Alum or hydrated aluminum sulphate is widely used precipitating
phosphates and aluminum phosphates (AlPO4). The basic reaction is:
Al3+ + HnPO43-n ↔ AlPO4 + nH+
The dosage rate required is a function of the phosphorous removal
required. The efficiency of coagulation falls as the concentration of
phosphorous decreases. In practice, an 80-90% removal rate is
achieved at coagulant dosage rates between 50 and 200 mg/l.