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Calcium phosphate (Ca₃(PO₄)₂) scaling is a common issue in open cooling tower systems, especially when phosphate-based corrosion inhibitors or phosphate-rich makeup water are involved.

• Dispersants Effective Against Calcium Phosphate
  1. For controlling calcium phosphate scale, you want dispersants that can:
  2. Threshold inhibit precipitation
  3. Disperse any precipitated solids to prevent deposition
  4. Some commonly used dispersants that are effective for calcium phosphate
• Polyacrylic Acid (PAA)
  1. Mode of Action: Disperses particulates and provides threshold inhibition for calcium salts.
  2. Molecular Weight: Typically low (~2,000–10,000 Da) for dispersing action.
  3. Pros: Cost-effective, widely used.
  4. Cons: Less effective at higher temperatures or pH.
• Polymaleic Acid (PMA)
  1. Mode of Action: Good threshold inhibitor for calcium salts, including CaPO₄.
  2. Better thermal stability than PAA.
  3. Often blended with PAA or phosphonates.
• Acrylic/Maleic Copolymers (AA/MA)
  1. Mode of Action: Combines properties of both PAA and PMA.
  2. Effective for dispersing and inhibiting both calcium phosphate and calcium carbonate.
  3. Stable over a wider range of pH and temperature.
• HEDP + Polymer Blends

1. 1. HEDP (1-Hydroxyethylidene-1,1-diphosphonic acid) is a phosphonate that:

1. Inhibits scale formation (threshold inhibition)
2. Can chelate calcium to some extent

1. 1. Blended with PAA or AA/MA copolymers for improved performance.

6. Choosing the Right Dispersant
   1. Depends on:
      7. pH (CaPO₄ tends to precipitate more at pH > 7.5)
      8. Phosphate levels in your system

• Calcium hardness

1. Temperature of the water
2. If you’re targeting CaPO₄ specifically
   1. a blend of AA/MA copolymer with a phosphonate (like HEDP)

is often the most robust and reliable option.

7. Dosage
   1. PAA 5 – 20 ppm
   2. PMA 5 – 15 ppm
   3. AA/MA CoPolymer 5 – 15 ppm
   4. Phosphonate(A) & Polmer Blend(B) (3 – 10)  & ( 5 – 15 )