metals, TDS, pathogens, cysts, iron, chlorine, salt and fluoride. This results in water that’s pure and safe to drink minus most of the pollutants commonly found in our waters.
  • Cost effective: While it might seem like a big outlay at the start, once installed the RO system will continue to filter gallons upon gallons of water. Compared to buying pure bottled water, this is a cost effective system that adds up to a few pennies per gallon.
  • Easy to maintain: While it might sound complicated, an RO filter is quite simple with only a few parts overall. This means fewer repairs and minimal maintenance. The main ongoing cost is the replacement of filters which would need to be done every 6 to 12 months, depending on the type of filter, the quality of the feed water and your usage. Apart from this, an annual cleaning of the unit is recommended.
  • Easy to use: RO systems are typically easy to install, like a DIY project. Once installed, they are easy to use. In fact, there’s nothing for you to do apart from opening a tap.
  • Energy efficient: RO systems use no energy (unless they have a UV filtration stage) as they work with water pressure. Once installed, they continue to work on their own.

Reverse Osmosis Cons:

  • Removes essential minerals: One drawback of RO is that it also removes certain beneficial minerals from the water such as magnesium and calcium. These minerals are typically found in hard water and occur naturally. They make the water taste clean and crisp and contribute to your daily mineral intake, albeit very minimally. However, there are plenty of other ways to get those minerals into our body, via diet and supplement, so not having them in your water isn’t a health issue. Also, many sophisticated RO systems now include a remineralization stage which introduces the minerals back into the water.
  • Wastewater: While RO is energy efficient, it’s not always sustainable. This is because for every gallon of pure water it filters, at least 1 gallon of water is wasted. For many, this wastewater is a turn-off. HHowever, this water can be reused for activities like washing your car or using in the garden. Always check the wastewater first to determine whether it’s usuable for the activities you have in mind.
  • Replacements: This isn’t exclusive just to RO systems, but to any type of filtration you use in your home. All filters need replacing after a while, as the filters will stop working effectively. This is an ongoing cost and one you’ll need to commit to.
  • Slow filtration: Because RO systems work by pushing tap water through a semipermeable membrane, it takes time to filter water and store it in the tank. If you use up a lot of filtered water, you might have to wait while the filter does its job.  
  • Clogging: This is not common but is something to consider. Sometimes the filters can get clogged up with tiny impurities, which can slow down the filtration process and stop water from going through. This is why many quality RO filters come with a pre-filter, which removes large particles before they go into the system and cause issues.  

Reverse Osmosis Wastewater

The wastewater from RO systems is disposed of and the user typically has the option of what to do with it. You can either let it go down the drain or use it for landscaping or artificial lakes. While it’s not safe to drink because of the high amounts of contaminants in it, brine is usually perfectly safe for non-drinking purposes.

If you want to reduce the amount of wastewater, you can do that by getting a permeate pump for your RO system. However, first ensure that the system you have is designed to use such a pump. The role of a permeate pump is to regulate how much brine is produced during the process. It can lower the amount of wastewater from one fourth to about one fifth (from 25% to 20%) of the total amount of water used.

Additionally, you should also get an automatic shut off valve (an ASO valve) which stops the flow of water through the RO system once the storage tank is full. This won’t reduce the percentage of brine but will remove the risk of wasting both clean and wastewater down the drain.

Should You Get A Reverse Osmosis System For Your Home?

The answer to that depends entirely on where you live and what the quality of the tap water in your home is like. While not every source of drinking water needs to pass through an RO filter, if you don’t trust your drinking water or your source of water is dubious, you could benefit from an RO system in your house.

To give you an example, the EPA recommends that the amount of TDS in water shouldn’t be more than 500 parts per million, but the drinking water in many states has much higher TDS content than that. Having a trustworthy filter means you can get rid of these impurities and have a safe glass of water to drink. So, before you decide whether you want an RO filtration system or not, it’s best to research the water quality in your state and city first.

Reverse osmosis is useful mostly for drinking water, but it’s also a good filter option if you have hard water in your home. Most RO filtration systems are adapted to work under the sink, as countertop filters or in your refrigerator. However, there are also whole house RO systems that remove all the impurities from your water supply and sends clean water to all your water outlets.

Overall, RO systems are perfect if you want pure drinking water. They’re well worth the investment and effort to install them and once installed, they do the job for years.

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