The effect of composting on the organic colloidal fraction from domestic sewage sludge

Abstract

Due to the frequent use of domestic sewage sludges as organic additives to soils with low humus content, the chemical characteristics of the extractable polymers from composted and uncomposted sludges were described. The alkali-soluble, acid-insoluble sludge fraction (humic-like) was isolated after exhaustive lipid extraction, and analyzed by chemical degradation followed by combined gas chromatography mass spectrometry. It was observed that this sludge fraction contained an important amount of lipid compounds (more than 40% by weight). Most of this lipid material can be physically removed, but the residual polymer fractions were also found to be highly aliphatic in nature. The high yields upon degradation of several types of alkanoic acids and the relatively high proportion of polypeptides (30% by weight), as well as carbohydrates, suggested that the humic-like fractions from both the composted and uncomposted sludges consisted of slightly altered microbial and residual biopolymers. The effects of composting sludge mainly concern the selective biodegradation of the less resistant moieties. The great differences between the chemical nature of the humic-like fraction of the composted sludge and that of the soil humic acids are considered to reflect the low proportion of lignified materials in the original sludge.