Here are the Top 10 Water Treatment Chemicals, widely used across drinking water, municipal wastewater, and industrial applications—along with how each works:

1. Alum (Aluminum Sulfate – Al₂(SO₄)₃)

• Function: Primary coagulant

• How it works:

• Dissolves in water to form aluminum hydroxide flocs (\ce{Al(OH)3}) via hydrolysis.

• These flocs enmesh suspended particles (sweep flocculation) and partially neutralize negative charges.

• Best for: High-turbidity surface water; low-cost option.

• Limitations: Narrow pH range (6–7.5); produces large volumes of sludge; adds sulfate.

2. Ferric Chloride (FeCl₃)

• Function: Coagulant

• How it works:

• Hydrolyzes to form ferric hydroxide flocs (\ce{Fe(OH)3}), which trap particles via sweep flocculation.

• Also effective at precipitating phosphorus as ferric phosphate.

• Best for: Wastewater with high phosphorus; broader pH range (4–9) than alum.

• Limitations: Corrosive; stains surfaces; increases chloride content.

3. Polyaluminum Chloride (PAC or PACl)

• Function: Advanced inorganic coagulant

• How it works:

• Contains pre-formed polynuclear aluminum species (e.g., (\ce{Al13O4(OH)24^{7+}})) that provide strong charge neutralization and some sweep flocculation.

• Best for: Cold or low-turbidity water; wider pH range (5.5–8.5); less sludge than alum.

• Advantage: Faster settling, lower alkalinity consumption.

4. PolyDADMAC (Polydiallyldimethylammonium Chloride)

• Function: Cationic polymeric coagulant

• How it works:

• A strong cationic polyelectrolyte with permanent positive charges.

• Neutralizes negative charges on colloids (e.g., organic matter, clay) via electrostatic attraction—no metal hydroxides formed.

• Best for: Color/NOM removal in drinking water; low-sludge applications; pH 4–9.

• Limitation: Not effective for phosphorus removal.

5. Cationic Polyacrylamide (CPAM)

• Function: Flocculant / sludge conditioner

• How it works:

• Long polymer chains bridge destabilized microflocs into large, fast-settling aggregates (polymer bridging).

• Often used after a primary coagulant (e.g., alum or PAC).

• Best for: Sludge dewatering (centrifuges, belt presses); clarifier performance boost.

• Note: Not a standalone coagulant.

6. Chlorine (Cl₂, NaOCl, Ca(OCl)₂)

• Function: Disinfectant & oxidant

• How it works:

• Releases hypochlorous acid (HOCl) in water, which penetrates cell walls and destroys pathogens.

• Provides residual protection in distribution systems.

• Best for: Municipal drinking water disinfection.

• Drawback: Forms disinfection by-products (DBPs) like trihalomethanes (THMs) with organic matter.

7. Activated Carbon (Powdered – PAC or Granular – GAC)

• Function: Adsorbent

• How it works:

• Removes contaminants via physical adsorption onto its highly porous surface.

• Targets: taste/odor compounds (e.g., geosmin), synthetic organics, pesticides, chlorine by-products.

• Use: PAC added during treatment; GAC used in filtration beds.

8. Lime (Ca(OH)₂) or Soda Ash (Na₂CO₃)

• Function: pH adjuster & softener

• How it works:

• Raises pH to optimize coagulation or prevent pipe corrosion.

• Lime also precipitates hardness ions (Ca²⁺, Mg²⁺) in softening processes.

• Common in: Corrosion control (e.g., lead/copper stabilization); softening plants.

9. Orthophosphates (e.g., H₃PO₄, Na₃PO₄)

• Function: Corrosion inhibitor

• How it works:

• Forms an insoluble protective layer (e.g., lead phosphate) on pipe interiors, preventing leaching of lead/copper.

• Critical in: Distribution systems with aging metal pipes (e.g., post-Flint crisis protocols).

10. Ozone (O₃)

• Function: Oxidant & disinfectant

• How it works:

• Powerful oxidizer that destroys bacteria, viruses, and micropollutants (e.g., pharmaceuticals, pesticides).

• Breaks down organic molecules, improving biodegradability and reducing DBP precursors.

• Limitation: No residual protection → often paired with low-dose chlorine.

• Used in: Advanced drinking water plants; bottled water industry.

Quick Reference Table

Final Notes:

• Combinations are common: e.g., PAC + CPAM for enhanced clarification; ozone + chlorine for disinfection with residual.

• Selection depends on: Source water quality, regulations (e.g., DBP limits), cost, and sludge handling capacity.

• Trend: Shift toward polymeric coagulants (PolyDADMAC, PAC) and advanced oxidation to reduce sludge and chemical residuals.

These 10 chemicals form the backbone of modern water treatment—ensuring safe, clear, and reliable water from source to tap.