What is the scale inhibition efficiency of HEDP?

This is a key question when evaluating HEDP for water treatment. The scale inhibition efficiency of HEDP is excellent, but it's crucial to understand that there is no single, fixed number for its efficiency. The performance depends heavily on several water quality and operational factors.

However, to give you a clear and useful answer, we can look at typical performance under standardized and real-world conditions.

Executive Summary

Under optimal conditions, HEDP can achieve scale inhibition efficiency of over 95% for common scales like calcium carbonate. It is considered one of the most efficient and cost-effective threshold inhibitors available.

Detailed Breakdown by Scale Type

HEDP's efficiency varies depending on the type of scale it is fighting.

1. For Calcium Carbonate (CaCO₃) Scale

This is HEDP's primary and most effective application.

Typical Efficiency: > 95% under controlled laboratory conditions (e.g., according to standards like GB/T 16632-2019).

Mechanism: HEDP works through a combination of threshold effect and crystal distortion. Even at very low doses (1-10 mg/L), it can prevent the crystallization and precipitation of calcium carbonate by adsorbing onto the growing crystal faces, making them distorted and unable to form a hard scale.

2. For Calcium Sulfate (CaSO₄) Scale

HEDP is also very effective against gypsum scale.

Typical Efficiency: > 90%

Mechanism: Similar to its action on carbonate, it effectively inhibits the formation of CaSO₄ crystals.

3. For Barium Sulfate (BaSO₄) Scale

This is a much tougher scale to inhibit.

Typical Efficiency: Low to Moderate (e.g., 30-60%)

Reason: Barium sulfate crystals are extremely hard and have a very low solubility. HEDP is not the best choice for barium-dominated scaling problems. Specialty polymers are often required.

4. For Calcium Phosphate (Ca₃(PO₄)₂) Scale

HEDP has a dual role here.

Inhibition Efficiency: Good (e.g., 80-90%) at low to moderate phosphate levels.

Important Caveat: HEDP itself contains phosphorus. While it is an effective inhibitor, its degradation or hydrolysis at high temperatures can ultimately release orthophosphate, which can contribute to phosphate scaling. This needs to be managed in system control.

Key Factors Influencing HEDP's Efficiency

The following factors will determine the actual efficiency you achieve in your specific system:

Dosage:

There is an optimal dosage range, typically 1-10 mg/L (ppm) of active HEDP.

Too Low: Insufficient inhibition, scale will form.

Too High: Does not linearly improve efficiency and can be wasteful. At very high concentrations, it can even form a soluble complex with calcium that precipitates at a different ratio (HEDP-Ca sludge).

Water Chemistry:

Calcium Hardness & Alkalinity: Higher levels of Ca²⁺ and HCO₃⁻ require a higher dosage of HEDP to achieve the same level of inhibition.

pH: HEDP performs best in a pH range of 7.0 to 9.5. Its effectiveness decreases outside this range, especially in highly acidic conditions.

Temperature:

HEDP has excellent thermal stability for a phosphonate (up to 110°C in aqueous solution, ~250°C dry). However, very high temperatures can slowly degrade it over time, reducing its efficiency.

Presence of Oxidizing Biocides:

This is a critical operational factor. Strong oxidizing biocides like chlorine and bromine will oxidize and degrade HEDP, significantly reducing its effectiveness. If using chlorine, the HEDP dosage may need to be increased, or a more oxidant-stable inhibitor like PBTCA should be considered.

Typical Performance Data Table (for reference)

The following table provides a generalized snapshot under specific test conditions:

Scale Type Test Conditions (Approx.) HEDP Dosage Typical Inhibition Efficiency

Calcium Carbonate (CaCO₃) [Ca²⁺]=250 ppm, [HCO₃⁻]=250 ppm, 70°C, pH 8.5 3 - 5 mg/L > 95%

Calcium Sulfate (CaSO₄) [Ca²⁺]=2000 ppm, [SO₄²⁻]=2000 ppm, 50°C 5 - 7 mg/L > 90%

Calcium Phosphate (Ca₃(PO₄)₂) [Ca²⁺]=250 ppm, [PO₄³⁻]=5 ppm, 70°C, pH 8.5 5 - 8 mg/L > 85%

Barium Sulfate (BaSO₄) Supersaturated solution, 70°C 10 mg/L ~50%

Conclusion

HEDP is a highly efficient scale inhibitor, particularly for calcium carbonate and calcium sulfate, where it can consistently achieve over 90-95% inhibition at low dosages.

However, to get the best results:

A water analysis is essential to determine the scaling potential.

System conditions must be considered, especially pH, temperature, and the presence of oxidizing biocides.

It is almost always used in a synergistic blend with polymers like Polyacrylic Acid (PAA) or Polymaleic Anhydride (HPMA) for superior performance. The polymer provides superior crystal dispersion, while HEDP provides excellent threshold inhibition.

For a precise efficiency number for your application, laboratory testing using your actual makeup water is highly recommended.