Mechanism: Gentle and prolonged mixing (typically 20-30 minutes) encourages collisions between the neutralized particles. Flocculants, often long-chain polymers, act as "bridges" between these particles, linking them together into larger, more robust flocs. The mixing energy must be carefully controlled; too little mixing prevents sufficient collisions, while too much can shear apart the delicate flocs.

Common Flocculants (Coagulant Aids):

Synthetic Organic Polymers:
- Anionic Polymers: Negatively charged, used when particles have a net positive charge (less common as primary flocculants in raw water).
- Cationic Polymers: Positively charged, often used to enhance coagulation and flocculation of negatively charged particles.
- Non-ionic Polymers: No charge, rely solely on bridging mechanisms.
Polymers significantly improve floc strength, size, and settling rate, leading to better clarification and reduced coagulant usage.
Activated Silica: A colloidal form of silica that enhances floc density and toughness, particularly with alum.
Bentonite Clay: Can be used as a weighting agent to improve the settleability of light flocs.

1.3. Sedimentation (Clarification)

Objective: To remove the large, heavy flocs by gravitational settling.

Mechanism: Water flows slowly through large sedimentation basins, reducing the water velocity to a point where the gravitational force on the flocs exceeds the drag force of the water, allowing them to settle to the bottom.
Outcome: Produces "clarified water" with significantly reduced turbidity, suspended solids, and associated contaminants. The settled material is called "sludge," which requires further treatment and disposal.

2. Disinfection: Pathogen Inactivation

Disinfection is arguably the most critical step in ensuring public health by inactivating pathogenic microorganisms (bacteria, viruses, protozoa, helminths) to prevent waterborne diseases.

Key Considerations: Disinfectant effectiveness ($CT$ value = concentration $\times$ time), contact time, pH, temperature, and presence of interfering substances.
Common Disinfectants:
- Chlorine ($Cl_2$), Sodium Hypochlorite ($NaClO$), Calcium Hypochlorite ($Ca(ClO)_2$):
  - Mechanism: When chlorine compounds dissolve in water, they form hypochlorous acid ($HOCl$) and hypochlorite ion ($OCl^-$). $HOCl$ is a much stronger disinfectant than $OCl^-$. The relative proportion of $HOCl$ to $OCl^-$ is pH-dependent (more $HOCl$ at lower pH). These powerful oxidants penetrate microbial cell walls, denature enzymes, and disrupt nucleic acids, leading to cell death.
  - Advantages: Highly effective against most bacteria and viruses, relatively inexpensive, provides a measurable and persistent residual disinfectant throughout the distribution system, protecting against recontamination.
  - Disadvantages: Can react with natural organic matter (NOM) in water to form potentially harmful disinfection byproducts (DBPs) such as Trihalomethanes (THMs) and Haloacetic Acids (HAAs), which are regulated carcinogens. Can cause taste and odor issues at high concentrations.
- Chloramines ($NH_2Cl$, $NHCl_2$, $NCl_3$):
  - Mechanism: Formed by carefully controlled reaction of chlorine with ammonia. Monochloramine ($NH_2Cl$) is the primary species used. They are weaker oxidants than free chlorine but are more stable and produce significantly fewer DBPs (THMs, HAAs) because they react less aggressive