(501b) Comparative Study of Sustainable Aviation Fuel Produced from Wastewater Grown Algae Vs. Farmed Algae

Algae are a promising biomass source for low-carbon fuels owning to their higher growth rate, high CO₂ uptake high lipid content, comparatively higher area-specific yield, and negligible competitiveness with farmland and food supplies. Biocrude can be produced from algae using various techniques among which hydrothermal liquefaction (HTL) has shown the highest carbon efficiency leading to comparatively lower costs of biocrude generation. However, the yield of biocrude and the overall cost of biofuel is highly dependent on the source and cost of the algae. Algae cultivation can be used to treat wastewater (WW), removing nutrients such as nitrogen and phosphorus. Seeing the efficacy of algae for wastewater treatment in reducing nutrients from water resource recovery facilities (WRRFs) as well as recent stringent government policies on the levels of N, P, and chemical oxygen demand (COD) levels in the treated water discharge has made the use of algae treatment more favorable.¹

We present a comparative study to produce sustainable aviation fuel (SAF) from WW-grown algae and farm-cultivated algae. While the fuel yield from algae from WRRFs is lower than the farmed algae, co-products such as struvite fertilizers may generate extra revenue. WW-grown algae could also be provided as a zero-cost feedstock because HTL reduces waste solids from the WRRF, Moreover, the CO₂ captured from air by algae during their growth brings in additional opportunities for low-carbon products and carbon credits. Assuming zero algal feedstock cost, the minimum selling fuel price (MFSP) of biofuel (which includes both gasoline and jet cuts) from WW-grown algae came out to be $2.61 gasoline gallon equivalent (GGE) relative to farm grown algae that ranged from $4.48-8.05 GGE.² However, the previous analysis assumed that algae cultivation wholly replaced the traditional wastewater treatment process, which is not feasible. Additional scenarios were considered to leverage existing WWT assets, while boosting the availability of HTL feedstock via algae cultivation. In the first scenario, wet-sludge is mixed with WW-grown algae for HTL leading to higher biocrude yield as well as lower biosolids processing and disposal cost. While in the second scenario, the wet sludge is processed via anaerobic digestion (AD) followed by their subsequent treatment of AD centrate via algae cultivation. We investigated other treatment streams at the WRRF for their viability for algae cultivation and blending algal and wastewater solids as combined feedstocks to maximize biocrude production and solids reduction.

References

(1) Clippinger, J.; Davis, R. Techno-Economic Assessment for Opportunities to Integrate Algae Farming with Wastewater Treatment; National Renewable Energy Lab.(NREL), Golden, CO (United States), 2021.

(2) Zhu, Y.; Schmidt, A. J.; Valdez, P. J.; Snowden-Swan, L. J.; Edmundson, S. J. Hydrothermal Liquefaction and Upgrading of Wastewater-Grown Microalgae: 2021 State of Technology; Pacific Northwest National Lab.(PNNL), Richland, WA (United States), 2022.