Sustainable solutions for pulp mill sludge management
Authors:   , Rodrigues Bruna Virgínia Cunha 
Published: January 2025

The pulp and paper industry has a crucial role in the global market, impacting the economy, environment, and society. Chemical and mechanical processes are used for pulp production. The kraft method is the most common chemical process in the world. The mechanical process is commonly used to produce high-yield pulp. The high water consumption in pulp and paper mills results in a high generation of effluent. Due to the high organic and inorganic matter, the effluent is treated in the effluent treatment plant (ETP) before discharge in the river. During the effluent treatment, the sludge is generated. Usually, two types of sludge are generated in the pulp and paper mill: primary sludge (PS), which comes from primary effluent treatment, and secondary sludge (SS), generated during secondary or biological treatment. Because of its characteristics, secondary sludge is difficult to handle and is often mixed with primary sludge to make it easier to manage. The sludge generated in the pulp and paper industry accounts for the largest proportion of solid waste generated at the mill and its main destination is the industrial landfill. Due to major environmental, social, and economic problems, landfills have been restricted. The transition of the world market towards bioeconomy has required sustainable management of all co-products generated in the production chain. The sludge generated in the pulp mill is rich in lignocellulosic materials presenting the potential for energy generation. There is a growing demand for the reduction of fossil fuels and the implementation of new renewable fuel sources. Energy generation using sludge from the pulp industry has emerged as an alternative for managing this biomass, also providing support for the Sustainable Development Goals (SDGs). This study aims to assess the technical and environmental potential of generating energy from PS and SS generated in pulp mills. The work is divided into four chapters. Chapter 1 provides a comprehensive review of current practices for sludge disposal, along with the latest innovations in sludge management found in the literature. The growing restrictions on landfills underscore the urgent need to explore and implement more sustainable methods for sludge treatment. Chapter 2 presents the characterization of PS, SS, and the mixture of PS and SS (MS) generated in a thermo-mechanical pulp mill, assessing their energy potential and the possible effects of their combustion in the boiler. The high moisture content in the sludges created a negative energy balance, requiring a pre-drying step prior to combustion. Chapter 3 assessed the technical feasibility of producing pellets using PS and SS. The effect of moisture content and the blends of PS and SS on the final quality of the pellets was also determined. The pelletizing process standardized the dimensions of sludge, increasing the energy efficiency in the boiler and reducing costs in transportation and storage. Chapter 4 presents the environmental viability through life cycle assessment (LCA) of the energy generation of sludge pellets. This study highlights and compares the environmental effects of burning and producing pellets with sludge disposal in landfills. The results of this chapter showed that energy generation using pellets sludge had a better environmental performance than landfilling. Optimizing energy generation at the mill with a new renewable fuel source contributes to more sustainable production in the pulp and paper industry. Keywords: bioeconomy; pulp and paper industry; renewable fuel; sludge; waste management.