Screening and extraction of heavy metals from anaerobically digested sewage sludge

Abstract

Heavy metals extraction from anaerobically digested sewage sludge is an important step towards achieving a safe usage of the large amounts of sewage sludge on agricultural land. The extraction reduces heavy metals accumulation in soil and their bio accumulation in plants, animals and humans through food-chain thus reducing both human health problems and adverse environmental impacts.Chemical extraction of heavy metals from anaerobically digested sewage sludge using Citric acid was studied using the full factorial design. The three factors considered were pH, Hydrogen peroxide dosage, and extraction time at two different levels: pH (3.0 and 5.0), hydrogen peroxide dosage (1g/l and 5g/l), and extraction time (1day and 10 days) with the objective of obtaining the models for the heavy metals extractions. The results were analyzed statistically using the Student’s t-test, analysis of variance, F-test, to define the most important process variables affecting the heavy metal extraction efficiency.Lead had the highest extraction at 99.90%, followed by Nickel at 99.87%, Copper at 99.80% and Zinc at 99.68%. The most significant effect was ascribed to extraction time followed by Hydrogen peroxide dosage and the interaction of the two. The pH effect and the interaction between pH and time also had an influence in extraction efficiency of heavy metals. There was no significant interaction between pH and Hydrogen peroxide dosage in the extraction of heavy metals Zn, Pb, Ni and Cu under tested conditions.