Aerobic fermentation to treat and upcycle hydrothermal liquefaction aqueous phase

Abstract

Hydrothermal liquefaction (HTL) is a promising wet thermochemical process for converting biomass feedstocks into biofuels. HTL aqueous phase (HTLAP), as one of major by-products of HTL, retains a significant portion of the biomass energy and requires treatment before discharge to the environment. Developing a method to treat HTLAP and recover its energy content is crucial for the economic viability of HTL, while the complexity, variability, microbial toxicity, and recalcitrance of HTLAP make it a challenging task. Biological treatment methods, such as algae cultivation, anaerobic digestion (AD), and aerobic treatment are environmentally friendly and straightforward options for treating HTLAP. Compared to algae cultivation and AD, aerobic treatment can tolerate higher concentrations of HTLAP and generally requires a shorter incubation time. Despite these advantages, aerobic treatment is less studied in literature. Therefore, it is of interest to develop an aerobic fermentation process for HTLAP that can effectively remove pollutants while producing valuable bioproducts. This work addresses the need to advance HTLAP treatment technology by developing an effective aerobic fermentation process while further promoting the sustainability of HTL for biofuel production. It begins with a comprehensive review of HTL technology, HTLAP characteristics, existing treatment technologies, and highlighting the benefits and drawbacks of each technology. The research journey of this dissertation embarks upon a comparative evaluation on the biological degradation of HTLAP with various bacterial and fungal strains. The study emphasized the significance of pH adjustment and mineral supplementation in enhancing COD removal during HTLAP fermentation. Bacterial strains tolerated a 10× dilution of HTLAP, while most fungal strains required a higher dilution of 20×. However, the fungal strains demonstrated effective degradation of a wide range of compounds, particularly phenolic compounds, achieving removal efficiencies between 60-100%. Among them, the filamentous fungi A. niger and T. versicolor were identified as the most promising strains, achieving approximately 50% COD removal in HTLAP fermentation. The research also highlighted the recalcitrance of sludge HTLAP, which was attributed to the presence of N-heterocycles, as these compounds were removed by less than 40% across all microbes tested. Building on these findings, we further evaluated three filamentous fungal strains in the fermentation of corn stover HTLAP. Among the tested fungi, A. niger showed the best biodegradation performance, converting approximately 50% of its consumed organic carbon into oxalic acid as a value-added product. To enhance HTLAP biodegradation, R. jostii was co-cultured with A. niger during fermentation. The oleaginous bacterium R. jostii consumed organic acids that were left behind by A. niger and accumulated cellular lipids. Together, this co-culture achieved consistent COD removal of approximately 70% in diluted HTLAP with COD values between 3.8 and 7.8 g/L. These findings demonstrated the promising fermentation capabilities of A. niger and R. jostii co-culture, highlighting their potential for HTLAP valorization to improve the sustainability of HTL processes. Lastly, we assessed the impact of HTL operating conditions on the chemical composition and biodegradability of the resultant HTLAP in A. niger and R. jostii fermentation. This study elucidated the variation in chemical composition of HTLAP derived from different HTL conditions and established a quantitative relationship between HTL operating parameters (temperature, retention time, and feedstock loading) and the biodegradability of HTLAP, highlighting that HTLAP derived from higher severities exhibited lower biodegradability. This reduced biodegradability was attributed to the increasing concentration of specific phenolic compounds in HTLAP produced at higher severities. Additionally, the research demonstrated the versatility of the A. niger and R. jostii co-culture in fermenting HTLAP derived from a wide range of HTL conditions, achieving COD removal rates of 45-70% depending on the specific HTL parameters used. In conclusion, this dissertation advances HTL technology by developing a promising aerobic fermentation process for HTLAP upcycling. The aerobic fermentation process outlined in this dissertation represents a significant step forward in making HTL technology more economically and environmentally sustainable.