Alum Recovery from Water Clarifier Sludge as Coagulant and Phosphorus Removal by Recovered Alum

To recycle water clarifier sludge as a coagulant, we extracted alum from the sludge (hereafter called recovered alum (RA)) and attempted to obtain a high Al concentration and improve RA quality. RA was obtained by acidifying the sludge with concentrated sulfuric acid to adjust its pH below 2, and then centrifuging the mixture and decantating the supernatant. RA contained Al at 0.75-9.4% as Al2O3. To obtain a high Al concentration, it was better to use the sludge with a low water content. As impurity concentration increased with Al concentration, we purified RA by coagulating it with the addition of sodium hydroxide and resolubilizing it with the addition of dilute sulfuric acid. We were able to remove T-N, Fe, and Mn in the crude RA at 43.6%, 45.4% and 76.5% respectively by purification. When we used RA to model effluent containing 0.85 mg P·L-1 and underground water samples containing 1.14-1.52 mg P·L-1, the phosphorus removal efficiency of RA was essentially comparable with that of commercial alum. RA removed over 90% phosphorus when the PO43-:Al molar ratio was 1:5. As a conclusion, RA is a useful coagulant as long as we pay attention to the dose of contaminants.