STABILISATION DE LA MATIERE ORGANIQUE AU COURS DU COMPOSTAGE DE DECHETS URBAINS : Influence de la nature des déchets et du procédé

Composting is a process of biological treatment of organic wastes which reproduces the natural process of organic matter humification in soil. Today in France, only 7 % of the urban wastes are composted, although the organic part represents 50 % of the 47 million tons produced annually. Composts are mainly used in agriculture to increase or to maintain soil organic matter concentration. Their behavior after soil incorporation depends on the stability of their organic matter (OM).In this work, we studied the evolution of compost OM characteristics during composting, in relation with their origin (nature of the composted wastes and process of composting) and evaluated the potential storage of OM in soil with their use. The relation between the residual biodegradability of compost OM and the potential availability of nitrogen in soil was also studied.Ten urban composts made from municipal solid wastes, biowastes, green wastes or sewage sludge, coming from industrial composting plants were sampled after three, four and six months of composting. To avoid the influence of composting process, eight composts elaborated in laboratory reactors from variable proportions of green wastes, biowastes, and papers-cardboards, were followed during three months. A range of five classes of compost OM stability was defined on the basis of mineralization kinetics of compost carbon during incubations in a reference soil. Compost OM was characterized by chemical (humic substances) and biochemical (lignin, cellulose, hemicellulose and soluble fraction) fractionation and by analysis in infrared spectrometry (FTIR). The degree of compost maturity was defined from the level of compost OM stability and the range of stability previously defined was used as reference for the validation of various indicators of maturity.During the composting process, all composts evolved towards products with similar characteristics and independent from the composted waste and process that only influenced the rates of stabilization. In most cases, the cellulose fraction rapidly decreased when nitrogen was not limiting. The slower degradation of lignin as compared to total organic matter led to its relative enrichment in compost OM. The IR spectra confirmed the increase of aromaticity during composting.When greenwastes were dominant in the composted wastes, their initial high lignin concentration explained the fast stabilization of OM during composting and little evolution was observed between three and six months of composting. On the contrary, composts made from municipal solid wastes, initially richer in cellulose materials (papers-cardboards) with a relative lack of nitrogen, were not stabilised after three months of composting, and required at least six months of composting, with turning, to be stabilized. The influence of biowastes on compost OM stabilization was not pointed out, probably because they only represent a weak proportion of the dry mass of the initial mixtures.From the biochemical fractionation results, an indicator of the proportion of stable OM in compost was calculated (Biological Stability Index, BSI). This stable fraction of compost OM contributes to soil OM upkeep or increase after compost application. For greenwaste composts, BSI little increased during maturation and the simultaneous decrease of total compost OM made lightly decreased their efficiency to soil OM upkeep. The reverse was observed for municipal solid waste composts. Their important stabilization during maturation made increase their BSI. Of this important stabilization resulted a relative increase of their efficiency for soil OM upkeeping between three and six months of composting.A small potential availability of compost nitrogen was estimated from incubations of soil-compost mixtures. The organic fraction of nitrogen of stabilized compost mineralized slowly. On the contrary, a strong immobilization of soil nitrogen occurred after incorporation of unstable compost, followed by a faster mineralization than observed with stable compost.Among the tested indicators of maturity, the humification ratio (RH=CHA / CFA) was the more reliable chemical indicator. RH values smaller than 1 indicated a lack of maturity, and RH larger than 1.3 corresponded to stabilized composts with high maturity levels. The self-heating test was the most relevant to be used on composting platforms. These two indicators allowed a correct estimation of compost OM stabilization level, from which recommendations could be made on the optimal period for soil incorporation, taking into account the risks of nitrogen immoblization.Le compostage des déchets organiques reproduit le processus naturel de transformation dans le sol, des matières organiques fraîches, d'origine animale et végétale, en matière organique humifiée, communément appelée humus. C'est une pratique ancienne, dont on trouve des témoignages dans les lointaines civilisations romaine et même égyptienne. Mais l'évolution des déchets suit étroitement celle des sociétés, et les déchets du passé qui étaient essentiellement organiques (effluents d'élevages, déchets alimentaires, résidus végétaux...) renferment aujourd'hui une gamme très hétérogène de produits (déchets verts, déchets alimentaires, effluents d'élevages et industriels, papiers, emballages plastiques et métalliques,...). Aux composts « artisanaux » destinés à un usage très local se sont ajoutés les composts industriels visant à digérer les déchets des collectivités et des industries agro-alimentaires, pour les valoriser en agriculture. Cette diversité des déchets, accentuée par la diversité des procédés de compostage conduit à une grande variabilité des composts produits