Abstract Background Traditional sludge treatment methods face significant challenges, including threats of secondary pollution, high carbon emissions, economic constraints, and limited scalability. These drawbacks have prompted research into alternative solutions for sludge resource utilization. In this study, sludge cakes produced via catalytic wet air oxidation (CWAO) were applied in the papermaking process to replace conventional inorganic fillers. Results Waterworks sludge (WS) and civil sludge (CS) were found to have the lowest effect on enhancing the tensile strength of the paper, while kitchen waste sludge (KS), paper sludge (PS), and post‐membrane thickened liquid sludge (MS) showed superior improvement in tensile strength thanks to their organic composition. The integration of cationic etherified starch (CES) with the three aforementioned sludge types enhanced the tensile strength, with the utmost strength reaching 0.358 kN/m at 3 wt.% CES and 30 wt.% PS. According to SEM characterization, the inorganic components in the fillers filled the voids among the fibers, while the organic components acted as connection links between the fibers and fillers. Conclusion This is the first study that provides a comprehensive guide for replacing conventional inorganic fillers with CWAO‐treated sludge, supported by CES additive, to facilitate resource valorization and cost reduction in the papermaking industry. Despite multiple benefits related to lowering the consumption of conventional fillers and reduced sludge handling costs, this approach is only appropriate for end‐use applications that do not involve direct contact with food or medicine, and the addition of CWAO‐treated sludge and CES must be determined according to the required tensile strength. © 2026 Society of Chemical Industry (SCI).