(682e) Anaerobic Treatment and Valorization of Aqueous Products from Hydrothermal Liquefaction of Wet Organic Wastes

Proper management of wet organic wastes (e.g., food waste) is receiving growing attention due to their increasing quantity. However, most of this waste stream is currently sent to the landfill, contributing to a significant amount of greenhouse gas (GHG) emissions. Four states (Connecticut, Massachusetts, New Jersey, and Vermont) in the US have enacted policies limiting food waste to landfills; thus, additional handling and disposal routes are of urgent interest. Hydrothermal liquefaction (HTL) is an emerging technology that leverages the unique properties of water at elevated temperatures and pressures to convert wet organic wastes into crude fuel and chemical products. While suggested to be a cost-effective waste management approach with much lower GHG emissions for certain feedstocks (e.g., sewage sludge compared to landfill, incineration, and land application; Figure A), there are still needs to further improve the economic performance to address the financial risks associated with commercial deployment, especially considering the capital-intensive nature of such systems.

One strategy to enhance cost competitiveness is to improve the valorization of HTL coproducts. Unlike the main energy-dense biocrude product, HTL aqueous product (HTL-AP) has received much less attention, largely due to its complexity that prevents low-cost valorization yet necessitates proper treatment before discharge or recycling. At the same time, the high organic contents (in terms of chemical oxygen demand, COD) of HTL-AP makes anaerobic digestion (AD) a potential route for simultaneous energy production and treatment. This presentation will discuss findings from the anaerobic treatment of HTL-AP of different feedstocks (ground coffee and food waste from the university dining hall) and reaction conditions (temperature and homogeneous additives), where HTL experiments have shown great variation in yields of biocrude and hydrochar (residuals; Figure B). Two mesophilic anaerobic systems were explored, including one batch AD system and one sequencing batch bio-electrochemical AD system (BEAD) where electricity was used to overcome the inhibitory effects from the complex aqueous products (e.g., ammonia, aldehydes, N-heterocyclic compounds, aromatic compounds). The batch experiments were performed in 250 mL reactors with HTL-AP diluted to a COD of 1 g·L^-1. For the sequencing experiments, three parallel tests were set up in 1 L reactors: one control test with acetate, one AD test with acetate-HTL-AP mixture but 0 voltage, and one BEAD test with acetate-HTL-AP mixture and 0.5 V of voltage. Methane yields from both batch and sequencing systems were monitored daily. Finally, cost and environmental impact implications of the experimental findings were interpreted through processed-based techno-economic analysis (TEA) and life cycle assessment (LCA) on QSDsan, an open-source platform for quantitative sustainable design of sanitation and resource recovery technologies. Two alternatives of an integrated hydrothermal waste valorization system consist of HTL, biocrude upgrading, and co-product valorization were considered: one employing chemical valorization of HTL-AP through catalytic hydrothermal gasification, while the other employing biological valorization through AD (Figure C).