Sustainability 2023 , 15 , 6915
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the resulting pulp is then a chlorine-free pulp [40,46]. Green solvents such as ionic liquids have also been explored in the literature as pulping agents to replace the harsh chemicals used in wood pulping [47–51]. Wastewater from the PPI is treated either by aerobic or anaerobic biological treatment, with aerobic being the most commonly used to reduce its BOD [52]. Aerobic treatment requires oxygen, which is supplied by aeration equipment. Anaerobic treatment is carried out in the absence of oxygen and has several advantages over aerobic treatment, including lower sludge production, but it is not commonly used in the PPI due to the formation of hydrogen sulfide [53]. The most common methods of disposal of the final paper sludge are landfilling and incineration [54]. When paper sludge is incinerated for energy recovery, another waste stream of the PPI, known as paper sludge ash, is produced. While paper sludge remains the main waste stream of the paper industry, paper sludge ash is also produced in large amounts [54]. Paper mill sludges are a mixture of several substances, including inorganic solids, chemical additives, and paper fibers [55]. From an environmental viewpoint, incineration and landfilling of sludge have a negative impact on the environment due to air and water pollution and land usage [56]. To reduce its environmental impact, the PPI explored paper waste as a raw material to produce paper, which seems to be environmentally beneficial [22]. However, many concerns have surfaced about the chemicals that are added to recycled paper to improve process efficiency and product quality [57]. Furthermore, recycled paper might have more harmful chemicals present in it due to cross-contamination with other wastes [23]. Moreover, waste recycling does not form a complete cycle as the amount of collected paper waste does not meet the required demands of paper products considering that recycling is not 100% efficient, as some of the paper waste degrades during the process [23]. Recycling also does not significantly reduce the greenhouse gas emissions of the PPI because it is not usually properly performed [58] and is still an energy-intensive process [59]. Due to the increase in global recycling rates, the amount of paper sludge from deinking and repulping also increased [60]. Deinking sludge contains ink particles, short cellulosic fibers, coatings, and deinking additives. Most deinking sludge is dewatered and incinerated for volume reduction and energy recovery, and very small amounts of it are used for the manufacturing of cement and bricks [61]. Figure 1 shows a summary of the main waste streams that are generated during paper’s lifecycle, starting with wood pulping to paper making, consumption, and recycling.
Figure1. Possible waste streams generated during the lifecycle of paper from wood pulping to paper production, consumption, and recycling. Overall, the PPI is a main consumer of wood, electricity, and water. It has a cur- rent objective of producing affordable and high-quality pulp while preserving natural resources [62]. Several parts of the world cannot keep up with the wood demand of the PPI
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