Sustainability 2023 , 15 , 6915
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major drawback of first-generation bioethanol is the usage of food crops for feedstock, which results in less available food and an increase in food prices. Bioethanol is currently the most produced biofuel globally, with the United States producing the majority of it [89]. To convert paper waste into bioethanol, cellulose and hemicellulose are hydrolyzed to simple sugars, either chemically or using enzymes. Enzymatic hydrolysis is used more commonly as it is more specific and milder [90]. The simple sugars are then converted into bioethanol by fermentation [91]. Most of the processes that are used currently for bioethanol production from biomass involve certain pretreatments before hydrolysis and fermentation. Pretreatments include the usage of acid, alkali, or other organic solvents, and choosing the correct pretreatment is crucial in bioethanol production as it affects the cellulose conversion rates and enzyme activity [87]. Pretreatment is essential to gain good yields of sugars from cellulose and hemicellulose, and it increases the digestibility of cellulosic biomass [92]. The chosen pretreatment should not degrade the sugars into enzyme activity inhibitors, use minimal energy, and require fewer chemicals. For example, pretreated paper sludge yields 90% or more sugar, whereas untreated sludge yields 20% or less [93]. The presence of barrier components such as lignin and the crystalline regions of cellulose decreases the effectivity of enzymes [92]. Newspaper and cardboard contain a high amount of barrier components, whereas hygiene paper and office paper have minimal amounts of them [94]. Using paper waste as a raw material for bioethanol production provides an alternate energy source and reduces the usage of fossil fuels, solving environmental issues such as global warming. It also solves issues related to biofuel production from biomass, such as land use, deforestation, threats to global food security, and the limited amount of available biomass [95]. It is, however, not as effective in fighting global warming as renewable energy sources such as solar and wind as it still emits greenhouse gases [96]. Paper waste has an advantage over other lignocellulose materials in bioethanol pro- duction as it does not require additional pretreatments since the majority of lignin is already removed in various paper manufacturing processes [97]. Moreover, the degradable nature of paper waste makes it suitable for bioethanol production. In addition to that, wastepaper is easily available at low costs and contains high levels of carbohydrates. Bioethanol produc- tion from wastepaper also provides an alternative to recycling since recycling has several limitations, such as additional energy usage in recycling processes and a limited number of recycling cycles [98]. The paper sludge from the papermaking process contains short-length fibers and has high amounts of lignocellulose and carbohydrates, which can also be used to produce bioethanol rather than incinerating or disposing of it. However, the presence of ash and calcium carbonate block the bioconversion of fibers as they increase the pH value of sludge, making it unsuitable for enzyme activity [99]. Treatments for calcium carbonate removal involve the usage of acids and produce CO 2 , causing environmental issues. To overcome this issue, simultaneous saccharification and fermentation (SSF) can be used to neutralize the calcium carbonate present in sludge without needing pretreatments [100]. 4.2. Biohydrogen and Biogas The biohydrogen produced from biomass waste is a renewable source of energy and does not have a negative environmental impact. It has the potential to overcome several environmental issues and the global energy demand [101]. A major barrier in biohydrogen production is the cost, as it has complications in terms of storage, compression, distribution networks, and lack of durable fuel technologies [102]. Hydrogen fuel cells are also costly to set up, delicate, and do not function for extended periods of time [103]. Thermochemical pulping (TMP) is a papermaking process by which wood fibers are treated using hot steam under pressure. TMP results in anaerobically-treated wastewater to produce biohydrogen in a process called dark fermentation (Figure 2) [85]. Dark fermentation is a temperature- sensitive process since a slight shift in temperature can significantly increase or decrease the hydrogen yield; hence, the temperature of wastewater has to be controlled efficiently [104]. A group of researchers used recirculated two-phased anaerobic digestion to produce bio- hydrogen and biomethane simultaneously from municipal solid waste, 50% of which was
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