Hydrothermal Liquefaction as Resilient Water Infrastructure for the Future

The Great Lakes water Authority has been involved with hydrothermal liquefaction (HTL) research to evaluate one of the potential ways for extracting energy resources from waste biosolids. GLWA has also been involved with research efforts investigating pyrolysis and anaerobic digestion. This presentation discusses the operational parameters, constraints, and drivers around resource recovery from wastewater biosolids. We present the potential for energy resource recovery based on a typical treatment plant facility and then describe some of the technology and operational constraints which need to be addressed before full-scale implementation can be achieved. The discussion will primarily focus on biocrude generation from HTL.

Wastewater treatment continues to evolve in response to tightening regulations, rising energy and biosolids costs, and increasing concern over emerging contaminants. Hydrothermal liquefaction (HTL) is a promising thermochemical technology that can be integrated into, or in some cases replace, portions of conventional biological treatment processes used in most wastewater treatment plants.

This presentation provides a concise overview of HTL as applied to wastewater solids management, including examples of pilot-scale and near-commercial systems that are operating or under construction. HTL equipment is presented in the context of a conventional wastewater treatment facility, illustrating multiple options for integration into existing process trains. Key material and energy inputs and outputs of the HTL process are described within this framework.

Two commercial deployment models for HTL implementation—utility-owned and third-party deployment models—are then examined, with a comparison of their technical, operational, and economic characteristics. A lifecycle perspective is used to illustrate overall cost drivers as well as typical life cycle cost and carbon balance, together with the demonstrated reduction of recalcitrant compounds, including pharmaceuticals and emerging contaminants such as PFAS and PFOS.

The presentation concludes with a discussion of the principal benefits and remaining challenges associated with HTL in wastewater treatment, including economic performance, regulatory considerations, energy recovery, carbon impacts, and the role of HTL as resilient water infrastructure.

Hydrothermal liquefaction (HTL) is a promising alternative management strategy for biosolid residuals from wastewater treatment. Interest in HTL has grown over the last decade as municipalities seek alternatives to land application, due in part to concerns over the fate of per- and polyfluoroalkyl substances (PFAS). Managing municipal biosolids onsite with HTL involves heating dewatered biosolids (c.a. 20% dry solids) to sub-critical conditions (300 – 350°C, 20 MPa), where thermal depolymerization reactions transform carbohydrates, proteins, and fats into a biocrude suitable for upgrading to liquid transportation fuels (about 50% of feed mass). The remaining solids are largely solubilized into an aqueous phase (AP) product, which could potentially be recycled back into the treatment plant headworks for further treatment, contributing 10-20% of the total influent load into the facility. The AP contains complex organic compounds, including aromatic nitrogen species, which have been shown to inhibit nitrogen oxidizing bacteria and could negatively impact treatment plant operations. Here we focus on two aspects of HTL: 1) Management of the high-organic AP (50 – 100 g/L COD) treated by wet-air oxidation (WAO), as well as 2) the fate of PFAS. Efficacy of a range of wet air oxidation treatment intensities was assessed by a biological nitrogen oxidation rate inhibition assay. Due to the recalcitrance of the aromatic nitrogen species, treatment of the AP via WAO requires high reaction severity, with the most severe treatment level resulting in little difference in nitrogen oxidation relative to experimental controls. The fate of PFAS was explored by spiking feed with PFOA, PFOS, and PFHpA, and tracking their distribution in HTL products in a bench-continuous trial. Wet-air oxidation (WAO) treatment of the AP required high reaction severity due to the recalcitrance of aromatic nitrogen species, and the most severe treatment showed minimal improvement in nitrogen oxidation relative to experimental controls. HTL removed 94% of targeted PFAS (PFOA, PFOS, PFHpA) from the feed in a bench-continuous trial; however, only 20% of the removal was measured as inorganic fluorine, and the overall fluorine mass balance was low (50%), suggesting the need for additional measurements of gas-phase HTL emissions to check for volatile fluorinated species.

Hydrothermal liquefaction (HTL) is a technology capable of recovering nutrients, removing emerging contaminants, and converting sludge into biocrude oil for biofuel production. This study presents the first feasibility assessment of implementing HTL at the Water Resource Recovery Facility in Detroit, incorporating economic, environmental, and social considerations. Initial operating conditions were identified for an on site HTL demonstration unit, and the feasibility analysis was repeated using field data from that unit.
Currently, Detroit processes 75% of its sludge (240 dry tons/day) through a biosolids drying facility (BDF) and incinerates the remaining 25%. Two alternatives were evaluated: anaerobic digestion plus BDF (AD+BDF) and HTL plus BDF (HTL+BDF). In the HTL scenario, adapted from a 2023 design case, 110 dry tons/day are processed through HTL and 210 dry tons/day through the BDF. HTL biocrude was assumed to be refined off site.
Both alternatives reduce the levelized cost of disposal (LCOD) compared to current practices. Although AD generates substantial RIN revenue, HTL offers higher potential fuel revenue due to higher carbon conversion efficiency, with further LCOD reductions expected at larger scales. Environmental results show AD+BDF reduces non renewable resource use, while HTL+BDF provides greater energy recovery and lower greenhouse gas emissions.
The sludge used in the field data had higher ash and lower lipid content than expected, reducing biocrude yields and increasing costs. Even so, HTL remains significantly less costly than current practices and comparable to AD, supporting its viability despite Detroit’s atypical sludge characteristics.

Hydrothermal liquefaction (HTL) is a promising technology for recovering nutrients, removing emerging contaminants, and converting sludge into biocrude oil for biofuels. This abstract summarizes results from a three month operation of the Genifuel HTL demonstration unit at the Great Lakes Water Authority (GLWA), conducted as part of an HTL feasibility study.
The unit processed 50 L/day of wet feedstock per run and completed six runs with various sludge and FOG blends at 350 °C and 200 bar. Due to GLWA’s combined sewer system and ongoing grit removal upgrades, sludge used in testing contained substantially higher ash and lower lipid content than assumed in earlier modeling, resulting in lower than predicted biocrude yields. A full mass balance was completed for one sludge only run. Biocrudes exhibited low carbon content, high moisture, and elevated inorganic levels, with silica concentrations exceeding 1000 ppm.
Biocrude from the 3% FOG run underwent upgrading, including water removal, acid washing, and hydrotreating under 1500 psig hydrogen. Reactor plugging occurred after 92 hours, attributed to high silicon (16,955 ppm) in the recovered solids. Distillation of upgraded products yielded ~60 wt% diesel range material with cetane values around 40 and elevated cloud/pour points.
HTL aqueous phase products (HTLaq) were analyzed for organics and inorganics. Dominant constituents included acetic acid, glycerol, ethanol, and acetone, with sodium and potassium as major inorganic species. Two treatment strategies—anaerobic digestion (AD) and supercritical water oxidation (SCWO)—were evaluated. An EGSB AD system achieved 60–80% COD removal, while a CSTR reached ~45% after operational adjustments. SCWO tests showed limited oxidation under tested conditions.
Overall, the demonstration confirms the need for improved grit removal and potential coprocessing with low ash wastes to support reliable HTL implementation at GLWA. HTLaq can be managed through AD with dilution or SCWO, enabling integration of HTL into WRRF operations.

Hydrothermal liquefaction (HTL) is a promising technology for conversion of wet biowastes, such as food waste, sewage sludge, agricultural waste, and algae into transportation fuels and other chemicals. HTL contributes to net-zero carbon goals and avoids competition for agricultural land and food resources. While HTL has a high energy and carbon recovery efficiency, the biocrude must be upgraded prior to use as a drop-in fuel. This presentation describes the development of a pilot-scale continuous plug flow HTL reactor, the production of biocrude oil from food waste, and upgrading to sustainable aviation fuel (SAF). The HTL biocrude oil was pretreated to remove impurities such as water, salt, and ash. Then, catalytic hydrotreating was performed to obtain a SAF precursor. The SAF precursor was evaluated using Tier α/β screening test following FAA recommendation and compared to ASTM D7566 specifications. Our results show that the HTL derived SAF met the major jet-fuel and handling properties, including surface tension, density, viscosity, LHV, flash point, DCN, and freeze point, proving a new paradigm for a circular bioeconomy via HTL pathway.

Circular economies for cities will rely on WRRFs to recover wasted resources. Biosolids are an important resource for recovering nitrogen and phosphorus at WRRFs that would otherwise be wasted to a landfill (56% of US biosolids are land applied).The application of biosolids has faced recent regulation because of per- and polyfluoroalkyl substances (PFAS). Hydrothermal liquefaction (HTL) is gaining momentum and popularity in wastewater research as a process to transform sludge into value-added products such as biocrude oil and biochar, with a residence time of only 15-30 minutes. Additionally, HTL also shows promise for treating PFAS. However, HTL produces an aqueous byproduct (HTL-AQ) which would normally be recycled to the influent at a WRRF which could contain PFAS. This study seeks to determine the extent of PFAS degradation and fate into the HTL-AQ.
We investigated the transformation of an influent feed sludge spiked with PFAS in a pilot-scale HTL system (100 kg/d). The feed was reacted for 30 min at 350 DegC at 220 bar for 9.5 hours. The effluent components HTL-AQ, off-gas, biocrude, and the biochar were analyzed using EPA methods 1633, 537, OTM 45 and OTM 50. All effluent components were separated using centrifugation, filtration, solvents, and rotary evaporation.
The total mass of the individual PFAS components within the HTL-AQ was compared to the total of the component in the spiked feed. Figure 1 displays the overall transformation and redistribution of PFAS from the HTL reaction to the HTL-AQ. About 3.5% of the total influent PFAS was redistributed to the HTL-AQ. In the HTL-AQ, mostly sulfonated compounds remained along with 5:3 FTCA, PFHxA, and PFPeA which are compounds usually associated as products of other, larger compounds. Other studies have demonstrated the difficulty to remove sulfonated PFAS which requires precursors for degradation. Overall, the HTL-AQ would be further diluted with other recycle streams and the influent thus minimally impacting the influent by less than 1% of a typical WRRF influent.

Bamboo is a promising bioenergy crop due to its rapid growth, high biomass yield, and capacity for carbon sequestration. In this study, Phyllostachys nuda bamboo was evaluated as a candidate for biofuel production using hydrothermal liquefaction (HTL). Yields of biocrude, char, aqueous phase, and gas products were measured over a range of reaction temperatures and residence times in a batch reactor. The optimal biocrude carbon yield was measured at 300 °C and 10 minutes, corresponding to a value of 49.5 wt% with 25.7 wt% char carbon yield. To provide insight into reactor design, the kinetic severity factor (KSF) was used to correlate biocrude yields with the combined effects of reaction temperature and residence time, with the observation of broad maximum in the range from 6.25 to 7 followed by decreasing yields for a severity factor greater than 7. Gas chromatograph with mass spectrometry detection and Fourier Transform Infrared Spectroscopy (FT-IR) of the biocrude indicated that phenolic compounds were the dominant chemical product class at low severity, consistent with lignin decomposition, while biocrude produced at higher severity contained products of secondary reactions and deoxygenation. A techno-economic assessment performed at a realistic scale of 10,000 dry ton/day projected a minimum fuel selling price of $2.75/gallon diesel equivalent after upgrading. Sensitivity analysis identified biocrude yield and feedstock cost as the most influential parameters on economic performance. A life cycle assessment demonstrated that bamboo-derived biodiesel emits 6 g CO2/MJ lower greenhouse gas (GHG) emissions, corresponding to an attractive carbon abatement cost of $36.2/t CO2, a value which is much less than most estimates corresponding to direct air capture of CO2. Collectively, the results presented here establish bamboo as a compelling feedstock for sustainable liquid fuel production via HTL.

There is a growing need for new wastewater sludge disposal technologies that maximize resource recovery, reduce operational costs and reduce environmental burden. Hydrothermal liquefaction (HTL) is well suited for processing wastewater sludge, enabling recovery of minerals and producing valuable biocrude oil. The aqueous byproduct of HTL processing is another potentially valuable product and/or environmental burden. In the current work, the suitability of HTL aqueous byproduct as a feedstock for supercritical water oxidation (SCWO) for energy recovery and/or final disposal of HTL aqueous waste is considered. Aqueous samples from a pilot-scale HTL demonstration processing actual wastewater sludge are used in the study. The technical approach uses laboratory-scale batch reactor experiments and analyzes the effects of the reactor operating conditions on the resulting SCWO products. All experiments are conducted at a reactor temperature of 375 ^oC, pressure of 196 bar and residence time of 15 minutes. The results are discussed in the context of conversion of the aqueous samples to SCWO products and the energy budget of the SCWO processing.

Hydrothermal liquefaction (HTL) and Hydrothermal Carbonization (HTC) are promising thermochemical processes for recovering energy and nutrient-rich products from wet biomass waste feedstocks. HTL operates under subcritical water conditions (280°−370 °C) and HTC (160°−250 °C), facilitating the conversion of biomass organic macromolecules into four distinct product phases, i.e., biocrude, hydrochar, aqueous, and gas phases. While biocrude and hydrochar products offer promise as energy fuels, hydrothermal processes generate an aqueous phase/process water byproduct that poses environmental and economic challenges. The aqueous phase (AP) retains valuable nutrients such as NH3−N, phosphorus, magnesium, calcium, etc., which can be recovered via successive unit operation techniques. To address this, my ongoing research looks from a systems perspective advancing further recovery of nutrients from the aqueous phase through oxidative pretreatment and spontaneous supersaturation/crystallization as fertilizer without any chemical additives. My recent work, which I intend to present, demonstrates that oxidative pretreatment of the aqueous phase not only decreases recalcitrant organics but also enhances the purity of the fertilizer product. While prior studies have partly shown such valorization treatments, my work distinguishes itself by systematically revealing the role of process parameters and envisioning an innovative integrated energy system that unites hydrothermal treatment, oxidation, and crystallization to optimize energy and nutrient recovery. This systems approach contributes to the overarching goal of advancing a circular bioeconomy, transforming waste into renewable fuels and sustainable products.

Hydrothermal liquefaction (HTL) offers a promising route for converting wet biomass into sustainable biocrude. However, effective valorization of its carbon-rich aqueous phase (HTL-AP) byproduct remains a major barrier to industrial deployment. This work develops the first lumped-parameter kinetic framework for the wet air oxidation (WAO) of HTL-AP. By enabling predictive process design, this study contributes to scalable and economically viable biorefinery infrastructure.
A four-lump global kinetic model was established on a carbon basis, representing heavy macromolecular species, light intermediates, acetic acid, and CO₂. Kinetic parameters were obtained by fitting batch WAO data collected over 260-320 °C across varied initial loadings. The model accurately captures the degradation and formation pathways of all lumps, including the transient buildup and subsequent oxidation of acetic acid, and reproduces temperature-dependent COD and TOC removal. This new framework offers operational insights to achieve strategies aligned with sustainable process intensification.
To elucidate the molecular basis of lump formation and transformation, high-resolution FT-ICR-MS characterization was conducted. HTL-AP was found to be dominated by O/N-rich macromolecules (200-800 Da) with high aromaticity and double-bond equivalence. During WAO, these heavy species depolymerize partially oxidize, but also crosslink, producing thousands of highly oxygenated intermediates. The sharp early reductions in COD and TOC are related to this effect, though new and even heavier intermediate molecules can also be observed. Over longer reaction times, high-mass/high-DBE formulas decline markedly at both 280 and 325 °C, consistent with progressive oxidation. These molecular insights reinforce the proposed reaction network and highlight WAO’s robustness for treating complex and toxic waste streams.
Together, this mechanistic and kinetic framework advances sustainable chemistry by enabling rational design of HTL-AP valorization strategies that reduce hazardous organic waste, enhance process circularity, and support the development of resilient, low-carbon biorefinery infrastructure.