Improved digestibility and biogas production from lignocellulosic biomass: Biochar addition and microbial response

Abstract Biochar addition could facilitate the microbial communities’ abundance and degradation of lignocellulosic biomass in anaerobic digestion (AD). The effect of biochar from walnut (Juglans regia L.) shell has not been studied in AD so far. Therefore, different concentrations of walnut shell biochar (WNSB) (1–8 g/L) were added to the AD of lignocellulosic biomass, including corn (Zea mays L.) straw (CS), wheat (Triticum aestivum L.) straw (WS), and rice (Oryza sativa L.) straw (RS) after optimizing the C/N ratio by addition of cow dung (CD). The addition of WNSB improved the biogas production by 38.26 %–87.55 %. The maximum biogas increments of 56.97 % were recorded for CS followed by WS (74.15 %) and RS (87.55 %) after 2 g/L addition of WNSB. The highest chemical oxygen demand removal was 75.4 %, 82.2 %, and 78.2 % for CS, WS, and RS with 2 g/L WNSB, respectively. Scanning electron microscopy examination of biochar samples indicated that the large surface provided an appropriate environment for microbial colonization and these microbes assisted in interspecies electron transfer (IET). Sphaerochaeta, an organic decomposer, was dominant in the RS reactor (2 g/L); and Methanosarcina was found to be responsible for the high biogas production in the RS rector (2 g/L). Optimization of the C/N ratio in the AD, along with biochar addition, facilitated IET and improved the digestibility of lignocellulosic biomass for biogas production.