Application of nanofiltration membrane in the recovery of aluminum from alkaline sludge solutions

Large amounts of aluminum hydroxide (Al(OH)3) exist in water purification sludge (WPS) because of the added aluminum coagulant in water treatment process. Notably, Al(OH)3 is an amphoteric compound, can be dissolved in its basic condition using sodium hydroxide to form aluminate ions (Al(OH)4-). However, in a process in which pH is increasing, the humid acid can be dissolved easily from WPS and will inhibit the recovery and reuse of the dissolved aluminate ions. This study attempts to fix this problem by a novel approach to separate Al(OH)4 - ions using nanofiltration (NF) technology. Sludge impurity in a alkaline solution is retained by the NF membrane, such that the process recovers Al(OH)4 - ions, and significantly decreases the organic matter or heavy metal impurities in the permeate solution. The Al(OH)4- ion is an alkaline substance. Experimental results confirm that a recovered coagulant of Al(OH)4- ion can effectively remove kaolin particles from slightly acidic synthetic raw water.