Second life for digestate - Mechanisms of interaction between organic matter and phosphates during hydrothermal liquefaction of digestates – D2LIFE

The D2LIFE project aims to understand the interactions between organic matter and phosphates during the hydrothermal liquefaction of residual biomass (e.g. digestate, sewage sludge, manure, agro-waste). This knowledge will contribute to the development of a hydrothermal treatment process that integrates the recovery of phosphorus (P) with the production of bio-fuels, by concentrating P in a solid phase mainly composed of Ca phosphates which will be used as fertilizer. The objective of the project is to identify and describe the effect of the main operating parameters of the hydrothermal process (e.g. temperature, pressure, reaction time, addition of reagents) on the kinetics and pathways of P conversion, and on the partition of P among the aqueous, oily and solid phases resulting from biomass liquefaction. The methodological approach of the project is based on hydrothermal liquefaction experiments carried out in a batch reactor using a real substrate (digestate from anaerobic digestion of sewage sludge), followed by modeling of the experimental results. The aqueous, oily and solid phases resulting from digestate liquefaction will be separated and characterized in order to quantify the main forms of P (organic and inorganic). The results will allow to identify the effect of the main operating parameters of the hydrothermal process on the conversion pathways and on the partition of P among the different phases, and finally to develop a model that describes P conversion during hydrothermal liquefaction of digestate. Finally, a techno-economic analysis of the process will be carried out to identify the optimal operating parameters based on exergetic criteria, conversion yields, and by-products (Ca phosphates and bio-fuels) recovery performance. The results of the D2LIFE project will have an impact in the field of bio-waste and residual biomass management, and will contribute to the development of a sustainable process chain for the valorization of digestate.