Alum — Frequently Asked Questions

In two ways at once. Fine suspended particles carry a negative surface charge that keeps them mutually repelled and permanently in suspension; trivalent aluminium ions neutralise that charge so the particles can approach and aggregate. At the same time the dissolved aluminium hydrolyses and precipitates as a voluminous gelatinous hydroxide floc that physically entraps particles as it settles, an effect called sweep flocculation.

Because hydrolysis consumes alkalinity and drives pH down. Aluminium hydroxide is only minimally soluble across a narrow band around pH 6 to 7, and outside that band residual dissolved aluminium in the treated water rises. Lime or soda ash is therefore usually dosed alongside alum to hold the pH in the effective window, which adds chemical handling and cost.

Because PAC arrives pre-hydrolysed, so it consumes far less alkalinity and moves pH much less, works across a wider pH range, performs better in cold water and produces less sludge per unit of turbidity removed. Alum remains cheaper per tonne and is still widely used where its pH effect can be managed economically.

As solid lump, kibbled or granular material, and as a liquid solution typically around 8 percent Al2O3. Liquid is easier to dose accurately and avoids dissolution equipment, but costs more to freight because most of it is water. Solid suits sites with make-up facilities and high consumption.