Enhancing the anaerobic digestion of papermaking black liquor with three-dimensional iron-carbon electrolysis and assessment of microbial community changes

Abstract

Anaerobic digestion (AD) of black liquor (BL) in papermaking is of great significance for energy recovery and environmental sustainability. Unfortunately, inhibitory compounds, such as aromatic compounds in BL hinder microbial activities if at high concentration, thus reducing the efficiency of AD. The AD experiments with different gradients of organic loading (1, 2, 4, 6, and 8 g COD/L) revealed that the maximum methane production inhibition rate of BL reached 40.74% at 2 g COD/L. A three-dimensional iron-carbon electrolysis (3D-ICE) pretreatment was performed for various time intervals (15, 30, 60, and 120 min) before AD (2 g COD/L) to explore the effect of 3D-ICE pretreatment on improving the AD efficiency. The results showed that the cumulative methane production changed from 114.10 mL CH4/g COD to 181.55, 139.40, 95.64, and 96.10 mL CH4/g COD, respectively. Moderate electrolysis (15 min) enhanced the conversion of toxic and/or refractory substances into readily degradable ones and improved the biochemical characteristics of BL by degrading the refractory nitrogenous compounds and reducing the secretion of extracellular polymeric substances. Furthermore, it was found that the dominant bacteria (e.g. Aminicenantales, Mesotoga) related to hydrolysis and fermentation of complex compounds such as aromatic compounds and acetotrophic methanogenic archaea (e.g. Methanosaeta) were significantly enriched, which was conducive to the conversion of organic matters in BL into methane. This also indicated the positive effect of 3D-ICE. Generally, 3D-ICE combined with AD is considered as an effective way to recover energy from BL.