CARBON-CONTAINING AND NITROGEN-CONTAINING COMPOUNDS IN COMPOSTED URBAN REFUSES

The composition of the organic matter of four composted urban refuses (one of them amended with CaCO3 in the composting process) from Spanish industrial composting plants was studied. Fundamental components and C-bearing compounds (humic substances) were assessed by classical fractionation methods; N-bearing compounds were determined by acid step-wise hydrolyses. Cellulose and hemicelluloses largely predominated over lignin, which was the second important component, followed by water-soluble compounds, the content of lipids being very low. Water-soluble compounds represented 3-14% of the organic C. Alkali-soluble compounds were quite low since humic compounds (humic and fulvic acids) only represented about 17% of the organic C (24% in the amended compost); humic acids predominated over fulvic acids, the most polymerized compounds being higher than the less polymerized ones. The predominant fraction (about 70% of the organic C) was the insoluble one which comprised unhumified compounds, insolubilizable humin, microbial humin (1.4-9.0% of the organic C) and residual humin (20-40% of the organic C). The extraction percentage was very low whereas the degree of humification was close to 50%. About 85% of the organic N was hydrolysable. Hydrolysable unidentified-N was the predominant organic N form, followed by alpha-aminoacid-N. NH4+-N from organic compounds and hexosamine-N were very small and amide-N was not detectable. According to the percentage of humification the four composted refuses were stabilized materials.