Correcting High Total Phosphorus in Recirculating Cooling Water

High total phosphorus in a recirculating cooling water system can lead to calcium phosphate scaling, fouling, and accelerated microbiological growth. Solving the problem requires a systematic approach.

1. Identify the source

Phosphorus may enter the system from three main sources:

Phosphorus-based corrosion inhibitors (e.g., orthophosphate, polyphosphate, or zinc phosphate blends) overdosed.

Makeup water contaminated by fertilizer runoff, sewage, or industrial discharge.

Microbial control agents containing phosphonates (organic phosphorus).

Test the makeup water and review chemical feed logs to pinpoint the origin.

2. Reduce or replace chemical treatment

If phosphate inhibitors are the cause:

Gradually decrease the feed rate while monitoring corrosion coupons to ensure protection is not compromised.

Consider switching to a non‑phosphorus program, such as polymer‑based or all‑organic corrosion inhibitors (e.g., molybdate or azole blends with dispersants).

For phosphonate-based biocides, replace them with non‑phosphorus alternatives like DBNPA, isothiazolone, or chlorine dioxide.

3. Increase blowdown and improve side-stream filtration

Dilution is a direct but water‑intensive method. Increase the blowdown rate to lower phosphorus concentration to the target limit (typically below 2–3 mg/L as P). To conserve water, pair this with:

Side‑stream sand or microfiltration to remove suspended phosphorus‑rich solids.

Use of a reverse osmosis recovery system for blowdown water.

4. Chemical precipitation

When immediate reduction is needed, add a metal salt coagulant:

Aluminum sulfate (alum) or ferric chloride at a dose of 1.5‑3× the phosphate concentration (molar basis). Adjust pH to 6.5‑7.0 for optimal precipitation of aluminum phosphate.

Follow with a cationic polymer to enhance flocculation and removal via sedimentation or filtration.

5. Improve operational control

Automate phosphorus analyzers to enable real‑time feedback control of inhibitor feeds.

Maintain proper pH (e.g., 7.5‑8.5 for most phosphate programs) to prevent calcium phosphate from precipitating.

Increase scale inhibitor dispersant dosage temporarily if soft scale is already forming.