Turbidity removal from synthetic turbid water using coagulant recovered from water treatment sludge: A potential method to recycle and conserve aluminium

Abstract

The water treatment sludge (WTS), a byproduct of coagulation-flocculation-sedimentation process in drinking water treatment contains significant concentration of aluminium (Al). The WTS is mostly disposed on land or in sewers without treatment. This practice not only poses a serious threat to environment but also results in permanent loss of metals like Al. In order to prevent the adverse effects, studies are being conducted to find alternative use of WTS, and one of the focus areas is to recover coagulant (a solution containing Al) from the WTS by leaching with high strength acid and reuse it as coagulant for treating industrial wastewater. However, the use of the recovered coagulant (RC) for treatment of drinking water is not yet reported. The main concern being the presence of other metals in the RC along with Al. The RC if added to remove turbidity from drinking water may result in elevated concentration of metals in treated water which is objectionable.In order to address this issue, the present study proposes a method to recover coagulant from WTS, and reuse it safely as a coagulant in drinking water treatment without hampering the treated water quality. Specifically, the study used low strength nitric acid to leach Al from WTS. The leaching studies were performed using a Box-Behnken Design (BBD) with three factors: acid strength (M), reaction time and sludge dose. Each factor was set at three levels: acid strength (0.025 M, 0.1625 M and 0.3 M), reaction time (30 min, 75 min and 120 min) and sludge dose (10 g/l, 12.5 g/l and 15 g/l). As Al is the main element responsible for coagulation, leaching efficiency of Al (AlLE) was fixed as the response variable. Performance of the RC in removing turbidity was evaluated on synthetic turbid water and was compared with turbidity removal obtained with 1% (w/v) solution of commercial alum.The results indicate that a maximum concentration of 432 mg/l of Al could be achieved in the RC with optimum factor settings (obtained from statistical analysis) of 0.25 M acid strength, 11.17 g/l sludge dose and 120 min reaction time. The addition of 1 ml/l of the RC in synthetic turbid water resulted in 74% removal of turbidity (with effluent turbidity concentration < 20 NTU) which was similar to removal efficiency obtained with addition of 0.6 ml/l of 1% (w/v) commercial alum solution.The environmental significance of the study is that, it shows that the RC has potential to be used as a coagulant in drinking water treatment. This will not only prevent the permanent loss of Al due to disposing the WTS in landfills but also minimize the requirement of fresh Al salts for coagulation thus resulting in conservation of a natural resource.