How to Use HEDP (1-Hydroxyethylidene-1,1-Diphosphonic Acid) in Water Treatment
Overview
HEDP is the most widely used phosphonate-based scale and corrosion inhibitor for cooling water and boiler systems. It prevents CaCO₃ and CaSO₄ scale by threshold inhibition — substoichiometric doses (2–10 ppm) inhibit crystal growth of thousands of ppm of hardness. Also provides mild anodic corrosion inhibition for carbon steel.
Preparation & Dissolution
HEDP is supplied as 60% liquid (most common) or 90% powder.
60% liquid: Use directly or dilute into treatment program blend. Specific gravity ~1.43.
90% powder: Dissolve in clean water at room temperature. Dissolves readily in 10–15 minutes with mild agitation. Can be pre-blended with zinc sulfate and polymer dispersant to create a complete cooling water treatment formulation.
Dosing Guide
| Application | Dose (as 100% HEDP) | Notes |
|---|---|---|
| Cooling water scale control | 2–5 ppm | Maintain residual continuously |
| Cooling water + corrosion | 3–8 ppm + 1–2 ppm Zn²⁺ | Synergistic with zinc |
| Low-pressure boiler | 5–10 ppm | < 20 bar boilers only |
| RO pretreatment | 2–4 ppm | As part of antiscalant blend |
| Oilfield squeeze treatment | 500–2000 ppm | Squeeze into formation near-wellbore |
Application Procedure
- Determine makeup water hardness (Ca, Mg) and system cycles of concentration (CoC)
- Calculate concentrate hardness = makeup × CoC
- Select HEDP dose from dosing chart (higher hardness = higher dose)
- Set metering pump to continuous dosing into makeup water line or recirculating line
- Verify HEDP residual by phosphonate field test kit (target 2–5 ppm as PO₄)
- Adjust blowdown to maintain target CoC (typically 3–5)
Monitoring & Control
| Parameter | Frequency | Target |
|---|---|---|
| HEDP residual (as PO₄) | 2x weekly | 2–5 ppm |
| Conductivity/TDS | Daily | Per CoC target |
| Ca hardness (circulating) | Weekly | Check vs. predicted by CoC |
| Corrosion rate (coupon/probe) | Monthly | < 3 mpy carbon steel |
| Scale on heat exchanger | Quarterly inspection | No visible deposit |
Common Mistakes
- Using HEDP with high chlorine: Free chlorine > 1 ppm oxidizes HEDP, destroying its inhibiting power. Use PBTCA instead if chlorine dosing is continuous.
- Assuming HEDP handles silica: HEDP does NOT inhibit silica scale. If SiO₂ > 150 ppm in concentrate, add a silica dispersant (AA/AMPS copolymer).
- No zinc synergist: HEDP alone provides limited corrosion protection. Adding 1–2 ppm Zn²⁺ dramatically improves carbon steel corrosion control.
- Using in high-pressure boilers: HEDP breaks down above 200°C. Use only for boilers < 20 bar.
Storage & Handling
- Shelf life: 24 months (60% liquid), 36 months (90% powder)
- Temperature: Store 0–40°C. Liquid HEDP freezes at approximately -5°C but thaws without degradation.
- Container: HDPE drums or IBC. Compatible with most materials.
- Safety: Acidic (pH ~2 at 60%). Causes skin irritation. Wear gloves and goggles. Not classified as hazardous for transport.
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