Stabilization of Organic Matter During Composting: Influence of Process and Feedstocks

Information on compost organic matter stability is necessary to anticipate the effects of compost application on soil properties. Such information will soon be required by French legislation. The main objectives of the present work were: (1) to study the influence of the composting process and the composted waste on the kinetics of stabilization of organic matter during the composting process; (2) to validate physicochemical indicators of compost stabilization and also rapid tests used by the compost producers on the composting plants in order to be able to follow compost stabilization during the process. Ten compost windrows were sampled after 3, 4 and 6 months of composting at plants where green wastes (GW), biowastes (BIO), sludge (SG) or municipal solid wastes (MSW) were composted. In some plants, the composting process was accelerated by enforced aeration. The stability of the organic matter was assessed by laboratory incubations during which organic matter mineralization was followed. Maturity tests were run on compost samples which were then analyzed. For the GW, BIO and SG composts, the stabilization of organic matter occurred more rapidly than for the MSW composts and 3 to 4 months were enough for satisfying organic matter stabilization. For the MSW composts, 6 months were necessary to reach stabilization similar to the GW or BIO composts. The self-heating test was highly significantly correlated with the compost organic matter stability as evaluated from the incubation results and should be recommended to the compost producers rather than the Solvita test. Among the physicochemical characteristics of the composts, total organic C (TOC) appeared to be well correlated with organic matter stability of the composts and a better indicator than the C/N ratio, pH, electrical conductivity and N-NO3−/N-NH4+ ratio. The humic and fulvic acids (C-HA + C-FA) represented 9 to 27 percent of TOC and the humification index (C-HA/C-FA) was also well-correlated with organic matter stability.