What Is the Hardness Level of RO Water in PPM?
Written by: Gene Fitzgerald // Last Updated: May 8, 2023
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One of the benefits of reverse osmosis is that it can soften water on top of removing various contaminants from it.
While using a dedicated water softener might be better in some cases, you should still not underestimate the effectiveness of reverse osmosis for that purpose.
However, what exactly is the hardness level of reverse osmosis water? Let’s take a look.
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Key Takeaways
- The hardness of reverse osmosis water depends on the hardness of the input water.
- Generally, RO reduces hardness by about 95 to 99%. So if the hardness of your feed water is 500 ppm, then you can expect the hardness of your RO purified water to range somewhere between 5 and 25 ppm.
What Is the Hardness Level of RO Water in PPM?
So, what is the hardness level of RO water in ppm (parts per million)?
Well, there is no way to define the actual hardness level of reverse osmosis water. That’s because it’s relative to the hardness of the feedwater. The harder your input water is, the harder the filtered water will be.
Generally speaking, reverse osmosis effectively removes over 95% of contaminants, including elements that make water harder. This means that the filtered water will also have its hardness reduced by roughly 95%. In some cases, the effectiveness of reverse osmosis can go up to as high as 99%.
Bottom line is, the hardness of your filtered water will be somewhere in the range of 1% to 5% of the hardness of the original water. So if we assume feed water of 500 ppm hardness, the RO purified water will read somewhere around 5 to 25 ppm.
What Is Water Hardness (Hard Water)?
Water hardness is a measure of the concentration of certain dissolved solids in your water. It’s mainly caused by calcium and magnesium compounds as well as a variety of metals.
The more of them there are, the harder your water is. You can sense hardness in the taste of water when you’re drinking it, too. Not only that, but hard water can leave residue on appliances and even your clothes when you’re washing them.
Using harder water means that you would have to treat appliances like your washing machine, your water kettle, and others more often to avoid scale buildups from accumulating
Hardness in PPM (Parts Per Million) vs GPG (Grains Per Gallon)
There are two main units of measurement used for indicating water hardness: Grains per gallon (gpg) and parts per million (ppm). A third unit is milligrams per liter mg/L which more or less equals parts per million.
PPM and gpg are directly convertible. You would often see water hardness measured in gpg. However, some might list it in ppm instead. The conversion rate is about 17.1:
1 gpg = 17.1 ppm
This can be useful when comparing systems made by different manufacturers who use different units to indicate the performance of their equipment.
The USGS defines these water hardness categories:
- 0-60 ppm (0-3.5 gpg) calcium carbonate = soft water
- 61-120 ppm (3.6-7 gpg) calcium carbonate = moderately hardwater
- 121-180 ppm (7.1-10.5 gpg) calcium carbonate = hard water
- 181+ ppm (10.6+ gpg) calcium carbonate = very hard water
Does That Mean RO Systems Soften Hard Water?
Yes, reverse osmosis systems do soften hard water. While that is not the direct intention of those systems, it’s a natural side effect of how they operate. Reverse osmosis removes most impurities from your water at a high level of effectiveness. This includes compounds which contribute to water hardness.
Does this mean that you can simply use a reverse osmosis system and skip using a water softener? Yes and no. It mainly depends on the quality of your water and the kinds of contaminants you’re dealing with specifically. If you’re primarily concerned with softening your water, then you might want to still get a dedicated water softener. You can then run the softened water through a reverse osmosis system to further purify it. This will also get rid of the extra sodium that gets added by the softening system.
It’s also important to consider that water that’s too hard can quickly lead to a drop in the performance of your reverse osmosis system. All those particles will end up clogging your RO membrane in no time. With that in mind, using a combined approach is often the best idea.
Further Reading:
What Is the Maximum Hardness Level a Reverse Osmosis System Can Handle?
A reverse osmosis system works best with water with at most 10 grains per gallon hardness. Anything more than that will still work, but your reverse osmosis system will get clogged up very fast. This will lead to increased maintenance requirements and higher long-term running costs.
10 gpg is not a universal value, though. Some reverse osmosis systems might be able to handle harder water. Always check the manufacturer’s guidelines in this regard to ensure that you’re staying on the safe side. You might sometimes have to add additional filtration stages to ensure that everything runs smoothly.
How Do You Know How Hard Your RO Water Is?
It’s easy to test your water for hardness with simple household supplies. All you need is a jar or bottle as well as some regular dish soap. Fill the bottle with water halfway through and add a few drops of dish soap. Shake the mixture well and let it sit for a few minutes. Observe how the foam behaves.
If you see a lot of foam forming with a relatively clear layer of water beneath it, this indicates that your water is likely pretty soft. On the other hand, if foaming is inconsistent or you see working as directly beneath it, this points to hard water.
If you want a more precise answer, you can purchase a hardness test kit from most hardware stores. This will give you a more in-depth overview of your situation. As an extra benefit, some of these testing kits can also identify the presence of other contaminants in your water.
If you have any questions about the hardness of RO water in ppm please don’t hesitate to leave a comment below!
About the Author Gene Fitzgerald
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