Composting of domestic refuse and sewage sludge. II. Evolution of carbon and some “humification” indexes

Abstract

The evolution, during composting of city refuse (R-pile) and city refuse+sewage sludge co-composting (R+S-pile), of total organic matter (TOM) and some forms of carbon: total organic carbon (C t), oxidizable carbon (C o), alkaline-extractable carbon (C ex), humic acid-like carbon (C ha), and fulvic acid-like carbon (C fa), was evaluated. The results were correlated with some maturity parameters, namely, C/N ratio in the solid phase (C/N s), C/N ratio in the water-soluble phase (C/N w), and cation-exchange capacity (CEC), to examine the relationships between the mineralization of the organic matter, biological stability of compost, and “humification”. The regression analysis was carried out considering jointly all the data points of both the R-pile and the R+S-pile, to extrapolate new maturity criteria irrespective of the initial composition of the urban raw material used for composting. The results show that the CEC/C t and C ha/C fa ratios may also be considered to be good maturity parameters. A value higher than 1.9 for both ratios, in addition to a value lower than 6 for the C/N w ratio, may be considered as a suitable indicator of a high degree of maturity in composts of urban origin (municipal solid waste and sewage sludge composts).