Sustainability 2023 , 15 , 6915
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mill sludge enhances its quality due to the practical physical properties it possesses. Paper sludge compost increases plant growth, and sludge containing calcium carbonate is used to lime soils with lower pH and also provides a source of Zn to marginal soils [186,187]. It is also easily stored without requiring large volumes as compared to raw paper mill sludge. However, the use of paper mill sludge as compost is still not widely welcomed in the agricultural field due to the heavy metals present in paper mill sludge, which can potentially leach into the soil, posing a health hazard [177]. However, a study conducted by Rosazlin et al. (2011) on a compost of recycled paper mill sludge mixed with empty fruit bunches showed that the compost displayed advantageous properties such as no toxicity on plants, 100% seed germination, and high nutrient content. Additionally, the heavy metal concentration in the compost was also within limits [188]. Overall, and as summarized in Figure 3, there are different possible routes for the utilization of paper waste to return its value to the economy and reduce its environmental impact. It is important to mention that some of these routes are also used for the utilization of other kinds of waste, such as electronic waste [189,190]. The greenness and efficiency of the conversion of paper waste into energy are significantly dependent on the form of energy (biofuel, biogas, hydrochar, or heat), the conversion method (SSF, carbonization, anaerobic digestion, or incineration), and the used processing conditions (temperature and others). In terms of the conversion of paper waste into high-value materials such as nanocellulose and soil amendments, the methods vary significantly in terms of efficiency and greenness as they can be chemical, biological, or physical, operating at different severities. In the literature, the comparison in terms of greenhouse gas emissions has mostly focused on the incineration, anaerobic digestion, and composting of different waste streams, including municipal solid waste, food waste, sewage sludge, and others [191–193]. The literature, in general, considers composting to generate less CO 2 emission. Anaerobic digestion comes second, followed by incineration. For example, one study estimated that the greenhouse gas emissions caused by composting municipal solid waste could be around 100 kg CO 2 − eq /ton waste, compared to around 125 kg CO 2 − eq /ton waste using anaerobic digestion, and around 1380 kg CO 2 − eq /ton waste using incineration [194]. These three methods, indeed, are still better options than waste landfilling [195]. In terms of cost and operation, composting is also the cheaper option and the easier to establish and manage [192]. However, several studies have highlighted that the use of an integrated waste management system combining the different methods is the right option to recover the maximum material and energy potential of waste [195].
Figure3. Possible routes for the conversion of paper waste into high-value materials and energy. 5. Conclusions The pulp and paper industry (PPI) uses massive amounts of natural resources and pollutes the environment, which has raised major concerns, for instance, regarding de-
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