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.
