Laboratory evaluation of water washing as a post-processing strategy for food waste-derived hydrochar intended for use as a soil amendment

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Hydrothermal carbonization (HTC) is an advantageous and potentially environmentally beneficial approach for the conversion of wet biomass and waste streams into a value-added solid product referred to as hydrochar. Although hydrochar composition and properties generated from the carbonization of a variety of feedstocks have been well-studied, there is a need for understanding whether hydrochar requires further refinement and/or treatment prior to being used in environmental applications, particularly when using the hydrochar as a soil amendment. In this study, work focused on understanding potential issues associated with using hydrochar generated from the carbonization of food wastes as a soil amendment and understanding how waste composition may influence the need for hydrochar post-processing. To accomplish this, laboratory-scale experiments investigating the carbonization of different components and mixtures of food waste (e.g., grains, vegetables, fruits) at different reaction temperatures (200 °C and 250 °C) for a time of four hours were conducted. The hydrochar was separated from the process water, dried, and subjected to sequential deionized water washing to understand the release of phytotoxic compounds from the hydrochar. Results of this study indicate that post-processing food waste hydrochar by water washing can sufficiently remove leachable phytotoxic compounds while preserving carbon and nitrogen solid content and maintaining a recommended C: N ratio for carbonaceous soil amendment application. The cumulative washing ratio required to remove phytotoxic effects depends on feedstock and HTC processing temperature.