## 1 Introduction
Various methods for pretreatment are developed in mechanical, thermal, biological, and chemical pretreatments, for this study the mechanical pretreatment was given to the substrates [7]. The objective of mechanical pretreatment is the reduction of size and crystallinity of lignocellulosic material. Physical pretreatment is given by grinding, hammer mill, knife mill, ball milling. For the reduction of particle size, the grinders are used to reduce the crystallinity of the substrate and increase the porosity of the substrate so that it could be easily degraded by the anaerobic bacteria. The decrease in particle size leads to an increase of available specific surface and a decrease in the degree of polymerization (DP) [8]. The milling causes also shearing of the biomass, pore-volume, or porosity of lignocellulosic material and enhances the initial enzymatic hydrolysis rate [9]. An increase in digestibility takes place when the pore size of the substrate is large to easily accommodate both large and small enzymes [10]. For a long time, there has been a growing attention for the effect of pretreatment on lignocellulosic residues for enhanced methane production. Accordingly, in the present study, we had investigated the potential of mechanically pretreated lignocellulosic residues i.e., wheat straw, rice straw and sugarcane bagasse present in Pakistan and to analyze their potential for biogas production through the anaerobic co-digestion process. Thus far, previous studies had focused on anaerobic co-digestion of wheat straw, rice straw with cow or cattle manure [11; 12; 13]. The chemical pretreatment and analyzed the optimal proportion of treated wheat straw and cow manure for efficient methane production, co-digested rice straw and dairy manure and assessed a wide range of feeding regimes on biogas productivity and yields in rice straw anaerobic co-digestion. Accordingly in the present study we used mechanical pretreatment described that mechanically pretreated lignocellulosic waste can work at high solids loading which is ideal if the recommended combination is used as a substrate in a small scale anaerobic digester [14; 15]. Furthermore, physical properties like porosity and durability of wheat straw, sugarcane bagasse, and rice straw have already been studied. Wheat straw is reported more porous and denser; the cellular size of wheat straw is greater than those of rice straw and sugarcane bagasse, so more bacteria could adhere to its surface and increase the production of biogas [16; 17]. Wheat straw is a potential residue to produce methane due to its permeable nature [18]. In the present study, wheat straw was co-digested with the other lignocellulosic residues to determine the best ratio and residue with which it could give the highest methane production. Lignocellulosic biomass waste has been a problem to manage as well as its anaerobic digestion is slow because of the high lignin content to increase the degradation rate cost-effective pretreatment strategy is required to enhance the process efficiency [19]. Bagasse produced by 70 sugar industries in the country has been found to be sufficient for the generation of 5700 GWh of electricity. Major crop residues include cotton stalks, wheat straw, rice straw, sugarcane trash, and corn stalk having production of 49.4, 34.581, 16.75, 7.83, and 5.325 million tons, respectively [20]. Lignocellulosic biomass has been used to produce Butanediol having potential use in cosmetic products, pharmaceuticals, antifreeze agents, synthetic rubber, fuel additives, and flavoring agents in food products [21]. However, bioplastics from lignocellulosic have an attraction to address environmental pollution [22]. Besides the other agricultural residues usage in developing countries like Pakistan, where 70% of the population do not have access to clean fuel for cooking, their priority is biogas production from these agricultural residues to access clean energy.