A critical review on biofuels generation from pulp-paper mill sludge with emphasis on pretreatment methods: renewable energy for environmental sustainability

The pulp and paper mills generates substantial amounts of sludge, posing significant environmental challenge. Addressing this issue, this review explores the dual objective of renewable energy generation and mitigating the environmental impact of pulp-paper mill sludge (PPMS), filling a critical gap in the existing literature. PPMS is recognized as a promising source of fermentable sugars, mainly glucose, and holds potential as a feedstock for biorefinery applications to produce various renewable biofuels, such as biomethane, biohydrogen, bioethanol, biobutanol, and biodiesel, using biorefining concepts. These efforts align with the the United Nations Sustainable Development Goals (UN SDGs) by resource recovery and reducing environmental impact. The present article provides insights on renewable energy generation from PPMS. Despite its promise for bioenergy production, numerous bottlenecks have been identified, including high ash content and the presence of toxic inhibitors, such as phenolics, lignin, chlorolignin, and ligno-carbohydrate complexes which impact the hydrolysis of cellulosic fibers and limit the amount of energy recovery. Various pretreatment methods, such as mechanical, thermal, thermochemical, chemo-mechanical, enzymatic, and microbial are emphasized for their ability to enhance sludge solubility by modifying its structure, thereby releasing fermentable sugars. Pretreatment can decrease crystallinity of cellulose, increase accessible surface area, reduce lignin content, and improve bioenergy recovery in the form of oil and gases from PPMS. Finally, the article suggested future research directions in feedstock pretreatment, catalyst development, and optimization of the overall production system. These areas could address existing gaps in the literature and make the process more feasible and practical for real-world applications. In conclusion, bioenergy recovery from PPMS not only helps reduce the demand for fossil fuels in the near future but also provides a sustainable solution for managing paper sludge, in alignment with the principles of a circular economy. • PPMS is a low-cost substrate of fermentable sugars to produce, biomethanol, bioethanol, butanol. • Thermal pre-treatment positively impacted biomethane production from PPMS. • PPMS contains inhibitors like phenolics, sulfite, wood extracts, organic acids, phenols, furfural, and lignosulfonates, which significantly affect process efficiency and recovery yield. • Acid pretreatment are promising method to remove ash (CaCO3) from PPMS. • Ligno-carbohydrate complexes in pulp-paper sludge hindering microbial valorization.