INTERNATIONAL JOURNAL of RENEWABLE ENERGY RESEARCH T.A.Shah et al., Vol.8, No.3, September, 2018 Effect of Alkali Pretreatment on Lignocellulosic Waste Biomass for Biogas Production Tawaf Ali Shah* + , Shehbaz Ali* + , Asifa Afzal* + , Romana Tabassum* +‡ * National Institute for Biotechnology and Genetic Engineering (NIBGE), Faisalabad, Pakistan + Pakistan Institute of Engineering and Applied Science (PIEAS), Islamabad, Pakistan ‡ Corresponding author: Romana Tabassum (tawafbiotech@yahoo.com, romanatabassum@yahoo.com) ‡ Postal address, National Institute for Biotechnology and Genetic Engineering (NIBGE), Jhang road 3800, Faisalabad, Pakistan Tel: +923444092987, Received: 21.03.2018 Accepted:14.05.2018 Abstract- Converting lignocellulosic waste biomass into biogas is a multi-step process; the rate limiting reaction is lignin removal. The objective of the current study was to evaluate alkali treatment for lignin removal and potential of biogas yield of pretreated waste biomass compared to untreated one. Three alkali reagents at various dosages: NaOH (1,2,3, and 5%), KOH (1,2,3, and 5%), and Ca(OH) 2 (0.5%) were tested at three different heating processes, water bath, autoclave and short time microwave. NaOH with short time microwave heating process had the highest delignification of 70-86% compares to other alkalies and heating processes. However, an opposite effect of high alkali dosage was observed on holocellulose. The highest cumulative biogas of 560 mL/gVS was obtained from 2% NaOH pretreated wheat straw, which was 2-times higher than the cumulative biogas produced from the same untreated substrates. In the present study, 2% NaOH alkali and microwave heating are determined optimum for lignin removal from waste biomass and enhancement in biogas production. Keywords Anaerobic digestion; alkaline pre-treatment; biogas; delignification; lignocellulosic waste. 1. Nomenclature MW=Microwave, AUTO= Autoclave, WT= Water bath NmL/gVS= Normalized biogas in mL per gram volatile solid, CH 4 = Methane OBA= Online Biogas App, AD = anaerobic digestion, MC= Moisture content, TS= Total solid, VS= Volatile solid. 2. Introduction Fossil fuels are continuously depleting and their consumption releases greenhouse gases that raise environmental problems. Therefore, interest is developed in looking for alternative energy. Biogas production from lignocellulosic biomass (LB) through anaerobic digestion (AD) is focused on sustainable bioenergy production process [1]. The agriculture waste residue is composed of lignin, hemicellulose, cellulose, and some extractable components. The percentage of these components varies among crop residue, but in general, hardwood biomass contains 40-50% cellulose, 15-25% lignin, 22-35% hemi-cellulose, and 2-7% extractives, whereas softwood biomass contains 20-31% lignin, 24-32% hemicellulose, 40-45% cellulose, and 1-7% extractives [2, 3]. Cellulose is homopolysaccharide chains of glucose units. Hemicellulose is heteropolymers of pentose sugars that is the outer surface layer of the biomass cell wall [4]. Lignin is the most complex hydrocarbon polymer and contains multiple phenylpropane units, crosslinking of these phenylpropane units and hydrophobic nature of lignin make the LB structure more resistant to microbial and enzyme degradation [5]. The degradation resistance of lignin significantly decreases the yield of biogas in AD process [6]. One of the initial rate-limiting steps is to choose a suitable pretreatment method to remove lignin and make the cellulose accessible to hydrolytic enzymes [7, 8]. The second concern is contingent on testing of high concentration of chemicals in biomass pretreatment. Several methods, steam explosion, hydrothermal process, acid treatment, alkalies treatment, ammonia fiber explosion etc, have been reported for biomass pretreatment with low and high solid concentration [9-14]. For lignin removal among all of the listed methods and many others, the preferred selection is alkaline pretreatment method. The lignin can be released by different alkalies, especially, KOH, Ca(OH) 2 and NaOH [15]. Treatment of waste biomass with these alkalies decreased the degree of polymerization, remove lignin, and make cellulose more accessible to enzymatic and microbial degradation. However, the high concentration of alkalies increase the cost of