Characterization of Solid Digestates: Part 2, Assessment of the Quality and Suitability for Composting of Six Digested Products

The aim of this research was to characterize six organic solid digestates in order to discuss their potential direct agricultural utilisation or their further valorization through an aerobic post-treatment. The studied digestates came from four different sources: waste water treatment sludge, food-processing wastes, agricultural solid wastes and source-selected organic fraction of municipal solid wastes. Physico-chemical, biochemical and biological characteristics showed that a direct use of digestates and application on land as soil amendment or fertilizer is not recommended according to current standards. A post-treatment appeared thus to be necessary in order to obtain commercial amendments or fertilizers. A composting post-treatment of these digestates would assure their full biological stabilization as these organic residues still contain residual biodegradability and present sometimes phytotoxic risks. The prediction of operational parameters showed that the composting post-treatment of these digestates will probably lead to a limited rise in temperature, require moderate aeration supply and last less than a typical composting treatment of organic wastes.