Evaluation of leached metals in recovered aluminum coagulants from water treatment slurry

Abstract

The water treatment industry consumes large quantities of coagulant and produces huge amounts of slurry. The cost of alum used in water treatment, stringent regulations and negative impacts of sludge disposal are the motive to do integrated research studies on the technical feasibility of aluminum coagulant recovery from sludge using acidification. This work studied the leaching of iron, manganese, and chromium as the most extracted metals with aluminum during sludge acidification; furthermore, these metals have a great impact on the recovered coagulants' efficiency and treated water quality. The sludge used was collected from El-Sheikh Zayd water treatment plant in Egypt, then dried and ground; afterward, the effect of acid concentration, sludge mass, temperature, mixing speed and mixing time was studied. In addition, it was noticeable that the efficiency of sulfuric acid in leaching iron, manganese and chromium is higher than that of hydrochloric acid. Also, higher leaching for the three metals was obtained in all the experiments using higher acid concentration, elevated temperature, and rotational speed. Finally, the leached metals in recovered aluminum coagulants will not limit its application to water and wastewater treatment, as their concentrations are still very low if compared with aluminum, even with the highest leaching efficiency.