Harnessing Municipal Solid Waste: Enzymatic Pathways to Bioethanol Sustainability: A Review
Abstract
Waste management problems and the need for renewable energy can be addressed by utilizing municipal solid waste for bioethanol production as a renewable feedstock. The enzymatic hydrolysis process of turning solid waste into fermentable sugars for the subsequent production of bioethanol is the main focus of the current study. Enzyme access is significantly facilitated by effective pretreatment, particularly the alkali process with NaOH, which breaks the resistant lignocellulosic structure. Hydrolysis is possible under moderate circumstances (40–50°C, pH 4.5–5.0) thanks to fungal-derived cellulolytic enzymes from Aspergillus and Trichoderma strains. Using ethanologenic yeasts such as Saccharomyces cerevisiae and Pichia stipitis, the sugar-containing hydrolysate is then fermented, with optimised procedures producing ethanol. It has been discovered that integration approaches to the process, like simultaneous fermentation and saccharification, increase efficiency compared to independent operating steps. Despite promising results, problems with process optimisation, biomass recalcitrance, and enzyme cost persist. Enzymatic hydrolysis is used in this study as an example of a possible method for turning municipal waste into bioethanol; however, further technological advancements are required to increase the economic feasibility and commercial use of this environmentally friendly bioconversion process.
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Introduction
Numerous studies on the production of bioethanol from lignin-based biomass, including solid waste from municipalities, have been spurred by the growing demand for renewable energy sources. MSW is a good feedstock for the production of bioethanol because it contains avast volume of organic waste, which is high in cellulose, hemicellulose, and lignin.(Kumar et al.2020 and Srivastava et al. 2017).Due to its many benefits over chemical hydrolysis methods, including high specificity, lower energy inputs, and environmentally friendly processing, enzymatic hydrolysis is an important step in the conversion of MSW to fermentable sugars. (Patra et al. 2017 and Banerjee et al. 2019). Alkaline, acidic, and steam explosion pretreatments are a few of the pretreatment techniques that have been employed to maximise the enzymatic hydrolysis efficiency of biomass derived from MSW. (Singh et al. 2019 and Chen et al. 2017).
One easily accessible and under-utilised resource that has become a promising option for the production of renewable biofuels is municipal solid waste. The accumulation of MSW has become a significant environmental issue due to global urbanisation, necessitating the development of environmentally friendly waste management solutions. Pollution and climate change are caused by conventional waste treatment methods like incineration and landfilling. The renewable energy production and the removal of environmental pollution are two benefits of using MSW as bioethanol through enzymatic hydrolysis. (Sharma et al. 2019). Furthermore, governments and research institutions worldwide have recognised the potential of waste-to-energy technologies and have been investing in the advancement of enzymatic hydrolysis and biomass pretreatment techniques (Lyndet.al 2017).
Hemicellulases and cellulases are essential for the hydrolysis process by enzymes because they breakdown structural carbohydrates into monomeric sugar, which microbes then ferment to produce ethanol.(Sarkar et al. 2012 and Zhang et al. 2018)Recent developments in microbial fermentation, genetic engineering, and enzyme science have significantly increased the efficiency of producing bioethanol from MSW.(Gupta et al. 2020 and Singhania et al. 2013). While Taherzadeh et al. and Karimi et al. (2007) studied enzyme-based hydrolysis techniques, authors such as Lynd et al. and Wyman et al. have contributed to the explanation of the enzymatic hydrolysis of complex polysaccharides to fermentable sugars. However, the main obstacles to scaling up this technology are problems like enzyme inhibition, restricted substrate accessibility, and higher production costs (Wyman et al. 2005and Banerjee et al. 2010).
Furthermore, studies by Zhang et al. 2018 and Lynd et al. 2017, and Sun et al. and Cheng et al. (2002) have demonstrated the impact of different pretreatment techniques on the efficiency of enzymatic hydrolysis. As a more sustainable alternative to chemical pretreatment, biological pretreatment such as microbial degradation via ligninolytic fungi has also been studied. The development of metabolic engineering of fermentative microorganisms is also noteworthy because, as Singhania et al.'sresearch shows, it has made it possible to produce more ethanol from mixed sugar substrates. The second crucial area of research is process optimisation, where it has been discovered that simultaneous saccharification and fermentation (SSF) increases ethanol yields by lowering process costs and end-product inhibition. (Miller et al.1959).
Lignocellulosic biomass Temperature, chemicals, Breaking of cell wall structure pressure Pretreated lignocellulosic material Hydrolysis of cellulose Enzymes, temperature Hydrolysis solution Yeast Fermentation of sugars Fermentation solution Distillation and purification Temperature, molecular sieves Fuel grade bioethanol FIGURE 1: Schematic representation of the biomass conversion into bioethanol.
All things considered, the transition to using MSW for the production of bioethanol is a significant step towards a circular economy, in which waste is recycled to create beneficial biofuels. The recent developments in the enzymatic hydrolysis of MSW are examined in this review, with a focus on process integration, fermentation tactics, enzyme optimisation, and pretreatment methods for effective bioethanol production. This research aims to contribute to the ongoing efforts to make bioethanol a viable and scalable alternative to fossil fuels by addressing current issues and identifying potential technological advancements. FIGURE 2: General overview of bioethanol production from lignocellulosic biomass.
Conclusion
One promising method for producing bioethanol sustainably is the enzymatic hydrolysis of MSW. The effectiveness of enzymatic hydrolysis and optimised pretreatment techniques insignificantly increasing sugar recovery and ethanol yield is covered in this work. However, to make it appropriate for large-scale implementation, technical and financial challenges such as substrate heterogeneity, process scale-up, and enzyme cost must be resolved. To make it more effective and economical, future research should concentrate on developing enzyme engineering, optimising microbial fermentation, and integrating bioprocesses.
In addition to producing energy, waste management and bioethanol production can have two benefits: reducing landfill disposal and promoting the circular economy. Enhancing the economic feasibility of bioethanol production will require improvements in microbial strains, process technology, and enzyme recycling. To facilitate the transition to cleaner biofuels, governments and industry must collaborate in supporting research and development projects. If these obstacles are removed, enzymatic hydrolysis of MSW would contribute significantly to the attainment of global renewable energy goals, reduce reliance on fossil fuels, and reduce environmental pollution, all of which would contribute to a cleaner, more sustainable world.
