## 8 Conclusions and Future Perspectives  
This mini-review highlights that SDB in the last five years have demonstrated high performance in adsorption, AOP catalysis, biological processes, and membrane enhancement across various wastewater treatment systems. Despite these advances, real wastewater testing and long-term application studies remain limited. Several innovative trends are worth noting, as follows:  
**(1) Integrated and hybrid treatment systems:** The synergistic integration of SDB within hybrid treatment architectures rather than standalone applications is the priority of the next research study. The MFC-CW system discussed earlier is a prime example of the convergence of technologies enabled by SDB. More attention should be paid to hybrid systems, such as SDB-packed photoreceptors (combining adsorption with photocatalytic degradation) or SDB-coupled electrochemical reactors (combining adsorption with electro-oxidation). Additionally, biochar characteristics can be fully utilized to try the dynamic sorption-oxidation processes, where contaminants are first captured by SDB and then destroyed in situ by an oxidant or electron transfer in one reactor. This could significantly increase removal efficiency for trace contaminants. **(2) PFRs in biochar:** Recent advances highlight the exploitation of intrinsic PFRs embedded within SDB for advanced contaminant degradation. Pyrolysis synthesis endows these materials with stabilized carbon-centered and semiquinone-type radicals, which demonstrate dual functionality: the catalytic activation of oxidants such as PAA and initiation of non-photochemical oxidation pathways through radical-mediated electron transfer mechanisms. Future research could aim to optimize pyrolysis conditions to enhance the yield and stability of PFRs in SDB. If biochar with consistently high PFR content can be reliably produced, it may serve as a metal-free catalyst operating through a distinct non-radical mechanism, such as singlet oxygen generation, which exhibits reduced sensitivity to water matrix constituents. **(3) Targeting emerging contaminants:** Beyond conventional pollutants, SDB is increasingly applied to address emerging micropollutants, including pharmaceuticals, personal care products, PFAS, and microplastics. Similar to microplastic removal, sludge biochar functions as a filtration medium and exhibits catalytic potential for fragment degradation when combined with UV or oxidants. Although current data remain limited, SDB's multifunctionality positions it as a promising candidate for advanced treatment systems targeting recalcitrant contaminants resistant to conventional biological processes. The real wastewater should be considered to capture the complexity of wastewater matrices (e.g., co-existing ions and organic matter). Thus, validation under real-world conditions remains a critical future research priority. **(4) Resource recovery and circular economy approaches:** Current research increasingly emphasizes not just removing contaminants but also recovering resources, and SDB plays a crucial role. Phosphorus-saturated SDB demonstrates potential for agricultural reuse as slow-release fertilizers, closing nutrient loops while reducing synthetic fertilizer dependence. Some studies have started to evaluate the agronomic value of spent sludge biochar loaded with nutrients. Similarly, biochar that adsorbs metals could be processed to reclaim metals or used in construction materials, though caution is needed regarding leaching. Another resource aspect is energy: using SDB integration into MFCs or anaerobic digesters enhances biogas yields and electricity generation. Emerging lifecycle assessments highlight the environmental benefits of these approaches. This paradigm shift redefines SDBs from disposable adsorbents to circular economy enablers, transitioning from single-use functions to multi-phase resource cascades. **(5) Scale-up and practical implementation:** Over the past five years, research in this area has been primarily at the laboratory or pilot scale.