The Department of Energy Bioenergy Technologies Office (BETO) invests in research and development of new pathways for commercially viable conversion of biomass into drop-in ready transportation fuels, fuel blendstocks and products. The primary emphasis has been on terrestrial and algae feedstocks, but more recently BETO has begun to explore the potential of wet wastes for biofuel production, with focus on wastewater residuals, manure, food waste, and fats, oils and grease. A recent resource analysis estimates that 77 million dry tons per year of these wastes are generated annually, 65% of which are underutilized for any beneficial purpose. 1 Approximately 14 million dry tons of the total resource is wastewater residuals (sludge and biosolids) generated at the nation's wastewater treatment plants (WWTPs). 2 Conversion of this resource into transportation fuels could significantly contribute to the creation of a new domestic bioenergy and bioproduct industry, while providing an economically and environmentally sustainable alternative for current waste disposal practices.

A preliminary process model and techno-economic analysis (TEA) was completed for fuel produced from hydrothermal liquefaction (HTL) of sludge waste from a municipal wastewater treatment plant (WWTP) and subsequent biocrude upgrading. The model is adapted from previous work by Jones et al. (2014) for algae HTL, using experimental data generated in fiscal year 2015 (FY15) bench-scale HTL testing of sludge waste streams. Testing was performed on sludge samples received from Metro Vancouver's Annacis Island WWTP (Vancouver, B.C.) as part of a collaborative project with the Water Environment and Reuse Foundation (WERF). The full set of sludge HTL testing data from this effort will be documented in a separate report to be issued by WERF. This analysis is based on limited testing data and therefore should be considered preliminary. In addition, the testing was conducted with the goal of successful operation, and therefore does not represent an optimized process. Future refinements are necessary to improve the robustness of the model, including a cross-check of modeled biocrude components with the experimental GCMS data and investigation of equipment costs most appropriate at the relatively small scales used here. Environmental sustainability metrics analysis is also needed to understand the broader impact of this technology pathway.

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Hydrothermal liquefaction (HTL) liquifies wet feedstocks to produce a biocrude under moderate temperatures (300−450 °C) and high pressures (>2500 psi). The biocrude can be upgraded to transportation fuels (predominantly diesel) using typical refinery unit operations (e.g., hydrotreater and distillation). HTL of wet-wastes is a promising route to produce environmentally friendly and cost-competitive fuels; however, the feedstock significantly impacts the product quality and the process yield. Consequently, it is important to rigorously compare different feedstocks to determine the critical material attributes, which impact the biocrude yield and quality. A few published comprehensive studies evaluate the performance of numerous different wet-waste HTL feed types, processed using the same reactor configuration and analytical approach. This is particularly true for continuous flow HTL. HTL studies generally investigate one or a few surrogate feedstocks or model compound materials or attempt comparative reviews by collecting the results of numerous different research groups. Such an approach involves numerous assumptions that can significantly compromise the legitimacy of the data compared and the conclusions drawn. This work investigates HTL of 13 different real-world wet-waste feedstocks, belonging to multiple different classes of municipal wet-waste, including food waste, biosolids, sludge, fermentation residues, manure, and blends thereof. The biocrude carbon yields obtained throughout the study ranged from 39.7 to 74.3%. The biocrude yield varied through a range of different process changes, such as increasing reactor flow rate and feed solid loading, and addition of Fe-based additives. By analyzing a broad range of materials, and through the comprehensive characterization profiles prepared for both the feedstock and products, this study has produced a significant volume of data, which was then analyzed using robust statistical methods.

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