Purifying drinking water with hypochlorous acid

Purifying drinking water with hypochlorous acid

Have you ever tried to describe the taste of water to someone? It’s not an easy task! However, it’s common to arrive in a new place and notice that the water tastes different from what you’re used to. In many cities, that distinctive "city water" taste could be due to high levels of chlorine necessary for safe disinfection. While the taste might be unpleasant, the results of drinking contaminated water are sure to be even less enjoyable!

Access to clean drinking water is a foundational aspect of human health and wellbeing. The majority of the drinking water in the world needs to undergo some form of disinfection process before it is suitable for regular consumption. While there are several methods for disinfecting water, chlorine is the most common and universally utilized method. Most municipal water systems use chlorine gas or hypochlorite tablets (either sodium or calcium) to purify their water supplies.

You can also use hypochlorous acid to purify water. Hypochlorous acid is the most available form of chlorine because it resides at a low pH and is therefore much more effective than bleach or hypochlorite. Add hypochlorous to drinking water in the same way that you would add hypochlorite. Naturally, the pH will rise to pH of the water, which will be around 7.5. This means that about half of the chlorine will be hypochlorous and half will be hypochlorite. This is basically the same outcome that would be achieved by traditional hypochlorite dosing.

The good news is that most drinking water purification protocols call for very little chlorine – usually between 1 and 3 parts per million. So, a little hypochlorous goes a long way. For example, if you made 1,000 ppm in the Hypo 7.5, that would be enough to dose 7,500 liters at 1 ppm.

Residual Chlorine

Because a large amount of the free available chlorine present in water is expected to be used up during the disinfection process, most public water agencies require a 'residual chlorine' level to be maintained. In the United States, the EPA requires that public drinking water contains no more than 4ppm residual chlorine levels. This is to help maintain water purity once it leaves the disinfection plant and enters the pipelines.

If you're using hypochlorous to disinfect water that's not for immediate use (eg; storage tanks), it's a good idea to start out by adding a higher chlorine concentration than the recommended base level, as some of the chlorine will be used up during disinfection and will also begin to decompose over time. This will ensure you have a safe residual chlorine level post-disinfection. Test the pH and ppm of your water regularly after disinfection to monitor the remaining FAC and re-administer hypochlorous if needed.

If you want more information about water purification with hypochlorous or any form of chlorine, we recommend checking out “White’s Handbook of Chlorination and Alternative Disinfectants”. It contains over 1,000 pages of great information and serves as the bible of water purification.

Photo by Karim Ghantous on Unsplash


UNHCR Emergency Handbook. (2020). Unhcr.org. https://emergency.unhcr.org/entry/32947/emergency-water-standard

Drinking Water Chlorination: A Review of Disinfection Practices and Issues. (2017). Water Quality and Health Council. https://waterandhealth.org/safe-drinking-water/wp/

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