Two PAC products can be labeled “28% Al₂O₃” and still behave completely differently. The hidden parameter doing the work is basicity. If you specify PAC on Al₂O₃ alone, you are leaving half the performance on the table.
What Basicity Actually Measures
PAC is not a single molecule — it is a mix of polymeric aluminum species. Basicity (or “hydroxylation”) describes how much of the aluminum has already reacted with hydroxyl groups before the product even reaches your water.
A useful definition:
Basicity (%) = [OH⁻] / (3 × [Al³⁺]) × 100%
Real commercial PAC is more complex than this formula suggests, but the concept holds: higher basicity means more pre-hydrolyzed aluminum.
Why That Matters
When PAC hits raw water, it continues to hydrolyze. Pre-hydrolyzed (higher-basicity) species are already partway through that reaction. Consequently:
- High-basicity PAC forms flocs faster, works in colder water, and consumes less raw-water alkalinity.
- Low-basicity PAC behaves more like traditional alum — cheaper, but more dependent on the water’s own alkalinity and pH.
The Three Tiers (Roughly)
| Tier | Typical range* | Best fit |
|---|---|---|
| Low basicity | ~30–40% | High-alkalinity, high-turbidity waters; cost-sensitive plants |
| Medium basicity | ~40–60% | General-purpose municipal and industrial workhorse |
| High basicity | ~60–85% | Cold water, low-alkalinity source, rapid floc needed |
*Ranges vary by manufacturer. Always read the supplier’s spec.
The Trap: “Higher Is Better”
It is not. A high-basicity product in a high-alkalinity, high-turbidity stream can actually over-floc, produce delicate flocs, or cost more than necessary. Conversely, a low-basicity PAC in low-alkalinity water will fail because the water cannot support further hydrolysis.
Coagulation depends on raw-water pH, alkalinity, turbidity, organic load, and temperature — not on one spec.
How to Choose
- Analyze raw water: pH, alkalinity, turbidity, temperature.
- Shortlist 2–3 PAC grades across basicity ranges.
- Jar-test them side by side.
- Measure: floc size, settling rate, residual turbidity, pH drift, sludge volume.
- Pick the lowest total-cost winner — not the highest-basicity label.
Pairing with PAM
Basicity also changes how much flocculant you need. Denser, faster-forming flocs may need less PAM; weaker microflocs may need more. Optimize the pair together.
FAQ
Is basicity the same as pH?
No. Basicity is about the polymer’s hydroxylation. Product pH is a separate spec.
What basicity do drinking-water plants use?
Often medium-to-high, because they face variable seasonal raw water — but always confirmed by jar test.
Does high basicity mean low insoluble matter?
Not necessarily. Check the spec sheet; they are independent parameters.
Conclusion
PAC basicity is the second most important spec (after Al₂O₃) and the most misunderstood. Treat it as one input in jar testing, not as a quality ranking. Get the water analysis, run the comparison, and let performance decide.
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