Valorisation of sewage sludge for sustainable bioenergy and bioresources recovery through hydrothermal treatment

Abstract

Amid rising global concerns over water and energy security, wastewater treatment plants (WWTPs) face a critical challenge in transforming sludge from an environmental burden into a valuable resource. The intricate composition of sludge, rich in organic matter and nutrients yet complex to harness, demands innovative solutions. This study developed an integrated hydrothermal treatment (HT)–based system to enhance energy recovery and nutrient valorisation from waste-activated sludge (WAS), thereby promoting sustainable wastewater management. Hydrothermal treatment promoted this transformation by breaking down complex organic matrices under high temperature and pressure, thereby making nutrients more bioavailable. At the optimised HT condition (220 °C, 20 min), soluble chemical oxygen demand (SCOD) increased more than tenfold (from 284 ± 15 mg/L to 3161 ± 45 mg/L), while phosphate and ammonium rose from 64.04 ± 12 mg/L and 10.3 ± 5.2 mg/L to 172.8 ± 21 mg/L and 151.6 ± 17 mg/L, respectively. However, this solubilisation also released inhibitory compounds, including phenolics (from 59.8 ± 1.2 to 119.4 ± 1.4 mg/L) and heavy metals, potentially constraining downstream recovery. Magnetic biochar (MBC) synthesised from sugarcane bagasse (550 °C) was applied for post-HT detoxification. Under the optimised RSMCCD conditions (pH 6.54, 5 g/ L, 35 min), removal efficiencies reached 37.9 % for phenolics, 60.4 % for As, and over 75 % for Cu, Ni, and Pb. Despite effective contaminant removal, nutrient co-adsorption resulted in 50.3±1.0 % phosphate and 47.2±1.5% ammonium loss. To mitigate these trade-offs, struvite precipitation was optimised (pH 9.24, MgCl2·H2O 10-25 mL/L sludge), yielding 79.5% phosphate and 32% ammonium recovery with 69.02% purity. Phosphorus fractionation revealed a 70% increase in water-soluble phosphorus, confirming enhanced bioavailability. Concurrently, HCl-extractable phosphorus (HCl-P) declined by 23% as HT effectively disrupts mineral-bound phosphorus complexes, facilitating the release of otherwise immobilised phosphorus. Four AD scenarios consisting of untreated (S1), hydrothermally treated (S2), adsorbed (S3), and struvite-precipitated (S4) sludges were subsequently evaluated for biomethane potential. HT-treated sludge achieved an over 270% increase in methane yield, with methane production escalating from 2.3 mL CH4/g VS to 72.7 mL CH4/g VS and reduced the lag phase from 3.5 to 1.2 days. Scenario 4 (HT + AD + adsorption + struvite) was the most economically efficient option, with a profitability improvement of over 200%, an ROI of 45.04 %, and an NPV of approximately US$2.5 billion over a 30-year lifespan, as determined by the technoeconomic evaluation. Sensitivity analysis has revealed that energy recovery efficiency, market value of struvite, and taxation are the most significant economic variables. A break-even point in a full-scale operation was estimated to be within five years. The life cycle assessment (LCA) reported decreases in global warming potential (GWP) of 21.3 % (S2), 37.4 % (S3), and 8.53 % (S4) compared to conventional sludge disposal (S1). These positive gains affirm the co-benefits to the environment and economy that accompany the integration of these processes. Despite the limitations of bench-scale HT-integrated sludge valorisation, including heat losses and the scarcity of LCA datasets, the study provides a validated framework for upscaling HT-integrated sludge valorisation systems. The results advance circular economy efforts as WWTPs are converted into bioresource recovery hubs that mitigate eutrophication, increase renewable energy production, and contribute to meeting SDGs 6 (Clean Water) and 13 (Climate Action). This work helps fill the gap between waste and resource recovery, ensuring the sustainability of next-generation wastewater treatment and representing a sustainable, energyefficient, and economical future for sludge management.