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of using these types of pulps is that methane production remains stable despite the shift in raw material or the pulp being in the bleached or unbleached form [113]. 4.3. Heat by Incineration Paper waste has three main disposal methods, landfilling, incineration, and compost- ing. Landfilling is the most common disposal method among the three. The effectiveness of landfilling is severely reduced due to the extensive land usage, especially in countries such as China, which is one of the largest producers of solid waste globally [114]. Landfilling emitsCH 4 as solid waste decomposes anaerobically. Paper waste can be utilized as renew- able energy for power generation and minimizes the greenhouse gas emissions released from landfilling [115]. In the past, landfilling sites were available in close proximity to urban areas, which limited the development and need for incineration facilities in countries such as Canada. In addition to that, waste incineration was believed to cause significant harm to human health and the environment by local communities [116]. Paper waste incineration produces heat, which can be utilized or further processed to produce elec- tricity. Sometimes waste is incinerated solely for the purpose of disinfection and volume reduction [117]. Waste incineration involves the combustion of solid wastes, releasing gases and energy simultaneously. Incineration of waste can reduce volume by 90% and weight by 70%. While incineration recovers a high amount of energy, it also releases a lot of toxic pollutants such as heavy metals, CO 2 , and persistent organic pollutants into the environment [118]. Technologies for waste incineration are developing rapidly. China currently uses three waste incineration technologies, which are stroker grate, fluidized bed, and rotary kiln [114]. The majority of the European incineration plants use moving toward grate technology due to its advantages, such as not requiring shredding or pretreatment, over other incineration technologies, such as the rotary kiln or fluidized beds. However, stroker gate technology has high maintenance costs compared to other technologies. The main challenge in incineration plants is to maintain a constant temperature in the com- bustion chamber and constant energy output, as increased temperature could corrode or decrease the life of essential plant components [119]. Waste incineration can fulfill the energy demands of smaller states as energy from incineration can be used to generate electricity or power water treatment plants [120]. Among different waste disposal techniques, landfilling is the worst option due to land usage and the release of greenhouse gases. Incineration is considered a better option since much of the energy can be recycled by waste combustion [18]. Among recycling and incineration, the best choice is dependent on factors such as waste handling, loss of quality in paper reprocessing, and energy recovery efficiency. In most cases, incineration coupled with advanced technology is the preferred method of paper waste management [121]. In the past, greenhouse gases and heavy metals released by the incineration process were a major concern. This was solved by the introduction of flue gas treatment systems, which then caused a shift in focus to manage the solid residues of waste incineration [117]. Following the incineration of waste, bottom ash, fly ash, and boiler ash are collected in the form of residues, the majority of which is bottom ash. Bottom ash contains brick, ceramics, and unburned organic matter such as wood, plastic, and fibers [122]. Bottom ash is commonly disposed of in landfills, but it can be recycled to produce construction materials such as concrete and cement. However, due to the leaching of heavy metals and toxic compounds, the recycling or disposal of bottom ash is environmentally concerning [123]. 4.4. Nanocellulose Paper recycling results in the collection of shortened fibers in the form of recycled paper sludge, which is not suitable for paper production. While recycled paper sludge has been used to produce bioethanol, production is limited since bioethanol, in its final form, is a low-cost product [124]. Currently, sludge is disposed of through incineration and landfilling, posing environmental hazards. The conversion of paper sludge into high-value materials, such as cellulose nanoparticles, minimizes the issues related to paper waste
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