Effect of corn stover hydrochar on anaerobic digestion performance of its associated wastewater

Hydrothermal carbonation (HTC) is an effective method to enhance the fuel quality of biomass in a subcritical water environment, but generates large amounts of wastewater (HTCWW), which was converted through anaerobic digestion (AD) into methane in this study. However, the toxic and refractory substances contained in HTCWW tended to cause operation instability of the AD system. The solid product in HTC of corn stover (CS), named CS hydrochar, was modified with KOH immersion and then added to the AD reactor to improve the methanogenic performance. The results showed that the optimum dosage of modified hydrochar (MCH) was 15 g/L, and the COD removal rate was increased by 19.3% and methane yield was increased by 42.3%-301 mL/g-COD, as the pore and the oxygen-containing functional groups of MCH provided colonization points for microorganisms, and also enhanced the electron transfer efficiency among methanogenic archaea. In addition, the increased alkalinity of MCH due to alkaline modification increased the pH buffering capability, and accelerated the consumption of acetic acid and butyric acid in the early AD stage (0-8 days) and propionic acid in the late AD stage (12-18 days), which then alleviated the organic acid accumulation and reduced the lag period by 2 days. The adverse effects of toxic and refractory substances of HTCWW on the AD performance were also decreased due to the adsorption of MCH at the beginning of the AD process, and latterly the adsorbed substances could be degraded by the microorganisms colonized on the MCH surface. The finding of this study showed AD is a feasible method to recover organic energy contained in HTCWW, and the associated hydrochar can be used as an effective promoter for the AD of HTCWW.