Chlorine Chemicals Used for Water Disinfection

Chlorine chemicals are very effective against bacteria, viruses and fungi that contaminate water. Four types of chlorine chemicals are commonly used in agriculture: Sodium Hypochlorite, Calcium hypochlorite, Gaseous chlorine and Chlorine dioxide.

Sodium hypochlorite - NaOCl

Sodium hypochlorite is a yellowish liquid with an active chlorine concentration of 10-15% pH around 13.0. It is not very stable, and when it comes in contact with air, light or high temperatures, the chlorine evaporates and therefore its concentration in water decreases. The chemical reaction with water is:

NaOCl + H2O <--> HOCl + Na + OH-

Due to its high pH,  sodium hypochlorite increases water pH. As described in the previous article (chlorine disinfection), the reaction of sodium hypochlorite with water results in two forms: HOCl (hypochlorous acid) and OCl-.

The ratio between HOCl and OCl- depends on the pH. HOCl is a much more effective disinfectant than OCl (100 time more effective), and since this form is predominant in a pH range of 3.0-6.7, the treated water should be acidified.

It is possible to acidify the water using sulfuric acid, and it is recommended to maintain the pH in the range of 5.8-6.5 in order to ensure effective disinfection while keeping the pH in a favorable range for irrigation.

The amount of sodium added to water using sodium hypochlorite, generally does not significantly affect water quality, since disinfection is achieved at relatively low concentrations of chlorine (usually <10 ppm). The addition of sodium can be considerable in water reutilization systems, in which sodium concentrations in the water increase gradually.

Advantages - Easy transport and storage, highly effective when correctly used.

Disadvantages - Corrosive, precautions should be taken in handling, evaporates/disintegrates upon contact with air, light and high temperatures, Short shelf-life.

A sodium hypochlorite disinfection system

Calcium hypochlorite - Ca(ClO)2

Calcium hypochlorite is more stable than sodium hypochlorite, and contains a higher chlorine concentration (30-75%).The chemical reaction with water is:

Ca(OCl)2 + 2H2O <--> 2HOCl + Ca+2 + 2OH-

Like sodium hypochlorite, calcium hypochlorite also increases water pH, so it is advised to acidify the water during the disinfection process.

Calcium hypochlorite is available as white powder or tablets, therefore it should first be dissolved in water, and only then it is injected into the treated water. Calcium hypochlorite isn't highly soluble, so it dissolves better in soft or medium-hard water. It is recommended to dissolve it in warm water, in order to improve its solubility.

In any case, all forms of calcium hypochlorite contain insoluble residues that form sediments in the solution. It is important to eliminate the sediment before injecting the calcium hypochlorite solution into the treated water tank to avoid clogging.

Advantages - Easy transport and storage, more stable than sodium hypochlorite, does not add sodium to the water, highly effective when correctly used.

Disadvantages - low solubility, might cause clogging, the final chlorine concentration in water is dependent on the extent of dissolution achieved.

Gaseous Chlorine - Cl2

This is chlorine in its pure form, which is a very strong oxidizing and disinfecting agent. It is stored and transported in cylinders, as pressure-liquefied gas. The chemical reaction with water is:

Cl2(g) + H2O <--> HOCl + H+ + Cl-

Unlike sodium or calcium hypochlorite, gaseous chlorine decreases water pH.  

Advantages - highly effective disinfectant. Reduces water pH so additional acidification is not necessary, uniform and rapid dissolution in water.

Disadvantages - Requires skill and strictest precautions in handling.

Gaseous chlorine disinfection system (cylinders on the left)

Chlorine dioxide - ClO2

Chlorine dioxide is very different from the other forms of chlorine described above. It cannot be compressed or commercially stored, because it becomes volatile under pressure. Therefore it is always produced on site as part of the disinfection process.

Sodium chlorite and hypochlorous acid are stored in separate containers and are injected into a chlorine dioxide generator where they react and produce chlorine dioxide.

The concentrations of chlorine dioxide needed to achieve effective disinfection are lower than the concentrations needed when using other chlorine forms.

Advantages - A powerful oxidant, effectiveness of disinfection does not depend on pH or organic load in the water, no byproducts (such as chloramines) are formed, rapid and uniform dissolution in water.

Disadvantages - Requires skill, very strict precautions in handling.