Optimizing chemical extraction of heavy metals from anaerobically digested sewage sludge using response methodology

Abstract

The presence of heavy metals in digested sewage sludge presents an environmental concern of great proportions owing to the large per capita sludge volumes involved globally. Besides, sludge having high heavy metal concentration is also not suitable for agricultural purposes hence the need for heavy metal removal prior to disposal or use at the farm level. Determination of the optimal conditions for heavy metals extraction from anaerobically digested sewage sludge is thus warranted. The goal of this study was therefore to employ a response surface methodology to optimize chemical extraction of heavy metals (Zn, Cu, Pb, and Ni) using the full factorial design. The three factors considered were pH, hydrogen peroxide dosage, and extraction time. The results were analyzed statistically using analysis of variance, F-test, and lack of fit to identify the most important process variables affecting the heavy metal extraction efficiency. For the heavy metals studied, 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 the heavy metal extraction efficiency. There was no significant interaction between pH and Hydrogen peroxide dosage in the extraction of heavy metals under the tested conditions. The optimal conditions of heavy metals extraction obtained were pH (3), extraction time (10 days) and Hydrogen peroxide of (1g/l). The percentage extraction at these conditions was above 98% for all the heavy metals. These results are promising for the management of digested sewage sludge.