Can HEDP be used with fungicides?

The answer is: It depends on the type of fungicide.

HEDP's compatibility varies significantly depending on whether the fungicide is oxidizing or non-oxidizing. The short summary is:

With Non-Oxidizing Fungicides: Yes, generally compatible. HEDP can be used in the same system.

With Oxidizing Biocides (like chlorine): Compatible with major limitations. HEDP will degrade when in contact with these oxidizers.

Here is a detailed breakdown:

1. Compatibility with Oxidizing Biocides (e.g., Chlorine, Bromine)

This is the most critical and common scenario, as oxidizing agents are the most widely used biocides in industrial water systems.

Compatibility: Poor. They chemically react.

Mechanism: Oxidizing biocides like sodium hypochlorite (chlorine) attack the carbon-phosphorus (C-P) bond in the HEDP molecule, particularly the one near the hydroxyl (OH) group. This breaks the molecule apart.

Consequences:

Loss of HEDP Efficacy: The degraded HEDP loses its ability to inhibit scale and prevent corrosion, leaving the system vulnerable.

Formation of Orthophosphate (PO₄³⁻): The breakdown of HEDP releases orthophosphate into the water. This causes two major problems:

Calcium Phosphate Scale: It can react with calcium ions to form a very hard and tenacious scale (calcium phosphate), which is difficult to remove.

Microbial Nutrient: Orthophosphate is a food source for bacteria and algae, potentially worsening microbial growth and counteracting the biocide treatment.

Practical Recommendation:

Staggered Feed: Do not feed HEDP and oxidizing biocides at the same time. The best practice is to feed HEDP continuously or intermittently, and then add the oxidizing biocide as a "shock dose" during a period when the HEDP feed is temporarily stopped (e.g., for a few hours).

Control Residuals: If staggered feed is impossible, the free oxidant residual (e.g., chlorine) must be kept very low (typically <0.5 ppm).

Consider Alternatives: For systems requiring constant chlorination, a more oxidant-tolerant scale inhibitor like PBTC is often a better choice.

2. Compatibility with Non-Oxidizing Fungicides/Biocides

This group includes most other industrial fungicides, and compatibility is generally good.

Common Types:

Isothiazolinones (e.g., CMIT/MIT)

Glutaraldehyde

Quaternary Ammonium Compounds (Quats, e.g., Didecyldimonium Chloride)

DBNPA (2,2-dibromo-3-nitrilopropionamide)

Compatibility: Generally Good to Excellent.

Mechanism: These biocides work by interfering with specific microbial processes (e.g., disrupting cell walls, inhibiting metabolism) and do not h3ly react with the HEDP molecule.

Practical Recommendation:

They can typically be fed into the same system without significant chemical interaction. HEDP is often dosed continuously for scale inhibition, while the non-oxidizing biocide is dosed periodically as a "shock" or "batch" treatment to control microbial populations.

It is always a good practice to confirm compatibility with the specific product supplier, as formulations can contain other ingredients that might cause issues.

Summary Table

Biocide/Fungicide Type Compatibility with HEDP Key Recommendation

Oxidizing Biocides (Chlorine, Bromine) Poor Use staggered feed or very low oxidant residuals. Consider PBTC as an alternative.

Non-Oxidizing Biocides (Isothiazolinones, Glutaraldehyde, Quats) Good Standard practice. Can be used in the same system with appropriate dosing schedules.

Overall Conclusion

Yes, HEDP can be used with fungicides, but the success of the treatment program hinges on selecting the correct type and implementing the proper dosing strategy.

For non-oxidizing fungicides, it is a standard and generally safe practice.

For oxidizing biocides (like chlorine), extreme care must be taken to prevent HEDP degradation, typically by using a staggered feeding program.

For any specific application, it is highly recommended to consult with your water treatment chemical supplier to confirm compatibility and optimize the dosing protocol for your system's unique conditions.