The Role of Coagulants and Flocculants in Water Treatment

Have you ever wondered how muddy river water is turned into crystal-clear tap water? The secret lies in a chemical process called Coagulation and Flocculation. Before water can be filtered or disinfected, we must first remove the suspended solids that make it cloudy (turbidity).

Since these tiny particles are often too small to settle by gravity alone, we use chemistry to help them out.

The Science: Zeta Potential

Why do dirt particles stay suspended? It's due to Zeta Potential.

• Dirt particles are naturally Negatively Charged.
• Because they all have the same charge, they push each other away (like two North poles of a magnet).
• Coagulants are Positively Charged. When you add them, the Zeta Potential drops to zero, and the particles can finally touch.

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Comparison: Choosing the Right Chemical

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The Recipe: How to find the Dose? (The Jar Test)

You can't guess how much chemical to add. You must perform a Jar Test:

1. Prepare: Take 6 jars of 1 Liter of raw water.
2. Dose: Add increasing amounts of chemical (e.g., 10ppm, 20ppm, 30ppm...).
3. Flash Mix: Stir fast for 1 minute (Coagulation).
4. Gentle Mix: Stir slowly for 15 minutes (Flocculation).
5. Observe: See which jar produces the largest floc and clearest water.
6. Scale Up: Use that dosage for the main plant.

Mixing Energy (G-Value)

Mixing is just as important as the chemical.

• Rapid/Flash Mix: Needs high energy (G > 700 s⁻¹) to dispersed the chemical in seconds.
• Flocculator Mix: Needs low energy (G < 50 s⁻¹). If you mix too fast here, you will "Shear" the flocs, breaking them back into tiny pieces.

Step 3: Sedimentation

Once large, heavy floc has formed, the water enters a sedimentation tank (or clarifier). The water slows down, allowing the floc to settle to the bottom as sludge, which is then removed. The clear water on top moves on to filtration.

Common Chemicals Used

Inorganic Coagulants

• Alum (Aluminum Sulfate): The most traditional and widely used. Cheap but effective range is pH limited.
• Iron Salts (Ferric Chloride): Effective over a wider pH range but can stain equipment.

Organic Coagulants/Flocculants

• Polymers (Polyelectrolytes): Long chains of synthetic organic molecules. They are extremely effective at bridging particles together to form massive, fast-settling floc.