Understanding Water Washing Needs of Hydrochar Generated from the Hydrothermal Carbonization of Food Wastes to Establish Minimum Post-Processing Needs for Soil Amendment Application

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. Understanding the need for and extent of hydrochar post-treatment is critical in identifying environmentally beneficial and economically attractive strategies for hydrochar use. In this study, work was 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 DI water washing to understand the entrainment of phytotoxic elements. The progressively extracted supernatants were characterized by common water quality indicators including pH and total volatile acids, and phytotoxicity using grain barley seed germination tests. Results of this study show 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. A general minimum water washing ratio of 1:25 (wt./wt.) is recommended to remove greater than 90% of acidic and salinity content associated with food waste hydrochar phytotoxicity and reduce and/or eliminate phytotoxic effects on seed germination.