1 Introduction

Waste activated sludge (WAS) is an inevitable byproduct of the wastewater biological treatment process, mainly including municipal and industrial sludge (Zhang et al. 2021). With increasing economic and societal development, wastewater treatment technology has improved, but it has also increased WAS (Li et al. 2024). For example, in Germany and the UK, 1.85 million tons (Mt) and 1.14 Mt of dry matter sludge are generated annually, respectively (Xiao et al. 2023). In 2019, in China about 60 Mt of WAS (calculated based on 80% water content) was generated (Guo et al. 2021). According to estimates, by 2025, global sludge production (calculated based on 80% water content) is expected to reach 224 Mt (Jiang et al. 2023). The massive output of WAS puts tremendous pressure on the sustainable development of the wastewater treatment industry (Kumar et al. 2021). WAS can be considered both a polluting substance and a resource. On the one hand, it contains harmful substances such as pathogenic microorganisms, heavy metals, and toxic organic matter. If WAS is not appropriately treated, it can cause secondary pollution in the natural environment and cause damage to human health. On the other hand, sludge is rich in organic matter, making it a potentially affordable and environmentally friendly carbon source for bioconversion and resource utilization, promoting the utilization of resources (Li et al. 2019). Hence, it is vital to treat the increasing amounts of WAS safely and effectively. In recent decades, especially in the context of global low-carbon emissions goals, various WAS treatment technologies have been developed, among which anaerobic digestion (AD) has emerged as a strategy worldwide for generating renewable energy from organic waste (Masebinu et al. 2019; Xu et al. 2020). Through AD, it is possible to simultaneously achieve the green and safe treatment of WAS while generating energy.

AD is an effective biological treatment technology stabilizing organic compounds in waste/wastewater while generating methane (Baek et al. 2018). The technology is suitable not only for the anaerobic stabilization of WAS from wastewater treatment plants (WWTPs) but also for co-digestion processes with other organic waste (Nghiem et al. 2017). AD achieves organic waste reduction and resource utilization through the synergistic metabolism of various microorganisms (Alvarado et al. 2022). The currently recognized four basic steps of AD, shown in Fig. 1, divide the AD process into hydrolysis stage, acidogenesis stage, acetogenesis stage, and methanogenesis stage (Appels et al. 2008). In AD, organic waste is hydrolyzed into simple organic compounds, which are decomposed into volatile fatty acids (VAFs). These compounds are then converted into acetic acid, H2, and CO2, and ultimately to methane (Park et al. 2022). However, researchers have noted that while the AD process of WAS is environmentally friendly, there are challenges, such as low methane production, inhibitor generation, system acidification, low facility processing load, and low operational efficiency. (Bin Khawer et al. 2022; Jin et al. 2022a). Therefore, combining AD with other approaches to resolve the above deficiencies is a key focus and challenge in current AD research.

Fig. 1
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Subsequent steps in the anaerobic digestion process

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Biochar is a carbon-based material made from the high-temperature pyrolysis of biomass feedstocks. Biochar is widely applied in agriculture, environmental remediation, construction, and other fields (Chen et al. 2019a). In recent years, using biochar as an additive for enhancing the AD efficiency of WAS has received much attention from researchers (Chiappero et al. 2020; Song et al. 2021). It can increase the decomposition efficiency of organic matter and reduce the accumulation of intermediate metabolites, leading to increased methane production. Notably, the applications of biochar are closely linked to its physical and chemical properties. Researchers have explored various activation and functionalization methods to optimize the characteristics of biochar, such as its pore structure, specific surface area (SSA), surface functional groups, and conductivity. Numerous studies (Nie et al. 2024; Zhong et al. 2023) have demonstrated that introducing functional biochar into AD systems for WAS treatment can significantly enhance the efficiency of the process. This enhancement was due to the multiple effects of functional biochar in these systems. For example, during the AD process, functional biochar not only facilitates electron transfer between microorganisms and methanogens but also has a buffering effect, promoting microbial immobilization, alleviating inhibition from harmful substances, and regulating the activity of functional microorganisms. These combined effects lead to a significant enhancement in the performance of AD.

In addition, functional biochar can effectively promote carbon neutrality, particularly in waste reduction, resource utilization in sludge treatment, and reduction in greenhouse gas emissions (Liu et al. 2023a; Mayilswamy et al. 2023). A circular economy strategy based on functional biochar ensures the sustainability of WAS anaerobic technology and empowers individuals to use “waste to control waste.” In this circular economy model, “waste” is transformed into energy or other valuable products, achieving maximum reduction, resource utilization, and waste remediation. This approach promotes sustainable development from economic, societal, and environmental perspectives.

Energy recovery from WAS during AD is critical for WWTPs. Biochar is vital in improving the stability and efficiency of AD of WAS. However, the impact of biochar on AD performance is closely linked to its physical and chemical properties (Zhang et al. 2022a). It is imperative to comprehend the effects of functionalized biochar on AD performances and develop effective preparation strategies. To the best of our knowledge, despite increasing research on the connection between biochar and AD of WAS, there is still a lack of comprehensive summary and discussion regarding the promoting effects and mechanism of functional biochar on the AD of WAS. This paper provides a comprehensive review of the process of biochar functionalization and assesses the effects of different preparation methods. In addition, the impact of functional biochar application in AD of WAS and related mechanisms are comprehensively reviewed and discussed. Meanwhile, this paper comprehensively assesses how functional biochar contributes to the circular economy of sludge AD. Finally, an operational model for the circular economy based on functional biochar in engineering practice is emphasized, and resource and energy recovery from WAS are achieved. This review aims to provide an overview of the application of functional biochar in the AD of WAS and to construct a sustainable circular economy model that integrates fields such as energy, the environment, and the economy.

2 Review methodology

The following criteria were considered for the selection of scientific literature. A systematic search strategy was used in this study, including searching and screening. The search phase entailed searching for publications directly linked to research on biochar application in AD of WAS through the Web of Science. The search involved carefully collecting the original database, which comprised 590 articles, on the target review topic. These articles were published from January 1, 2000 to April 1, 2024. Based on this preliminary selection, the review questions were identified. Herein, different combinations of additional keywords were used for specific questions: function* biochar, modified biochar, anaerobic digestion, sludge, methane, electron transfer, DIET, microbial immobilization, heavy metals, carbon neutralization, resource utilization, and circular economy. After screening the abstracts of all identified reference sources, 126 relevant references were analyzed for completeness and included in this review. This review is structured according to the content outline. In each section, the main findings and research gaps of the specific topic are summarized and finally condensed into perspectives and conclusions.

3 Functionalized modifications of biochar

Biochar, a highly aromatic carbon-rich solid particle derived from biomass pyrolysis, is widely regarded as a viable alternative for waste management (Kasera et al. 2022). Almost all types of organic waste, from kitchen waste to agricultural and forestry waste, livestock manure, and even organic sludge from wastewater, can be utilized as raw materials for the preparation of biochar (Fig. 2). The biomass feedstock, preparation technology, and preparation parameters all influence the elemental composition and physicochemical properties of biochar. (Jayakumar et al. 2023). Biochar produced using pyrolysis often has low porosity, poor conductivity, and limited types and quantities of surface functional groups (Xie et al. 2022). These inherent properties, to some extent, constrain the widespread application and utilization efficiency of biochar in various fields. The performance of biochar is closely related to its physicochemical properties, such as pH, cation exchange capacity (CEC), SSA, and pore structure. Appropriate activation or functionalization methods to improve the properties of biochar have become a popular research field (Liu et al. 2022b; Joshi et al. 2023). Figure 3 shows the effects of the different modification methods on the properties of biochar. Modification can produce biochar with diverse physicochemical properties that can be “tailor-made” for specific applications (Table 1). In recent years, researchers have focused on the activation and functionalization of biochar, especially for its application as an additive in AD (Xu et al. 2023), pollutant removal (Qiu et al. 2022), agriculture (Zhou et al. 2022), and bioenergy (Kumar et al. 2020). Previous reviews have focused primarily on the application of biochar in the AD of WAS (Kumar et al. 2021; Deena et al. 2022). The following sections discuss various methods of activation and functionalization of biochar to improve its surface and physicochemical properties or to utilize its surface as a platform to support functional materials.

Fig. 2
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Different feedstocks for biochar preparation

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Fig. 3
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Effect of different modification methods on the properties of biochar

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Table 1 Physicochemical properties of functional biochar modified by various methods
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3.1 Acid–base activation

Acid and alkali treatments can significantly enhance the physical and chemical properties of biochar. The acid treatment introduces acidic functional groups to the surface of biochar, improving its physicochemical characteristics, which includes increases in the number of surface functional groups and CEC (Zhang et al. 2023a). H2SO4, HCl, HNO3, H2C2O4, H3PO4, and C6H8O7 are commonly used acid treatment agents. For example, Jin et al. (2022b) noted that treating woody and straw biochar with 1.00 M HCl significantly increased methane production and reduced the lag phase during the AD of WAS. Cao et al. (2018) employed mass ratios of 85% (w/w) H3PO4 for the acid activation of lignocellulose, successfully forming oxygen-phosphorus functional groups, which improved the thermal stability and surface acidity of the biochar. Ateş (2021) reported that biochar with high SSA (1984.00 m2 g−1) and PV (2.70 cm3 g−1) was obtained by microwave and ultrasound irradiation of raw tea waste in the presence of H3PO4 for 30 min followed by pyrolysis at 500 °C.

After alkali treatment, the SSA, surface basicity, and quantity of oxygen-containing functional groups on the biochar increase, enhancing its aromaticity, hydrophobicity, and π–π interactions (Zhang et al. 2024). Overall, alkali-treated biochar exhibits greater surface area, carbon content, and PV than acid-treated biochar. NaOH and KOH are common activation agents. Recently, Ngo et al. (2022) utilized 8 M HNO3 and 0.4 M NaOH for the acid–base treatment of wood biochar and reported that the ammonium adsorption capacity and porosity of the functional biochar significantly improved. Wang et al. (2020) reported that rice husk biochar contains more oxygen-containing functional groups after pyrolysis at a relatively low temperature, and KOH activation further optimized its pore structure. Liu et al. (2023b) successfully prepared alkaline biochar by pyrolyzing a mixture of KOH and raw biochar with different mass ratios at 800 °C. The results showed that the adsorption performance of KOH-biochar increased with the increase of the modifier KOH concentration.

3.2 Oxidative modification

The efficacy of biochar in improving the quality of digestate is closely associated with its characteristics, such as surface functional groups, SSA, and porosity. When oxidants are introduced, the aldehyde, hydroxyl, and carboxyl groups on the surface of biochar may be partially oxidized, increasing the electron transfer rate, porosity, and SSA of the biochar and facilitating the AD process (Wang and Wang 2019). For instance, biochar modified with KMnO4 has MnOx and oxygen groups loaded on its surface, increasing the number of adsorption sites on the biochar surface (Wang et al. 2015). Research by Wang et al. (2023a) indicated that the modification of biochar with 0.10 M KMnO4 resulted in a significant improvement in surface properties. The modified biochar had enhanced SSA, more oxygen-containing functional groups, and more hydroxyl groups. Jiang et al. (2022b) showed that, compared to unmodified biochar, H2O2-oxidized biochar had higher phenolic and lactone functional group contents, which increased 1.20 times and 5.10 times, respectively. Furthermore, H2O2 (30%, w/w) oxidation significantly increased the redox capacity of biochar, particularly its electron transfer activity, enhancing its methanogenic potential. Results revealed that H2O2-oxidized biochar showed 197.80 ± 4.40 mL g−1 COD removal during methane production, 58.70% higher than the unmodified biochar.

Biochar has extensive potential for various applications. Appropriate modification and functionalization can significantly enhance the physical and chemical properties of these materials. However, each modification method has advantages and disadvantages. For example, acid–base modification can increase the number of surface functional groups and the porosity of biochar. Still, it may result in losing some of its original hydroxyl functional groups. It is crucial to finely tune the application parameters of biochar and ensure that the biochar is environmentally safe after modification. Notably, high concentrations of corrosive chemicals such as acids and alkalis may have adverse environmental impacts (Kumar et al. 2020). Redox-driven modification can increase the number of adsorption sites, oxygen-containing functional groups, and hydroxyl groups on the biochar surface, but it may also disrupt the biochar structure and microscopic pores (Pan et al. 2022). Therefore, thorough research and evaluation are necessary to choose the most suitable activation and functionalization approaches for specific applications before selecting a modification method.

3.3 Magnetic modification

Magnetic biochar has attracted much attention recently due to its high carbon content, large SSA, and ease of magnetic separation. It provides a new route to overcome challenges related to biochar recovery from solution. Li et al. (2020b) employed an environmentally friendly, efficient, cost-effective ball milling method to prepare magnetic biochar. By ball milling biochar with Fe3O4 nanoparticles, the functional groups and C = C aromatic bonds in the biochar were increased, and the biochar was endowed with magnetic properties. Wang et al. (2023c) successfully prepared nanomagnetic iron-loaded biochar through a series of hydrothermal and pyrolysis processes using solid waste and Fe(NO3)3 as raw materials. The modified magnetic biochar exhibited a larger surface area, higher electrochemical response, and lower electron transfer resistance. In addition, Fenton sludge, a waste containing high concentrations of ferric ions and organic pollutants, has been used to prepare magnetic biochar. Wang et al. (2021b) found that using iron-rich Fenton sludge as a feedstock and subjecting it to pyrolysis at 400 °C can transform amorphous iron oxides into magnetite, while the redox functional groups in biochar are preserved. Therefore, this novel magnetic biochar possesses high capacitance and good conductivity and significantly enhances methane production when added to AD systems. Despite the significant advantages of magnetic biochar in terms of application and recovery, the metals and oxides loaded in biochar may cause secondary pollution in the environment. Therefore, minimizing the potential toxicity of magnetic biochar to the environment has become a critical issue (Qu et al. 2022).

3.4 Modification with functional materials

To improve the performance of biochar in AD applications, it is essential to appropriately functionalize biochar. A coating of functional materials on the surface of biochar can enhance properties such as adsorption capacity and EC. The current research focuses mainly on three functionalized material types: metal ions, organic functional reagents, and carbon-based materials.

3.4.1 Metal-based modifications

Loading metal ions onto biochar is a common method for biochar modification. Since biochar surfaces are mostly negatively charged, the loading of metal ions can alter the surface charge, which may further enhance the magnetic properties, active site activity, chemical adsorption properties, electron transfer capacity, and complexation properties of biochar (Liu et al. 2022c). Utilization of metal-based biochar has been proven to be an efficient strategy for enhancing the AD efficiency of WAS. For instance, previous studies (Zhang and Wang 2020) have shown that metal ion-loaded biochar can be used to modulate the AD of WAS for methane production, as well as to facilitate the immobilization of heavy metals in sludge and reduce its risk to the environment. Biochar-supported metals can be prepared through two standard methods: prepyrolysis and postpyrolysis. Prepyrolysis involves immersing the raw material in a solution of metal salts before anaerobic pyrolysis, while postpyrolysis involves pyrolysis followed by immersion in a metal salt solution. Common metal ion modifications include Fe3+, Mg2+, Al3+, Ca2+, and Mn2+, among others (Wang et al. 2021b). For example, Zhang et al. (Zhang and Wang 2020) reported that the modification of biochar with manganese oxide significantly increased the number of oxygen-containing functional groups on its surface. Additionally, Wu’s research group (Lu et al. 2022) successfully synthesized iron-loaded biochar and observed a significant increase in biochar conductivity. During the AD process of sludge, the iron-loaded biochar demonstrated outstanding performance. Due to its excellent conductivity, the biochar increased the concentration of coenzyme F420 in the codigestion system, stimulating the activity of methanogenic bacteria and enhancing the efficiency of methane production.

3.4.2 Modification with organic functional materials

In organic modification, using surfactants or polymeric materials allows for the adjustment of the SSA and functional groups of biochar, thereby expanding its range of applications (Wei et al. 2023). Macedo et al. (Macedo et al. 2021) modified sugarcane bagasse biochar using an isothiocyanate one-step method. They found that this modification increased the S and N contents of the biochar surface, particularly the number of isothiocyanate groups, which led to a significant increase in the biochar adsorption capacity. Truong et al. (Truong et al. 2020) applied the industrial byproduct polyethyleneimine to modify biochar to enhance biochar adsorption capacity for organic compound removal. Compared to unmodified biochar, the organic-modified biochar had a larger surface area and a more positive charge, making it more suitable for adsorbing organic compounds. The primary purpose of organic modification is to enhance the performance of biochar by introducing specific functional groups. However, traditional organic modification methods have drawbacks, such as high preparation cost, toxicity, volatility, and potential secondary pollution. Therefore, researchers are exploring cost-effective and environmentally friendly organic modification methods. In recent years, several innovative technologies and green materials have been employed in organic modification to reduce costs, minimize toxicity, and mitigate environmental pollution. For instance, using bioactive substances extracted from plants (Jiang et al. 2022a) and microorganisms (Zhang et al. 2020) as modifiers can significantly reduce costs and environmental impact.

3.4.3 Modification with conductive carbon materials

Conductive carbon materials, such as carbon fibers, carbon nanotubes, carbon cloth, graphene, and granular activated carbon, have been confirmed to reduce the amount of energy required to generate extracellular conductive pili and cytochrome c for electrical connection between cells (Wang et al. 2018; Nabi et al. 2022). Combining biochar with conductive carbon materials can accelerate the methanogenic metabolism process by direct interspecies electron transfer (DIET) and bypass the electron carrier shuttle service, improving the AD systems (Qi et al. 2021). Conductive carbon materials-coated biochar is usually prepared by dip-coating followed by pyrolysis. Inyang et al. (2014) placed 10 g of biomass into 100 mL suspension of carbon nanotubes and stirred it at 500 rpm for 1 h. The carbon nanotube-coated biochar was then produced by pyrolyzing the treated feedstock at a temperature of 600 °C for 1 h at 10 °C/min. A comparison of untreated biochar and its characterization indicated that the addition of carbon nanotubes improved the physicochemical properties (e.g., surface area, porosity, and thermal stability) of biochar. Zhang et al. (2012) successfully developed a novel engineered graphene-coated biochar derived from cotton wood. Herein, 10 g of the biomass was immersed in the graphene suspension and dried at 80 °C. The graphene-treated biomass was slowly pyrolyzed at 600 °C under N2 for 1 h to produce graphene-coated biochar. The results indicated that during the annealing process, graphene molecules are encapsulated on the surface of the biochar. Thermogravimetric analysis revealed that the graphene coating significantly improved the thermal stability of the biochar, increasing its capacity for carbon sequestration in large-scale land applications.

4 Application of functional biochar in anaerobic digestion

Critical indicators of AD process performance include the efficiency of intermediate metabolite conversion, organic matter degradation, and methane production. While progress has been made in optimizing the AD of WAS, the inherent characteristics of AD limit its scope and efficiency of application to some extent. The application of functional biochar is expected to enhance the AD performance of WAS, promoting environmental and energy sustainability. The functional biochar obtained after modification promoted the conversion of intermediate metabolites and the degradation of organic matter, and increased methane production to varying degrees (Table 2).

Table 2 Roles of functional biochar in improving anaerobic digestion performance
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4.1 Intermediate metabolites conversion

During AD, the conversion of intermediate metabolites such as VFA, ammonia, and sulfide is a crucial parameter for assessing system stability. Excessive accumulation of TAN and high concentrations of organic acids can inhibit microbial activity and methane production. The addition of functional biochar alleviates the accumulation of intermediate metabolites and facilitates VFA co-oxidation (Wang et al. 2021a). For example, Lu et al. (2022) prepared an iron-containing biochar (Fe-BC) and conducted batch AD experiments under mesophilic conditions (36.00 ± 0.50 ℃). Their results showed that adding Fe-BC reduced the concentration of VFA, promoting the conversion of VFA to methane and enhancing the pH stability of the whole AD system. Similarly, Zhang and Wang (2021) investigated the impact of biochar-supported nano zero-valent iron (nZVI-BC) during the codigestion of sludge and kitchen waste. The concentration of VFAs was significantly lower in the treatment group than in the control group (11.59–45.80%). Jin et al. (2022b) compared the efficacy of two methods, water-washing and acid-washing, on wood-biochar (W-BC) and straw-biochar (S-BC) in regulating the AD of sludge. Water-washed and acid-washed treated W-BC reduced the concentration of short-chain fatty acids (SCFAs) by 7.70% and 13.50%, respectively, and reduced the accumulation of SCFAs more effectively than the unmodified W-BC. Jin et al. (2024b) prepared biochar from iron-rich residual fermented sludge and applied it in AD. On the first day, the SCFAs content was 106 and 152 mg COD/g VSS in the control and sludge biochar groups, respectively. The results indicated that sludge-based biochar could promote the conversion of soluble organics to SCFAs.

Li et al. (2017) also confirmed that the quantity of hydroxyl, amino, and carboxyl groups present affects the adsorption capacity of biochar for ammonia nitrogen (AN). Zhong et al. (2023) synthesized nitrogen-doped biochar-loaded magnetite (NBM) and verified its effectiveness in optimizing the AD of WAS. Their study confirmed that adding 5.00 g/L−1 NBM could accelerate the conversion of organic substrates into VFAs and enhance hydrolysis efficiency. Li et al. (2019) and Zhang et al. (2019) also conducted in-depth studies on the role of biochar composite materials in optimizing the AD of WAS. Their results showed that adding an appropriate quantity of composites could alleviate the inhibitory effects of VFAs and AN, significantly improving the stability and microbial activity of the system and enhancing methane production. Liu et al. (2022a) synthesized magnetic straw-based biochar (MSBC) through a green approach and investigated its impact on the AD of sludge. MSBC significantly changed the composition and concentration of VFAs, greatly increasing the production of total VFAs. Notably, the highest VFAs content in the control group (~ 2158 mg L−1) and MSBC group (~ 2463 mg L−1) appeared on days 9 and 6, respectively. This indicated that MSBC could optimize the hydrolysis and acidification process of sludge and enhance the efficiency of converting intermediates to methane.

Therefore, biochar plays a crucial mediating role in AD, facilitating the efficient transfer and conversion of substances. The functionalization of biochar can significantly improve its efficiency and role in substance conversion. Specifically, the functional groups on the biochar surface catalyze the transformation of certain substances, enabling the adsorption and slow release of excess substances such as AN, metal ions, and sulfides. Furthermore, loading conductive materials onto biochar can facilitate the conversion of VFAs. During AD, the digestion substrate is hydrolyzed into VFAs, which methanogens cannot immediately utilize. Under these conditions, conductive materials act as catalysts to accelerate the conversion process of VFAs. Conductive materials can stimulate the acetate fermentation step during AD, generating more acetate and accelerating methanation. Hence, by employing functional biochar, the conversion efficiency of VFAs during AD can be effectively improved, and methane production can be promoted.

4.2 Organics removal

Most organic matter in WAS consists of microbial cellular material, which is difficult for anaerobic microorganisms to decompose and utilize (Tang et al. 2020). The organic matter removal rate is a crucial indicator for evaluating the AD biodegradation process of WAS. Notably, functional biochar can enhance organic matter removal and optimize dissolution and hydrolysis processes during the AD of sludge, significantly improving the conversion efficiency of organic matter. For instance, adding 5.0 g g−1 VS nZVI-BC enhanced the degradation efficiency of organic matter, and the degradation rate of TCOD was improved by 34.93%. (Zhang and Wang 2021). Wang et al. (2021b) reported that magnetic biochar could achieve up to 87.80% COD removal during the AD of WAS. Moreover, Zhong et al. (2023) reported that the removal efficiency of SCOD could be improved by 15.00% via a novel NBM (5.00 g L−1). Zhang et al. (2023b) investigated the impact of different dosages of magnetic biochar on COD removal in AD reactors. Their results indicated that low dosages of magnetic biochar promoted the consumption of SCOD and metabolites, while continuous increases in the dose hindered the consumption of metabolites. On the other hand, Wang et al. (2023b) modified hydrochar at room temperature using KOH, H2O2, and HCl to increase oxygen-containing functional groups and enlarge pore sizes, among which KOH modification was effective. The COD removal of CH5 and MCH5 reached 46.80% and 58.90%, respectively, after 16 days of modified material addition, showing that MCH was more effective at reducing organic matter content.

In summary, the resistance of microbial cell walls in sludge to breakage hinders the hydrolysis of complex organic compounds, which is a critical factor limiting the rate of AD in sludge. However, with functional biochar, COD removal can be effectively facilitated, which enhances the decomposition of organic matter, increasing methane production. The efficiency of functional biochar for COD removal in AD can be attributed to the presence of functional groups, high EC, magnetism, and buffer capacity. This provides new perspectives for selecting suitable functional biochar to improve the efficiency of AD of WAS.

4.3 Methane yield

AD of WAS can generate methane, providing supplemental energy for WWTPs and contributing to carbon neutrality goals. However, the methane output during AD of WAS is usually low. Using biochar prepared from a wide range of organic wastes is recognized as an effective strategy for enhancing the methane output of AD. However, owing to the complex nature of WAS (e.g., low organic mass, high sand content, and excessive heavy metal and organic pollutant content), biochar is not so effective at boosting methane production from the AD of WAS. Therefore, to increase methane yield during AD, various functional biochars have been developed for application in AD. For instance, Zhang and Wang (2020) successfully prepared a Fe–Mn oxide biochar and investigated the role of MnFe2O4 biochar in the AD of WAS. Their results indicated that the highest cumulative methane production was obtained when the dose of MnFe2O4 biochar reached 1.50 g, resulting in a 55.86% increase compared to that of the control. Another study showed that adding 15 g L−1 of iron-based biochar increased methane production rate to 2.0 mL h−1, twice that of the control (Che et al. 2022). This enhancement may be attributed to the iron oxides and zero-valent iron generated on the surface of the biochar, which drive methane production. Zhang and Wang (2021) assessed the effect of nZVI-BC on the anaerobic codigestion of sludge and food waste. Through the application of kinetic models, they revealed that when the nZVI-BC amount was increased from 0 to 3.0 g g−1 VS, the methanogenic potential and the maximum methanogenic rate increased from 251.52 mL g−1 VS and 15.16 mL g−1 VS d−1 to 359.91 mL g−1 VS and 22.72 mL g−1 VS d−1. At the same time, the estimated lag period decreased from 1.97 to 1.56 d. The lower lag period indicated that supplementation of nZVI-BC favored microbial growth, which resulted in improved biodegradability and methane production.

Zhong et al. (2023) synthesized NBM and applied it to enhance the AD of WAS. A total of 5.00 g L−1 NBM increased the cumulative methane production by 1.75-fold compared to the control. Li et al. (2019) investigated the impact of adding manganese oxide-modified biochar (MBC) on methane generation during the AD of WAS. It was found that MBC could enhance methane production and promote the decomposition of intermediate acids. MBC increased cumulative methane production by 121.97% relative to the control. Zhang et al. (2019) successfully prepared nZVI-BC and employed it in the AD of sludge. This additive enhanced the stability of AD. Additionally, the methane content in the biogas increased by 29.56%, and the cumulative methane production increased by 115.39%. Liu et al. (2022a) synthesized a novel MSBC and investigated its impact on the AD of sludge. Their results indicated that the maximum net cumulative methane yield in the MSBC group was 241.68 mL g−1 VS, which was 45.36% higher than that in the control group (166.26 mL g−1 VS), demonstrating that adding MSBC could significantly increase the methane production in the WAS AD process. The fitting results from the modified Gompertz equation showed that adding MSBC increased the maximum methane yield from 12.69 mL g−1 VS d−1 to 16.58 mL g−1 VS d−1, indicating that MSBC not only increased the methane yield but also increased the methane production rate. The study showed that the dose with the best results was 16 mL∙g-VS−1 Fe-BC with an increase in methane yield of 49.7%. However, the cumulative biogas production decreased when 24 mL∙g-VS−1 Fe-BC was added (Lu et al. 2022). The reasons for this result may be that, firstly, the high concentration of Fe-BC made the digestion system high in solids and reduced the mass transfer efficiency. Secondly, the iron in the digestion system was 1423 mg mL−1, enough to destroy the bacterial cell membrane and inhibit microbial activity (Wei et al. 2016).

In conclusion, functional biochar can significantly enhance methane production during the AD of WAS. Due to their unique properties, different types of biochar have varying effects on methane production enhancement. However, the addition of excess biochar might reduce methane generation. In addition to considering the optimal dosage, it is crucial to consider the cost and experimental conditions during practical applications.

5 Mechanisms of anaerobic digestion promotion with functional biochar

Functional biochar has emerged as a research hotspot for optimizing methane production and enhancing the efficiency of AD of WAS. Its core advantages lie in its physical properties and functional groups. Through an in-depth understanding of these characteristics and their relationship with methane generation, efficient biochar additives can be precisely selected and tailored to improve the methane yield of AD. EC, redox capacity, porosity, adsorption, and functional groups are crucial for enhancing methane production (Fig. 4).

Fig. 4
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Mechanisms of anaerobic digestion promotion via functional biochar

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5.1 Electron transfer

Electron transfer, the fundamental driving force for biological activity, plays a crucial role in the progression of WAS. This process largely depends on the interactions between bacteria and archaea, which maintain the stability and function of AD through efficient electron transfer in a synergistic symbiotic community. Mediated interspecies electron transfer, which includes interspecies hydrogen transfer (IHT) and interspecies formate transfer, is an essential pathway for electron transfer. Specifically, IHT represents a more direct and readily accessible pathway for methanogenic bacteria to participate in AD reactions (Lovley 2017; Baek et al. 2018; Wang and Lee 2021). However, despite IHT being a typical mechanism in AD, DIET provides up to an 8.57-fold greater electron transfer rate than IHT. This significantly enhances the activity and efficiency of AD (Storck et al. 2016). DIET involves the direct transfer of electrons between specific microorganisms, which can be achieved by conductive structures within the microorganisms (e.g., conductive hyphae or specific proteins) or by introducing an external conductive medium (e.g., biochar) (Gahlot et al. 2020). In the promotion of DIET, the primary mechanism of action of biochar is its role as an electron conductor (Xu et al. 2022b). Microorganisms involved in DIET can directly attach to the surface of biochar, facilitating long-distance interspecies electron transfer. This not only enhances the efficiency of electron transfer but also optimizes the overall performance of AD of WAS.

Wang et al. (2021b) prepared iron-enriched biochar from Fenton sludge and successfully retained redox-active quinone/hydroquinone groups at suitable pyrolysis temperatures. The electron transfer capacity of biochar was positively correlated with the content of reducible functional groups. Therefore, optimizing the types of oxygen-containing functional groups on biochar is crucial for promoting DIET. Zhang and Wang (2020) loaded MnFe2O4 onto biochar and found that it was more effective than raw biochar at promoting methane production. Their results confirmed that MnFe2O4-biochar does not directly stimulate the activity of methanogenic bacteria but improves digestion performance by helping to establish DIET in the AD system, increasing methane yield. A study by Ren et al. (2020) confirmed that prepared sludge-based hydrochar effectively enhances hydrogenotrophic methanogenesis. They found that in this process, DIET converts H+, electrons, and CO2 into methane. Further investigations showed that the conductivity and total redox properties of hydrochar did not affect methane production, but the abundance of surface oxygen-containing functional groups had an effect. The coenzyme F420 is a unique enzyme cofactor utilized by methanogens as an electron transfer carrier that plays a vital role in the reduction of CO2. Lu et al. (2022) demonstrated that prepared Fe-biochar enhanced the activity and conductivity of coenzyme F420 in AD. Iron is an essential cofactor for many enzymes in methanogenic bacteria involved in AD. With a significant difference in redox potential between iron and biochar, numerous microgalvanic cells are formed in the digestion matrix, and chemical energy is converted into electrical energy, which lays the foundation for the DIET system.

Functional biochar can activate DIET and enhance methane production. However, certain metal-based biochars may have inhibitory effects on microbial methane generation. Therefore, it is essential to explore the physicochemical properties of various biochars and consider their adaptability under specific operational, substrate, and inoculation conditions. In the future, more research should be conducted on the key elements affecting electron transport efficiency, such as critical microorganisms and functional groups in DIET, to reveal the interactions of biochar with these elements.

5.2 pH stability

During AD, the pH stability of the environment significantly impacts degradation efficiency. The application of biochar has been proven to enhance the pH stability of AD systems. The three main reasons are the following. First, the ash fraction of biochar contains alkaline or alkaline earth metals, providing excellent pH stability. Chiappero et al. (2021) analyzed various biomass wastes (including cork, sludge, and rice husks) subjected to pyrolysis and activation treatments. They revealed that biochar derived from rice husk had the highest pH. This difference was primarily attributed to the high alkali content of the rice husks. They also found that high pH in biochar is often associated with high ash content and a low number of acidic functional groups. Second, the surface of biochar possesses alkaline functional groups, which can provide pH buffering through chemical reactions. Moreover, biochar has redox properties and can act as an electron donor or acceptor. Therefore, biochar can neutralize acid shock through chemical reactions, stabilizing the pH of the AD environment. In AD, excessive accumulation of VFAs leads to a decrease in the pH of systems. Under this condition, the alkaline functional groups on biochar (such as amino groups) can adsorb H+ and accept electrons to buffer the system. It has been reported that the organic functional groups on biochar (such as quinones, phenols, ketones, and phenazine) can react with VFAs to maintain the stability of systems and thus improve the efficiency of methane production (Jiang et al. 2022b; Wang et al. 2019). Interestingly, biochar treated with HNO3 oxidation may inhibit acid generation and methane fermentation. This could be due to the inhibitory compounds that are generated or dissolved after HNO3 oxidation, as well as the impact of HNO3/NO2 functional groups on pH (Jiang et al. 2022b). Finally, adding biochar promotes the enrichment and growth of useful microorganisms during the methane generation process, enhances the degradation of organic acids, and prevents the accumulation of VFAs from impacting system pH.

5.3 Microbial immobilization

The stable operation of AD heavily relies on the functional microbial community (Guo et al. 2015). Due to its unique pore structure, high surface area, and abundant functional groups, biochar offers an ideal environment for immobilizing active microorganisms. Biochar particle size is critical for determining its possible relationships with microorganisms involved in the AD process. Biochar particles may provide a habitat for microbial growth. Small-sized particles may result in improved microbial attachment. He et al. (2020) used biochar-biofilm consortia as a recyclable inoculant for the AD process. They found that biochar with particle sizes smaller than 5 μm was more effective at initiating methanogenesis than biochar with particle sizes larger than 1 mm. Additionally, 20 times more microorganisms could be enriched by the smaller biochar than the larger biochar. An et al. (2022) explored the potential impact of various characteristics of biochar on microbial community composition through Spearman’s correlation analysis. Specifically, Thermomonas and Bacillus were significantly positively correlated (p < 0.05) with specific capacitance (SC) and total PV but negatively correlated with average pore diameter. Moreover, Pseudomonas, an electroactive microorganism, positively correlated with SC. Most microorganisms showed a positive correlation with SSA, showing that a larger surface area provides more growth and attachment space for microorganisms.

As research has advanced, more studies have emphasized exploring how biochar enhances AD efficiency from the perspective of microbial community regulation (Zhang et al. 2022a). In particular, functional biochar has a well-developed pore structure and abundant oxygen-containing functional groups, providing more space for microbial attachment and growth and helping promote microbial community reproduction. Bu et al. (2022) proposed a mechanism for enhancing methane production using conductive biochar with redox potential. First, functional biochar rapidly optimizes the abundance of key microbial species at the beginning of the AD process. Second, functional biochar significantly increases the number of specific microorganisms present, such as Methanosarcina, Fermentimonas, Desulforhabdus, Longilinea, and Clostridiales. These microorganisms primarily participate in symbiotic metabolism, butyric acid decomposition, and direct methanogenesis (Chen et al. 2019b; Qin et al. 2020). In summary, biochar enhances methane production by creating distinct ecological niches, allowing microorganisms to position themselves selectively and more effectively and improving methane yield (Bu et al. 2022). Wang et al. (2021b) improved methane production in AD using magnetite-contained biochar and found that the increased methane production was attributed to the fact that iron oxides can enrich Fe(III)-reducing microorganisms capable of utilizing a wide range of organic matter, thereby accelerating the decomposition of complex organic matter through dissimilatory iron reduction, and thus providing a more significant amount of substrate for methanogenesis (Bird et al. 2011; Kato et al. 2012). Li et al. (2022a) showed that electroactive bacteria were enriched in magnetic Fe3O4 biofilms, and the existence of DIET was further empirically demonstrated by the study of the CO2 reduction pathway and the abundant presence of type IV bacterial appendage genes in electroactive bacteria. Similarly, Di et al. (2022) studied nano-Fe3O4 biochar and reported that its porous structure could result in the immobilization and enrichment of microorganisms. The material significantly increased the relative abundance of Methanomicrobia and Firmicutes by 52.62–87.70% and 28.67–54.44%, respectively. According to Li et al. (2022b), the SSA of biochar with multiple functional groups and a graphene structure did not affect microbial enrichment. However, the surface pH of acid or alkali treated coffee grounds biochar prepared at 700℃ is unfavorable for microbial enrichment (Li et al. 2022b). Furthermore, the enrichment of anaerobic microorganisms might be influenced by the large pore structure on the biochar surface and by the biochar ZP.

However, the conditions and mechanisms of the pore structure and functional groups of functional biochar on microbial immobilization as well as microbial community interactions remain clear. Future studies need to investigate microbial community characteristics in conjunction with metagenomics to develop functionalized biochar that can immobilize target microorganisms and regulate microbial community changes to improve AD efficiency.

5.4 Alleviation of inhibition

During AD treatment of WAS, interference from inhibitors such as heavy metals (e.g., Cd2+, Ni2+, Cu2+, Cr3+, Zn2+) (Zhou et al. 2021), organic compounds (e.g., pesticides, antibiotics, VFAs) (Xiang et al. 2023), AN, and sulfides (Li et al. 2020a) are commonly encountered. It is widely believed that the concentrations of VFAs, AN, and sulfides are the primary factors affecting the efficiency of AD treatment. However, insufficient attention has been given to heavy metals, pesticides, and antibiotics. Excessive amounts of these substances could become significant factors limiting the AD of WAS. This section primarily focuses on the mechanisms by which functionalized biochar mitigates AD inhibition by heavy metals and pharmaceutical pollutants.

Heavy metals in wastewater treatment are subject to migration and enrichment during treatment. Although AD can alter their state, the risk of toxicity remains. High concentrations of heavy metals have toxic effects on microorganisms, which can substitute for ions in biological enzyme systems, leading to a loss of enzyme activity. According to a study by Ruan et al. (2023), heavy metal immobilization in dyeing sludge improved, as did AD efficiency, with magnetic biochar application. This can be attributed to the Fe3O4 loaded on the surface of the magnetic biochar; the iron could exchange with heavy metal ions. Zhang and Wang (2020) showed that MnFe2O4-biochar enhances methane production compared to raw biochar at immobilizing heavy metals and reducing the inhibition of ammonium in the AD system. The primary reasons for the reduced bioavailability of heavy metals in the presence of MnFe2O4-biochar might be precipitation, adsorption, reduction, and the formation of metal chelates with functional groups on the biochar. MnFe2O4-modified biochar enhances the ability of biochar to immobilize heavy metals by increasing the number of active sites and functional groups present. Similarly, MnO-modified biochar prepared by Li et al. (2019) had a large surface area and negatively charged functional groups, which could adsorb heavy metals and form chelates. Moreover, MnO has positively charged mineral surfaces and facilitates the effective removal and oxidation of heavy metals in sludge.

Pharmaceutical pollutants typically exhibit both biotic and abiotic toxicity and can inhibit microbial activity and even lead to growth inhibition or death. Fe3O4-modified biochar has been demonstrated to enhance methane yield in the presence of erythromycin (Zhang et al. 2023c). Good AD performance in systems with high concentrations of Fe3O4-modified biochar can be explained by the presence of the dominant bacterial groups Lentimicrobium sp. and Methanosarcina sp. compared to blank groups. This finding suggests that these materials have outstanding detoxifying effects under erythromycin stress. Additionally, Fe3O4-modified biochar effectively reduced the abundance of representative antibiotic resistance genes (ARGs), mitigating environmental risks. Xu et al. (2022a) reported that biochar with large SSA and a well-developed pore structure can effectively adsorb ARGs, providing a good survival environment for microorganisms. Furthermore, phenolic compounds and organic acids released from biochar might partially inhibit microbial activity. Therefore, reduced microbial activity could further decrease microbial contact, limit the exchange of genetic elements, and consequently minimize gene transfer in bacteria carrying ARGs. Dong et al. (2023) successfully implanted various heteroatomic nitrogen and sulfur species into lignin biochar to degrade 4-nonylphenol (4-NP) in WAS. Synergistic effects of pyrrole-N, thiophene-S, and lattice oxygen were found for the degradation of 4-NP through the activation of hydroxyl radicals generated by hydrogen peroxide and singlet oxygen.

6 Carbon neutralization during anaerobic sludge digestion

In the realm of AD for sludge treatment, biochar not only paves the way for achieving carbon balance but also presents new challenges and opportunities. These challenges and opportunities encompass various facets of innovation and improvement, offering a complex and promising research landscape. The introduction of biochar into the AD process of WAS reflects the concept of using “waste to control waste,” in which organic waste is converted into a stable carbon source to improve the resource utilization efficiency of sludge. In this process, the unique porous structure and surface activity of biochar provide microorganisms with habitat sites and enhance their ability to decompose organic matter, improving the efficiency of AD and presenting new possibilities for optimizing the energy supply chain. Biochar also has significant potential for environmental remediation. Its stable carbon structure effectively sequesters a substantial amount of carbon, reducing carbon dioxide emissions, realizing carbon emission reduction, and helping to achieving carbon neutrality. Furthermore, biochar can adsorb harmful substances, such as heavy metals, in WAS, mitigating environmental risks in wastewater treatment. Therefore, biochar exhibits substantial environmental advantages and economic value in sludge treatment via AD. The following sections detail specific approaches to achieve carbon neutralization in the AD of WAS.

6.1 Resource utilization after sludge treatment

After AD treatment, sludge volume can be reduced to 30–50% of its original size, enhancing dewatering and facilitating the separation of solid and liquid components. This treatment method not only increases the stability of the sludge and reduces associated odors but also effectively results in the recovery and utilization of valuable resources such as VFAs and CH4. This provides a new way to reduce organic waste and stabilize sludge and plays a crucial role in lowering biotoxicity, opening a new pathway for the utilization of resources present in sludge.

6.1.1 Utilization of resources in fermentation liquid

Recently, VFAs generated from the AD of WAS have received increasing attention from industry and academia due to their substantial potential to produce biodegradable plastics and for carbon source applications in the biological treatment of wastewater. Parchami et al. (2020) attempted to replace exogenous organic matter in the denitrification stage of WWTPs by using VFAs obtained from the AD of WAS as an added carbon source and evaluated the benefits and costs. Their results showed that replacing 300 kg of methanol per hour with VFAs can save up to 140 euros and lead to effective resource recovery and energy substitution, and the overall treatment efficiency was improved.

Polyhydroxyalkanoates (PHAs) are polymers that store carbon and energy in various microorganisms. Due to their biodegradable properties and because they are sources of renewable resources, PHAs have been viewed as biomaterials with the potential to replace traditional plastics. However, high production costs are an obstacle to their widespread use. Using VFAs as a carbon source to produce PHAs is a practical approach. Therefore, using sludge AD instead of synthetic feedstock has become a popular research topic for reducing the production cost of PHAs. Crutchik et al. (2020) demonstrated the economic feasibility of adjusting wastewater treatment processes to produce PHAs. They proposed directly utilizing 25% secondary sludge for PHA accumulation, which could reduce costs by approximately 15.90–19.00%. However, WAS contains many proteins and large organic molecules, which lead to microbial inhibition by ammonia. This process slows the hydrolysis of organic matter, affecting the efficiency of AD. Functional biochar can enhance microbial electron transfer capacity, promote microbial diversity, reduce microbial inhibition by ammonia, and increase VFA production. Zhai et al. (2020) suggested that converting WAS into biochar rich in functional groups and applying sludge biochar to AD enable the utilization of resources from two materials, significantly boosting VFA production.

6.1.2 Soil amendment

The land application of organic waste has been a popular research topic. Compared with traditional inorganic fertilizers, bioorganic fertilizers can significantly improve the physical and chemical properties of soil and enhance soil water retention capacity, fertility, and permeability. In particular, the digestate from AD of WAS is rich in organic matter and includes a variety of essential plant nutrients; as a result, it presents substantial potential for energy recovery and resource reuse. The study (Tambone et al. 2010) found that the digestate generated from the AD of sludge is a high-quality fertilizer containing essential nutrients such as N, P, and K that plants easily absorb. Therefore, digestate is an excellent alternative to inorganic fertilizers and is effective at promoting the short-term recycling of soil organic matter. The other (Chavez-Rico et al. 2022) indicated that, compared to composting, AD retained C, N, and P in the byproducts more efficiently and increased the concentration and availability of these nutrients in the water-soluble fraction. This contributed to the growth of soil microbial biomass, enhancing soil carbon stability. However, due to the potential presence of harmful components such as heavy metals in digestate, land application may threaten environmental safety and human health. There is a lack of information on the degree of land contamination from biochar application. Therefore, research on digestate safety is crucial. Several studies [e.g., (Ruan et al. 2023; Li et al. 2019; Zhang et al. 2019)] have demonstrated that adding functional biochar to AD of WAS can effectively immobilize heavy metals and reduce environmental risks. Athanasios et al. (Balidakis et al. 2023) further assessed the effectiveness of sludge amended with biochar as a soil amendment. Research has shown that adding biochar significantly improved soil organic matter and fertility. Additionally, biochar improved the levels of N and P nutrients and micronutrients, such as Cu and Zn, in soil. Most importantly, the use of biochar allows the availability of heavy metals in sludge to be reduced in amended soil, ensuring its safety for agricultural applications.

6.2 Optimization of circular economy in sludge anaerobic digestion

AD of WAS can generate biomass energy, which is, to some degree, a form of resource recovery. However, the current linear economy model is overly simplistic, lacks sustainability, and poses risks of resource wastage and environmental damage from waste. Future research solutions will undoubtedly focus on a circular economic model in which organic waste is seen as a viable resource. This approach can meet the energy needs of communities and industries, improve waste management efficiency, and increase the effective utilization of resources. For example, through the development of strategies such as biomass energy, circular agriculture, and waste recycling, “wastes” are transformed into energy or other valuable products to maximize the reduction, resourcing, and safety of wastes, driving sustainable development in the economic, societal, and environmental realms.

Jiang-flavour Baijiu is taken as an example. The Baijiu industry is essential in Guizhou Province and the Chishui River Basin. However, the substantial wastewater generated during the brewing process and the resulting WAS pose significant environmental challenges. These sludges account for 3% of the wastewater volume, and it is projected that by 2025, the amount of sludge generated from the treatment of distillery wastewater in Guizhou Province will reach 250,000 tons. In this context, treating the sludge as a resource to achieve efficient wastewater treatment in the Jiang-flavour Baijiu industry is particularly important. Figure 5 shows a concept model of waste resource conversion based on functional biochar applicable in the Baijiu brewing industry. This model is conducive to ensuring resource use and energy recovery from WAS and is also suitable for use with digestate resources. The use of biochar in this model includes straw, distiller’s grains, and WAS, and the average cost of biochar feedstock is $59.44 per ton, considering all expenditures. Meanwhile, the market price of biochar ranges from $120 to $480 per ton (Singh et al. 2022). AD of the remaining WAS coupled with biochar produces about 296.0 Nm3 t−1 of dry sludge of methane, which provides energy for the workshop and WWTPs. AD of WAS produces a fertilizer substitute of about 30.05 kg t−1 dry sludge that can improve the soil for growing wheat and sorghum (Huang et al. 2023). Recycling methods allow for the utilization of by-products of the processes employed, thereby minimizing or eliminating waste. This means that the circular economy model of anaerobically digested sludge can be effectively integrated with many societal production activities and generate significant economic benefits. Of course, safety testing and certification are prerequisites for the use of digestate, which need to be fully verified in practical applications. In brief, an innovative circular economy model can solve problems related to secondary pollution by applying digestate as a fertilizer and making biogas production more environmentally friendly and sustainable.

Fig. 5
figure 5

Circular economy model for the Baijiu industry under biochar treatment and anaerobic digestion

Full size image

7 Future perspectives

Solid waste management provides scientific and technological solutions for achieving carbon neutrality, green development, and a circular economy. WAS has attracted widespread attention because of its potential energy and resource value. Strategies for dealing with WAS should follow the core concepts of reduction, stabilization, safety, and resource reuse. AD efficiently recovers energy resources from WAS and stabilizes the pollutants contained therein. Biochar is regarded as an ideal additive for enhancing the efficiency of AD of WAS due to its substantial potential. However, biochar does not have consistent effects as its functionality and performance vary depending on its properties. Therefore, it is crucial to explore the functionalization of biochar to support the AD process better.

  1. a.

    In optimizing the selection of biochar feedstock and preparation methods, researchers could leverage technologies such as machine learning to fine tune the biochar production process. This approach helps achieve cost savings and enhances production efficiency and the quality of biochar, catering to various application requirements.

  2. b.

    The mechanism by which functional biochar enhances the AD of WAS has not been fully elucidated. It is crucial to gain insight into the relationship and contribution of the characteristics of functional biochar (e.g., electronegativity, functional groups, SSA, etc.) to the facilitation of electron transfer, pH stability, and microbial immobilization in sludge AD with the help of surface chemistry, electrical signal detection techniques, and metagenomics. This will reveal the critical biochar factors affecting the AD of WAS process and provide a theoretical basis for optimizing the design and application of functional biochar.

  3. c.

    Functional biochar improves the synergy of sludge and kitchen waste codigestion. However, due to the diversity of waste compositions and reaction pathways/thermodynamics, the effect of biochar on the codigestion of sludge with various other wastes should be further investigated to promote the commercialization and widespread application of this technology.

  4. d.

    There is extensive literature on the stable and effective operation of laboratory-scale AD reactors for WAS amended with functional biochar. However, research at the pilot and industrial scales has received less attention, which has limited the widespread application of this technology in the field. Therefore, there is a need for further research and an in-depth understanding of the performance of functional biochar in large-scale treatment, taking into consideration both practical challenges and economic factors.

  5. e.

    The application of digestate, a byproduct of functional biochar-mediated AD of WAS, to agricultural production requires a comprehensive risk assessment. Although previous studies have shown that digestate application significantly improves soil conditions and that biochar effectively immobilizes heavy metals in digestate, the ecological risk assessment of digestate in agricultural production has not been adequately investigated.

  6. f.

    Biochar and AD process performance can be further optimized through life cycle analysis, environmental safety, and energy and material balance assessments. This type of optimization can ensure that biochar can contribute to achieving sustainability goals within the context of a circular economy framework and practical applications in the AD of WAS.

8 Conclusions

This review details the preparation of functional biochar and provides insights into the role of functional biochar in enhancing the AD of WAS and related mechanisms. These findings emphasize the benefits of functional biochar in reducing intermediate metabolite accumulation, increasing organic degradation, and improving methane production. The promoting effect of biochar on AD was mainly attributed to its electron transfer capacity, buffering effect, favorable growth environment provision, alleviation of impacts from inhibitory substances, and promotion of functional microbial colonization. Moreover, the value of functional biochar in utilizing digestate, removing pollutants, and fixing carbon was highlighted. More importantly, a sludge energy recovery model based on functional biochar was proposed, demonstrating the potential to achieve circular economy goals and environmental sustainability in practical engineering applications. Future research should focus on optimizing the preparation of functional biochar and the large-scale application of AD of WAS through technical and economic analysis, feedstock and environmental risk assessment, and whole life cycle analysis. Responding to the need for sustainable development strategies, this review provides essential references and insights for applying functional biochar in AD of WAS, offering new ideas for sustainable development in energy, environment, and agriculture.