Chlorine as a disinfectant

List several alternative chemicals that compete with chlorine as a disinfectant. 

Water chlorination is carried out by using both free and combined residuals. The latter involves chlorine application to produce chloramine with natural or added ammonia. Anhydrous ammonia is used if insufficient natural ammonia is present in the wastewater. Although the combined residual is less effective than free chlorine as a disinfectant, its most common application is as a post-treatment following free residual chlorination to provide initial disinfection. 

The first use of chlorine as a disinfectant in America was in New Jersey in the year 1908 (Leal, 1909). At that time George A. Johnson and John L. Leal chlorinated the water supply of Jersey City, NJ. 

Reactions with inorganics. Reducing substances that could be present in the raw water and raw wastewater and treated water and treated wastewater are ferrous, manganous, nitrites, and hydrogen sulfide. Thus, these are the major substances that can interfere with the effectiveness of chlorine as a disinfectant. The interfering reactions are written as follows with ferrous ... 

Bromine is sometimes used in place of chlorine as a disinfectant in swimming pools. If the effectiveness of a chemical as a disinfectant depends solely on its strength as an oxidizing agent, do you expect bromine to be better or worse than chlorine as a disinfectant, at a given concentration ... 

Chlorine dioxide is a strong oxidizing agent, bactericide, and antiseptic. It is used in bleaching cellulose, paper pulp, leather, flour, fats and oils, textiles, and beestpwax, and in deodorizing and purifying water. It is currently considered as an alternative to chlorine, as a disinfectant for public water supplies in the United States. It is also used in the manufacture of many chlorite salts. 

Before leaving this sub-section on chlorine as a disinfectant, we should note that the majority of the Asian cities still do use chlorine to disinfect drinking water. The benefits in term of protection from water-borne diseases in Asia far overweigh the possible hazard from, for example, 10 or 20 ppb of trihalomethanes. Disinfection with chlorine may possibly pose a cancer risk in old age no disinfection in the Asian region does mean a substantial chance of dying from typhoid fever or cholera as a child or young adult. However, as we have seen in Table 1, there are different methods of disinfection and we consider some of these alternatives to chlorination. 

Disinfection of water supplies commonly uses oxidizing agents such as chlorine or ozone to kill microbial pathogens (bacteria and viruses). Chlorine gas forms chloric(i), HOCl, acid in water. The chlorate (i) ions, CIO, formed in water are responsible for its germicidal properties. Use of chlorine as a disinfectant is of concern due to its ability to oxidize other species, forming harmful by-products (for example, trichloromethane, CHCI3). 

A major problem with the use of chlorine as a disinfection agent is by-product production from the reaction of chlorine or bromine generated from chlorine with organics in water. The most common such by-products are the trihalomethanes including chloroform, HCCI3, and dibromochlo-romethane, HCBr2Cl, noted as water pollutants in Chapter 4, Section 4.13. Humic substances are common precursors to chlorinated by-products their removal before water chlorination prevents the formation of organochlorine by-products. 

Monochloramine is approximately 200 times less effective than free chlorine as a disinfectant. However, it is still used as an alternative to chlorine because chloramines do not react as readily with organic materials to form trihalomethanes (THMs). In theory, due to the less aggressive nature of chloramines, the tolerance of polyamide composite membranes to chloramine is about 300,000 ppm-hrs. However, chloramines are usually in equilibrium with free chlorine, making it difficult to use chloramine in RO pretreatment, as the free chlorine will degrade polyamide composite membranes. 

A limit of 100 pg/1 of THM in potable water is being proposed in the USA this could be achived by replacing chlorine as a disinfectant for water by other agents, by a radical change in chlorination practices, or by adsorption of THM after chlorination by active carbon or synthetic resins. If the latter procedure is adopted, the spent carbon or resin itself then becomes a waste to be treated. 

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