removal of heavy metals in anaerobically digested sewage sludge from kariobangi sewage treatment works using citric acid and chemical oxidation

Abstract

Sewage sludge from municipal wastewater treatment plants has high content of organic matter and substantial N, P and K concentrations that suggest its use preferentially as a fertilizer in agriculture however the presence of heavy metals in the sludges restricts their use. In this connection the objective of this study was to characterise and chemically extract heavy metals in anaerobically digested sewage sludge from Kariobangi Sewerage Treatment Works in Nairobi. In addition, the study sort to determine the optimal values of pH, Hydrogen Peroxide dosage and extraction time for effective heavy metals extraction. Using the full factorial design with three factors at two levels, the chemical extraction was undertaken with the help of Citric acid and Hydrogen peroxide. The two levels considered were: pH (3.0 and 5.0), hydrogen peroxide dosage (1g/l and 5g/l), and extraction time (1day and 10 days). The results were analyzed statistically using the Student’s t-test, analysis of variance, F-test, and lack of fit to define the most important process variables affecting the heavy metal removal efficiency. The results revealed that the sewage sludge had high amounts of N, P and K in addition to heavy metals of which the concentrations of Lead and Zinc were higher than the National Environment Management Authority maximum allowable values. The Student’s t-test, analysis of variance and F-test revealed that for heavy metals Zn, Pb, Ni and Cu the most significant effect was 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 removal efficiency of the heavy metals. 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 Zn, Pb, Ni and Cu extraction obtained using response surface methodology from anaerobically digested sewage sludge were pH value of 3, extraction time of 10 days and Hydrogen peroxide dosage of 1g/l. The percentage extraction at these conditions was Zn (98.4%), Pb (98.8%), Ni (98.6%) and Cu (98.2%). This percentage extraction reduced the heavy metals concentration to below the National Environment Management Authority maximum allowable values. Chemical oxidation with Citric acid acidification was found to bean excellent heavy metals extraction method and sewage treatment plants should consider applying it in their anaerobic treatment processes.