Can hydrothermal carbonization be used as a mean to increase the circularity of existing food waste anaerobic digestion plants?

Large-scale anaerobic digestion of food waste is often characterized by important material losses due to the pre-treatment steps required to remove impurities. Hydrothermal carbonization was used to increase the circularity of anaerobic digestion plants by converting the reject fractions, that would otherwise be discarded, into hydrochar and process water. The viability of this conversion was tested experimentally with samples collected from two commercial plants, and experimental results were used to verify the energetic performance of the process and realize a life-cycle assessment. While the hydrochar could be valorized as a fuel to replace coal, due to the high energy content (>20 MJ kg−1 for the light rejects) or as a soil amendment, the process water can be converted into additional methane. For every reject fraction considered, an increase in the methane potential of the process water was shown, when compared to the initial fraction, showing a solubilization of the organic materials. This high methane potential leads to an additional production of energy for anaerobic digestion facility. Overall, the conversion of the reject fractions by hydrothermal carbonization improves the environmental performance of anaerobic digestion plants for every scenario considered (1.2- to 1.6-fold reduction for climate change impact category), with the biggest gain when the hydrochar is used as a fuel to substitute coal.