Disinfection is the essential final barrier in water treatment designed to destroy pathogenic micro-organisms and prevent waterborne disease transmission. The inactivation rate of pathogens follows Chick's Law of disinfection kinetics, which states that the rate of microorganism destruction is directly proportional to the concentration of active organisms remaining at any time $t$:
$$\frac{dN}{dt} = -k \cdot N$$
Integrating this relationship over time yields the concentration reduction expression:
$$\ln\left(\frac{N_t}{N_0}\right) = -k \cdot t \quad \implies \quad N_t = N_0 \cdot e^{-k \cdot t}$$
Where $N_0$ is the initial pathogen count, $N_t$ is the pathogen concentration at time $t$, and $k$ is the reaction rate constant. Expanding this to account for chemical disinfectant concentration $C$ leads to the empirical Watson-Chick Model:
$$k = k' \cdot C^n \quad \implies \quad C^n \cdot t = \text{Constant}$$
Where $n$ represents the coefficient of dilution. The value $C \cdot t$ (Concentration $\times$ Contact Time) serves as the primary hydraulic design metric for municipal contact basins.
When gaseous chlorine ($\text{Cl}_2$) hydrolyzes in water, it forms Hypochlorous Acid ($\text{HOCl}$) and Hypochlorite Ions ($\text{OCl}^-$), collectively termed Free Available Chlorine:
$$\text{Cl}_2 + \text{H}_2\text{O} \rightleftharpoons \text{HOCl} + \text{H}^+ + \text{Cl}^-$$
$$\text{HOCl} \rightleftharpoons \text{H}^+ + \text{OCl}^-$$
$\text{HOCl}$ is up to 80 times more effective as a germicide than $\text{OCl}^-$. At $pH < 7.5$, $\text{HOCl}$ remains the dominant species, whereas at $pH > 8.5$, $\text{OCl}^-$ dominates. When ammonia is present, chlorine reacts to form chloramines (Combined Available Chlorine). As chlorine dosage increases, combined residual builds up until it reaches the Breakpoint Chlorination point, beyond which organic compounds break down completely and free residual chlorine begins to accumulate linearly.
Traditional free chlorination of raw surface water containing natural organic matter (NOM) generates harmful carcinogenic Disinfection By-Products (DBPs), such as Trihalomethanes (THMs) and Haloacetic Acids (HAAs), which pose long-term public health risks in distribution networks.
To strictly limit DBP formation under updated drinking water standards, modern Indian municipal plants are transitioning to alternative advanced disinfection strategies. Facilities now deploy Primary Ozonation or Ultraviolet (UV) Irradiation to achieve rapid pathogen inactivation without chemical residuals, followed by a minor dosage of **Chloramines (Secondary Chloramination)**. Chloramination maintains a long-lasting residual across extensive distribution pipelines while significantly minimizing DBP synthesis.
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