Process water from hydrothermal carbonization: from waste to liquid fertilizer and soil health amendment in circular bioeconomy

Abstract Hydrothermal carbonization (HTC) transforms wet or dry biomass into hydrochar, generating a nutrient-rich process water, hereafter termed HTC-PW, which is often overlooked as waste. This review synthesizes current knowledge on HTC-PW composition, including varied pH (3.5–9.2), high organic content (TOC 4,000–31,700 mg L−1), and nutrients such as NH₄⁺–N (up to 4,400 mg L−1) and potassium (5,870–6,330 mg L−1), derived from feedstocks such as sewage sludge and food waste. Process controls such as temperature and residence time tune HTC-PW properties for agronomic use, enabling enhanced partitioning of elements between solid and liquid phases. Pathways include direct fertigation, co-application with biogas slurry, and conditioned recovery, such as struvite precipitation yielding 92–99% P and 43–88% N. Performance metrics demonstrate yield increases of 6.7–29.2% and improved nutrient use efficiency of 15–30% in crops such as rice, alongside microbiome shifts favoring bacterial communities for better nutrients cycling. Beyond fertilization, valorization routes encompass anaerobic digestion for biogas (250–350 mL CH4 g−1 COD, with 70–85% COD removal) and catalytic reforming for H₂. Risks such as salinity (EC 5–24 mS cm− 1) and context-dependent N2O responses (suppression under inhibitory organics versus pulses under high NH4⁺ loading) necessitate bioassays and regulatory compliance, while techno-economic analysis and life-cycle assessment indicate scenario-dependent benefits, including economic savings where avoided wastewater-treatment credits apply and 20–50% reductions in global warming potential when mineral fertilizer substitution is credited. Gaps in long-term trials and scalability are identified, with future directions emphasizing machine learning for predictive optimization of HTC-PW properties and applications. Overall, current evidence supports HTC-PW primarily as a nutrient-rich liquid amendment (fertilizer-like input) that alters soil DOM and microbial processes, while direct evidence for consistent improvements in soil physical structure remains limited and warrants targeted measurement in future field trials. Graphical Abstract