Managing dissolved impurities in cooling water is essentially managing a set of interrelated chemical reactions. Clear categorization tells us where the problems lie. From calcium and magnesium ions to silicates, from chloride to sulfate — this article walks through the common dissolved impurities in industrial cooling water, their hazards and control strategies.
1. What are dissolved impurities in cooling water?
Dissolved impurities are ions or small molecules that cannot be removed by filtration because they are fully dissolved in water. They mainly include:
- Scale-forming ions: Ca²⁺, Mg²⁺, HCO₃⁻, CO₃²⁻, SO₄²⁻, SiO₂
- Corrosion-driving ions: Cl⁻, SO₄²⁻, dissolved O₂
- Nutrients: NH₄⁺, NO₃⁻, PO₄³⁻ (fueling biofilm)
- Metal ions: Fe²⁺/Fe³⁺, Mn²⁺, Al³⁺
2. Scaling impurities — hazards & control
1) Calcium carbonate (CaCO₃)
The most common scale. Its thermal conductivity is only 1/50 of steel — 1 mm of scale can cause 5–10% more energy consumption.
2) Calcium sulfate (CaSO₄)
Low solubility; it scales easily at high cycles of concentration and is nearly impossible to remove by acid cleaning. Strict control of the [Ca²⁺]×[SO₄²⁻] ion product is required.
3) Silica scale (SiO₂)
Above pH 8.5, amorphous silicates can form and combine with Mg²⁺ to produce magnesium silicate that is hard to remove. High-silica water is best treated with the TRISPE® SI001 dedicated silica scale inhibitor.
| Scale type | Safe ion product | Control method |
|---|---|---|
| CaCO₃ | LSI ≤ 2.5 (after dosing) | Scale inhibitor + pH control |
| CaSO₄ | [Ca²⁺][SO₄²⁻] < 500,000 | Limit cycles + high-performance dispersant |
| SiO₂ | < 150–180 mg/L | Dedicated silica inhibitor + control Mg²⁺ |
| Ca₃(PO₄)₂ | [Ca²⁺][PO₄³⁻] related | Use non-phosphate formulas or calcium phosphate inhibitors |
3. Corrosion impurities — how to control
1) Chloride Cl⁻
Causes pitting on carbon steel and stress corrosion cracking on 304/316 stainless steel. Typical limits:
- Carbon steel: Cl⁻ < 300 mg/L
- 304 stainless: Cl⁻ < 200 mg/L
- 316 stainless: Cl⁻ < 500 mg/L (below 50 °C)
2) Dissolved oxygen O₂
DO in cooling water approaches saturation; film-forming corrosion inhibitors (molybdate, zinc, HEDP, etc.) are used to form a dense protective layer.
4. Nutrients & biofilm
NH₄⁺, NO₃⁻ and PO₄³⁻ entering the loop promote algae and bacteria, forming biofilm that causes:
- Heat exchanger plate blocking and loss of heat transfer efficiency
- Deposit under-corrosion (DUC), leading to perforation
- Cooling tower fill aging and bonding
Recommended strategy: alternate oxidizing / non-oxidizing PT-BIO® biocides + keep total phosphorus < 1 mg/L.
5. Monitoring metal ions
Fe²⁺ will oxidize to Fe(OH)₃ sludge inside the system, acting as both a source of blockage and a catalyst for microbial growth. Keep total iron < 1 mg/L and install iron-removal equipment on the make-up side.
Operating tip: Cooling water management shouldn't rely on a single parameter; build a joint monitoring framework covering scale-forming ions, corrosion ions and biological nutrients. Aqua-Link offers monthly full-parameter analysis services so that plants move from reactive treatment to proactive control.
6. Conclusion
Dissolved impurities are invisible yet are the root causes of scaling, corrosion and biofouling in circulating water. Only by understanding each impurity's origin, concentration limit and control chemistry can a cooling system run reliably long-term.