The study is centered on a real-world case from Meir Medical Center (MMC), one of Israel’s largest regional hospitals, which services a diverse population and produces approximately 160 tons of mixed non-hazardous waste per month. A detailed waste survey was conducted in collaboration with MMC’s operations department, identifying food waste, paper towels, and cardboard as the major contributors to the organic fraction. These were selected as feedstocks for a systematic evaluation of HTL performance.
HTL offers distinct advantages over conventional incineration and anaerobic digestion. It operates in aqueous conditions at subcritical temperatures (typically 240–320°C) and elevated pressures (20–30 MPa), allowing it to process high-moisture content waste without the need for drying. Through hydrolysis, decarboxylation, and depolymerization, organic macromolecules are broken down into a hydrophobic biocrude phase, a solid carbon-rich biochar, an aqueous phase with dissolved organics, and gases. This process mimics the natural formation of fossil fuels but occurs over a matter of minutes instead of millions of years.
A series of batch experiments were conducted to optimize process conditions. A key focus was placed on the role of catalysts in enhancing conversion efficiency and product quality. Six catalysts were tested: dolomite (MgCO₃+CaCO₃), dolime (MgO+CaO), calcium hydrogen phosphate (CaHPO₄), potassium carbonate (K₂CO₃), potassium nitrate (KNO₃), and potassium phosphate monobasic (KH₂PO₄). Catalyst concentrations were fixed at 5% (by dry weight of feedstock), and reaction conditions included a 20-minute residence time and a feedstock load of 20% dry weight. Experiments were carried out at temperatures ranging from 220°C to 313°C.
Calcium hydrogen phosphate (Cat.3) delivered the highest dry mixed solids (DMS) yield of 53.5%, with a biocrude yield of 21.95%. Potassium nitrate (Cat.5) showed similar performance, with slightly lower DMS yield and energy recovery. Dolime (Cat.2) increased biochar yield significantly but suppressed biocrude formation, while potassium carbonate (Cat.4) performed poorly in all metrics. Catalyst-free (Cat.0) experiments served as benchmarks.
The use of acetone as a co-solvent was introduced as an innovation to improve phase separation during post-processing. Traditionally, biocrude is extracted from the solid HTL product using organic solvents. When acetone was added prior to the reaction, it altered the phase behavior, reducing the biochar-to-biocrude ratio from 1.5 to 0.44 and doubling biocrude yield (up to 38.2%). This also led to a substantial increase in energy recovery, from 33.4% to 67.6%. However, it also resulted in higher pressure in the reactor, indicating a need for careful safety assessment in scaled systems.
Further optimization included a devolatilization process for biochar at 300°C under vacuum, which reduced its mass by 25.3% and removed volatile organics, as confirmed by clear filtrates during ethanol and acetone washing. Thermogravimetric analysis (TGA) and scanning electron microscopy (SEM) revealed significant morphological changes, including particle agglomeration and increased ash content (22.5% vs. 5.5% in feedstock). Energy-Dispersive X-ray Spectroscopy (EDS) confirmed retention of catalyst elements such as phosphorus and calcium in the final char.
The biocrude product exhibited excellent fuel characteristics, with high heating values (HHV) ranging from 33.0 to 38.5 MJ/kg—representing a 75% improvement over the original food waste feedstock (20.66 MJ/kg). Simulated distillation and gas chromatography-mass spectrometry (GC-MS) analyses identified oleic acid and other long-chain fatty acids as major components. However, high acidity and olefin content (acid number = 100.6 mgKOH/g; bromine number = 45.4) indicated the need for upgrading through catalytic hydrotreating before the biocrude can replace diesel or fuel oil in existing systems.
To improve system efficiency for real-world applications, pretreatment of mixed waste (including paper towels and cardboard) was introduced. Shredding and preheating (140°C, 30 min) prior to HTL improved biocrude yield by over 50% and increased energy recovery by 30%. This approach addresses one of the key challenges in biomass processing—heterogeneity—by producing a more uniform and reactive feedstock.
An extensive economic analysis was conducted comparing current waste management costs at MMC with the projected operational savings and revenue from HTL implementation. Four scenarios were evaluated:
Food waste and organic waste from hospital departments, focusing on biochar production.
The same waste streams with a shift toward biocrude production.
Inclusion of acetone as a co-solvent to boost biocrude yield.
Addition of cardboard and paper towel waste, combined with pretreatment to maximize total energy product output.
Under Scenario 4, annual savings were projected at $131,362, primarily through reductions in disposal costs and increased revenue from energy products (10.13 tons of biochar/month @ $351/ton and 2.88 tons of biocrude/month @ $810/ton). Discounted savings over 10 and 20 years reached $1.01 million and $1.64 million, respectively. Moreover, social benefits, which included avoided emissions from transport and landfilling, raised the total value to $2.17 million over 20 years.
Sensitivity analyses demonstrated the impact of changes in feedstock composition, market prices of bioproducts, and discount rates. Notably, the inclusion of paper and cardboard (currently categorized as “other waste”) had the largest effect, significantly increasing yields and revenue potential. Market prices for upgraded biocrude and premium biochar could potentially double or triple, further improving economic feasibility.
The study concludes that catalytic HTL, combined with co-solvent and pretreatment strategies, is a viable solution for hospital organic waste valorization. It offers a dual benefit: (1) reducing environmental impacts by diverting waste from landfills and (2) producing high-value energy carriers that can offset fossil fuel dependence. The case of Meir Medical Center demonstrates real-world potential for applying this technology in healthcare systems globally.
Recommendations for future research include:
Pilot-scale deployment to validate system reliability and throughput.
Development of on-site upgrading methods for biocrude.
Life cycle assessment (LCA) to quantify environmental benefits across the supply chain.
Policy frameworks and incentives to encourage adoption in public hospitals and municipal systems.
By transforming an underutilized waste stream into valuable resources, this study supports the transition toward sustainable, circular healthcare systems and provides a replicable model for other institutions seeking to minimize their ecological footprint.