Work out alum dose for coagulation instantly with clear inputs, formula shown and shareable results.
Aluminium sulphate hydrolyses to aluminium hydroxide floc, consuming alkalinity and depressing pH — about 0.5 mg of alkalinity as calcium carbonate per mg of alum. If residual alkalinity falls below about 20 mg/L the coagulation pH drops out of the optimum window and lime or soda ash must be added.
Alum dose
Alum (kg/day) = flow (m3/day) x dose (mg/L) / 1000
Alkalinity balance
Residual alkalinity = raw alkalinity - dose x consumption factor
Residual aluminium in treated water is regulated. Coagulation pH must be controlled and residual aluminium monitored.
By jar testing on the actual raw water across a dose and pH matrix, repeated as raw water quality changes. Dose requirements vary several-fold between clear and turbid seasons.
Aluminium hydroxide solubility is minimum around pH 6.0-6.8, where floc forms best and residual dissolved aluminium is lowest. Outside that window both treatment performance and residual aluminium compliance suffer.