Valorization of hydrothermal carbonization process water from food waste through anaerobic digestion: effect of operating conditions and variability of the feedstock

Hydrothermal carbonization (HTC) is a promising process to convert food waste into a carbonaceous material, provided that the process water (PW) generated is properly handled. To do so, anaerobic digestion (AD) was shown to be an interesting process that can both produce additional energy in the form of biomethane and reduce the organic contamination. Despite many recent research studies on the combination of these two processes, the effect of pre-processing of the substrate before the HTC and the post-processing of the slurry remains unclear. In addition, a clear understanding of the influence of the substrate composition on the PW and its methane potential during AD is also lacking. The objective was to assess the influence of different HTC operating conditions, as well as pre- and post-processing steps on the methane yield obtained by the AD of food waste aqueous phase. Temperature and food waste composition were shown to be the most important parameters to explain the variability of the PW composition. However, the total recovery of organic compounds in the PW, expressed as the COD, was mainly affected by the post-processing of the HTC products. The separation of the products by filtration with large pore sizes can increase the COD of the PW by 26%, thus increasing also the methane production and the methane yield. Similarly, the methane production obtained from the PW was mainly affected by the temperature (36% of the variance) and the substrate composition (30%), and to a lesser extent by the residence time, the pressure and the substrate pre-processing. Overall, it was possible to correlate the methane yield to the PW composition or the HTC conditions with linear regression models having R2 values higher than 0.87. Process water composition is influenced by temperature, residence time, and pressure. Food waste composition impacts process water methane yield. Hydrochar separation affects methane yield of process water. Methane yield of process water is well predicted by COD.