Hydrothermal liquefaction (HTL) is a thermochemical conversion process capable of transforming high-moisture waste streams, such as municipal sludge, into unconventional and high-value products. As a byproduct of wastewater treatment, sludge represents a critical interface in the water-energy nexus, with high potential for valorization through targeted reaction engineering strategies. In this study, we investigate how upstream wastewater treatment design affects sludge composition and HTL product outcomes. Two wastewater treatment plants (WWTPs)3 in Lawrence, Kansas, serve as case studies. The Kansas River WWTP, an older facility, uses traditional methods to remove contaminants and adds calcium oxide to supply alkalinity, while the Wakarusa River WWTP, completed in 2018, employs more advanced biological processes such as enhanced biological phosphorus removal (EBPR). Sludge samples from both plants were characterized for organic and inorganic content and subjected to HTL under standardized conditions. Results indicate that EBPR sludge contains 10% more carbon in its structure than the Kansas WWTP and a wider range of phosphorus species, likely due to microbial polyphosphate accumulation that occurs in the EBPR tanks. This variability may contribute to improved biochar quality, particularly when used as a fertilizer. Meanwhile, sludge from the lime-added plant shows increased biochar yield by and biocrude fuel quality, with the possibility of reducing the oxygen content in the crude, due to the presence of calcium carbonate formed during lime addition in the plant. The inorganic matrix, particularly CaO and phosphate speciation, influences both the catalytic behavior during HTL and the partitioning of nutrients and energy-rich organics across product phases. These findings demonstrate the importance of understanding site-specific treatment chemistries and sludge properties to optimize HTL performance. The work supports the development of sustainable sludge-to-energy pathways and highlights the role of chemical reaction engineering in converting water treatment residuals into valuable products.
