This study evaluates the prospective techno-economic and environmental performance of four hydrothermal liquefaction (HTL)-based technological pathways for converting sewage sludge into biofuels at facilities co-located with wastewater treatment plants. Two sewage sludge carbon-accounting cases based on IPCC guidelines are considered: a 100 wt% biogenic carbon case and a case containing 7 wt% fossil carbon in the sewage sludge stream. Four technological configurations differing in their energy sources for HTL and biocrude upgrading are considered. Technology 1 relies on natural gas for process energy and hydrogen production, while Technology 2 utilizes onsite biomethane. Technology 3 combines natural gas-based HTL with electricity-driven hydrogen production, whereas Technology 4 integrates biomethane-based HTL with electricity-driven hydrogen production. Their performance is assessed under three prospective background climate scenarios: REMIND-SSP2-Base (3.5 °C), Pkbudg1150 (below 2 °C), and Pkbudg500 (below 1.5 °C), for 2030, 2040, and 2050. An integrated techno-economic and environmental objective function (OF), applied within a technological superstructure, is used to evaluate and rank 36 scenarios for each carbon-accounting case. Results consistently identify Technology 4 as the optimal configuration across all scenarios. For a first-of-a-kind (FOAK) plant in 2025, maximum credits reach 697 € t−1 DSS for 100 wt% biogenic sludge and 510 € t−1 DSS for fossil-containing sludge under Pkbudg500. For an nth-of-a-kind (NOAK) plant in 2050, sensitivity analysis considering 10%, 50%, and 100% utilization of European sludge availability yields credits of 969–1008 € t−1 DSS and 732–776 € t−1 DSS, respectively. These findings highlight the potential of biomethane-integrated HTL with electricity-driven upgrading for economically and environmentally sustainable sewage sludge valorization within a circular economy framework.
The surging global demand for lithium-ion batteries (LIBs) underscores the urgent need to secure sustainable lithium supply chains for the transition to a green economy. Over the past few years, hydrometallurgical routes have been widely explored to treat spent LIBs, valorise their components, and recover critical raw materials. However, achieving battery-grade metal purity requires advanced separation and purification technologies. Nanofiltration (NF) is emerging as a promising alternative due to its ability to discriminate between single-charge (Li+) and multi-charge (e.g., Co2+, Al3+) metals. Nevertheless, NF performance depends heavily on solution composition; specifically, the choices of leaching acid and neutralising alkali directly impact lithium recovery and purity. This study utilizes the Solution-Electro-Diffusion-Film model to assess the technical and economic viability of NF under different operational scenarios, comparing HCl and H2SO4 for leaching, alongside NaOH and Mg(OH)2(s) for neutralisation. The transport model was first calibrated using lab-scale experimental data, and subsequent predictive simulations evaluated single-pass and two-pass configurations under varying operating pressures and permeate recovery rates. The results reveal a distinct techno-economic trade-off: while chloride-based systems minimise operational costs, they yield lower-quality products. Conversely, sulphate-based systems incur higher costs and slightly lower Li+ recoveries but provide superior permeate quality, thereby significantly reducing the need for expensive downstream refining steps and offering a clearer path toward circular battery recycling.
