## 1 Introduction  
With the acceleration of global urbanization, sewage sludge has led to significant environmental concerns, with its annual production on the order of tens of millions of tons per year (dry solids). For example, [PERSON] et al. [1] report that municipal WWTPs worldwide produce about 45 million dry tons of sewage sludge annually. This figure underscores the urgency of sustainable valorization routes such as biochar conversion [2]. Traditionaldisposal methods such as landfill and incineration not only take up land resources but also release greenhouse gases (e.g., CO\({}_{2}\), CH\({}_{4}\)) and toxic substances (e.g., dioxins). Although incineration and landfilling remain common in some regions, land application in agriculture continues to be a significant final disposal pathway globally, especially in Europe and parts of Asia, due to its nutrient recycling potential. However, concerns over contaminants and microplastics have led to more stringent regulations in recent years. Biochar materials have been widely reviewed for environmental applications. However, sludge-derived biochar (SDB) possesses distinct mineralogical and catalytic properties that set it apart from conventional biomass biochar. The data illustrate SDB's consistently high performance in contaminant adsorption and removal processes. Table 1 shows a comparative overview of SDB versus other common biochar types (e.g., wood and straw biomass), indicating the unique benefits over wood- or crop-based biochar. The advantages are largely attributed to the mineral-rich, catalytically active surface of SDB and its dual functionality as both an adsorbent and a reactive medium. Such features distinguish SDB as a value-added material for sustainable wastewater treatment. The conversion of sludge into high value-added materials (i.e., SDB) via pyrolysis, gasification, or hydrothermal carbonization has attracted significant attention, as these thermochemical processes not only mitigate sludge disposal challenges but also yield carbonaceous adsorbents or catalysts enriched with minerals (e.g., Fe, Ca, P) and functional groups [3]. Pyrolysis remains the most commonly reported route in the 2020-2025 literature, hence our primary focus. The performance of SDB is closely linked to its pyrolysis conditions. Studies have indicated that pyrolysis temperatures between 500 and 700 \({}^{\circ}\)C generally produce biochar with greater surface area and aromaticity, making them more effective for catalysis and adsorption [4], while lower temperatures (300-500 \({}^{\circ}\)C) preserve more oxygen-containing functional groups [5], enhancing nutrient binding capacity. Mineral crystallization, porosity formation, and redox behavior are strongly shaped by the type of inert atmosphere (e.g., N\({}_{2}\) or CO\({}_{2}\)) and the applied heating rate during pyrolysis [6]. SDB is a promising material for wastewater treatment owing to its unique physicochemical properties, including a high specific surface area (300-1000 m\({}^{2}\)/g), abundant surface functional groups (e.g., -COOH, -OH), and a tunable pore structure. These attributes confer distinct advantages in adsorption, catalytic oxidation, and electrochemical processes. Despite the proliferation of general biochar reviews, a focused synthesis on SDB-specific advances, particularly covering the recent surge in multifunctional wastewater treatment applications (2020-2025), remains lacking. Thus, this mini-review provides a critical analysis of recent trends, application challenges, and future research directions for SDB. In summary, this mini-review provides a comprehensive review of the progress of SDB in wastewater treatment applications between 2020 and 2025, focusing on the mechanisms of pollutant removal, modification strategies for performance enhancement, and emerging trends that may influence future research directions. This review summarizes key findings by application category, highlighting performance, modifications, and emerging trends.