In the present world, the increase in the human population is proportional to the increase in energy demand. Energy extracted from fossil fuel is mostly used which creates issues such as scarcity, expensive in price, and causes air pollution mainly affects the environment . Previous studies also suggested using biogas as a clean energy source for power generation, cooking, and heating. Furthermore, anaerobic digestion for biogas production is a potential option that may both augment and reduce the use of non-renewable energy sources like fossil fuels . Cotton production is predicted to climb 4.7 percent globally by 2021/2022, according to a forecast by the United States Department of Agriculture. However, there is a 6.5 percent reduction in comparison to 2019/2020. China is predicted to be the largest producer, with 29.0 million bales of cotton, the highest level in the last six years, while India is estimated to produce roughly 475 kg/hectare, a 1% decrease from the previous year. India was first in cotton-producing countries in 2015, and second in 2019 and 2020, followed by the United States, Brazil, and Pakistan . In India, surplus crop residue is composted, which takes up a lot of space, and then burned. Crop residues produce greenhouse gases when burned, contributing to increased air pollution . Agricultural crop residues are used to produce biogas via anaerobic digestion to address this problem. Agricultural crop residue such as cotton waste was generated around 11.4 Mt in India . Appropriate technologies are employed for the production of clean energy to properly utilize waste and meet current energy requirements by reducing pollution. For the maximum degradation of organic waste, anaerobic digestion is the best technique opted for the generation of energy (Methane) in a more economical and environment-friendly manner [7, 8].
The alternative new renewable energy extraction can be done from biomass which could be a better solution. Biomass is the best alternative source that produces renewable energy and replaces fossil fuel products. Lignocellulose biomass comprises a large fraction of cellulose (40-50%), a smaller fraction of hemicellulose (20-40%), a very low fraction of lignin, and a small quantity of other alternatives. Cellulose is a glucose polymer with an average molecular weight of around 1,00,000, whereas hemicellulose is a heterogeneously branched polysaccharide with an average molecular weight of less than 30,000. Lignin has complex and cross-linked structures with high molecular weight. Lignocellulose biomass is difficult to degrade, naturally. For better degradation, a proper treatment method is followed . The anaerobic biodegradability has been enhanced with the aid of treatment methodologies such as mechanical, thermal, chemical, ultrasonic, biological and microwave approaches through the anaerobic digestion process [9, 10]. Biogas is produced through an anaerobic digestion process on various biomass materials such as wheat straw , pearl millet straw , corn stover [13, 14], rice straw, yard waste, and leaves , and also with other wastes such as food waste with cow dung , food waste with fresh septic tank sludge , etc.
Anaerobic co-digestion is a promising technique to enhance the digestion performance and biodegradability of lignocellulose biomass . Among all the pre-treatment processes of biomass, the chemical pre-treatment process has been reported to offer maximum methane yield through the anaerobic digestion process with the effective breaking of chemical bonds among cellulose, hemicellulose, and lignin through microorganisms . In the chemical treatment methods, the following chemicals are commonly used such as H2SO4, HCl, H2O2, CH3COOH, NaOH and Ca(OH)2 for the biodegradability of lignocellulosic biomass materials through an anaerobic digestion process . NaOH and Ca(OH)2 are used as pre-treatment chemicals on giant reed materials for the enhancement of biogas production through anaerobic digestion . Three plant species such as hay, straw, and bracken were pre-treated using Ca(OH)2, maleic acid, and ammonium carbonate, and subsequent anaerobic digestion was carried out under mesophilic temperature for 40 days in a batch digester . The highest methane production was achieved by pre-treating corn stalk with Ca(OH)2 through an anaerobic co-digestion process with levulinic acid wastewater . Pre-treated grass silage with NaOH increases biodegradability as well as biogas production through anaerobic digestion . Sunflower stalks are pre-treated using various chemicals like NaOH, H2O2, Ca(OH)2, HCl, and FeCl3 to enhance methane production through anaerobic digestion . Anaerobic co-digestion of cattle manure and corn stover was compared with that of ammonia solution and NaOH pre-treated samples which also results in higher methane yield under mesophilic conditions . Pre-treatment of corn stalk using H2SO4 and H2O2 yields higher biogas production on the anaerobic co-digestion with swine manure . Anaerobic co-digestion of cocoa pod husk with swine manure yields maximum biogas production where cocoa pod husk was pre-treated by using H2SO4 and H2O2 . It was reported that methane production is directly proportional to the enzymatic degradation of the lignocellulose biomass, whereas methane potential is inversely related to the lignin content present in the lignocellulose biomass . Remarkable biogas production was achieved on pre-treating NaOH with corn stover [13, 14, 30] and Asparagus stover  in the anaerobic digestion process. Pre-treatment of rice straw with H2O2 used in biodegradation enhances biogas production . Co-digestion of wheat straw with chemicals like KOH, Ca(OH)2 results in higher biodegradability with the increase in biogas yield . The main advantage of chemical pre-treatment on lignocellulosic biomass is to improve the effective biodegradability and upsurge the bioenergy production .
Naturally, agricultural biomass waste like cotton stalks was generally burnt and lead to air pollution. Cotton stalks can be effectively converted into useful energy through anaerobic digestion by various treated chemicals such as KOH, NaOH, Ca(OH)2, alkali Hydrogen per-oxide (AHP), H2SO4, H3PO4, and steam explosion method . Wheat straw was pre-treated with H2O2 and also co-digested with cattle manure for the enhancement of biodegradability and also methane yield . Cotton wastes such as cotton stalks, cottonseed hull (CSH), and cotton oil cake can be effectively converted for biogas production in the presence of basal medium through anaerobic digestion . For sustainable environments, energy extraction has been done by performing effective pre-treatment studies to improve energy production .
Biochemical methane potential (BMP) test was conducted in an anaerobic co-digestion of cow dung (CD) with cottonseed hull (CSH) at different ratios (CD: CSH) i.e., 0:100, 50:50, 25:75, 75:25 and 100:0. Maximum biogas yield was obtained from the anaerobic co-digestion at the ratio of 75:25 among other proportions. Mono-digestion of CSH results in less biogas yield compared to the co-digestion of CD and CSH at the ratio of 75:25. From the previous studies, it is found that the optimum biogas yield is obtained from the anaerobic co-digestion of CD with CSH at the ratio of 75:25, and verified both experimentally  and using kinetic studies .
From the above literature review, it is found that the importance of chemical treatments on different biomass and the triggering of biogas generation through anaerobic digestion. Besides, the chemicals used for treatment break the bonding between the complex biomass structures in generating the anaerobes at a rapid rate. The CSH has the potential to generate biogas during mono- and co-digestion with CD. Very few literatures have reported on the biogas generation from anaerobic mono- and co-digestion of CD with chemically treated CSH.
The performance of the digester with CD and CSH in the ratios of 75:25 and 0:100 with pre-treatment with various chemicals such as sulfuric acid, hydrochloric acid, hydrogen peroxide, acetic acid, sodium hydroxide, and calcium hydroxide at different concentrations for the enhancement of biogas yield has been investigated in this paper. The main objective of this paper is to find out the maximum biogas yield among all chemically pre-treated CSH and the best concentration in each acid and alkaline. The best-treated chemical for CSH for the complete replacement of CD in the existing gobar gas plant is also suggested.