Understanding the dechlorination process is a complex task and requires an understanding of several other related concepts such as what chlorination is or what it is used for. In addition, the importance of defining in more detail what chlorine is and what its derivatives are should not be overlooked.

What Is Water Chlorination

Water chlorination is a chemical disinfection technique that deactivates micro-organisms that can be dangerous to health. To chlorinate water, chlorine or one of its derivatives is added to the water. When added to water, the oxidizing power of chlorine attacks the DNA of microorganisms to deactivate them and prevent them from reproducing. In some cases, a filtration step must be added to remove dead bodies from the water.

Chlorine can be added to water in any form: liquid, gaseous or solid. Typically, the most common chlorination techniques are the addition of liquid chlorine and the addition of a chlorine derivative, chloramine.

What Is Water Dechlorination

Dechlorination is a process used to remove chlorine residues from a chlorination stage. Whether it is for the purpose of extracting free chlorine, total chlorine, combined chlorine or any other type of chlorine derivatives, dechlorination techniques generally make it possible to drastically reduce the presence of these chemical compounds that can affect the water.

Traces of chlorine can come from your installations when you disinfect water or equipment. They can also come from your water supply. In fact, if the municipal water treatment plant to which you are connected use chlorine or a derivative, residues may be found.

There are several reasons for adding a dechlorination step. One reason is to reduce the taste and odour caused by the presence of chlorine.

• For example, the strong odour of municipal swimming pools and the particular taste of the water are caused by a high concentration of chlorine to continuously disinfect the water.

In addition to affecting the aesthetic characteristics of the water, the presence of chlorine in the water can impact the integrity of the water treatment equipment. Indeed, the oxidizing power of chlorine can accelerate the wear of the systems. In addition to premature system wear, chlorine residues will have a negative impact on filtration membranes (microfiltration, ultrafiltration, nanofiltration & reverse osmosis).

Chlorine and its derivatives

To begin with, no matter what type of chlorine is added to the water, when it is added to the water, it falls into three categories: free chlorine, combined chlorine and total chlorine.

What is chlorine?

Chlorine, symbol Cl, is a chemical element in the halogen group that is formed from two covalently bonded chlorine atoms with a valence of 7. In its natural state, chlorine is a yellowish gas that is toxic and corrosive. Thanks to technological advances, chlorine is now available in three forms: gaseous, liquid and solid.

Free Chlorine

This is the amount of chlorine ions available to attack contaminants in water. In other words, free chlorine is defined as the amount of chlorine available to disinfect water.

This means that the presence or absence of free chlorine means the ability or inability of chlorine to perform disinfection. This is why it is this parameter that must be monitored during chlorination.

Combined Chlorine

Combined chlorine is defined as chlorine ions that have combined with the various contaminants in the water. This means that the combined chlorine has already used its oxidizing capacity to remove contaminants.

This is why combined chlorine occurs during water disinfection as the chlorine ions combine.

Total Chlorine

Simply put, total chlorine is the sum of the free chlorine and the combined chlorine in the water. If you know the amount of free and combined chlorine in your water, here is the formula to calculate the amount of total chlorine in your water

• Free Chlorine + Combined Chlorine = Total Chlorine.

There are many technologies for identifying chlorine, but the least expensive and therefore the most common are those that allow you to know the total amount of chlorine in the water. On the other hand, although it is relatively easy and inexpensive to identify the amount of total chlorine present in water, it is sometimes useless to know. Without information on the concentrations of free chlorine and combined chlorine, it is virtually useless to know how much total chlorine is in the water, since it is impossible to know if the chlorine present is capable of treating the contaminants in the water or if it has already been used.

Chlorine Dioxide

Chlorine dioxide (ClO2) is a synthetic gas that results from the amalgamation of chlorine and oxygen atoms. When added to water, ClO2 is a free radical that reacts strongly with reducing agents and dissociates into chlorine gas, oxygen gas and heat. When it breaks down, chlorine dioxide produces chloride (Cl), chlorite (ClO) and chlorate (ClO3).

When added to water, chlorine dioxide does not hydrolyze, which means that it remains as a dissolved gas in solution. Furthermore, compared to chlorine, chlorine dioxide has a solubility about 10 times higher.

Chloramine

Also known as monochloramine and with the formula NH2CL, chloramine is an oxidizing agent most often used in dilute solution. NH2CL is used in dilute solution since in concentrated form it is highly unstable and even more unstable as a pure liquid. As an example, NH2Cl decomposes violently at temperatures above -40°C. 

Chloramines are made by adding ammonia to water containing free chlorine. The free chlorine can be in the form of HOCl or OCl, depending on the pH of the solution. For the reaction to be ideal, the water must be alkaline and have a pH of 8.4. The importance of the pH of the solution is explained by the fact that its value will determine the type of chloramine that will be formed. Indeed, three different types of inorganic chloramines can be formed by this mixture. Trichloramine which is usually formed when the pH is below 3. Dichloramine will form when the pH is between 3 and 7. Finally, monochloramine, the desired result, is formed when the pH is above 7.

More technically, the synthesis is performed in dilute solution, as the reaction occurs when HClO is attacked by the nucleophile NH3. This synthesis will allow generating monochloramine in solution for use in water treatment. For information, the preparation of the pure compound is made by contacting fluoramine with calcium chloride: NH2F + CaCl2 -> NH2Cl + CaCIF.

Compared to chlorine, chloramine has two advantages. Firstly, water treated with chloramine have a more pleasant smell and taste than water treated with chlorine. However, the main advantage of chloramine is that it is more stable than chlorine. This means that the treatment is much longer-lasting, and therefore less likely to recontaminate.

Chlorine Extraction Techniques

Now that we understand a little more about how the chlorination steps and the different types of chlorine work, let's look at the technologies for dechlorination. Typically, the most common technologies for chlorine removal are activated carbon, sodium metabisulfite, UV irradiation or in some cases, the addition of hydrogen peroxide.

Activated Carbon

In order to understand how activated carbon removes chlorine and chloramine from water, it is important to understand that when either of these two products is added to water, hypochlorous acid (HOCl) is formed. This eventually dissolves to form hypochlorite ions (OCl-). Activated carbon dechlorinates the water through the oxidation potential of chlorine and its derivatives. Oxidation reactions occur on the surface of the carbon when water comes into contact with the carbon bed.

The oxidation reactions are as follows

• Coal + HOCL ->C*O + H + Cl-
• Coal + OCL- -> C*O + Cl-
  • The symbol "*" means the compound oxidized by the chlorine which then dissolved into hypochlorite ions.

This is why, over time, carbon beds become more efficient at removing chlorine as they capture organic matter. The capture of this organic matter promotes the formation of bacteria as the carbon bed becomes a fertile ground for the bacteria.

When charcoal is used for chloramine removal, the principle is essentially the same, but the chemical reaction is different. This is caused by the weaker oxidizing power of chloramine. As an aside, when chloramine is formed, several derivatives can be formed. The type of chloramine formed will change depending on the pH of the water. The vast majority of the time, it is monochloramine that is formed.

Finally, it should be noted that standard activated carbons are not particularly effective in removing chloramines. This is due to the fact that the oxidation potential of chloramine is much lower than that of chlorine. This is why it is advisable to use activated and catalytic carbons for the treatment of chloramines.

Sodium Metabisulfite

With the formula N2S2O5, sodium metabisulfite is an inorganic compound used in the water treatment community as a reducing agent. When added to water, SMBS produce sulfur dioxide and acts as an oxygen scavenger to remove dissolved oxygen from the water.

• Its oxygen scavenging characteristic means that its presence creates an anaerobic environment that promotes biological fouling of membranes if too much SMBS is added.

Typically, when sodium metabisulfite is used in water treatment, it is with the objective of directly reducing free chlorine and chloramines present in the water.

Depending on the type of contaminants to be removed, the chemical reaction caused by sodium metabisulfite varies.

When the removal of free chlorine is the objective, the following reaction will occur:

• Na2S2O5 + 3H2O + 2Cl2 -> NaHSO4 + 2HCl

If the objective is the extraction of chloramines, the following reaction will occur

• Na2S2O5 + 9H2O + 2NH3 + 6Cl2 -> 6NaHSO4 + 10HCl + 2NH4Cl

Since the reactions generated by the addition of sodium metabisulfite are practically instantaneous, it is necessary to ensure that the quantities added are sufficient for the entire extraction of the chlorine residues. Typically, the amounts required for dechlorination with SMBS look like this.

• 1.38 mg of sodium metabisulfite will remove 1.0 mg of chlorine
• 1.86 mg of sodium metabisulfite will remove 1.0 mg of chloramine

Since a known amount of SMBS will treat a known amount of chlorine or chloramine, it is understandable that in a water supply where the presence of these contaminants is variable, SMBS are not ideal since the amounts required for treatment can change significantly. Remember that too much sodium metabisulfite increases the chance of bioburden and too little will not remove all the chlorine or chloramine.

Ultraviolet Irradiation

Increasingly popular, the use of ultraviolet light for dechlorination is a technique that has the advantage of performing water disinfection at the same time, is chemical-free and requires very little maintenance. On the other hand, to carry out dechlorination effectively, a large amount of ultraviolet light is required and it can generate a lot of heat.

In short, when the UV rays come into contact with the HOCl and OCL molecules, a photolysis reaction occurs to dissociate the atoms forming the contaminating molecules.

Costs related to dechlorination technologies

Obviously, the costs associated with the purchase and use of different dechlorination techniques vary greatly depending on the different situations

• To be noted that the activated carbon compared here is standard carbon.

The annual costs related to the use of the different dechlorination techniques include the purchase of chemicals if necessary, the maintenance of the equipment, the cost of energy used to operate the systems and any other associated expenses.

A Dechlorination Technic for Every Situation

The three dechlorination techniques that have been discussed are the most common and effective. However, they are not the only ones; one can think of the addition of hydrogen peroxide. It is important to remember that dechlorination is not only useful for improving the aesthetic characteristics of the water, but it also has advantages in terms of equipment integrity, since the oxidizing power of chlorine derivatives can damage the various equipment.

In short, we hope that these explanations have helped you better understand the dechlorination process and the different technologies. If you have any questions, do not hesitate to write to us and we will be happy to answer your questions.