Fertilizer potential of hydrothermal carbonization aqueous phase aerobically fermented by Saccharomyces cerevisiae

In this study, the hydrothermal carbonization aqueous phase (HTC-AP) from municipal sludge (MS) was converted into liquid fertilizer via aerobic fermentation (AF) using Saccharomyces cerevisiae (30 °C, 180 r/min, 6 % inoculum, 72 h). The fermented HTC-AP with an initial chemical oxygen demand of 50 g/L exhibited the optimal performance. It achieved 69.5 ± 2.1 % carbon reduction, significant total nitrogen and total phosphorus removal (70.9 ± 5.0 % and 94.3 ± 0.8 %, respectively), and nitrogen (976.7 ± 39 mg/L) and phosphorus (27.6 ± 24 mg/L) retention. The seed germination tests demonstrated that fermented HTC-AP achieved a germination index above 70 %, indicating no phytotoxicity and enhancing seed germination. In addition, transcriptomics revealed the downregulation of glycolysis genes (PFK1, TDH1) under inadequate carbon and the upregulation of fatty acid degradation (ALD6) acted as the alternative energy. This approach not only addresses the management of HTC-AP but also provides a sustainable pathway for converting sludge into agricultural resources, thereby having potential possibility to align with circular economy models in future.