Water disinfection

What is the disinfection of water?

Drinking water contains pollutants that have high amounts of minerals and metals before entering the water supply network, as well as pathogenic microorganisms, most of which are destroyed in different ways.

Since different treatment methods do not have a great effect on the destruction of pathogenic bacteria in water, therefore different methods, equipment and materials are used in the chemical treatment stage to destroy these bacteria. It is noteworthy that the purpose of disinfecting water is to remove and destroy pathogenic bacteria, not all living bacteria in water.

The method of disinfection in water treatment:

Today, the term chlorination is synonymous with disinfection because chlorine is commonly used to disinfect water and wastewater (disinfection method in water treatment). But it should be kept in mind that although chlorine is considered as an antiseptic, it is not always used as an antiseptic. Disinfectants are used to destroy or inactivate pathogenic microorganisms (pathogens) such as bacteria, algae, viruses, etc.

Methods of water disinfection:

• Physical method, including ultraviolet and gamma rays
• Chemical method, through penetrating into the cell and disrupting cellular activity

Effective factors in the function of disinfectant:

Several variables, such as the type and amount of the desired disinfectant, have a complex effect on the disinfectant function. The type and concentration of microorganisms, contact time and water quality also affect the function of the disinfectant. Therefore, in order to determine the required amount of disinfectant, a laboratory study is the best in many cases, and this study is especially necessary for water because it is affected by the quality of water, and of course, the required amount must be such that the possible changes in quality Include water up to the point of consumption.

a) The effect of water quality on disinfectants:

The function of the disinfectant depends on the quality of the incoming water, including turbidity, organic matter, pH and water temperature. According to the conducted research, the particles that are the cause of cordite can protect microorganisms from the effect of disinfectants like a shield. Organic substances stick to the cell surface and in this way they can reduce the effect of disinfectants and, moreover, the combination of disinfectants and organic substances may result in compounds that have less or no germicidal properties. As a result of the use of disinfectant compounds such as iron, manganese, hydrogen sulfur, efficacy is reduced because they are easily oxidized. Disinfectants such as chlorine first react with such inorganic compounds and then participate in the removal of microorganisms. Therefore, the need for chlorine increases due to the presence of these compounds.

Does water pH affect the water disinfection?

The type of reaction can be affected by the water pH, so the pH is effective in killing bacteria. For example, hypochlorite acid, is the most effective form of chlorine for killing bacteria which is suitable for pH<7.

b) The effect of disinfectant on the quality of treated water:

Disinfectants such as chlorine and ozone can oxidize with organic substances that are the source of color and lead to the removal of water color. However, some compounds resulting from the combination of chlorine with humid substances and some organic substances with low molecular weight are suspected to be carcinogenic. Adding ozone to water produces aldehyde, which is not known to be safe.

Chlorine dioxide produces chlorate and chlorite. Therefore, the impact of these by-products on human health or the environment in general should not be overlooked. Based on observations, the work of filters improves due to the presence of chlorine in the water.

Types of water disinfectants:

Chlorine disinfectant:

Chlorination is the most common method of water disinfection. As a result of adding chlorine to hydrolyzed water, it produces hypocaloric acid and hydrochloric acid. The disinfection power of chlorine is influenced by its chemical type (OCI- or HOCI), pH, temperature, organic substances in water and other characteristics of water. In general, in the short term the antiseptic effect of hypocaloric acid is greater than that of hypochlorite in, but in the long term, both have the same effect.

Industrially, hypochlorite acid is called "free chlorine", and hypochlorite ion is collectively called "available free chlorine", while chlorine in the form of chloramine is called "combined available chlorine".

The difference between free chlorine and combined chlorine:

Combined chlorine is more stable than free chlorine, and this is their main difference, although the disinfecting properties of combined chlorine are weaker than those of free chlorine. This point is important for water that needs a lot of chlorine. For this type of water, some ammonia is actually injected into the water or wastewater so that part of the injected chlorine becomes stable chlorine.

Chlorine dioxide disinfectant:

Chlorine dioxide is another chlorine compound that is used for water disinfection. By using this substance, carcinogenic substances that may arise due to the use of other chlorine compounds, are not formed. If the pH of the alkaline environment is high, chlorine dioxide will have a greater disinfecting effect. In addition, as a result of the reaction with sulfide compounds, chlorine dioxide can remove them from water and also helps remove unpleasant odors and tastes from water. Production of chlorine dioxide using the following reaction:

2NaCIO2+CI2   2CIO2+2NaCI

How to use chlorine dioxide disinfectant?

Because there is a risk of explosion when transporting chlorine dioxide, its production, like ozone, must be done at the place of consumption. Although chlorine dioxide has all the good characteristics of chlorine, it works better than chlorine to control the smell and taste.

Disinfection of pool water:

Different types of chlorine are used to disinfect pool water and drinking water and in different forms such as sodium hypochlorite, calcium hypochlorite (solid chlorine), hypochlorous acid or chlorine gas and chlorine dioxide in the disinfection process. However, unlike the very high oxidizing power and removal of microbes, as well as its cheaper price compared to other chemical compounds, this disinfection method also has disadvantages.

Disadvantages of using chlorine in water disinfection:

• Since chlorine gas is poisonous, it causes severe poisoning and suffocation.
• As a result of chlorine contact with the skin, it will cause severe burning.
• None of the chlorine compounds must be used in a closed space with other chemicals because there is a high possibility of explosion in them.
• In case of excessive use of chlorine, harmful side effects and compounds will remain, which requires the use of different methods to reduce these effects as much as possible.