Characterisation of sewage sludge-derived organic matter: lipids and humic acids

The distribution of humic substances and lipids extracted from two sewage sludges (S1 and S2) resulting from different wastewater and sludge processes was studied. The humic acids were investigated using elemental analysis, infrared spectroscopy and thermally assisted hydrolysis and methylation coupled to gas chromatography–mass spectrometry (THM-GC–MS). The influence of lipid extraction prior to humic fractionation was also tested. The main differences between the two sludges are evident from the analysis of lipids and the distribution of carbon in the humic fractions. The results reveal that the carbon is more extractable in the most matured sludge (S2). The chromatograms of the lipids show that the extract from the least matured sludge (S1) consists mainly of fatty acids whereas that from the most matured one is dominated by steroids. THM-GC–MS analyses show the aliphatic nature of the sludge derived humic acids as confirmed by elemental analyses and infrared spectroscopy. The aliphatic character is attributed to the presence of lipids presumably trapped in the humic structure.