PAC vs alum: an honest comparison

Alum wins on the invoice and loses on the plant. If PAC costs more per tonne but you dose a third to a half as much, the chemical cost per cubic metre is already close — before you count anything else.

Then add what alum brings with it. Alum consumes roughly two to three times more alkalinity, so many plants dose lime or soda ash purely to hold pH; that is a second chemical, a second dosing system, and a second line on the operating budget. Alum also produces more sludge, and that sludge holds more water, so you dewater and dispose of more of it.

There is also the sulphate question. Alum adds sulphate to the treated water. Where that water feeds reverse osmosis, or where the distribution network runs through concrete infrastructure, added sulphate is not neutral.

Where alum still wins: very high turbidity raw water where sheer bulk of coagulant is what is needed, plants with abundant natural alkalinity and no pH constraint, and sites with existing alum infrastructure where the switching cost outweighs the running saving. Those cases are real — just less common than the price-per-tonne comparison suggests.