Energies 2021, 14, 5940. https://doi.org/10.3390/en14185940 www.mdpi.com/journal/energies Article Anaerobic Biodegradation of Wheat Straw Lignin: The Influence of Wet Explosion Pretreatment Muhammad Usman Khan 1,2,3 and Birgitte Kiaer Ahring 1,2,4, * 1 Bioproducts, Sciences and Engineering Laboratory, Washington State University, Tri-Cities, Richland, WA 99354, USA; usman.khan@uaf.edu.pk 2 Bioengineering & Biological Systems Engineering, Washington State University, Pullman, WA 99163, USA 3 Department of Energy Systems Engineering, Faculty of Agricultural Engineering and Technology, University of Agriculture, Faisalabad 38000, Pakistan 4 Gene and Linda Voiland School of Chemical Engineering, Washington State University, Pullman, WA 99163, USA * Correspondence: bka@wsu.edu Abstract: Large amounts of lignin residue is expected in the future when biorefineries for producing biofuels and bio-products will increase in numbers. It is, therefore, valuable to find solutions for using this resource for the sustained production of useful bioenergy or bio-products. Anaerobic di- gestion could potentially be an option for converting the biorefinery lignin into a valuable energy product. However, lignin is recalcitrant to biodegradation under anaerobic conditions unless the structure is modified. Wet oxidation followed by steam explosion (wet explosion) was previously found to make significant changes to the lignin structure allowing for biodegradation under anaer- obic conditions. In this study, we examine the effect of wet explosion pretreatment for anaerobic digestion of wheat straw lignin under mesophilic (37 o C) conditions. Besides the biorefinery lignin produced from wheat straw, untreated lignin was further tested as feed material for anaerobic di- gestion. Our results showed that wet exploded lignin pretreated with 2% NaOH showed the highest lignin degradation (41.8%) as well as the highest methane potential of 157.3±9.9 ml/g VS. The un- treated lignin with no pretreatment showed the lowest methane yield of 65.8±4.8 and only 3.5% of the lignin was degraded. Overall, increased severity of the pretreatment was found to enhance an- aerobic degradation of lignin. Keywords: lignocellulosic materials; anaerobic digestion; wet explosion pretreatment; lignin; cellu- lose; hemicellulose 1. Introduction Energy security, as well as greenhouse gas emissions, are some of the greatest global challenges of the 21st century. Moreover, solar, wind, and hydropower are facing chal- lenges of variations in supply as well as storage [1]. Biomass-based bio-refineries are, therefore, emerging steadily over the world as a way to supply security and stability into the renewable energy system. Biomass-based bioenergy production is advantageous over other processes as it can be integrated into the existing energy infrastructure and energy supply chain. Lignocellulosic biomass including wood waste, crop residues, and energy crops can be used for producing different types of biofuels, such as bio-oil, biogas and bioethanol, to improve energy security with concomitant reduction of the dependency on the conventional fossil fuels [2–4]. Currently, the lignocellulosic bio refineries are mainly utilizing the carbohydrate fraction of biomass leaving the lignin fraction behind as a waste material. The paper in- dustries are further only utilizing the cellulose fraction of the biomass for paper produc- tion while producing steam or wood pellets from the lignin or discharging this fraction as Citation: Khan, M.U.; Ahring, B.K. Anaerobic Biodegradation of Wheat Straw Lignin: The In- fluence of Wet Explosion Pre- treatment. Energies 2021, 14, 5940. https://doi.org/10.3390/ en14185940 Academic Editor: Hwai Chyuan Ong Received: 26 August 2021 Accepted: 16 September 2021 Published: 18 September 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdic- tional claims in published maps and institutional affiliations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/lic enses/by/4.0/).