*5.4.1. Chlorine*

Chlorine is one of the oldest disinfection agents used, which is one of the safest and most reliable. It has extremely good properties, which conform to the aspects of the ideal disinfec‐ tant. Effective chlorine disinfection depends upon its chemical form in wastewater. The influencing factors are pH, temperature, and organic content in the wastewater [3]. When chlorine gas is dissolved in wastewater, it rapidly hydrolyzes to hydrochloric acid (HCl) and hypochlorous acid (HOCl) as shown in the following chemical equation:

+ - Cl +H O H +Cl +HOCl 2 2 «

Free ammonia combines with the HOCl form of chlorine to form chloramines in a three-step reaction, as follows:

> 3 2 2 2 2 2 2 3 2 NH +HOCl NH Cl+H O NH Cl+HOCl NHCl +H O NHCL +HOCl NCl +H O ® ® ®

Figure 51 illustrates the chlorination curve, where the formation of chloramines occurs at the breakpoint. The free chlorine residual first rises then falls until the reaction with ammonia has been completed. As additional chlorine is applied and ammonia is consumed, the chlorine residual rises again.

Dechlorination is a very important process, where activated carbon, sulfur compounds, hydrogen sulfide, and ammonia can be implemented to minimize the residual chlorine in a disinfected effluent prior to discharge. Activated carbon and sulfur compounds are the most widely used [3]. The commonly used sulfur compounds are sulfur dioxide (SO2), sodium metabisulfite (NaS2O5), sodium bisulfate (NaHSO3), and sodium sulfite (Na2SO3). The dech‐ lorination reactions with the abovementioned compounds are described in the following equations:

**Figure 51.** Chlorination curve [3].

$$\begin{aligned} \text{SO}\_2 &+ 2\text{H}\_2\text{O} + \text{Cl}\_2 \rightarrow \text{H}\_2\text{SO}\_4 + 2\text{HCl} \\ \text{SO}\_2 &+ \text{H}\_2\text{O} + \text{HOCl} \rightarrow 3\text{H}^+ + \text{Cl}^- + \text{SO}\_4^{2-} \\ \text{Na}\_2\text{S}\_2\text{O}\_5 &+ 2\text{Cl}\_2 + 3\text{H}\_2\text{O} \rightarrow 2\text{NaHSO}\_4 + 4\text{HCl} \\ \text{Na}\text{HSO}\_3 &+ \text{H}\_2\text{O} + \text{Cl}\_2 \rightarrow \text{NaHSO}\_4 + 2\text{HCl} \end{aligned}$$

## *5.4.2. Ozone*

Ozone (O3) is a very strong oxidant typically used in wastewater treatment. Ozone is able to oxidize a multitude of organic and inorganic compounds in wastewater. These reactions cause an ozone demand in the treated wastewater, which should be fulfilled throughout wastewater ozonation prior to developing an assessable residual. Ozone should be generated at the point of application for use in wastewater treatment as ozone is an unstable molecule [3]. Figure 52 illustrates the corona discharge method for making ozone. Ozone is generally formed by combining an oxygen atom with an oxygen molecule (O2) as follows:

#### *5.4.3. Ultraviolet light*

Ultraviolet (UV) radiation is a microbial disinfectant that leaves no residual. It requires clear, un-turbid, and non-colored water for its implementation. The commercial UV disinfection systems use low- to medium-powered UV lamps with a wavelength of 354 nm [3]. The UV dosage can be calculated as follows:

*D It* = ×

**Figure 52.** Schematic drawing of corona discharge method for making ozone [3].

where, *D* is the UV dose (mW. s/cm2 ); *I* is the intensity (mW/cm2 ); and *t* is the exposure time (s).

The advantages of UV radiation are: (1) directly effective against the DNA of many microor‐ ganisms, (2) not reactive with other forms of carbonaceous demand, and (3) provides superior bactericidal kill values while not leaving any residues. The advantage is often the disadvant‐ age, because power fluctuations, variations in hydraulic flow rates, and color or turbidity can cause the treatment to be ineffective [3]. Additionally, cell recovery and re-growth of the damaged organisms because of the inactivation of their predators and competitors has come to light.
