Aluminium Recovery from Water Treatment Sludge as Coagulant by Acidification

Aluminium in water treatment sludge can be recovered as coagulants by acidification method. However, there are some factors that affect efficiency in recovering aluminium, such as stirring speed during acidification, pH, and sludge characteristic. This study aims to find the optimum acidification stirring speed in recovering alum and to evaluate the performance of recovered coagulant compared to a pure coagulant. Aluminium recovery is carried out by adding sulphuric acid until the pH of the sludge reaches 2.5. The acidic sludge is stirred for 30 minutes, with acidification stirring speed is varied from 240-720 rpm, and left for settling until the supernatant is formed. The supernatant was used as a recovered coagulant, examined using a jar test to determine the optimum dose, and compared with the pure coagulant. The results show that the optimum acidification stirring speed was obtained at 540 rpm, where mass balance shows aluminium recovery efficiency of 16.5%. The optimum dose for recovered coagulant was 25 ppm, with a turbidity reduction of 93.38% in jar test. While the optimum dose for pure alum was 20 ppm, with turbidity reduction of 93.26%. Though potentially reduced higher turbidity, the recovered coagulant has slightly lower quality than the pure coagulant, which required a higher optimum dose.