Polyaluminium Chloride (PAC) — Frequently Asked Questions

Basicity is the degree to which the aluminium chloride has been neutralised with hydroxide, expressed as a percentage. Higher basicity means more pre-formed polymeric aluminium species, lower acidity and less alkalinity consumption in the water. High-basicity grades suit cold or soft water, while medium-basicity grades are common general-purpose coagulants. Choose basicity by jar testing on your own water.

PAC works over a wider pH range, typically about 5 to 9, forms flocs faster, performs better in cold water and consumes less alkalinity, so less caustic or lime is needed afterwards. It usually produces less sludge. Aluminium sulfate is cheaper per kilogram but may need a higher dose. Compare them on cost per cubic metre treated, not price per tonne.

Spray-dried PAC is made by spraying the solution into hot air, giving a white to light yellow powder with very low insolubles; it is used for drinking water and demanding applications. Drum-dried PAC is dried on heated rollers, giving a yellow to brown powder with more iron and insolubles at a lower cost; it suits wastewater and sludge treatment.

Yes, but only grades that meet the relevant requirements. Products used in public drinking-water supply in the UK generally need approval under Regulation 31 of the water quality regulations and must conform to BS EN 883. Ask whether the specific grade offered holds that approval before specifying it for potable use, and keep the documents on file.

Powder is normally dissolved in clean water to about 5–10% solution in a stirred tank made of plastic or lined steel, then dosed by metering pump. Use dust control and respiratory protection during make-up. Diluted solutions can hydrolyse over time, so prepare them fresh, typically for use within a few days. Confirm the optimum dose by jar testing.