A Mini-Review of Sludge-Derived Biochar (SDB) for Wastewater Treatment: Recent Advances in 2020–2025

Abstract

Sludge-derived biochar (SDB) synthesized by the pyrolysis of sludge is gaining enormous interest as a sustainable solution to wastewater treatment and sludge disposal. 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), receives little emphasis. In particular, a critical analysis of recent trends, application challenges, and future research directions for SDB is still limited. Unlike broader biochar reviews, this mini-review highlights the comparative advantages and limitations of SDB, identifies emerging integration strategies (e.g., bio-electrochemical systems, catalytic membranes), and outlines future research priorities toward enhancing the durability and environmental safety of SDB applications. Specifically, this review summarized the advances from 2020 to 2025, focusing exclusively on functional modifications, and practical applications of SDB across diverse wastewater treatment technologies involved in adsorption, catalytic oxidation, membrane integration, electrochemical processes and bio-treatment systems. Quantitative comparisons of adsorption capacities (e.g., >99% Cd2+ removal, >150 mg/g tetracycline adsorption) and catalytic degradation efficiencies are provided to illustrate recent improvements. The potential of SDB in evaluating traditional and emerging contaminant degradation among the Fenton-like, persulfate, and peracetic acid activation systems was emphasized. Integration with membrane technologies reduces fouling, while electrochemical applications, including microbial fuel cells, yield higher power densities. To improve the functionality of SDB-based systems in targeting contamination removal, modification strategies, i.e., thermal activation, heteroatom doping (N, S, P), and metal loading, played crucial roles. Emerging trends highlight hybrid systems and persistent free radicals for non-radical pathways. Despite progress, critical challenges persist in scalability, long-term stability, lifecycle assessments, and scale-up implementation. The targeted synthesis of this review offers valuable insights to guide the development and practical deployment of SDB in sustainable wastewater management.

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, Ferrentino 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]. Traditional disposal methods such as landfill and incineration not only take up land resources but also release greenhouse gases (e.g., CO₂, CH₄) 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 °C generally produce biochar with greater surface area and aromaticity, making them more effective for catalysis and adsorption [4], while lower temperatures (300–500 °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., N₂ or CO₂) 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 m²/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. Specifically, it emphasizes the adsorptive and catalytic properties of SDB, its integration with membrane technologies and electrochemical systems, and the various modification approaches explored to address specific wastewater treatment challenges. It aims to provide a comprehensive overview of existing knowledge and highlight potential directions for future research by synthesizing the latest research results. To obtain the most up-to-date knowledge, studies published between 2020 and 2025 pertaining to SDB in wastewater treatment were collected. Some classic publications that first studied or defined these specific technologies were certainly introduced regardless of the publication date. The relevant publications for each section were roughly collected from “Web of Science”, “Scopus”, and “ScienceDirect” databases based on the corresponding keywords. Search terms included combinations such as the following: (“sewage sludge biochar” OR “sludge-derived biochar” OR “sludge biochar”) AND (“wastewater treatment” OR “adsorption” OR “electrochemical” OR “catalysis” OR “nitrogen” OR “phosphate” OR “membrane”) using Boolean operators. Additionally, the reference lists of influential papers were tracked to identify any further relevant studies. By applying this search strategy and screening against the inclusion criteria, over 100 relevant studies were manually selected to offer useful information for SDB in wastewater treatment for this mini-review’s analysis.

Table 1. Comparative performance of sludge-derived biochar (SDB) with biochar derived from other substrates in some typical contaminants in wastewater applications.

Pollutant/Application	Sludge-Derived Biochar (SDB)	Other Biochar	References
Heavy metals (e.g., Pb2+)	~83% removal of Pb2+ at μg/L levels, even at pH 2, due to metal oxide sites	~0% removal in same low-concentration test (no capacity at pH 2) with wood biochar; heavy metal sorption capacity ranks SDB > agricultural waste biochar > wood biochar	[7,8]
Dyes (e.g., anionic dye)	~256 mg/g using KOH-activated SDB from dyeing sludge (high mineral content)	Typical inactivated sludge–rice husk biochar achieves 22–60 mg/g for various dyes	[9]
Antibiotics	SDB modified by NaOH shows tetracycline antibiotic uptake with ~379.8 mg/g	~7–12 mg/g (raw at 400–600 °C) using corn straw biochar, improved to ~31 mg/g after Ca(OH)2 modification	[4,10]
Nutrients	MgO-loaded SDB achieved maximum phosphate capture with ~97.45 mg/g	<1 mg/g P uptake using unmodified pine/wood biochar: (e.g., <0.7 mg/g at 50 mg/L initial P); activation with CO2 at 800 °C raises wood-based biochar capacity only to ~1.2 mg/g in the P removal	[11,12]

Table 1. Comparative performance of sludge-derived biochar (SDB) with biochar derived from other substrates in some typical contaminants in wastewater applications.

Pollutant/Application	Sludge-Derived Biochar (SDB)	Other Biochar	References
Heavy metals (e.g., Pb2+)	~83% removal of Pb2+ at μg/L levels, even at pH 2, due to metal oxide sites	~0% removal in same low-concentration test (no capacity at pH 2) with wood biochar; heavy metal sorption capacity ranks SDB > agricultural waste biochar > wood biochar	[7,8]
Dyes (e.g., anionic dye)	~256 mg/g using KOH-activated SDB from dyeing sludge (high mineral content)	Typical inactivated sludge–rice husk biochar achieves 22–60 mg/g for various dyes	[9]
Antibiotics	SDB modified by NaOH shows tetracycline antibiotic uptake with ~379.8 mg/g	~7–12 mg/g (raw at 400–600 °C) using corn straw biochar, improved to ~31 mg/g after Ca(OH)2 modification	[4,10]
Nutrients	MgO-loaded SDB achieved maximum phosphate capture with ~97.45 mg/g	<1 mg/g P uptake using unmodified pine/wood biochar: (e.g., <0.7 mg/g at 50 mg/L initial P); activation with CO2 at 800 °C raises wood-based biochar capacity only to ~1.2 mg/g in the P removal	[11,12]

2. Adsorptive Removal of Contaminants

2.1. Heavy Metals

Heavy metal (HM), e.g., cadmium, mercury, and chromium, poisoning in wastewater is a major contributor to global health and environmental problems [13]. Adsorption is considered a highly effective and versatile approach for removing both inorganic and organic contaminants from various water sources, including natural water bodies, drinking water, sewage, and industrial effluents. The advantages involve cost efficiency, ease of use, global availability of adsorbents, design flexibility, and environmental sustainability [14]. For heavy metal removal, SDB provides a cost-effective and environmentally friendly solution. Its porous structure has a large surface area for adsorption, and its surface functional groups, including carboxyl, phenolic, and hydroxyl groups, can form chelates with metal ions. The adsorption mechanisms include physical adsorption (metal trapped within the pores) and chemical adsorption (metal ions form complexes with the functional groups of the biochar) (Figure 1). Recent studies have shown that SDB is highly efficient in removing heavy metals, with removal rates typically exceeding 90% under optimal conditions [15,16]. SDB inherently contains metal oxide minerals and polar groups that confer strong metal sorption. The incorporation of inorganic modifiers, e.g., metal oxides, into SDB can substantially boost its HMs adsorption performance, primarily through improvements in the abundance of reactive functional groups, and introduce additional active binding sites. [17]. For instance, CaO-modified SDB achieved 99.74% Cd²⁺ removal, approximately three times higher than raw SDB under identical conditions. However, while surface activation increases adsorption capacity, it may also reduce material stability over repeated use cycles. Even without modification, SDB can effectively sequester metals such as Cu, Pb, Cr, and Cd through surface complexation and precipitation. Notably, iron-enriched SDB demonstrates exceptional Cr(VI) removal by facilitating its reduction to Cr(III) followed by adsorption [18]. Another effective strategy involves phosphate-based modification, where SDB treated with ammonium polyphosphate exhibits improved Cd adsorption due to the introduction of phosphate functional groups [19]. These studies underscore that chemical activation or doping can dramatically improve metal binding and thereby enhance heavy metal removal. However, the potential leaching of intrinsic metals (e.g., Fe, Zn, and Cu) or previously adsorbed contaminants under acidic or dynamic conditions necessitates a careful assessment of secondary pollution risks associated with SDB reuse.

2.2. Organic Pollutants

SDB has been extensively explored for removing organic contaminants, including dyes, pharmaceuticals, and industrial chemicals. Its porous carbon framework and oxygenated functional groups promote hydrogen bonding and π–π interactions with these contaminants [9,20]. Dye adsorption is one notable area: a sludge biochar removed various textile dyes with capacities of 20–60 mg/g (e.g., up to ~59 mg/g for Direct Red dye) [21]. SDB used for dye removal retained approximately 85% of its initial capacity after three cycles, underscoring the need for regeneration optimization [22]. Surface activation can greatly enhance these capabilities. For example, novel oxidative activation increased the adsorption of tetracycline antibiotics from 34 mg/g (original biochar) to 154 mg/g [9]. Researchers have also tailored sludge biochar with specific functional groups to broaden pollutant affinity. For instance, introducing nitrogen through ammonia or amino-rich additives can generate amine and cyano functional groups, enhancing the adsorption of both cationic and anionic organic contaminants [23]. Tannery SDB has been engineered as a versatile adsorbent capable of simultaneously removing dyes, phenols, and antibiotics [24]. Some studies demonstrate the effective removal of many pharmaceutical compounds through π-electron interactions and the partial adsorption of short-chain PFASs, while longer-chain variants necessitate SDB surface modifications [25,26]. These advances indicate that SDB can be engineered for the high-capacity removal of diverse organic contaminants from wastewater. Overall, electrostatic attraction, π-πinteractions, and hydrogen bonding are the three main adsorption mechanisms that have been widely investigated, especially for the removal of organic contaminants.

2.3. Nutrients (Phosphorus and Nitrogen)

SDB has gained attention for phosphorus and nitrogen removal from wastewater effluents, particularly as a pre-discharge treatment strategy to mitigate nutrient enrichment in receiving water bodies. Its inherent metal constituents (e.g., Fe, Al, and Ca) provide reactive sites that enable phosphate sequestration through ligand exchange and precipitation processes. Higher mesopore volume and exposed mineral phases could effectively bind phosphate. For instance, SDB pyrolyzed at 600 °C achieved ~87% phosphate removal efficiency from municipal wastewater, while lower retention for SDB occurred at 400–500 °C [27], primarily due to reduced mesopore volume and limited mineral phase exposure. This underscores the pivotal influence of pyrolysis temperature in optimizing adsorption properties. Enhancing SDB with metal-based additives can significantly improve its phosphate adsorption capacity. For example, SDB modified with MgO exhibited high phosphate uptake over a broad pH range, primarily due to the Mg-P mineral complex formation [11]. Impregnating sludge biochar with lanthanum represents another effective approach, as La-modified biochar efficiently removes trace phosphate from water through the formation of stable La-PO₄ complexes [28]. Beyond phosphorus, SDB can also aid nitrogen removal as an adsorbent via cation exchange and surface complexation, with reported capacities ranging from a few mg/g up to tens of mg/g depending on production conditions [29]. One key factor is pyrolysis temperature. SDB produced at ~500 °C was shown to adsorb more NH₄⁺ than that produced at 700 °C, due to better retention of hydrogen-containing functional groups that serve as cation-exchange sites [30]. Various post-processing modifications (e.g., acid/alkali activation or metal impregnation) are another strategy to greatly enhance NH₄⁺ uptake. For instance, introducing carboxylate and sodium functional groups raised the capacity from 5 mg/g (unmodified) to 17 mg/g [31]. Wu et al. [32] synthesized an Fe-enriched magnetic SDB (via FeCl₃ infusion and re-pyrolysis at 300–600 °C) that exhibited the highest NH₄⁺ sorption capacity at 400 °C, with 10.7% greater than the unmodified SDB. The NH₄⁺ removal mechanism for such Fe-modified SDB was dominated by ion exchange and electrostatic attraction. In some cases, in situ doping during sludge processing yields excellent results. Zhang et al. [33] intensified sludge anaerobic digestion with scrap iron additions, producing an iron-rich biochar at 600 °C that could remove over 91% of ammonium from the fermentation liquor. Adsorption is also maximized at near-neutral pH, since, under highly alkaline conditions, NH₄⁺ is converted to volatile NH₃ (reducing its availability for adsorption) [34]. Furthermore, pre-oxidizing the sludge feedstock can dramatically improve performance. Fenton pre-oxidation of sludge prior to pyrolysis at 400 °C produced a biochar with exceptionally high NH₄⁺ sorption (approximately 222 mg/g) due to increased porosity and surface charge sites [35]. Overall, recent studies confirmed that, while baseline SDB has moderate affinity for NH₄⁺, targeted modifications (chemical activation, metal doping, or mineral amendments) can greatly improve its ammonium removal efficiency and capacity. Such improvements are crucial for developing SDB-based technologies to recover nitrogen from waste streams in a circular economy context.

3. Catalytic Advanced Oxidation Processes (AOPs)

SDB has gained attention as a catalyst in advanced oxidation processes to destroy refractory organic contaminants [9,36]. Distinct from conventional carbon materials, SDB incorporates embedded metal oxides (e.g., Fe, Mn), enabling the generation of reactive oxygen species (ROS) through Fenton-like reactions and persulfate activation pathways.

3.1. Fenton-like (H₂O₂) Activation

Numerous SDB possess iron, which facilitates the catalytic decomposition of hydrogen peroxide into hydroxyl radicals (·OH). H₂O₂ activation by SDB highly relies on the iron speciation and carbon phase. For instance, SDB pyrolyzed at 400/600 °C tend to release Fe²⁺ for homogeneous H₂O₂ activation, whereas SDB pyrolyzed at 800/1000 °C first adsorb 4-chlorophenols rapidly and oxidize them [37]. Wu et al. [38] synthesized an iron-enriched SDB and applied it for triclosan degradation via Fenton-like reactions. The study revealed H₂O₂ activation was primarily driven by iron species (80.3%), with a smaller yet notable contribution from persistent free radicals (PFRs, 19.7%), ultimately improving the efficiency of pollutant degradation. Pristine SDB has also demonstrated the ability to activate H₂O₂ under near-neutral pH conditions. For instance, a study reported approximately 91.5% removal of ofloxacin, using raw municipal sludge biochar in conjunction with H₂O₂, demonstrating inherent iron species participation [39,40]. To optimize performance, researchers have also enhanced H₂O₂ activation by intentionally doping SDB with metal species (e.g., Fe, Mn, and Cu). For instance, in situ iron impregnation (e.g., using FeCl₃ pretreatment) produces an SDB that releases Fe²⁺/Fe³⁺ under reaction conditions, accelerating ·OH generation. One recent study used Cu-loaded sludge biochar to effectively oxidize bisphenol-A in high-salinity water, addressing the challenge of organic removal in saline industrial waste [41].

3.2. Sulfate-Radical AOPs

SDB has been extensively investigated as a heterogeneous catalyst for peroxymonosulfate (PMS) and peroxydisulfate (PDS) activation. Its carbon matrix facilitates single-electron transfer, resulting in the generation of sulfate radicals (SO₄·⁻) or even non-radical oxidants like singlet oxygen [42]. SDB catalysts have been shown to be very effective in sulfate-radical AOPs, typically removing the contaminants completely within minutes to hours. The PMS/SDB system achieved a 94.63% removal of sulfamethoxazole (SMX) within 30 min, exhibiting remarkable stability, interference resistance, and adaptability across different pH conditions [43]. A noteworthy advance was the preparation of metal-bearing SDB, i.e., by pyrolyzing Fe-rich SDB under reductive conditions. For instance, Wang et al. [44] obtained ZVI-SDB by one-step preparation with no external iron source that rapidly activated persulfate, achieving the highly efficient degradation of acid orange. Similarly, Wang et al. [45] reported that SDB produced from iron-rich sludge exhibits excellent performance in removing tetracycline (TC) from both synthetic and real pharmaceutical wastewater via combined adsorption and oxidant activation processes. The activation of PMS and H₂O₂ was primarily attributed to the presence of Fe(II), which facilitated the generation of ·OH and SO₄·^–. Such composite SDBs exploit the synergistic effects of added metal ions. Another study reported that SDB effectively activates PDS for efficient SMX degradation, highlighting that SDB catalysts can favor non-radical routes [46]. This non-radical dominant mechanism reveals the propensity of SDB to drive contaminant breakdown via electron transfer pathways, thereby mitigating performance limitations caused by radical scavengers in complex wastewater matrices. Thus, selecting appropriate modification strategies is critical to optimize the catalytic route for different pollutant classes.

3.3. Peracetic Acid (PAA) Activation

Peracetic acid (PAA) has emerged as an alternative to H₂O₂ and persulfate for the treatment of persistent contaminants in wastewater [47]. In a recent study, Zhou et al. [48] used SDB to activate PAA for the degradation of fluoroquinolone antibiotics (FQs) at neutral pH, where non-radical pathways were found to play a predominant role. π-conjugated structures and carbon-rich functional moieties on SDB’s surface facilitated electron transfer processes, thereby promoting singlet oxygen (¹O₂) generation. Simultaneously, the iron species present in the biochar can catalyze PAA activation, contributing to the formation of organic radicals, e.g., CH₃COO· and CH₃COOO·. Except for the traditional ROS, recent observations verified that the high efficacy in PAA-based systems was attributed to PFRs on the SDB surface that facilitate PAA decomposition. For example, Miao et al. [49] demonstrated that SDB efficiently activated PAA for the degradation of 4-chlorophenol, achieving a rate constant of 0.051/(M·min). The primary mechanism identified in the study indicates that the activation of PAA by SDB is largely governed by quinone/hydroquinone groups and residual metals on the SDB. These components play a crucial role in the formation of PFRs, which facilitate the activation process. Given the relatively recent exploration of PAA-based AOPs, the promising performance of SDB catalysts highlights their potential for advancing alternative disinfection and oxidation strategies in wastewater treatment.

3.4. Photocatalysis and Ozonation

In addition to peroxide and persulfate systems, SDB catalysts are used in photocatalytic and ozone treatment processes. Photocatalysts like TiO₂ and ZnO are limited in solar energy utilization due to their wide bandgap energies (3.2–3.6 eV), restricting their activity primarily to the ultraviolet range, which accounts for only 3–5% of the solar spectrum. Coupling these semiconductors with SDB has been shown to reduce the bandgap and improve responsiveness under visible light, thereby enhancing photocatalytic performance [50]. Under light irradiation, the carbon substrate can absorb light or act as a support for semiconductor particles. Simultaneously, the biochar can adsorb contaminants, increasing their local concentration and enhancing the photocatalytic efficiency. For instance, Zerga et al. [50] investigated a novel SDB/TiO₂ composite, highlighting the role of metallic elements present in sewage sludge in enhancing photocatalytic hydrogen generation from water. The composite achieved a hydrogen generation rate of 2523 µmol/(g·h), which was a 5.5-fold improvement compared to pristine TiO₂. During the co-synthesis process, structural defects formed in the SDB were conductive to the development of multilayered architectures, which in turn improved charge carrier mobility and facilitated the photo-reforming of glycerol. In micropollutant removal, SDB/TiO₂ composites have shown improved photocatalytic degradation of antibiotics, leveraging SDB’s adsorptive enrichment and electron shuttling to reduce charge recombination [51]. In ozonation, recent studies have demonstrated that SDB can function as an effective catalyst, promoting the decomposition of ozone into ·OH and thereby enhancing oxidative degradation efficiency [52,53].

Overall, SDB demonstrates multifunctional catalytic capacities, enabled by its diverse surface functionalities, intrinsic metallic species, and hierarchical porosity. These structural attributes facilitate efficient activation across various AOPs, including iron-mediated redox systems, persulfate activation, PAA catalysis, and light-driven or ozone-coupled degradation mechanisms. These advancements establish SDB as a low-cost and sustainable catalyst for advanced oxidation in wastewater treatment.

4. Electrochemical and Bio-Electrochemical Applications

Sludge biochar’s conductivity and surface chemistry have opened new applications in electrochemical water treatment and bio-electrochemical systems.

4.1. Electrode Material for Microbial Fuel Cells (MFCs)

Microbial fuel cells (MFCs) have emerged as a promising approach that integrates wastewater treatment with concurrent energy recovery [54,55]. One of the developments is using SDB to improve MFC anodes for wastewater treatment and energy recovery simultaneously. SDB has shown potential as an electrode material in MFCs due to its electrical conductivity, large surface area, and biocompatibility [56]. Zhu et al. [57] reported that SDB anodes made by pyrolyzing activated sludge achieved a power density of approximately 2.17 W/m² and a current density of 5.99 A/m² under an optimized temperature, which was about three times higher than a plain carbon cloth anode. The hierarchical porosity and enriched graphitic structure of SDB could facilitate electron transfer and support a thick electroactive biofilm. Enriching anodic microbial community electroactive bacteria and boosting key SDBs for extracellular electron transfer are one of major reasons for MFCs conducting with SDB anodes. In practical terms, using SDB as electrode material to create MFC electrodes can cut costs (replacing expensive carbon felt or graphite) while enhancing the treatment of organics. A recent study focused on utilizing SDB as electrode materials in microbial fuel cell-constructed wetland (MFC-CW) configurations, revealing its capacity to boost both denitrification processes and bioelectricity output. SDB could improve cathode electrochemical performance and anode biocompatibility, achieving 95.85% nitrogen removal and 9.05 mW/m² power density in SDB-based MFC-CWs [58]. Gupta et al. [59] constructed wetland-MFC systems that used SDB as both the filler medium and electrode, achieving >95% nitrogen removal along with power generation. These results highlight SDB as a highly effective anode material for bio-electrochemical treatment systems.

4.2. Capacitive Deionization (CDI)

Capacitive deionization (CDI), an emerging electro-sorption technology for desalination and ion removal, is also being explored for utilizing SDB as an electrode. Owing to its porous carbon matrix and intrinsic salts, SDB can function similarly to activated carbon electrodes. The extensive surface area of SDB creates numerous sites for ion adsorption, while its electrical conductivity ensures efficient charge transfer during the CDI process. Zhang et al. [60] used aerobic granular sludge as the precursor for preparing hierarchically porous biochar. The specific capacitance is enhanced by 12% compared to commercial activated carbon. Zhang et al. [61] developed an iron-enriched SDB from dyeing industry sludge, which exhibited a remarkable salt adsorption capacity of 83.0 mg/g under batch mode CDI conditions with 1000 mg/L of NaCl. The presence of iron species enhanced the electrode’s capacitive behavior and salt adsorption capacity, likely due to faradaic (pseudocapacitive) reactions. Furthermore, the electrode demonstrated excellent cycling stability, maintaining both salt adsorption capacity and charge efficiency over 200 cycles during the desalination/regeneration CDI process. Recent studies have shown that SDB-based CDI electrodes can achieve high salt removal capacities and regeneration efficiencies, offering a promising approach for sustainable water treatment. Ongoing research is optimizing electrical conductivity (by increasing the carbonization temperature or adding conductive additives) and the wettability of sludge char electrodes to further boost CDI salt uptake.

4.3. Electro-Fenton and Electrochemical Catalysis

The electro-Fenton process combines the electrochemical generation of H₂O₂ with a Fenton-like reaction to produce ·OH radicals to degrade contaminants [22]. SDB can be used as an electrocatalyst in these systems, contributing to the electrochemical generation of H₂O₂ and its subsequent activation [62,63]. For instance, a cathode coated with SDB has been utilized to facilitate in situ H₂O₂ generation from oxygen, which then initiates Fenton reactions for organic pollutant degradation, integrating both electrochemical and catalytic treatment approaches [64]. The quinone groups in the SDB promote electron transfer to oxygen, producing H₂O₂, while its iron content promptly converts the peroxide into ·OH radicals. This integrated approach enables pollutant mineralization without requiring external H₂O₂ addition. Given that the electro-Fenton membrane enabled excellent self-cleaning ability and stability, Yin et al. [65] designed Fe/N co-doping SDB ultrafiltration membranes, which achieved 100% tetracycline removal within 0.749 min under electro-assisted degradation, while also mitigating membrane fouling (shown in Figure 2). The abundant oxygen functional groups (C-O/C=O ratio) and nitrogen species (i.e., pyridinic and pyrrolic N) played a key role in boosting H₂O₂ formation and accelerating the Fe(III)/Fe(II) cycling process. An alternative approach involves utilizing SDB as a conductive adsorbent in electrochemical reactors, where it captures organic contaminants and subsequently transfers [62,63] them to the electrode for direct oxidation, effectively serving as a bridge between adsorption and electro-oxidation. Based on this concept, while still a lab-scale study, this could provide a basis for improving the contact between contaminants and electrodes in flow-through reactors.

In summary, the use of SDB in electrochemical applications is a growing trend. As a sustainable electrode material, it enhances adsorption and catalytic functions in electrochemical water treatment. Its applications range from boosting power generation and pollutant removal in MFCs to facilitating energy-efficient desalination through CDI. By repurposing sludge into a valuable energy- and resource-generating material, these advancements align with circular economy principles.

5. Enhancement of Biological Treatment Systems

SDB has been widely studied as an additive or support medium to enhance biological wastewater processes, including anaerobic digestion, activated sludge processes, and biofilm reactors. Its porous structure, nutrient content, and redox-active surfaces can stimulate microbial activity and biochemical transformations.

5.1. Anaerobic Methane Recovery

In anaerobic digestion systems, particularly in systems treating high-solid or inhibitory wastes, SDB could act as a conductive platform that supports electron transfer between microbial communities, strengthens symbiotic interactions, and boosts methane generation. Specifically, SDB provides surfaces for microbial colonization and supports electron transfer between syntrophic bacteria and methanogens through direct interspecies electron transfer (DIET). For instance, Zhao et al. [66] used iron-modified SDB that increases functional groups such as C=O, thereby mitigating salt inhibition and significantly increasing methane yields, which in turn enhances SDB electroactivity. Mechanistically, SDB can not only adsorb inhibitory by-products (e.g., long-chain fatty acids, ammonia) but also stimulate enzymatic activities involved in hydrolysis, acidification, and methanogenesis. In another case, Zhang et al. [67] reported that Fe₂O₃ nanoparticle–biochar composites improved methane production (by 42%) and reduced membrane fouling in anaerobic membrane bioreactors. Fe₂O₃-SDB composites facilitate enzyme activation (Cyt c and F₄₂₀) during methanogenesis by acting as electron donors and acceptors. From an overview of the studies [68,69,70], the consensus is that SDB is a powerful accelerator for anaerobic digestion, improving gas yield, methane content, and digester stability. SDB provides a conductive substrate that facilitates electron transfer between microbial species, enhances symbiotic interactions, and promotes biogas production.

5.2. Activated Sludge and Biofilm System

In aerobic treatment, SDB can serve as a microbial carrier to enrich beneficial communities [71]. Its surface offers attachment sites for biofilms, aiding in biomass retention within biological systems such as moving bed biofilm reactors and integrated fixed-film activated sludge. In the activated sludge system, the carbon skeleton of SDB supports larger floc growth, leading to both reduced membrane fouling and enhanced nitrogen removal [72]. Likewise, in denitrification, SDB can leach minimal amounts of organic carbon, supplementing low-C/N wastewaters and thereby enhancing nitrate removal [58]. Wang et al. [73] reported that SDB acts as an electron shuttle for enhancing transfer system activity as well as improving functional microbial communities, leading to elevated denitrification performance. Additionally, SDB enhanced ammonia removal by promoting the growth of nitrifying biofilms on its surface, where the biochar created microsites with optimal oxygen gradients and pH conditions for nitrifiers [74]. Studies in other remediation contexts have noted that SDB can help microorganisms effectively resist adverse external influences, increasing overall biological process robustness [75,76]. For instance, SDB mitigates oxytetracycline stress and its toxic by-products to the nitrifying microbiome due to the increased OTC removal with a novel biotransformation pathway [75]. In summary, these observations indicated that SDB can act as both a carrier and a mild slow-release carbon source in biological systems.

5.3. Constructed Wetlands (CWs)

SDB is gaining attention as a potential substrate or additive in treatment wetland applications. Compared to gravel, biochar provides greater surface area and adsorption capacity for contaminants in wetland beds. CWs with SDB remove 60–95% inorganic or organic contaminants, based on complexation, ion exchange, precipitation, and pore-filling [77]. Guo et al. [78] explored the effectiveness of SDB as a substrate in CWs for nitrogen removal efficiency and discovered that it promoted the enrichment of genes involved in nitrate metabolism. While a study of Shi et al. [79] indicated that denitrification in CWs benefits from SDB, it supplies additional electron donors, and its redox-active surface can facilitate a partial reduction in nitrates. Integrating wetlands with electrochemical components, such as MFC-CW hybrids, is an innovative strategy in which SDB serves both as a conductive medium and a biofilm support [58]. Even in traditional CWs, SDB aids in regulating ammonium and phosphate levels by adsorbing these nutrients when biological uptake is low, effectively buffering concentration fluctuations. As SDB gradually reaches saturation, it can be collected and repurposed as a nutrient-rich fertilizer, contributing to a sustainable nutrient recycling system [80]. Recent publications have highlighted the potential of SDB-based CWs in advancing carbon neutralization [81,82]. CWs are vital in mitigating greenhouse gases (GHGs), such as N₂O and CH₄ emissions from CWs, with hydraulic retention time and wetland type playing key roles in mitigating CH₄ emissions [83].

In summary, SDB revolutionizes constructed wetlands by synergizing adsorption, catalysis, and microbial enhancement, yet critical gaps persist. While its multifunctionality improves nitrogen removal and GHG mitigation, long-term stability under dynamic wetland conditions and potential ecological risks (e.g., metal/nanoparticle leaching) remain underexplored. The integration with hybrid systems (e.g., MFCs-CWs) showcases innovation but lacks cost–benefit analyses for scalability. Future research must prioritize standardized life-cycle assessments to balance environmental benefits with unintended consequences, ensuring SDB-driven CWs align with circular economy and carbon neutrality goals.

5.4. Suppressing Emerging Contaminants and Pathogens

An increasingly explored biological application involves incorporating SDB into biofilters or bioreactors to tackle micropollutants, such as pharmaceutical residues, and pathogens. SDB can capture trace organic micropollutants, prolonging their retention and enhancing their bioavailability for microbial degradation. Some studies on antibiotic removal have employed SDB-packed bioreactors where certain degrader bacteria are enriched on the SDB surfaces [84,85]. Results showed a faster attenuation of compounds like sulfamethoxazole and ciprofloxacin compared to biochar-free controls, indicating a synergy between adsorption and biodegradation. Additionally, SDB can be enhanced through physical, chemical, and biological modifications to improve its antimicrobial activity [86]. These modifications collectively enhance the antimicrobial properties of SDB by promoting microbial adsorption, inducing oxidative damage, releasing toxic metal ions, and altering surface charge to inhibit bacterial adhesion. SDB-based anti-bacterial materials may help reduce pathogenic bacteria in treated water, although this aspect is still being explored.

In summary, incorporating SDB into biological treatment systems enhances pollutant removal, improves process stability, and introduces additional functions such as adsorption and electron transfer that go beyond microbial capabilities. Whether applied in anaerobic digestion, activated sludge processes, or eco-engineered wetlands, SDB proves to be a versatile enhancer, showcasing how the integration of biological and physic/chemical treatment with biochar results in more robust and efficient wastewater treatment.

6. Sludge-Based Biochar (SDB) in Membrane Technologies

The incorporation of SDB into membrane technologies represents a promising development in wastewater treatment. Particularly, membrane-based water treatment can benefit from SDB through fouling mitigation and contaminant adsorption. Recent studies show that dosing SDB into membrane bioreactors (MBRs) improves performance on multiple fronts. Wang et al. [72] alleviated membrane fouling as well as improved nitrogen removal in a municipal wastewater MBR by adding 1 g/L of SDB. The SDB particles adsorbed soluble microbial products (SMPs), serving as nuclei for larger flocs that settle better and form a looser cake layer on membranes. Furthermore, the micro-scale anoxic niches within SDB-amended flocs could release labile carbon for biological nitrogen removal. Similarly, Huang et al. [87] found that the SDB addition significantly reduced membrane fouling, primarily due to the enlargement of floc size and the reduction in SMPs. These results suggest that biochar addition is a viable fouling control strategy in both aerobic and anaerobic MBR systems. However, the mechanical stability of SDB layers remains a concern under prolonged shear forces, as biochar particle detachment or compression could diminish performance over time.

In addition to direct addition, SDB can be blended with membrane materials to form composite membranes or used as a pre-treatment or post-treatment component in membrane systems. Researchers have created biochar-based membranes by either blending biochar into polymeric membranes or using biochar as a coating layer [55,88]. Such membranes combine size-exclusion filtration with adsorption, aiming to remove micropollutants more effectively. SDB-based membranes, created by integrating SDB into a polymer matrix, have shown effectiveness in removing micropollutants like nitrobenzene from water [51,89]. These membranes enhance adsorption and filtration properties, positioning them as a promising technology for micropollutant removal in wastewater treatment. In such cases, the porous surface of SDB traps dissolved contaminants that pass through the membrane pores, enhancing effluent quality beyond the membrane’s standalone performance. Furthermore, integrating SDB into the membrane matrix also improves both its thermal stability and mechanical strength [90,91]. Another emerging concept is the dynamic membrane or biofilm membrane where biochar is used as a filter aid. Powdered SDB can be added to the aqueous phase to form a pre-coat on a support screen, creating a “biochar dynamic membrane” that traps solids and contaminants. While still at the experimental stage, this approach could turn a conventional filter into a reactive adsorption barrier that is periodically renewable by replacing the biochar layer.

In summary, SDB is increasingly utilized in membrane technology for various functions: as an MBR additive to mitigate fouling and enhance treatment efficiency, as a functional filler in membrane fabrication for improved pollutant removal, and as a potential dynamic membrane layer. These approaches harness biochar’s adsorption capacity and surface reactivity to enhance membrane separation performance. While SDB has shown promise in membrane applications, its long-term durability and maintenance remain insufficiently studied. Most experiments report performance over short durations, with limited data on membrane integrity or fouling resistance over extended use. SDB-based dynamic membranes may require frequent replenishment due to surface abrasion or pressure drops, and binder-free composite membranes may face the detachment of biochar particles under high shear. A few studies indicate that biochar additions can delay membrane fouling and improve floc structure in MBRs [64,67], but detailed assessments of membrane reusability, cleaning protocols, and biochar degradation are lacking. Future research should address membrane longevity, structural reinforcement (e.g., polymer–biochar hybrids), and regeneration strategies to support sustainable operation.

7. Modification Strategies for Sludge-Derived Biochar

A recurring theme across applications is that the proper modification of SDB is key to maximizing its performance. Unmodified SDB (especially from low-temperature pyrolysis) are limited by a small surface area, poor pore structure, and a lack of active functional groups In contrast, modified SDB offers enhanced porosity, greater surface area, improved physicochemical stability, and a more active functional fraction, making it more effective for environmental applications and remediation [3]. In recent years, researchers have developed various enhancement techniques.

7.1. Thermal Activation and Pyrolysis Tuning

The properties of biochar-based materials are significantly affected by the pyrolysis temperature. Higher temperatures (≥600 °C) generally increase surface area, pore volume, and graphitic carbon content, at the expense of lower yield. Microwave pyrolysis at 800 °C increases SDB surface area from 5 to 214 m²/g, promoting graphitic domains for electron transfer [92]. Figure 3 graphically represents the presence of different types of functional groups at different temperatures. For SDB, increasing the temperature concentrates the catalytic active sites (by breaking down organic matter and exposing minerals). For instance, the PMS activation rate for sulfamethoxazole degradation increased by two times with the pyrolysis temperature from 500 °C to 700 °C [93]. Fast pyrolysis tends to produce smaller particles with more surface functional groups, while slow pyrolysis produces larger, more carbonized char. By adjusting these parameters, the pore structure and chemical composition of the SDB can be tuned to meet the needs of a particular application (adsorption vs. catalysis) [94]. The gas atmosphere during pyrolysis significantly impacts biochar properties: pyrolyzing sludge under CO₂ or steam can in situ activate it (etching pore structure), using an inert gas like N₂ prevents the oxidation of certain elements, preserving specific functional groups and chemical properties [95]. In summary, optimized thermal processing is usually the first step in producing high-quality SDB. It should be emphasized that higher temperatures (>700 °C) enhance aromaticity but reduce functional groups, necessitating balanced optimization.

7.2. Physical and Chemical Activation

The limited pore structure and small specific surface area of biochar derived from sludge restrict its potential applications. Pretreatment of sludge through activation is a promising approach to enhance the pore structure and functional group composition.

Using steam or CO₂ at high temperatures is an effective physical activation. It burns off residual tars and widens pores [96]. Excessive activation could reduce the yield of some functional groups; thus, conditions are required to be optimized to achieve a high surface area without destroying useful surface chemistry. The technique most frequently used to activate biochar is chemical activation [97]. Chemical activators, including H₃PO₄, KOH, NaOH, and zinc compounds, have been applied to sludge biochar to create hierarchical pores [98]. For instance, KOH-activated SDB showed 3.9–14.5 times higher in the surface area (often >500 m²/g) and enhanced the adsorption capacity for CO₂ capture, ranging from 136.7 to 182.0 mg/g [99]. However, these strong activators can be corrosive and pose secondary pollution, so recent research explored milder activators. Zhang et al. [100] designed SDB from cassava ethanol sludge for using the environmentally benign NaHCO₃ as an activating agent. NaHCO₃-activated SDB adsorbed 154.45 mg/g of TC, surpassing the raw biochar’s adsorption of 34.04 mg/g. The activator increased the content of C, H, N, and O and also effectively removed the ash from the biochar during the thermal activation process, which increased the surface area of BET and improved the adsorption capacity. Another study replaced KOH with CaO (a cheap, benign agent) to activate SDB, successfully achieving the Cd(II) adsorption rate of 99.74% [101]. CaO can enrich the calcium ion content and expand the specific surface area of SDB, thereby increasing its active sites and surface functional groups. Using steam or CO₂ at high temperatures is an effective physical activation. It burns off residual tars and widens pores [96]. Excessive activation could reduce the yield of some functional groups; thus, conditions are optimized to achieve a high surface area without destroying useful surface chemistry.

7.3. Heteroatom Doping

Sludge itself contains N and S from proteins and biogenic matter. Thus, SDBs naturally have N-doped carbon domains (pyridinic, pyrrolic, graphitic N) and some sulfur functionalities. These heteroatoms can modulate the electronic structure of carbon, creating basic sites or catalytic centers. Recently, heteroatom doping has been widely employed by researchers to impart unique properties to SDB, allowing it to be customized for specific wastewater treatment challenges. N, S, and P were introduced into the matrix of SDB during or after pyrolysis. In terms of nitrogen doping, by mixing sludge with urea, melamine, or ammonia treatment prior to pyrolysis, the biochar’s N content can increase, yielding more amine, amide, or graphitic-N sites [102]. Nitrogen doping introduces pyridinic-N sites, lowering the work function from 4.8 to 4.3 eV to enhance electron density [103]. In terms of sulfur doping, by impregnating sludge with sulfate salts, S-doped SDB can have an affinity for certain metals, which is attributed to the strong affinity between sulfur atoms and metals [104]. In terms of phosphorus doping, a phosphoric acid or phosphate addition produces P-functionalized SDB, which exhibits strong metal-chelating properties (e.g., with Cd and Pb) [105]. In addition, the electrical conductivity of SDB can be improved through phosphorus doping, which is beneficial for applications in electrochemical systems such as microbial fuel cells and capacitive deionization. Overall, heteroatom doping tailors the surface chemistry of sludge biochar to target contaminants. As for the concern of heteroatom leaching, it is also possible to immobilize the active element through biochar–mineral composites to provide a safer alternative.

7.4. Metal Loading and Composite Formation

To impart specific catalytic functions, sludge biochar is often used as a support for metal nanoparticles or as a matrix to embed metals in situ. Although the high ash content of sludge (rich in Fe, Al, and Ca) means that some metal presence is inherent, additional loading is required for superior performance. Through soaking sludge in FeCl₃ or FeSO₄ before pyrolysis, Fe-loaded SDB could be obtained. The yields of iron on carbon showed excellent performance for Fenton and persulfate activation [44]. Hashemi et al. [106] investigated the use of magnetic SDB for the removal of various dyes in wastewater. These methods offer advantages such as high efficiency, speed, simplicity, reusability, and environmental sustainability. Yu et al. [41] prepared Cu-doped SDB for bisphenol A degradation in high-salinity wastewater (100 mg/L, PDS = 0.5 g/L, catalyst = 0.7 g/L) by PDS activation. The removal process was found to be dominated by ¹O₂. In contrast to Yu’s study, Wang et al. [107] improved bisphenol S removal using PDS through CuFe₂O₄-doped SDB activation to produce reactive oxygen species, and the main reactive species was the sulfate radical. By endowing SDB with magnetic properties, the multifunctional adsorbents can use an external magnetic field to efficiently recover from treated water [108]. Magnetic SDB often refers to Fe₃O₄-laden sludge-derived biochar, which is retrievable by magnet and also catalytically active. Hashemi et al. [106] used magnetic SDB to remove various dyes in wastewater. Except for serving as recyclable adsorbents, magnetic SDBs have been recently used as electron shuttles as DIET facilitators in digesters for enhancing methane production [109]. The integration of magnetism into biochar through the doping of magnetic nanoparticles represents a significant advance in wastewater treatment technology. Beyond sole substrate, sludge is co-pyrolyzed with another waste to form composite biochar, such as red mud (Fe/Al source), clay minerals, or agricultural biomass [110]. The sludge-red mud magnetic biochar was used to adsorb TC and catalyze PDS. Unlike conventional SDB, the combination of Fe-driven radical activation and the nonradical pathway facilitated by carbon-based functional groups synergistically enhanced the persulfate-driven oxidation of TC. Alternatively, the incorporation of SDB with conductive nanostructures such as carbon nanotubes or graphene oxide enables the fabrication of hierarchical composite electrodes exhibiting enhanced electrical conductivity and robust electrochemical stability, which is particularly advantageous for energy storage applications, including supercapacitors and electrocatalytic systems [111]. In summary, the combination of sludge and dopants often yields synergistic effects. Researchers are increasingly leveraging SDB as a scaffold to anchor active nanophases (metals, metal oxides, etc.,), thus turning a waste into a value-added composite material.

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 photoreactors (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. While lab-scale studies demonstrate the high potential of SDBs for pollutant removal, practical deployment requires careful consideration. In the field trials, there are some challenges compared to the ideal lab-scale. For instance, variability in sludge composition can result in inconsistent biochar quality and contaminant removal performance. Furthermore, variable influent composition, fluctuating pH, flow rates, and biochar aging under continuous operation also impact the performance of SDB. Developments of flow-through system applications and standardized multi-cycle durability testing for used SDB materials should be prioritized. Bridging this lab-to-field gap through more pilot-scale, long-duration, and real wastewater studies is essential for accelerating the adoption of SDB in real wastewater treatment systems. Finally, regulatory acceptance is another crucial factor. SDB’s application, especially in agriculture or open environments, should not introduce any harmful leachates or by-products.