Residual Chlorine: Why It’s Vital to Measure Your Water’s Chlorine Levels?

Chlorine is known to protect against pathogens. But how much protection can it provide? Does the volume of chlorine matter? What about the residual chlorine? What role does it play in the treatment of water? Unfortunately, most homeowners have never given these questions significant consideration, even though they can have drastic consequences for your health and safety.

Why Use Chlorine?

Do you know why people use chlorine? Service providers get their water from natural bodies. But those water bodies have pathogens that cause diseases. People disinfect water using methods such as sedimentation and coagulation. However, chlorination is one of the most popular because the process is efficient and relatively inexpensive.

When chlorine was first introduced to the world in 1744, people thought that the terrible odors in the water caused diseases. Chlorine was first added to water in 1835 to remove those foul odors. Decades later, scientists discovered it disinfects water as well.

Today, many governments use the chemical to combat bacteria and viruses. Although it cannot eliminate every single water-related threat (some microbes are immune to the effects of chlorine), people still use it because it is cost-effective and easy to deploy.

Why Residual Chlorine Matters?

The volume of chlorine in your drinking water is essential. Even though chlorine disinfects water, the substance is still dangerous, especially if you consume it in large quantities.

The Environmental Protection Agency expects service providers to keep chlorine levels below 4mg/L. At this level, chlorine in drinking water is less likely to attract adverse reactions.

The EPA has admitted that their figure is a broad estimate that doesn’t apply to every individual. For some sensitive people, 4mg/L of chlorine is too much. Additionally, that figure highlights how much chlorine drinking water can hold without harming the consumer.

It doesn’t say anything about the amount of chlorine required to disinfect the water. This is where terms like ‘Residual Chlorine’ enter the equation. To understand the meaning of ‘Residual Chlorine,’ you must first understand the transformations the substance undergoes when you add it to water.

When a service provider adds the substance to water not all of it dissolves. This is because the substance will first react with the water’s organisms and metals. ‘Chlorine Demand’ is the term people use to describe this process.

‘Total Chlorine’ is the chlorine that remains after this reaction. But again, the ‘Total Chlorine’ will not disinfect the water. A portion of this remaining chlorine will react with the nitrates. They call it ‘Combined Chlorine.’

The only chlorine left at the end is termed ‘Free Chlorine.’ It does the difficult work of attacking and neutralizing dangerous organisms in the water.

When you add chlorine to a water supply, you have to realize that only the ‘Free Chlorine’ neutralizes pathogens. The rest is wasted by the reactions mentioned above. Therefore, the volume of chlorine you add should take those reactions into account.

The makeup of the water is just as important. If the water is completely pure, it doesn’t have metals or nitrates to intercept the chlorine, which means that all the chlorine added is ‘Free Chlorine.’ However, if the water has pollutants and metals, only a fraction will become ‘free Chlorine.’ Therefore, you must add more chlorine to compensate.

What about the residual chlorine?

This is technically free chlorine. When you chlorinate water, most of the substance will either react with the contents of the water or neutralize the pathogens. The residual chlorine is what remains.

It stays in the water for an extended period after the initial application of chlorine. It neutralizes pathogens that invade the water later on. Without that residual chlorine, you cannot trust your water supply. So yes, the initial chlorination process disinfected the water, but bacteria can still invade it down the line.

The presence of residual chlorine tells you that your water supply has long-term protection.

Why is Testing Necessary?

Testing isn’t particularly relevant for water coming out of your tap. Service providers expect residual chlorine to persist in the water they supply to homeowners.

Testing of this kind matters if you collected the water in a separate container and stored it. It also matters to people in emergencies who rely on water reserves. The quantity of residual chlorine can paint a relatively accurate picture of the level of protection you can expect to encounter.

Tap water that doesn’t have residual chlorine is dangerous because bacteria and viruses that invade the water after collecting and storing it can make you sick.

How to Test For Chlorine?

You can use numerous methods to test for residual chlorine. Pool test kits are popular. They include a tube that you fill with water before adding several drops of othotolidine. If the water has total chlorine, it will turn yellow.

People use these kits to test for chlorine in swimming pools. They’re popular because of their low cost and ease of use.

If you need something more accurate, buy a color-wheel test. It’s the superior option if you want more accurate readings.

The method uses powders or tablets to create colors that match the quantity of chlorine in the water. Though, in this case, you have wheels that measure the intensity of the color change.

You can’t ask for a more accurate testing method. But, because user error can ruin the results, many people opt for digital colorimeters. You can use one too, if you don’t trust your judgment.

The device uses light to read the intensity of the color change.


The next time you consult a water and HVAC service provider, ask them about residual chlorine. If you don’t trust the purity of your tap water, they will recommend a suitable testing method.

They can also investigate your plumbing system for potential sources of toxins and pathogens. Residual chlorine is supposed to protect your water after it passes through the service provider’s treatment plant. In other words, water that leaves the treatment plant is vulnerable to any pollutants it encounters along the way.

If you can’t identify and eliminate those pollutants, you can install additional filtration systems in your home that can purify the water before it reaches your tap.