Home Courses Treatment of Urban Sewage Course materials Lectures 5. Elaboration Question 6

5. Elaboration Question 6

Course subject(s) 5. Nitrogen and phosphorus removal

Question 6: Chemical phosphorous removal utilising alum

Determine the amount of liquid alum required to precipitate phosphorus in wastewater that contains 8 mg/l of P and a P reduction of 85% is required. Also determine the required alumn storage capacity, if a 30-day supply is to be stored at the treatment facility. The flowrate is 12,000 m3/d. Alumn information:


  • Formula for liquid alum Al2(SO4)3∙8H2O
  • Alum strength = 48%
  • Density of liquid alum solution = 1.2 kg/l

Phosphate precipitation with aluminium: Al3++HnPO(3−n)4←→AlPO4+nH+

Theoretical molar ration = 1 mol Me3+ / mol P, higher in practice (testing required).

P reduction (%) Typical molar Al/P ratio
75 1.4 : 1
85 1.7 : 1
95 2.3 : 1

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