Enhanced acidogenic fermentation of waste sludge via hydrothermal carbonization pretreatment: Process optimization, performance limitations, and carbon pathways

Waste sludge management poses major economic and environmental challenges. Valorization through acidogenic fermentation to generate volatile fatty acids (VFAs) offers a sustainable alternative, but the low biodegradability of waste sludge often requires pretreatment to enhance process efficiency. This study investigated hydrothermal carbonization (HTC) as a pretreatment to improve VFA production from waste sludge via subsequent thermophilic fermentation (50 °C). Biochemical methane potential tests were used to optimize HTC conditions (170–200 °C, 0–120 min), identifying 180 °C for 60 min as the most effective for enhancing sludge biodegradability. Continuous fermentation of HTC-pretreated waste sludge (applied either as whole slurry or as separated process liquid) was then carried out in completely stirred tank and sequencing batch reactors. HTC pretreatment increased VFA production and enriched the mixture in longer-chain acids (butyric, valeric, caproic; 30–40 % of total VFAs), which are advantageous for downstream applications such as polyhydroxyalkanoate synthesis. Nonetheless, moderate acidification yields (∼0.3 gCOD-VFAout/gCODs,out) indicated incomplete conversion, likely due to recalcitrant organics in the HTC-treated sludge, as confirmed by GC–MS analyses. Both process configuration and hydraulic retention time markedly influenced performance and microbial community composition. COD mass balance revealed substantial carbon partitioning into the hydrochar fraction, underscoring the importance of its valorization to ensure overall process sustainability. In summary, HTC shows potential as a pretreatment strategy for converting waste sludge into VFAs, but addressing the limitations imposed by recalcitrant compounds is crucial to maximize carbon recovery within a circular economy framework. • HTC pre-treatment at 180 °C for 60 min was identified as optimal for sludge biodegradability. • Thermophilic fermentation yielded a VFA mix rich in butyric, valeric and caproic acids. • VFA conversion efficiency was moderate due to recalcitrant organics formed during HTC. • Hydrochar valorization is crucial as it retains over 50 % of the initial sludge COD. • Integrated valorization of both hydrochar and process liquid is key for sustainability.