Sustainability 2023 , 15 , 6915
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length decreases, affecting the strength of the paper [78]. It is important to mention that the effects of recycling on mechanical and chemical pulp are different. The bonding potential of chemical pulp significantly reduces due to the repeated drying and rewetting processes during recycling, while mechanical pulp deteriorates less and demonstrates a slight improvement in bonding potential [59]. For example, a study showed that the tensile index of sheets prepared from chemical pulp decreased from 65 to 55 N.m/g after one cycle of recycling, while the tensile index of sheets prepared from mechanical pulp increased from 30 to 35 N.m/g after one cycle of recycling [79]. In terms of the available technologies for recycling, one of the leading technologies is in situ precipitated calcium carbonate (PCC), which utilizes the CO 2 obtained from other industries with natural limestone to produce the calcium carbonate needed for paper production. This technology increases the strength and opacity of paper, reduces wastewater discharge, and saves energy, all of which result in lower manufacturing costs and minimal harm to the environment [73]. Another technology is dry paper recycling, which is used to recycle paper in offices without the need to export the wastepaper to a recycling facility. Dry paper recycling involves three main steps: separating the paper fibers by applying impact force, producing a sheet of paper with the collected fibers, and binding the fibers together using powdered binders and heat [80]. This technology reduces CO 2 emissions by 22% and water consumption by 99%. However, power consumption and environmental burden are significantly high considering the use of cartridges containing bonding agents. Therefore, further improvements are required to enhance its greenness [81]. Recycled paper is used for various applications such as newspaper, envelopes, office and printing paper, cardboard, and insulation. The utilization of recycled fibers depends on the quality requirement of the end product [82–84]. Paper production cannot fully depend on recycled paper since fresh fibers are required to add strength to newly produced paper and improve paper quality since some wastepaper grades cannot be used to produce high-quality paper. For instance, newspapers are produced using a high percentage (80%) of recovered fibers and 20% of primary fibers, while printing paper and magazine paper use 100% primary fibers, which are chemically treated due to required whiteness and brightness [4]. Moreover, the availability of waste paper is an issue since some types of paper, such as sanitary paper and cigarette paper, disintegrate with use, which makes them unsuitable for recycling [85]. Furthermore, paper can be recycled up to seven times, after which the cellulose fibers are deemed unsuitable for paper production and are rejected [4]. Therefore, there is a strong need for routes other than recycling for the utilization of paper waste, which is discussed below. 4. Conversion of Paper Waste into Energy and High-Value Materials One major approach towards sustainability is the conversion of waste into energy and value-added products, which has shown significant development recently. While minimizing linear economies, a sustainable circular economy should encourage the steps of take, make, use, reuse, and recycle, as opposed to a linear economy that follows take, make, use, dispose, and pollute [86]. Below are examples of how paper waste and rejects can be converted into energy and useful products, including biofuels, biohydrogen, biomethane, heat, nanocellulose, hydrochar, construction materials, and soil amendments. 4.1. Biofuels Bioethanol is the only fuel so far that can be transported in liquid form and does not re- lease greenhouse gases into the atmosphere when burned as the CO 2 emitted is used up by plants in photosynthesis as long as the biofuel consumption rate is not more than the photo- synthesis rate [87]. First-generation bioethanol is produced from sugar and starch, whereas second-generation bioethanol is produced from waste containing lignocellulose. Bioethanol production requires high amounts of cellulose, making paper waste a potential raw mate- rial [88]. The commercial production of bioethanol currently involves only first-generation product, which is produced by food crops such as sugarcane and corn as a feedstock. A
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