Recovery of alum from wasted sludge produced from water treatment plants

Abstract

Alum is used in the coagulation of raw water in almost all water treatment plants world‐wide. The sludge produced from this process is usually thrown away i.e. wasted. In Egypt, thousands of tons of alum are used annually in water treatment plants across the country costing millions of dollars. This paper deals with the recovery of most alum available at the coagulation sludge. The recovery process involved the following steps: concentrating the sludge from 5% to 20% solids, acidification with sulfuric acid, separating the supernatent liquid, decolorizing it using charcoal, filtration, concentrating the solution by evaporation, and finally crystallization to obtain nearly pure alum crystals. The effects of operating variables on the efficiency of recovery were also studies. The recovered alum was tested in coagulation runs using a jar‐tester. The results were very promising.