How Does Reverse Osmosis Work? [Effective But Flawed]

Written by: James Smith

Updated: August 18, 2023

Reverse osmosis water filter

Reverse osmosis water filtration is an excellent choice for your home. However, there are a few drawbacks you should be aware of.

In this guide, I'll delve deep into the inner workings of reverse osmosis, its advantages and disadvantages, and what you should consider when opting for an RO system for your household.

So, join me on this journey through the fascinating world of reverse osmosis, and let's see if it can revolutionize your water quality!

What is Reverse Osmosis? A Quick Introduction

Reverse osmosis is an innovative water purification process that relies on a semipermeable membrane to eliminate contaminants from water.

By exerting pressure, water molecules are pushed through the membrane, leaving undesirable substances behind. 

Consequently, the water you obtain is cleaner and devoid of numerous common pollutants. It works just like a coffee filter, just much more effectively.

Basically anything bigger than the pore size is left behind, allowing clean water molecules to continue unobstructed.

Reverse osmosis membranediagram

The Reverse Osmosis Process Explained

Now that we have a basic understanding of reverse osmosis (RO), let's explore the process in greater depth.

A standard household RO system has 3 filtration stages. A sediment filter, a carbon filter, and a reverse osmosis membrane.

However, the units you'll commonly see advertised have 5, 6, or 7 filtration stages. I'll cover each and every filtration stage below.

Here's an outline of the general process:

  1. 1
    Initially, water undergoes pre-filtration, passing through a sediment filter that removes larger particles such as sand, silt, and debris. This protects the RO membrane and stops it form getting clogged up.
  2. 2
    Next a carbon filter (usually a carbon block) removes unpleasant tastes and odors while removing chlorine, chloramine, volatile organic compounds (VOCs) and other contaminants.
  3. 3
    A further carbon filter (either another carbon block or granular activated carbon like you'd find in a water filter pitcher) is usually implemented here to further refine the water.
  4. 4
    The heart of the reverse osmosis process takes place next, as water is forced through the RO membrane. During this phase, dissolved salts, heavy metals like lead and mercury), and  other contaminants like arsenic and nitrates are removed with an efficiency of up to 99%. 
  5. 5
    Afterward, the purified water goes through a final carbon filter, which polishes its taste and odor. This filter is usually made from coconut shell.
  6. 6
    In some cases, an additional remineralization stage is included to restore minerals, such as calcium and magnesium, that may have been removed during the reverse osmosis process. It gives the water that 'mineral' taste that makes it seem more natural.

Additional carbon filters may enhance the filtration removal and pH filters (usually containing calcium carbonate) can be used to raise the pH.

In all honesty, the pH filters aren't particularly effective and extra carbon filters is probably overkill.

A key point to remember is that the RO membrane works better with higher water pressure, so you don't want too many filters in line before reaching this stage.

How reverse osmosis works

The Benefits of Reverse Osmosis Filtration

Improved Water Quality

One of the most significant advantages of reverse osmosis is the superior water quality it delivers. By effectively removing contaminants like lead, arsenic, nitrates, and even certain microbes, RO filtration provides you with safe, clean water for drinking and cooking.

RO removes up to 99% of water contaminants which makes it the most effective form of home water filtration.

Better Taste and Odor

The reverse osmosis process also has a positive impact on the taste and odor of your water. By removing chlorine and other impurities, RO filtration ensures that your water tastes fresh and free of any unpleasant smells.

It should be noted that it's the carbon stages of the system that do this though. You can get the same effect with other types of water filter that use carbon filtration.

Low Maintenance

Reverse osmosis systems are generally low-maintenance, requiring only periodic filter replacements to maintain optimal performance. This makes them a convenient and cost-effective choice for many households.

How to remineralize reverse osmosis water

The Drawbacks of Reverse Osmosis Filtration

RO water filtration is incredibly effective but there are a couple of big drawbacks that need to be mentioned.

Wastewater Production

This first drawback is a big one, and a serious blight on reverse osmosis water filters. If you live in an area where water can be scarce in the summer then it's a big reason not to buy one.

During the RO process, a portion of the water is not able to pass through the membrane and becomes a concentrated solution of contaminants, which is then flushed away. 

This wastewater, also known as brine, can be significant, with some systems producing up to 3-4 gallons of waste for every gallon of purified water. However, modern RO systems with better efficiency are available, which can help reduce the amount of wastewater generated.

Highly efficient RO systems, often referred to as "water-saving" or "zero waste" systems, have significantly reduced wastewater production. These systems can achieve a 1:1 ratio of purified water to wastewater or even better, meaning that for every gallon of purified water produced, only one gallon of wastewater is generated.

However, it's important to note that actual performance can depend on factors such as water pressure, temperature, and the quality of the feed water. Also, a 1:1 ratio is still not very good and is still very wasteful!

Slow Filtration Rate

Reverse osmosis filtration is a slow process due to the high pressure required to push water through the membrane. This results in a lower flow rate compared to other filtration methods.

To compensate for this, RO systems typically include a storage tank to hold the purified water, ensuring that there is always a supply available when needed. However, the slow filtration rate can be a drawback for those who require a large volume of purified water quickly.

You'll find that RO systems are classified according to the number of filter stages but also the amount of water they can filter in a day (the flow rate). You need to pay special attention to this number (for example 50 gallons per day or 90 gallons per day).

You also need to pay attention to the size of the water tank. This is the amount of water that can be dispensed at one time. Four gallons is a typical standard size, which is quite a lot of water. But, these can be easily upgraded too.

Removal of Beneficial Minerals

During the filtration process, reverse osmosis systems effectively remove contaminants from water, but they also remove some beneficial minerals, such as calcium and magnesium.

These minerals contribute to water's taste and provide nutritional benefits. Some RO systems include a remineralization stage to add these essential minerals back into the water, but this feature may not be present in all systems, making it a potential drawback for those seeking both purified and mineral-rich water.

A common complaint about RO water (that which hasn't had minerals re-added) is that it just doesn't taste right. It lacks a bit of natural sweetness and texture that minerals provide.

Initial Investment and Maintenance Costs

Reverse osmosis systems can be more expensive than other water filtration options in terms of both initial investment and ongoing maintenance costs.

RO systems require periodic filter and membrane replacements, which can be quite costly if you have seven filter stages and you use a lot of water. Filter lifespans vary according to manufacturer, type, amount of use, and also water quality.

Generally, pre-filters and post-filters should be replaced every 6-12 months, while the RO membrane typically lasts 2-3 years.

It should be noted that these expenses may be offset by the long-term benefits of improved water quality and the potential savings on bottled water.

Space Requirements

RO systems can be relatively large, particularly when considering the need for a storage tank to hold the purified water. If you're talking something like six filtration stages with a four gallon then you need considerable kitchen space.

As a result, they may not be suitable for households with limited space under the sink or in other installation areas. For individuals with space constraints, a more compact filtration system may be a more appropriate choice.

Choosing the Right Reverse Osmosis System

When selecting an RO system for your home, consider factors such as your water quality, available space, and budget. It's essential to test your water first to identify the specific contaminants that need to be addressed.

This will help you choose a system that effectively tackles your water quality issues. Additionally, consider the size of the RO system and ensure that you have enough space for installation and maintenance.

Reverse osmosis water filtration offers a powerful and efficient solution for improving your water quality but it's not without its faults. I hope I've given you a balanced viewpoint on how reverse osmosis really works so that you can make an informed choice for your home.

If you've still got questions about reverse osmosis, please leave a comment below and I'll get back to you promptly.

About the Author James Smith

James is the chief water geek here at His mission is to empower the consumer and allow people to take control of their health. His passion for water health is contagious, hopefully unlike your tap water!

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