Can Tap Water Be Dechlorinated by Letting It Sit?

To ensure municipal tap water is safe, authorities frequently use chlorine or chloramine to disinfect it and remove harmful microorganisms. Although these chemicals are effective at purifying the water, they can leave an unpleasant taste and smell, and may cause problems when used for aquariums or delicate gardening applications. This raises an important question: Is it possible to dechlorinate tap water effectively by simply allowing it to sit?

Understanding Chlorine and Chloramine

Chlorine (Cl₂) has been the primary choice for water disinfection due to its potent ability to eradicate bacteria and viruses. Being a volatile compound, chlorine can evaporate quickly from water, which often improves the taste and smell of the water as its concentration decreases.

Chloramine (NH₂Cl), formed by combining chlorine with ammonia, presents a more stable alternative, allowing for longer-lasting water treatment as it evaporates much slower than chlorine. While chloramine offers the advantage of prolonged disinfection, its stability makes it more challenging to remove from water.

Dechlorination Techniques

1. Natural Evaporation

For chlorine, letting tap water sit out in an open container can be an effective dechlorination method. The chlorine, present as dissolved gas, naturally escapes into the air over time. Typically, it takes about 24 to 48 hours to remove chlorine completely, depending on factors such as the water’s initial chlorine concentration, the surface area exposed to air, and ambient temperature.

For chloramine, the process is markedly slower. Due to its stability, chloramine may require several days to weeks to decrease to insignificant levels by natural evaporation alone. This method is impractical for those requiring quicker dechlorination.

2. Accelerating Dechlorination

To speed up the removal of chlorine, we can increase the exposed surface area of the water to facilitate the escape of chlorine gas. This can be achieved by using shallow containers or employing a bubbler to agitate the water, which helps release more chlorine into the air.

For the removal of chloramine, chemical neutralisation is more effective. Adding compounds such as sodium thiosulfate to the water can break down chloramine quickly, a method favoured by aquarium hobbyists for making water safe for fish.

For those interested in the chemistry, sodium thiosulfate (Na₂S₂O₃) is commonly used to dechlorinate water because it effectively neutralises chlorine through a chemical reaction. The basic reactions that occur when sodium thiosulfate is used for dechlorination are as follows:

Reaction with Chlorine (Cl₂)

• Chemical equation: 2Na₂​S₂​O₃ ​+ Cl₂ ​→ 2Na₂​SO₄ ​+ S₂​Cl₂
• ​In this reaction, sodium thiosulfate reacts with chlorine, resulting in the formation of sodium sulfate (Na₂SO₄) and sulfur dichloride (S₂Cl₂).
• This reaction rapidly reduces the active chlorine levels in the water.

Reaction with Chloramine (NH₂Cl)

• Chemical equation: 4Na₂​S₂​O₃ ​+ NH₂​Cl + 5H₂​O → 4Na₂​SO₄​ + NH₄​Cl + H₂​SO₄
• ​In this reaction, sodium thiosulfate reacts with chloramine to form sodium sulfate, ammonium chloride (NH₄Cl), and sulfuric acid (H₂SO₄).
• This reaction not only neutralizes the chloramine but also results in the release of ammonia.

These reactions demonstrate why sodium thiosulfate is effective in removing both chlorine and chloramines from water, making it safer for aquariums and similar applications.

3. Alternative Dechlorination Methods

• Activated Carbon Filters – These are excellent at removing both chlorine and chloramine and also enhance the taste and odor of water.
• Water Conditioners – Readily available in pet stores, these products can instantly neutralise chlorine and chloramine, thus protecting aquatic life.
• Reverse Osmosis Systems – Though costly, these systems are highly effective at removing a wide spectrum of contaminants, including chlorine and chloramine.
• Distillation – By boiling water and collecting the resulting condensation, distillation produces nearly pure water. However, this method is energy-intensive and not suitable for large-scale use.

4. Boiling as a Dechlorination Method

To Remove Chlorine – Boiling tap water is highly effective for chlorine removal. The volatile nature of chlorine allows it to evaporate quickly at high temperatures. About 15 to 20 minutes of boiling should suffice to eliminate most of the chlorine.

To Remove Chloramine – While boiling also reduces chloramine levels, it does so less efficiently. Extensive boiling (over an hour) may be necessary to achieve significant chloramine reduction, which is less practical for large volumes of water.

In conclusion, while letting water stand can decrease chlorine levels, it does not effectively remove chloramines. For thorough treatment of both disinfectants, options such as activated carbon filtration or reverse osmosis systems may be considered, offering more immediate and comprehensive solutions. Understanding and managing the disinfectants in tap water is crucial to ensure its suitability for uses like drinking, gardening, or maintaining aquariums.

Methods for Testing Chlorine and Chloramine Levels

Testing water for chlorine and chloramine levels is essential for maintaining safe and healthy water quality in various settings, such as swimming pools, aquariums, and for municipal water supply management.

The common methods for measuring the levels of chlorine and chloramine in water are as follows:

1. Test Strips: These are simple, inexpensive strips that change color when dipped into water. They can measure free chlorine, total chlorine, and sometimes chloramine directly. They provide quick results but are less accurate than other methods.
2. Colorimetric Test Kits: These kits use a chemical reagent that reacts with chlorine or chloramine in the water, causing a color change. The intensity of the color is then compared to a color chart or measured with a photometer for more precise quantification. These kits can differentiate between free and total chlorine and may require separate reagents for chloramine.
3. Digital Testers: Electronic meters provide a digital readout of chlorine levels. Some advanced models can detect both free chlorine and total chlorine. They are more accurate than test strips and are easy to use but can be more expensive.
4. DPD (N,N-diethyl-p-phenylenediamine) Method: This is a specific test for both chlorine and chloramine. It involves adding a DPD reagent to the water, which reacts and forms a colored complex. The intensity of the color, measured using a photometer, corresponds to the concentration of chlorine or chloramine. This method can distinguish between free chlorine, combined chlorine (chloramine), and total chlorine.

How to Calculate Chloramine Levels in Water

To assess chloramine levels in water, it’s essential to measure both free chlorine and total chlorine. The difference between these two values indicates the level of combined chlorine, primarily chloramines:

Combined Chlorine (mostly Chloramines) = Total Chlorine – Free Chlorine

Understanding Chlorine Measurements

When testing for chlorine, there are three measurements that we need to be concerned with, because chlorine can be present in two states, both as free and bound chlorine. Depending of the test used, it may measure one or both separately, and also as a combination of the two, as described below.

• Total Chlorine measures all chlorine forms in the water, including free chlorine, combined chlorine (chloramines), and other bound chlorine compounds.
• Free Chlorine refers to the chlorine available for disinfecting water, which has not yet reacted with contaminants or ammonia.
• Combined Chlorine is calculated by subtracting the free chlorine from the total chlorine. This value largely consists of chloramines, formed when chlorine reacts with nitrogenous materials like ammonia.

In pool testing, it is essential to measure both free chlorine and total chlorine. For instance, a higher total chlorine reading compared to free chlorine suggests the presence of chloramines. Managing chloramine levels is crucial because high concentrations can lead to eye irritation and produce a potent chlorine smell, indicating reduced effectiveness in water sanitation..

Conducting the Tests

To measure these chlorine types, a test kit that can separately measure both total chlorine and free chlorine will be required. Many pool test kits and more advanced water testing kits include these capabilities.. Follow the kit instructions carefully, noting that the required reagents and timing may vary based on the type of chlorine being measured.

Using this method, we can estimate the level of chloramines in the water, assuming that other forms of combined chlorine are negligible, which is typically the case in municipal water treatment systems. Measuring both chlorine and chloramine levels is essential to assess the efficacy of our dechlorination processes.

References

• Water Disinfection with Chlorine and Chloramine, Public Water Systems, Drinking Water, Healthy Water, U.S. Department of Health & Human Services, Centers for Disease Control and Prevention (CDC). <https://www.cdc.gov/healthywater/drinking/public/water_disinfection.html&gt;
• Basic Information about Chloramines and Drinking Water Disinfection, US EPA. <https://www.epa.gov/dwreginfo/basic-information-about-chloramines-and-drinking-water-disinfection&gt;