Hydrothermal valorization of beach-cast brown seaweed Ascophyllum nodosum into bioactive compounds and hydrochar using severity factor as a design tool

• Hydrothermal processing valorizes beach-cast A. nodosum into high-value products. • The severity factor (R o ) was assessed as a robust tool for HTP development. • Fucoidan, antioxidant-rich extracts, and hydrochar were effectively recovered. • Liquid at log R o = 3.18–3.22 enriched with saccharides, phenolics, and carotenoids. • Hydrochar at log R o = 3.79 shows improved fuel properties compared to the feedstock. Beach-cast brown seaweed ( Ascophyllum nodosum ) is an abundant but underutilized biomass, often discarded as waste from coastal management. This study presents a hydrothermal processing (HTP) strategy under mild subcritical water conditions (100-240 °C and 1–33 bar) to valorize A. nodosum into liquid bioactive compounds and solid hydrochar. A key challenge in HTP scale-up is the variability in heating times across reactors, which complicates process optimization and control. To address this, the severity factor (log R o = 0.59–5.10) was evaluated as an integrated design parameter to combine final temperature (100-240 °C), heating time (31–99 min), and hold time (0–24 min) for waste brown seaweed valorization. This approach allows recovery of crude alginate (15.75 dry wt% at log R o = 1.99), crude fucoidan (39.94 dry wt% at log R o = 2.39), antioxidant-rich crude extract (53.84–55.07 dry wt% at log R o = 3.18–3.22), and hydrochar (29.53 dry wt% at log R o = 3.79) at the maximum yields and/or qualities. The crude extract obtained at log R o = 3.18–3.22 was enriched in saccharides, phenolics, and carotenoids, and concomitant antioxidant activities, demonstrating potential use as natural antioxidant ingredients. Hydrochar produced at log R o = 3.79 showed enhanced fuel properties (HHV = 21.9 MJ/kg, carbon content of 55.3 %, and energy yield of 40.6 %), suggesting its potential as a solid biofuel. This work demonstrates a scalable and sustainable valorization strategy for transforming coastal biomass waste into a broad spectrum of value-added products within a circular economy framework.