Does Reverse Osmosis Remove Blue-Green Algae?

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Reverse osmosis is well known for its thorough water purification capabilities.

It’s usually one of the first considerations of people who’ve just discovered that they’re dealing with cyanobacteria – also known as blue-green algae – in their water supply.

But is it really effective at removing those?

Key Takeaways

  • Reverse osmosis does remove blue-green algae and most of the cyanotoxins associated with them.

Does Reverse Osmosis Remove Blue-Green Algae from Water?

So, does reverse osmosis remove blue-green algae from water?

Yes, not only does reverse osmosis remove blue-green algae from water, but it’s also one of the most effective methods for accomplishing that.

And, reverse osmosis also removes cyanotoxins which are toxic substances released into water by blue-green algae. RO removes microcystins, cylindrospermopsin, and most likely anatoxins as well.

How Effective Is Reverse Osmosis at Removing Blue-Green Algae?

Reverse osmosis can remove blue-green algae and all the associated bacteria with an effectiveness of over 95%. In some cases, it can reach 99%, provided you have an adequate set of pre and post-filters, and that you take good care of your reverse osmosis system through frequent maintenance.

algae with bubbles in water

How Does Reverse Osmosis Remove Blue-Green Algae?

Reverse osmosis works by pushing water through a membrane lined with very fine pores at a high level of pressure. The main idea behind this setup is that the pores are so tiny that only water molecules can pass through them. Typically, pores of around 0.0001 microns in diameter must be used for optimal effects. All contaminants get rejected by the membrane and sent away with a separate wastewater stream. Unfortunately, this can waste a lot of water, but it also results in very thorough purification that effectively removes almost all impurities.

This includes blue-green algae. The bacteria are too large to pass through the pores of a reverse osmosis membrane, so they don’t make it to the other side.

Other Ways to Remove Blue-Green Algae from Water

If you’re looking for thorough water filtration that’s guaranteed to remove not only blue-green algae but most other types of contaminants as well, look no further than reverse osmosis. But if you’re primarily concerned about blue-green algae and want to focus on that, there are other options that can prove just as effective.

Microfiltration, Ultrafiltration, Nanofiltration

Microfiltration, ultrafiltration, and nanofiltration can all remove blue-green algae from water. Plus, nanofiltration can also capture extracellular microcystins which is one type of cyanotoxin.

Ozone, Chlorination, Potassium Permanganate

Other solutions against blue-green algae are ozonation, chlorination, and treatment with potassium permanganate. However, the processes don’t work against all types of cyanotoxins.

UV Radiation

UV radiation is generally a very effective method for removing various biological contaminants from water. This includes cyanobacteria. Just know that microcystins and cylindrospermopsin in particular may be relatively unaffected by UV light, at least in the doses provided by the typical household unit.


While it’s not exactly something that comes to most people’s minds when thinking of domestic water treatment, coagulation and sedimentation can also be effective treatment methods for removing blue-green algae and the toxic substances they produce.

The process requires a lot of care and attention to implement properly, which is why it’s not recommended for inexperienced users. It works by introducing a coagulating agent into the water, which mixes with contaminants and coagulates into a mass at the bottom of the container, which can then be removed.

What Is Blue-Green Algae?

One interesting fact about blue-green algae is that they’re not actually algae, at least not by the strict scientific definition of the word. Still, the name has stuck and is commonly used when referring to cyanobacteria.

These are bacteria commonly found in large bodies of water, responsible for the production of cyanotoxins. The bacteria go through cycles of growth referred to as “bloom periods”, during which they produce exceptionally large amounts of cyanotoxins and pose a particularly high risk to human health.


Cyanotoxins are very powerful toxins, extremely harmful to human health. In fact, some of the most dangerous poisons for humans are in this group. Various toxins fall in this category, including some neurotoxins and endotoxins. Some cyanotoxins produce immediate health effects, while others can lead to the development of long-term diseases, including even Parkinson’s and Alzheimer’s.

young woman drinking water

Are Blue-Green Algae Harmful?

Blue-green algae is just another word for cyanobacteria, which produce cyanotoxins. With that in mind, blue-green algae are extremely harmful and should be avoided at all costs. When it comes to your drinking water supply, you must ensure that you’re thoroughly purifying your water to eliminate all traces of blue-green algae. For the needs of the average household, a reverse osmosis system is typically the best choice.

It’s also important to remember that you don’t necessarily need to drink water contaminated with blue-green algae to become affected. Even taking a dip in a contaminated body of water can often be enough to result in severe health complications if you’re particularly unlucky.

How We Are Being Exposed: Algal Blooms

Algal blooms occur when a source of nutrients enters a body of water where blue-green algae is present, accelerating its growth. These blooms result in the release of large amounts of cyanotoxins in affected areas, and sometimes require special attention by responsible personnel.

In some parts of the world, algal blooms are a somewhat recurring event with a cyclic nature. Some algal blooms can result in an exponential growth of the affected algae population, sometimes multiplying by factors as high as one million. In some cases, an algal bloom can produce a permanent increase in the concentration of certain algae.

If you have any thoughts about the question, does RO remove blue-green algae from water, please don’t hesitate to leave a comment below!

About the Author Gene Fitzgerald

Gene Fitzgerald is one of the founders of BOS and currently head of content creation. She has 8+ years of experience as a water treatment specialist under her belt making her our senior scientist. Outside of BOS, Gene loves reading books on philosophy & social issues, making music, and hiking.
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