(77c) Synthetic Aviation Fuel from Co-HTL of Anaerobic Digestion-Derived Biosolids and Wastewater-Grown Algae: A Combined Experimental Study and Techno-Economic Assessment

Water resource recovery facilities (WRRFs) provide a unique opportunity to produce synthetic aviation fuel (SAF) from cost-advantaged feedstocks. The nutrient-rich wastewater (WW) serves as an excellent medium for microalgae growth, which helps in nutrient removal and meet effluent discharge limits. Algal biomass can be used to generate biocrude using hydrothermal liquefaction (HTL), which can then be easily upgraded to SAF. However, algal cultivation alone is not sufficient to provide biomass for fuel production and is often recommended to blend with other feedstocks to increase scale and to make biocrude cost-competitive with conventional fuel.¹ Biosolids are the nutrient-rich organic materials resulting from anaerobic digestion (AD) of wet sludge and are often disposed of in landfills. HTL of biosolids can serve as a great resource for biocrude generation and eliminate the need for transportation to landfills. Here, we report SAF production pathway from co-HTL of WW-grown algae and AD biosolids. Co-HTL of WW-grown algae with biosolids can increase the biomass availability for HTL resulting in increased biocrude production. Several blends of biosolids and WW-grown algae were processed via HTL. The obtained biocrude from the above process was then hydrotreated to form upgraded fuel. The key properties of SAF cut were analyzed using a low-volume testing methodology. Initial results indicate that the physical and chemical properties of SAF meet the specification for conventional jet-fuel.

A techno-economic analysis (TEA) was performed to assess the feasibility and economic potential of the produced fuel using this pathway. The process boundary and the flow of materials are shown in Figure 1. For this analysis, a WWRF with a 100 million gallons per day treatment capacity produces is considered. Such a site produces about 52 dry tons/day of biosolids and has a potential of producing about 25 dry tons/day of algae from its nutrient-rich AD centrate.² For additional revenue, co-products such as fertilizers in form of struvite and cement additives from HTL solids are also considered. The minimum fuel selling price (MFSP) of fuel from blended feedstock of WW-grown algae and biosolids was $8.30 per gasoline gallon equivalent. The impact of different blend ratios and scales on the MFSP of fuel was also forecasted. A sensitivity analysis was also performed to evaluate the impact of various parameters on MFSP. Plant specific parameters such as HTL scale and capital cost were found to have the most significant impact on the MFSP of fuel.

References

(1) Watkins, J. D.; Kumar, A.; Valdez, P. J. Whole Algae Hydrothermal Liquefaction and Upgrading: A review of progress and challenges and insight into the future; PNNL-37209; Pacific Northwest National Laboratory (PNNL), Richland, WA (United States), United States, 2025. https://www.osti.gov/biblio/2500903

(2) Kumar, A.; Watkins, J. D.; Cronin, D. J.; Fox, S. P.; Schmidt, A. J.; Valdez, P. J. 2024 Case Study: Hydrothermal Liquefaction of Biomass Sources at a Wastewater Treatment Facility; PNNL-37230; Pacific Northwest National Laboratory (PNNL), Richland, WA (United States), United States, 2025. https://www.osti.gov/biblio/2499474