Hydrothermal Liquefaction of Municipal Sludge: Feedstock Composition, Inorganic Interactions, and Co-Liquefaction as a Dewatering Alternative

Wastewater treatment plants in the United States generate several million dry tons of sewage sludge each year, and the conventional disposal pathways (landfilling, incineration, and land application) present challenges, particularly with the rise of contaminants like per- and polyfluoroalkyl substances (PFAS). Hydrothermal liquefaction (HTL) is a near-term solution for wet sludge conversion because subcritical water tolerates the 96–99% moisture content of waste activated sludge (WAS) without the need for extensive drying. Three decades of HTL research have established robust parametric windows but have largely treated sludge as an interchangeable input characterized only by its bulk biochemical composition, with mechanistic studies conducted under conditions that do not reflect how sludge is actually generated, stored, or delivered. This dissertation tests the hypothesis that HTL outcomes are also tightly coupled to upstream wastewater treatment decisions and to inorganic speciation, and that resolving these couplings opens process alternatives invisible under the conventional framing.Three experimental projects were conducted using sludges collected from a conventional activated sludge (CAS) facility practicing lime addition and an enhanced biological phosphorus removal (EBPR) facility, supplemented by controlled alkaline additive experiments and a co-liquefaction study. Reactions were carried out at 350 °C for 20–60 min at 10–20 wt.% solids in 75 mL Parr reactors, with products characterized by proximate analysis, ultimate analysis, FTIR, XRD, GC-MS, simulated distillation, ICP-OES, and solid-state 31P NMR. The first work in this dissertation demonstrated that wastewater treatment configuration and storage regime produce predictable shifts in HTL behavior. Lime addition raised the CAS feedstock Ca:P molar ratio to 3.13, drove quantitative phosphorus retention in the biochar, and suppressed biocrude carboxylic-acid content. EBPR sludges stored aerobically and anaerobically exhibited different phosphorus partitioning despite similar bulk Ca:P ratios; 31P NMR supported a mechanism in which polyphosphate-bound Mg2+ released during HTL of aerobically stored sludge destabilizes calcium phosphate phases and shifts phosphorus to the aqueous product. The second study separated cation effects from alkalinity by varying CaCO3, Ca(OH)2, and Na2CO3 at matched loadings. Calcium-based additives drove biocrudes toward amidation pathways, lowered O/C ratios, and quantitatively retained phosphorus in the biochar, whereas Na2CO3 promoted N-heterocycle formation in the biocrude and provided no enhancement in phosphorus recovery. The third study demonstrated that direct blending of dilute WAS with dry pistachio shells reaches the 20 wt.% solids HTL target without mechanical dewatering. Co-liquefaction produced a 23% synergistic enhancement in biocrude carbon yield over the weighted-average prediction and was evaluated through paired life cycle assessment and life cycle costing, which revealed a context-dependent trade-off between centrifuge-based and blending pathways that depends on the choice of functional unit. Together, these results reframe sludge HTL as a unit operation whose performance is set by upstream wastewater decisions and inorganic chemistry and provide a systems-level basis for designing HTL as an integrated component of water resource recovery facilities.

Name thesis:degree_name

Ph.D.

Discipline thesis:degree_discipline

Chemical & Petroleum Engineering

Grantor dc:publisher

University of Kansas

Year dc:date.issued

2026

Author dc:creator

• Poli, Joao Victor

Advisor dc:contributor.advisor

• Stagg-Williams, Susan M

• Calcium
• Hydrothermal Liquefaction
• Phosphorus
• Sludge

Language dc:language.iso

en

Dc Identifier Other

https://www.proquest.com/LegacyDocView/DISSNUM/32702845

OAI identifier oai:identifier

oai:kuscholarworks.ku.edu:1808/39522