*2.1.3. Process*

**2. Methods of disinfections**

4 Photocatalysts - Applications and Attributes

Chlorine is a greenish-yellow gas. By providing high pressure, the gas becomes liquid. It is toxic. Chlorine gas is mostly used as a water disinfectant. Introducing chlorine to water plays a very effective role for removing almost all pathogenic microorganisms. It can be used both as a primary and a secondary disinfectant. The gas is not applicable to be used in household system as it is very dangerous. It is lethal at concentrations as low as 0.1% air

• Chlorination is a cheaper source than UV or ozone disinfection methods used to treat water.

• The chlorine residuals left in the wastewater effluent can make the disinfection process longer even after initial treatment. They can be further used to evaluate the effectiveness [2].

Although chlorine gas is used in large-scale water distribution treatment plants and networks as a best method for treating water, still it have various limitations. These limitations might affect the applicability to a point of use (POU) treatment system. Objections against chlorina-

Regarding esthetic level, chlorination might be rejected as it imparts bad tastes and odors to the water. The developed countries might teach their people about the good impacts of

Limitations in using chlorine gas in a household context might include the distribution, procurement/manufacturing, dosing of chlorine, and accurate handling. The health hazards caused by chlorine are not only confined to its volatile nature. A great concern might be the byproducts and incompletely oxidized compounds present in chlorinated water that increases its toxicity. The most notorious byproducts of chlorination are chloro-organics and trihalomethane (THMs). Humic and fulvic acids are present in the water. When chlorine reacts with these acids, trihalomethane are formed. It has been identified in many studies that some of these chloro-organics are mutagens, toxins, or carcinogens. The well-known THM chloroform is an animal carcinogen. Some guidelines have been set by USEPA (United States Environmental Protection Agency) that THMs should not be greater than 0.10 mg/l. The high concentrations of THMs will lead to health

• It is very effective against a wide range of pathogenic microorganisms.

tion are because of the esthetic, logistic, and health-related concerns.

chlorination; however, less-developed countries lack this ability.

• Dosing rates are controlled easily as they are flexible.

**2.1. Chlorine Gas**

by volume [1].

*2.1.1. Advantages*

*2.1.2. Limitations*

complications [1].

Chlorine readily combines with all the water components, i.e., chemicals, small animals, microorganisms, plant material, odors, colors, and tastes. Sufficient quantity of chlorine is necessary to meet with the demand of chlorine in the water and provide residual disinfection.

Residual (free) chlorine is termed as the one that does not combine with other water components. The point at which free chlorine is available for continuous disinfection is termed as the breakpoint. The system at which free chlorine is supplied at a concentration of 0.3–0.5 mg/l is an ideal system. Simple test kits, most commonly the DPD (N,N-diethyl-p-phenylenediamine) colorimetric test kits are available for testing breakpoint and chlorine residual in private systems. The kit must test the amount of free chlorine, not total chlorine [3].
