Why Is My Reverse Osmosis Water Green?

Why Is My Reverse Osmosis Water Green?

Reverse Osmosis (RO) filters have made it possible for homeowners to obtain safer and better quality water regardless of the source of their domestic supply. However, the drawback is that RO systems are complex and sometimes things can go wrong, such as having green water come out of the faucet.

There are three main reasons why your RO water is green. The most likely reason is due to algae growth in the housing of the filter or in clear lines. The second reason for bluish-green water could be due to the corrosion of copper in your home’s plumbing. Lastly, greenish water is also a common occurrence after replacing an RO membrane.

Luckily, there are easy fixes to all of these possible scenarios, which we will discuss, at length, in this article.

Read my comprehensive article about reverse osmosis.

Reasons For Green Reverse Osmosis Water – How to Fix Them

RO purification systems are by far one of the most effective ways of purifying water and eliminating nearly all contaminants. This is why it is uncommon to find any discoloration or bad-tasting RO water.

If your RO water seems to have a tinge of green, it is most likely due to the three reasons mentioned above. Let’s look at these reasons in more detail and explore their possible solutions:

1. Algae growth

Algae or cyanobacteria are found in almost all types of water systems. This type of bacteria is essentially microscopic organisms that are naturally present in freshwater found in lakes, reservoirs, rivers, and your domestic water supply.

Reverse osmosis filters are extremely effective at eliminating algae and all other types of bacteria. However, these filters are also extremely effective at removing chlorine, the natural deterrent to algae growth.

As a result, it is possible for algae to find a way to grow or bloom inside your RO unit if the conditions are just right.

In order for algae to form it requires sunlight. If your reverse osmosis unit is in a place where it is exposed to the sun, you must ensure that the filter housings are not clear or transparent.

A clear filter would expose water to sunlight which can result in algae forming on the inner surface of the filter housing causing it to bypass the filter and give water that greenish tinge.

If your filters are colored, check your lines or tubing. If you have clear lines, it is possible for algae to grow within them especially when exposed to sunlight. If you see algae growing in the clear lines connecting to the faucet, it is highly likely that this is the source of the issue.

The solution:

To fix or stop algae growth, you need to make sure that your RO unit isn’t exposed to sunlight. If that isn’t a possibility, make sure you don’t use clear housing filters or clear lines/tubing.

It is also recommended to clean the housings with bleach or light washing detergent during maintenance or when changing filters, just to ensure that no algae are allowed to grow inside.

2. Corrosion of copper pipes and fixtures

If you find your RO water to have a bluish/greenish tinge, and you are positive it isn’t because of algae, it is likely the result of copper corrosion within your home’s plumbing.

The corrosion of copper can result in the release of heavy metals such as iron and magnesium in your water.

Although reverse osmosis systems are designed to filter out these metals, large quantities of heavy metals present in water can clog piping and lead to the growth of specific types of bacteria which can give water that distinct green coloring.

Read my article about reverse osmosis water and corrosion.

The solution:

The first thing to do would be to conduct a water test. If you find that there is a case of extremely high levels of heavy metals present in your RO water this is confirmation that your household plumbing is compromised and requires replacement.

3. Reverse osmosis membrane replacement

If you find your reverse osmosis water to be green after replacing an RO membrane, this is to be expected at least for the first 20 gallons or so.

This is because most RO membranes come with food-grade preservatives, such as sodium metabisulfite, that prevent microbial contamination. These preservatives get mixed in with water passing through the membrane discoloring the water and giving it that greenish-yellow color.

Read my article on replacing your reverse osmosis membrane.

The solution:

After replacing an RO membrane it is recommended to flush out at least 20 gallons of water. In some cases, the instructions might even suggest running the filter for at least 24 hours to ensure all preservatives are flushed out and your water is clear and as pure as possible.


How do I get rid of algae in my reverse osmosis filter?

The best way to eliminate algae growth in RO filters is to disassemble the filter by separating the housing from the filter cartridges. Using a soft detergent or bleach thoroughly clean the filter housing/cover to eliminate all algae spores. You can also clean the cartridges by washing them under running tap water. Make sure not to use any detergent, soap, or bleach when cleaning the cartridges. Just water.

How can algae grow in purified reverse osmosis water?

It is hard to comprehend just how algae can grow in water that is 99% free of all chemicals, metals, solids, toxins, and other contaminants. However, it does happen. Some types of algae only require sunlight to grow, even when no other impurities are present.

Is algae detrimental for health?

Water that has a greenish color, as a result of algae growth, may not be suitable for drinking, or for agricultural or recreational use. Water with high levels of algae can be bad for the skin and cause gastroenteritis resulting in headaches, vomits, diarrhea, and even high fever.

Final Thoughts

The good news is that even if your RO water seems to have a green color, it can only be due to these three possible reasons.

If for some reason, even after trying the above, your RO water is still green, call in a water treatment expert to take a look. They are best qualified to figure out the problem and offer a permanent solution.

Chief Guru

Richard Boch is a chemical engineer responsible for designing water filtration systems for industrial and residential customers. He has more than 20 years of experience with ion exchange, activated carbon, and reverse osmosis. Richard's expertise has made him a go-to source for municipalities and businesses looking to improve their water quality. When he's not working, Richard enjoys spending time with his wife and two young children. You can also follow him on LinkedIn, Twitter and Facebook.

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