Hydrothermal carbonization process waters: Linking composition with recovery, recirculation, and treatment routes

Hydrothermal carbonization process waters (HTC PWs) contain feedstock-derived carbon, nutrients, dissolved ions, and potentially recoverable organics, but also inhibitory and poorly biodegradable fractions. This review links feedstock composition and HTC conditions to PW composition and evaluates biological conversion, nutrient and organic recovery, process-water recirculation, and enabling treatment. A source-traceable compilation of 112 experimental records from 37 primary studies covered HTC temperatures of 150–280 °C and residence times of 5–1200 min; condition-specific TOC and COD values were available in 86 and 64 records, respectively, while nitrogen- and phosphorus-related metrics were less consistently reported. This heterogeneity, together with differences in analytical basis and sample handling, limits direct cross-study comparison. Bulk indicators such as COD, TOC, TN, NH 4 + -N, and TP are useful for screening but do not resolve biodegradability, inhibition, nutrient speciation, or salinity constraints. Readily biodegradable carbon can support methane production, fermentation, biomass cultivation, or carbon-source use, whereas phenolics, furans, nitrogen-containing organics, ammonium, and salts may restrict biological stability. Process-water recirculation can reduce freshwater demand and improve hydrochar or carbon recovery but may concentrate persistent solutes. We therefore propose a composition-response framework in which measured liquid fractions identify candidate routes and route-specific response tests confirm suitability; membrane separation, oxidation, and polishing are used as enabling operations when constraints remain. Evaluation should include hydrochar responses, resource inputs, product quality, and all residual streams. This framework shifts the management of HTC PWs from bulk-load removal toward integrated resource recovery and process-scale control.