Assuming higher is better
High basicity is not a premium grade. On heavy turbidity it underperforms a cheaper low-basicity product.
Technical guide
PAC basicity explained
Basicity is the single most misunderstood number on a PAC specification — and the one most likely to explain why a product that worked for someone else does not work for you.
What basicity actually measures
Basicity is the ratio of hydroxide to aluminium in the product, expressed as a percentage. It tells you how far the aluminium has already been hydrolysed — pre-polymerised — before it enters your water.
At low basicity, around 40%, the aluminium is only partly pre-hydrolysed. It completes hydrolysis in your water, which consumes alkalinity and depresses pH, but it also delivers strong charge neutralisation. That is what you want when you are fighting a heavy particulate load.
At high basicity, 70 to 90%, most of the hydrolysis has already happened during manufacture. The product barely touches your alkalinity and hardly moves pH, and it works through bridging and sweep mechanisms more than raw charge neutralisation. That suits low-turbidity, low-alkalinity water where alum and low-basicity products cause pH problems.
Choosing basicity by water type
| Your water | Suggested basicity | Why |
|---|---|---|
| High turbidity (>100 NTU) | 40–55% | Strong charge neutralisation is what removes heavy particulate load |
| Moderate turbidity, good alkalinity | 55–70% | Balanced performance; the general-purpose band |
| Low turbidity (<10 NTU) | 70–90% | Sweep and bridging mechanisms work better than charge neutralisation here |
| Low alkalinity water | 70–90% | Minimal alkalinity consumption avoids pH crashes and lime dosing |
| Cold water | 70–90% | Higher basicity holds performance better at low temperature |
| Industrial effluent, high COD | 40–60% | Aggressive charge neutralisation handles complex organic loads |
| RO / UF pre-treatment | 70–90% | Lower residual aluminium risk; gentler pH effect |
These are starting points for selection, not a substitute for a jar test. If in doubt, trial two basicities side by side on your actual water — the difference is usually obvious within one test.
What goes wrong
Common basicity mistakes
Switching without retesting
Changing basicity changes your optimum dose. Carrying the old setpoint across is the most common cause of a failed switch.
Ignoring it in quotes
Two quotes at the same Al2O3 and different basicity are not comparable products. Ask for both figures.
Not specifying on reorder
If you do not state basicity on the purchase order, you may not get what you got last time. Put it on the PO.
Blaming the product
A batch that suddenly underperforms is often a basicity shift, not a purity problem. Check the CoA before changing supplier.
FAQ
Common questions
It is the ratio of hydroxide to aluminium in the product, as a percentage — a measure of how far the aluminium has been pre-hydrolysed during manufacture. It governs pH depression, alkalinity consumption and how the coagulant works in your water.
Not universally. High basicity suits low-turbidity and low-alkalinity water. On high-turbidity or heavily loaded industrial effluent, a lower basicity product often performs better and costs less.
Roughly: 40–55% for high turbidity and industrial effluent, 55–70% for general municipal use, 70–90% for low turbidity, low alkalinity, cold water or membrane pre-treatment. Confirm with a jar test.
Yes, usually significantly. Always re-run a jar test after switching basicity rather than carrying the old setpoint across.
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