Chlorine is an active element that reacts with many chemical compounds in water and wastewater to form new and often less offensive components. Hydrolysis and ionization occur when chlorine gas is added to water which forms HOCl and OCl", the free available chlorine. Hypochlorite salts such as Ca(OCl)2 and Na(OCl) can be added to water to form free chlorine. HOCl is predominant at a pH <7.0 which is beneficial since its disinfection power is approximately 40-80 times of that of OCl".
HOCl reacts with ammonia in wastewater to form various chloramines (NH2Cl, NHCl2, and NCl3), the combined available chlorine (Sawyer and McCarty 1978). Since chloramines are less effective disinfectants, additional chlorine is needed to insure the presence of a free chlorine residual. Figure 7.30.1 is a schematic illustration of the stepwise reaction phenomena that result when chlorine is added to wastewater containing ammonia (breakpoint chlorination curve).
An even more important characteristic of chlorine is that it is toxic to most pathogenic microorganisms (see Figure 7.30.2). Chlorine acts as an oxidizing agent to change the character of an offending chemical. Chlorine
is a strong oxidizing agent because its atoms are constructed with three shells of electrons and the outer shell (with seven electrons) has a strong tendency to acquire an eighth electron for stability. Oxidation is a process in which an atom loses electrons (see Figure 7.30.3).
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