Combustion effect on the physical, chemical, mineralogical, and microstructural compositions of urban sewage sludge

Abstract

This paper investigates the combustion of the urban sewage sludge from 550 to 1000˚C with
the aim of studying the evolution that occurs on the composition of the sewage sludge during the
heat treatment process. Several analyses have been carried out: pH, X-ray fluorescence, and atomic
absorption spectrometry. The results of the experiments indicate that increasing the temperature
increases the basic character, the amount of major and minor elements in the obtained sewage
sludge ash. These phenomena are due to the richness of the sludge in the organic matter, which
led to a considerable volume reduction when the sludge was combusted. The X-ray fluorescence
analysis of the sewage sludge ash showed an increase in the amount of aluminosilicates, which
constitute the reactive part in a pozzolanic material. The atomic absorption spectrometer analysis of
the heavy metals in the ash showed that their respective concentrations depend on their melting and
boiling points. The calcium carbonates decomposition, observed by X-ray diffractometry, occurred
during the combustion of the sludge between 650 and 700˚C. The scanning electron microscopy
showed morphological changes in the sewage sludge ash when the temperature increases. The size
of the ash grains increased due to agglomeration, and sintering occurs. It can be concluded that
the combustion of the dried sewage sludge leads to an ash, whose properties are interesting for its
valorisation in building materials.