Chemical modification of straw hydrochar as additive to improve the anaerobic digestion performance of sludge hydrothermal carbonization wastewater

Hydrothermal carbonization (HTC) is a promising technology for treating highly wet organic solid waste such as sewage sludge (SS), but it generates a large amount of organic wastewater accompanied with solid fuel. In this study, hydrothermal carbonization wastewater (HTCWW) of SS was treated through anaerobic digestion (AD), and methanogenic performance was improved by adding straw hydrochar and its chemically modified products, which could reduce the adverse effects of refractory and toxic substances in HTCWW on microorganisms. After the hydrochar was modified through KOH, H2O2, and HCl immersion at room temperature, the abundance of oxygen-containing functional groups and the average pore size of the hydrochar increased, especially KOH, which had the most remarkable effect. By adding KOH-modified hydrochar at the optimal dosage of 20 g/L, the cumulative methane yield was 152 mL CH4/g-COD, which was 148.4 % higher than that of the control group (61.2 mL CH4/g-COD) without any hydrochar. At the beginning of AD, as some toxic substances, such as nitrogen compounds in HTCWW, was adsorbed by the added hydrochar, microorganisms could quickly adapt to the substrate environment. In addition, the hydrochar acted as a buffer to avoid the adverse effects of organic acid accumulation, thus reducing the lag period of AD by 0.2–1.3 days. In the middle and later stages of AD, hydrochar could provide colonization sites for the microorganisms, thereby enriching Metalosarcina and Metalosaeta, enhancing direct interspecific electron transfers, and further promoting the degradation of organic substances and generation of methane. The finding of this study showed that using modified hydrochar as the promoter of AD is a feasible method to improve the energy recovery of HTCWW, which will provide a novel paradigm for the clean and efficient utilization of wastes.