Hydrochar improves the dewaterability of digestate from anaerobic co-digestion of food waste and waste activated sludge: A neglected benefit

Although considerable effort has been devoted to have an in-depth understanding on the role of hydrochar on anaerobic co-digestion of food waste and waste activated sludge in enhancing biogas production, few studies have investigated on how it affects the digestate dewaterability. This work investigated the impact of different dosages of hydrochar (0, 2, 5, and 10 g L⁻¹) on the digestate dewatering performance and its physicochemical characteristics during the anaerobic co-digestion of food waste and waste activated sludge, as well as underlying mechanism. Experimental results indicated that adding hydrochar obviously improved digestate dewaterability, with the specific resistance to filtration reducing by 62.0%–65.6% and the capillary suction time decreasing by 6.9%–24.0%. Mechanistic studies revealed that 10 g L⁻¹ hydrochar reduced the protein content and enhanced the removal of tyrosine-like and tryptophan-like protein in extracellular polymeric substances. Fourier-transform infrared spectroscopy analysis demonstrated that adding hydrochar not only enhanced the removal of hydrophilic functional groups in extracellular polymeric substances, but also reduced the ratio of α-helix to (β-sheet + random coil). Consequently, extracellular polymeric substances hydrophobicity was preferred, resulting in better digestate dewaterability. Further microbial exploration indicated that 10 g L⁻¹ hydrochar enriched the protein-hydrolyzing microbes, which probably contributed to the lower hydrophilic extracellular polymeric substances content and components, as well as weaker protein secondary structure. Further dewatering test demonstrated that hydrochar (especially at high-dose) still caused a positive effect on the subsequent dewatering of digestate when adding the regulator FeCl_3. The current findings not only increased the appeal for the application of hydrochar in the anaerobic co-digestion of food waste and waste activated sludge, but also provided forward-looking insights on the neglected benefit of hydrochar on the subsequent dewatering process of the digestate.