Environmental assessment of the agronomic recovery of post-treated digestates

International audience; Objectives The environmental assessment of digestate (residue from anaerobic digestion) post-treatment pathways is seen from a new perspective: it is no longer a residue management but an agronomical pathway which is environmentally assessed. LCA is used to compare the post-treatment pathways of four types of digestates. Methodology Pathways are compared in terms of the agronomical value of the produced and spread products. The fertilizer value is based on the amount of available nitrogen for plants and the amendante value is based on the amount of organic matter that remains to the soil. A functional unit based on "to provide X mass of available nitrogen and Y mass of organic matter to the soil, from an annual production of Z mass of raw digestate" has been deployed to the various pathways (farm, territorial, biowaste) applied to each one of the digestates. Results For a given nitrogen fertilization and a given amendante value, adding available nitrogen via mineral fertilizing or organic matter via peat spreading has a low influence on the scenario hierarchy. Moreover, emissions that follow spreading are the main causes of the impacts of the pathways. This is not true regarding the resource depletion category, which is mainly due to the reagents production (for centrifugation and gas treatment). When a raw digestate direct spreading is compared to the spreading of both solid and liquid phases, establishing or not a raw digestate post-treatment does not discriminate the pathways concerning digestates issued from farm biogas plants. The results are more contrasted for the post-treatment of digestates from territorial pathways, when the spreading of both solid and liquid phases is compared to the spreading of a dried product and the liquid products issued from ultrafiltration and reverse osmosis. The environmental assessment is also balanced in the comparison of the spreading of the solid phase towards the spreading of a digestate biowaste compost. Conclusion LCA has shown its limits here as an evaluation tool for agricultural product-providing-pathways, due to the poor grasp of impacts and the benefits of spreading such products on agricultural soils via LCA. This study approaches the aim of a digestate post-treatment via the agronomical view. More investigations should be done on this interest to better reflect the environmental benefits of post-treatment. Authors would like to thank the project partners for having provided the data used in the LCAs.