Of all the advancements in science and technology achieved, treatment of water can be described as one of the greatest. Adequate clean water is essential for the survival of humans and good public health. In the absence of disinfection and filtration, consumers would be at the risk of contracting diseases that are waterborne. Chlorination is the addition chlorine to water to disinfect it and kill germs. Chlorine is a strong oxidant that is often used in treatment either through oxidation and disinfection ("Water Treatability Database | US EPA"). The common contaminants removed through the process of chlorination include iron, manganese, hydrogen sulfide and bacteria. This paper will primarily focus on how chlorine get rid of contaminants from water but also a brief background of the whole process of chlorination.
History of Chlorination
Sweden is the place that chlorine was first discovered in 1744. During this time, people believed that the odor coming from water was spreading diseases. In 1835, chlorine was applied for the first time to treat odor in water. However, the use of chlorine as a disinfectant was discovered later in 1890. This was a huge milestone in reducing the prevalence of diseases transmitted through water. With this discovery, Great Britain was the first place where chlorination began and later extended to the U.S in 1908 and to Canada in 1917 (Hancock, 2017). Chlorination up to today is the most common method of disinfecting drinking water in both small and large scale.
Chlorination Process
Chlorination can be conducted at any particular point in the process of water treatment. There is no specific point designed for the addition of chlorine. Pre-chlorination is the application of chlorine to water immediately after it has made entry into the treatment facility. During this stage, chlorine is directly added to the untreated water making its way into the treatment plant. It can also be added to the flash mixer which evenly distributes chlorine into the incoming water. This addition of chlorine is to get rid of algae and other forms of aquatic life to ensure that they do not interfere in the later stages of treatment (Hrudey, 2009). The purpose of chlorine addition in the flash mixer is to get rid of odor and taste, inhibit the growth of biological life in the sedimentation tanks and filtration tools. The contaminants inform of manganese, iron and hydrogen sulfide that may be present in water are oxidized so that it becomes possible to remove them as sediments (Xie, 2016).
Disinfection by use of chlorine can also occur before the filtration process or after sedimentation. This stage aims to control biological growth, oxidize manganese and iron, eliminate odors and tastes and get rid of coloration in water. Chlorination may also take place in the final stage of water treatment. This is the most common stage where chlorine is added in water treatment. The primary aim of chlorination at this step is to disinfect the water and maintain chlorine residuals that will be left in the water as it goes towards the distribution system (Hrudey, 2009).
The process of chlorination constitutes addition of chlorine to water. However, the chlorinating products do not always have to be pure chlorine. The process can also be conducted using chlorine-containing substances. The use of these substances is dependent on the pH levels and the size of the storage facility. The three most common forms of chlorine used for chlorination process are chlorine gas, calcium hypochlorite and sodium hypochlorite (Hancock, 2017).
How Contaminants are Removed
Addition of chlorine into the water during the treatment process results in the formation of hypochlorous acid (HOCL) and hypochlorite ions (OCL-) (Hancock, 2017). These are the primary disinfecting compounds in chlorinated water.
Cl2 + H2O = HOCL + H+ + OCL-.
Hypochlorous is the most effective disinfecting compound. The intensity of the compound in the water depends on the pH level before the addition of chlorine. Hypochlorous acid is dominant when the pH level is low. When there is a high level of pH, there will be a complete dissociation of chlorine, and therefore the hypochlorite ions become dominant (Hancock, 2017).
Chlorine can react with a variety of substances; there is a possibility of numerous impurities in the raw water which may increase the demand for chlorine. Chlorine will initially react with inorganic impurities such as magnesium, ammonia and dissolved iron prior to a reaction with organic compounds such as bacteria, viruses and other biological growths (Xie, 2016). Iron, for example, reacts with hypochlorous acid resulting in transformation into an insoluble state. The precipitate can be removed from the water through a filtration process.
2 Fe2 (liquid) + HOCL + 5H2O -> 2Fe(OH)3 (solid) + 5H+ + CL-
If the water being treated contains hydrogen sulfide (H2S), hypochlorous acid can also react with it. Hydrogen sulfide is a contaminant in water that produces a foul smell. It is also a toxicant. It reacts with hypochlorous acid to give hydrochloric acid and sulfur ions (Hancock, 2017).
H2S + HOCL -> H+ + CL- + S + H2O
Another common contaminant in water is bromine which can result in t formation of harmful substances. The reaction between bromine ions and hypochlorous acid forms hypobromous acid. Hypobromous acid has a high reaction rate than hypochlorous acid and can also be a disinfectant. Either of the acids when in water reacts with available organic materials to form halogenated byproducts (Hancock, 2017).
Br- + HOCl -> HOBr + Cl-
The reactions in the disinfection process result to several different by-products. Those produced from the reaction between inorganic substances and chlorine are not harmful, and their removal can be done through the filtration process. Other by-products such as chloramines produced in the reaction between ammonia and chlorine can be essential during disinfection as they possess disinfecting properties. The reaction between chlorine and organic materials can result in harmful by-products. These include trihalomethanes and haloacetic acids which in high concentrations are associated with causing some types of cancer (Xie, 2016).
Conclusion
The discovery of chlorine as a product that can remove contaminants from drinking water is one of the significant landmarks in the field of science and technology. Chlorination has become very beneficial to human health by reducing the rate of diseases transmitted through water or those caused by water contamination. The functioning mechanism of chlorine in the removal of contaminants in water is a reaction which forms insoluble products. The precipitates formed can easily be removed in the filtration process in the water treatment facility. Chlorine can be used in its different forms during the chlorination process. These forms are chlorine gas, sodium hypochlorite, and calcium hypochlorite. Hypochlorous acid is the stronger form of chlorine acid formed after the reaction between water and chlorine.
References
Hancock, N. (2017). What is Chlorination?. Safe Drinking Water Foundation. Retrieved 2 March 2018, from https://www.safewater.org/fact-sheets-1/2017/1/23/what-is-chlorination
Hrudey, S. E. (2009). Chlorination disinfection by-products, public health risk tradeoffs and me. Water Research, 43(8), 2057-2092.
Water Treatability Database | US EPA. Iaspub.epa.gov. Retrieved 2 March 2018, from https://iaspub.epa.gov/tdb/pages/treatment/treatmentOverview.do?treatmentProcessId=-1118142891
Xie, Y. (2016). Disinfection byproducts in drinking water: Formation, analysis, and control. CRC press.
