Influence of mixing conditions on coagulant recovery efficiency and quality

Abstract In this work, the influence of mixing conditions on aluminium recovery from alum sludge and on the recovered coagulant (RC) quality was investigated using acidic treatment. The response surface methodology with a face centred design was used to evaluate the effects of velocity gradient and mixing time on aluminium recovery efficiency, and on the chemical oxygen demand (COD), total suspended solids (TSS) and total phosphorus contents of the RC. No significant effects of both mixing intensity and mixing time on aluminium recovery efficiency were observed; however, these factors significantly affected the quality of the RC in terms of COD, TSS and total phosphorus. The higher velocity gradient increased COD and TSS and the longer mixing time increased phosphorus and TSS in the RC. The defined operational conditions (velocity gradient = 100 s−1 and mixing time = 5 min) for aluminium recovery were suggested on economic basis. At these optimal conditions, the RC presented the lowest values of TSS and total phosphorus (100 mg L−1 and 7.0 mg P L−1, respectively), and the following other characteristics: 1,001 mg Al L−1; 384 mg Fe L−1 and 1,100 mg COD L−1.