Recycling ferric water treatment sludge (Fe-WTS) as coagulation aid: A case study in residual aluminum control

To provide an insight into aluminum (Al) control during the treatment of micro-polluted water, recycling ferric water treatment sludge (Fe-WTS) as an alternative approach was innovatively investigated. Particular attention was paid to the recycling ratio of Fe-WTS when coagulated at an optimized dosage of both Al 2 (SO 4 ) 3 and polyaluminum chloride (OH/Al = 2.0, denoted as PAC 20 ) respectively. Coagulation performance, i.e., turbidity, organics and Al speciation were also examined. A morphological analysis, characterized via floc size and fractal dimension, was further conducted to explore the fundamental floc operational parameters and to elucidate the mechanism of different Al species removal. The results indicate that an appropriate recycling ratio yields improvements in residual Al control. In comparison, Fe-WTS is more effective in dissolved Al (Al D ) removal when coagulated by Al 2 (SO 4 ) 3 , while is proper to the reduction of colloidal Al (Al CO ) when coagulated by PAC 20 . Flocs properties are significantly correlated with Al speciation during the treatment. Specifically, floc size is proportional to Al CO removal rate, however, the value of floc fractal dimension is inversely proportional to Al D removal rate.