Barrios et al. Biotechnology for Biofuels and Bioproducts
(2025) 18:48
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point due to capillary effects, while NFBW is strongly bound to cellulose at the molecular level and does not undergo phase transitions [19]. Bound water is held tightly within the fiber matrix due to hydrogen bonding with cellulose, making it difficult to remove, especially during the falling rate drying period [20]. As fibers undergo refining, their ability to retain bound water increases due to more significant fibrillation and the exposure of more hydroxyl groups on the fiber surface [21]. This bound water significantly contributes to the energy required for thermal drying. The economic and environmental implications of dewatering processes in the P&PI are significant; however, comprehensive analyses addressing both decarbonization potential and economic implications of dewatering technologies are limited [22–24]. The P&PI stands as a significant contributor to greenhouse gas emissions, primarily from natural gas boilers and lime kiln operations during chemical recovery [25]. Among the industry’s processes, the dryer section of the paper machine is one of the most energy-intensive units, consuming 20–30% of total steam demand [26]. Innovative technologies, such as impulse dryers, shoe presses, and enzymatic treatments, have shown promise in enhancing dewatering efficiency and decreasing energy use [27]. For example, impulse dryers have demonstrated up to 10% decarbonization potential in the Austrian pulp and paper industry [23]. Enzymatic processes, in particular, offer additional environmental and economic advantages by reducing water retention in fibers and minimizing steam consumption [13, 16]. By integrating enzymatic treatments into production lines, mills can lower fossil fuel reliance, cut direct CO ᔐ emissions, and achieve cost savings, addressing both decarbonization potential and the economic trade-offs essential for sustainable pulp and paper manufacturing. Building on the findings reported from our previous work [16], this study aims to investigate the application of the cell-free enzyme pretreatment, comprising mild mechanical refining, the use of a commercial enzyme formulation, and treatment with a cationic biopolymer, on bleached hardwood pulp to evaluate its efficacy in enhancing press dewatering during papermaking. Hardwood pulps, which differ from softwood pulps in their higher hemicellulose content and smaller fiber dimensions, present unique challenges and opportunities in water removal and paper strength enhancement. The differences in hemicellulose content, particularly the presence of hexenuronic acid (HexA) groups, and the typically lower lignin content in hardwoods may impact the efficiency and outcomes of enzymatic treatments [28, 29]. In addition to measuring the moisture content after pressing and paper properties, this study includes
adsorbable organic halides (AOX) in effluents, reducing environmental impact. Cell-free enzymatic systems have also been used in fiber modification and refining processes. Applying cellulases, hemicellulases, and pectinases can selectively hydrolyze components of the fiber cell wall, thereby enhancing fiber swelling, flexibility, and fibrillation. This results in better fiber bonding and improved paper strength properties, such as tensile and tear strength [5, 6]. Moreover, the use of these enzymes can reduce energy consumption during the refining process, as enzymatically treated fibers require less mechanical energy to achieve the desired level of fibrillation [7–9]. The selectivity of these enzymes also minimizes fiber degradation, preserving the length and integrity of the fibers, which is crucial for maintaining the strength of the final paper products [10]. Enzymatic treatments are increasingly integrated into processes designed to convert lignocellulosic biomass into valuable bioproducts, such as bioethanol, bioplastics, and other chemicals, alongside traditional pulp and paper production [11, 12]. In addition to quality improvements, enzymatic treatments offer significant potential for energy savings, particularly in water removal during papermaking [13]. Water removal is a critical step, as thermal drying is one of the most energy-intensive processes in paper production [14, 15]. Previous research has shown the potential of enzymatic treatments, particularly those involving cellulases and xylanases, to enhance dewatering and improve the strength properties of paper made from southern bleached softwood kraft pulp, leading to significant energy reductions during papermaking [16]. Water in cellulosic fibers exists in different forms, each with distinct impacts on the dewatering and drying stages in paper manufacturing. Understanding these forms of water—free, bound, and hard-to-remove water (HRW)—is essential for optimizing energy consumption and enhancing the efficiency of the papermaking process. Mechanical dewatering and thermal drying are key stages where water removal significantly affects paper mills’energy demands and overall productivity. Changes in Water Retention Value (WRV) reflect how easily water can be extracted during mechanical dewatering, particularly in the forming and press sections of the paper machine. Enzymatic treatments, which modify the fiber structure and surface characteristics, have been shown to reduce WRV, enhancing dewatering efficiency [17, 18]. Mechanical dewatering is typically followed by thermal drying, where bound water, including freezing-bound water (FBW) and non-freezing-bound water (NFBW), becomes the focus. FBW, which exists in the outer layers of the fiber hydration structure, has a depressed melting
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