Dual-path exploration of anaerobic biotechnology under carbon neutrality goals: from wastewater methane production to systematic utilization of renewable energy

Abstract

This study investigates the application of anaerobic biotechnology in wastewater treatment and resource recovery within the carbon neutrality framework. It systematically elucidates the complete technological chain, from methane production to renewable energy utilization. In the context of intensifying global climate change, carbon neutrality has emerged as a critical strategic objective worldwide. The energy-intensive and high-carbon wastewater treatment sector has become a key focus for emission reduction. Distinct from conventional processes plagued by high energy consumption and carbon emissions, anaerobic biotechnology is garnering increasing attention due to its triple advantages in energy recovery, resource utilization, and emission reduction. By employing anaerobic microorganisms to convert organic matter into methane and other renewable energy sources, this technology not only reduces CO₂ emissions but also provides energy supplementation for wastewater treatment facilities. The research further examines specific applications of anaerobic biotechnology in methane synthesis, nutrient recovery from wastewater, and sludge treatment, along with synergistic mechanisms with other renewable energy technologies. While demonstrating significant potential, technology still confronts multifaceted challenges including process optimization, cost management, and social acceptance. Findings confirm that anaerobic biotechnology serves as a vital pathway for low-carbon transformation in wastewater treatment, offering multidimensional value for establishing circular economy systems and advancing sustainable development.

1 Introduction

The exacerbation of global climate change has made carbon neutrality a key strategic objective in several countries (Jin et al., 2021). Besides the reduction of greenhouse gas emissions, carbon neutrality requires the optimization of the energy mix and the sustainable use of resources (Springmann et al., 2018; Xu et al., 2024). The path to achieve carbon neutrality is complex and varied, involving not only the green transformation of the energy industry but also technological innovations in emission control and reduction in various high-emission industries (Abeydeera et al., 2019). Wastewater treatment, as a high-energy-consuming and high-emission industry, has become one of the key areas for reducing global emissions (Zhang et al., 2011; Górka et al., 2022). Efficient reduction of carbon emissions and achieving sustainable use of resources in the wastewater treatment process have become important issues that urgently need to be addressed (Xie et al., 2025; Zhao et al., 2021). While significant progress has been made in treating wastewater pollutants, conventional wastewater treatment processes are inherently energy intensive and produce large amounts of carbon dioxide and other greenhouse gases during the degradation of organic matter (Müller et al., 2022). Therefore, reducing emissions, reducing energy consumption and realizing resource recovery in the wastewater treatment process have become important issues in the context of carbon neutrality.

Among the wastewater treatment technologies, anaerobic digestion is one of the classic and evolving technologies with feasibility and reference, as well as its advantages. Wastewater treatment technology, as an advanced approach, is gaining attention from researchers and policymakers due to its potential advantages in energy recovery, resource utilization and emission reduction (He et al., 2017; Cheng et al., 2011; Plevri and Noutsopoulos, 2021). The core of anaerobic digestion technology lies in the utilization of anaerobic microbial communities for the degradation of organic matter and the generation of renewable energy sources such as methane through anaerobic digestion processes (Costanzo et al., 2021; Negri et al., 2020). Compared to traditional aerobic wastewater treatment technologies, anaerobic technologies can significantly reduce carbon dioxide emissions during the degradation of organic matter (Müller et al., 2022). Additionally, anaerobic technology can convert organic matter in wastewater into methane, reducing greenhouse gas emissions and providing a potential energy source for wastewater treatment plants, shifting the wastewater treatment process from energy consumption to energy production (Xu et al., 2024). This energy recovery and utilization align with the circular economy model advocated in the carbon-neutral strategy, which can effectively reduce the dependence on fossil energy in the wastewater treatment process.

This study is dedicated to the systematic investigation of the potential application of anaerobic technology in wastewater treatment and resource utilization and its environmental benefits. The second part of the paper focuses on the core aspects of the anaerobic digestion process, and analyzes its carbon neutralization pathway and carbon emission characteristics in depth; followed by a systematic overview of the mainstream anaerobic technologies in the third part of the paper, which lays the foundation for the subsequent analysis. Based on the clarification of the technical principles and carbon accounting methods, the study focuses on the specific strategies and effectiveness of anaerobic technologies for resource (e.g., energy and nutrient) recovery in wastewater treatment in Part IV, and explores their practice and potential as a renewable energy production pathway in Part V. The study also focuses on the carbon neutralization pathway and carbon emission characteristics of anaerobic digestion processes. By comparatively analyzing the significant carbon footprint of conventional treatment processes, this study aims to quantitatively reveal the unique carbon reduction advantages of anaerobic technology. Finally, the sixth section validates the effectiveness of the technology through a typical case study, while the seventh section rationally looks at the key challenges and directions for the future application of the technology.

2 Carbon neutralization and carbon emissions in anaerobic digestion of wastewater

2.1 Concepts and objectives of carbon neutrality

Carbon neutrality refers to the process of achieving net-zero carbon emissions by reducing carbon emissions and enhancing carbon sequestration. Rapid economic development and accelerated urbanization are not conducive to reducing carbon emissions. As a major agricultural country, China’s agricultural development has significantly contributed to the national economy; however, it has also been accompanied by an increase in greenhouse gases, including carbon dioxide and methane (Xu et al., 2024; Liu et al., 2024). Therefore, reducing energy consumption intensity, adjusting the industrial structure, and improving the economic policy framework are essential strategies for promoting the development of China’s low-carbon economy (Jin et al., 2021; Ma et al., 2019).

With advances in industrial energy technology, wastewater is no longer regarded as wastes; instead, many wastewater treatment plants now consider it a source of energy, nutrients, and clean water (Zhang et al., 2011; Fang et al., 2016; Wang et al., 2016). Achieving global carbon neutrality necessitates significant emission reductions across all sectors, and the wastewater treatment industry plays a crucial role in this effort due to its high energy consumption and substantial carbon emissions. The primary sources of carbon emissions from wastewater treatment plants mainly include microbial respiration and biogas combustion (Wang et al., 2016). The utilization of methane and carbon dioxide as energy sources in wastewater treatment through enhanced processes and the adoption of innovative technologies can significantly reduce greenhouse gas emissions and contribute to the global goals of carbon neutrality.

2.2 Carbon emission analysis of wastewater treatment processes in anaerobic technology

Anaerobic digestion is an energy-efficient option for reducing greenhouse gas emissions and recovering biofuels from waste materials. Greenhouse gas emissions from wastewater treatment primarily arise from the methane (CH₄) and carbon dioxide (CO2) produced during anaerobic digestion. Methane is the second most significant greenhouse gas, with a global warming potential (GWP) value of 25, and it has a considerable impact on global climate change (Wang et al., 2025). If these gases are not effectively treated, they are released directly into the atmosphere, leading to severe environmental consequences. Methane is a potent greenhouse gas, second only to carbon dioxide, and serves as a precursor to ozone (O₃) (Mar et al., 2022). Methane has a 100-year global warming potential (GWP100) of 27.9, i.e., each kilogram of methane generates 27.9 times more radiative forcing than 1 kg of carbon dioxide over a 100-year period. Although methane itself is not classified as an air pollutant that poses direct hazards to human health, it significantly contributes to air pollution as a precursor to tropospheric ozone. Therefore, it is possible to utilize the methane produced by anaerobic digestion both as an energy source and to reduce methane emissions while reusing wastewater. Carbon dioxide, a major greenhouse gas, is associated with various environmental issues, including global warming. Carbon footprint assessment methods, such as life cycle assessment (LCA) (Müller et al., 2022), enable the quantification of carbon emissions from wastewater treatment processes and provide a scientific foundation for implementing carbon reduction strategies.

2.3 Role of anaerobic biotechnology in carbon reduction

Anaerobic biotechnology can efficiently produce methane, which can be converted into renewable energy, thereby reducing carbon emissions (Yi et al., 2024). In addition, the carbon dioxide generated during anaerobic digestion can also be utilized through carbon capture technology, further minimizing the carbon footprint of the entire treatment process. Figure 1 can indicate the distribution of literature on carbon-neutral and carbon-anaerobic processes in recent years, clearly reflecting the trends in this research area. The keywords associated with these topics are mainly focused on anaerobic digestion, wastewater treatment and carbon dioxide. The occurrence of the terms anaerobic digestion and wastewater treatment indicates that carbon neutrality is closely related to these technologies and is the key to achieving carbon neutrality. The high frequency of these keywords highlights the importance of anaerobic digestion technology and its potential application in academia. Notably, the reuse of carbon dioxide generated during anaerobic digestion is regarded as a significant contributor to achieving carbon neutrality goals. This approach not only aids in the reduction of greenhouse gas emissions but also offers new perspectives and solutions for sustainable development and further promotes the efficient recycling of resources. Table 1 demonstrates the key parameters and performance of anaerobic technologies for wastewater treatment. Cruz-Salomón et al. (Wu et al., 2024) studied an expanded granular sludge bed (EGSB) reactor, which was operated at a buffer index of 0.23 ± 0.1, a pH of 7.22 ± 0.4, and a temperature of 26.6°C ± 1.4°C, The reactor successfully treated cheese whey wastewater, achieving a biochemical methane potential (BMP) of 92% and a chemical oxygen demand (COD) removal rate of up to 90%. This technology demonstrated significant efficiency in treating wastewater with high organic loads and contributed to enhanced methane recovery. In the field of wastewater treatment, achieving “carbon neutrality” refers to optimizing processes and resource recovery so that carbon emissions from the treatment process are equal to the amount of carbon recovered or offset, with net emissions converging to zero. Anaerobic digestion can convert organic matter into methane for energy recovery, significantly reducing direct carbon emissions. However, the actual realization of carbon neutrality still faces multiple challenges, including the efficient recovery and utilization of methane, the resourcing of sludge and residues, the control of indirect carbon emissions such as energy and pharmaceuticals, and the potential fugitive emissions of greenhouse gases. Currently, some advanced wastewater plants are close to carbon neutrality through process integration and energy efficiency improvement, but further breakthroughs in technology and management are needed for large-scale popularization. Therefore, anaerobic technology provides a powerful path to carbon neutrality in wastewater treatment, but full realization still requires system synergy and continuous innovation.

FIGURE 1

3 Overview of anaerobic technology

3.1 Basic principles and process flow

Anaerobic digestion is a process in which microorganisms break organic matter into methane and carbon dioxide under anaerobic conditions (Cruz-Salomón et al., 2020; Wang et al., 2024). Table 2 illustrates the anaerobic digestion process, which consists of four distinct steps, each carried out by different groups of microorganisms that produce various products. The first step is hydrolysis, which involves the breakdown of macromolecular organic matter into simple compounds that are more readily absorbed by anaerobic microorganisms. During acid production, the hydrolyzed compounds are further degraded into organic acids, which are subsequently converted to acetic acid, the main source of energy for anaerobic methanogenic bacteria, in the process of acetogenesis. Methanogenic bacteria produce gaseous metabolites in this step, completing the final step of methane generation. The step-by-step conversion process of anaerobic digestion of wastewater includes hydrolysis, acid production, acetic acid production and methane production, and each stage interacts with the other stages to jointly realize the degradation of organic matter and energy recovery.

Many countries utilize anaerobic digestion (AD) to treat wastewater and increase methane production, thereby improving economic viability (Gan et al., 2025). For example, in Malaysia, anaerobic digestion (AD) is an energy recovery method that can be used as an alternative to conventional sludge treatment technologies such as landfill and incineration. Given the large amount of food waste and sludge generated in Malaysia, anaerobic digestion not only significantly improves the efficiency of wastewater treatment, but also effectively addresses the growing food waste problem (Hanum et al., 2019). Brazil is a large tropical agricultural country with abundant natural resources. Researchers have studied the anaerobic digestion of cassava processing wastewater in Brazil to determine the optimal conditions for the production of biogas (especially carbon dioxide and methane) in order to maximize the economic benefits. Experimental results showed that a cassava wastewater to primary sludge ratio of 4:1 (i.e., 80% cassava wastewater and 20% primary sludge) resulted in the shortest fermentation time and highest methane content (Peres et al., 2019).

3.2 Technology classification

Figure 2 illustrates the growth in the number of citations for anaerobic digestion in publications over the past decade, indicating an increasing interest from the scientific community in this topic. Anaerobic biotechnology (AD) can be categorized into single-stage anaerobic digestion and multistage anaerobic digestion, depending on the process design. Single-stage anaerobic digestion refers to the scenario where all digestion reactions occur within the same reactor, while multistage anaerobic digestion involves different digestion steps carried out in multiple reactors (Van et al., 2020). Due to its operational simplicity and low cost, single-stage anaerobic digestion has been widely adopted worldwide (Jang et al., 2014). However, advancements in science and technology have led to a better understanding of the optimal growth conditions required for various digestion reactions. Although single-stage anaerobic digestion is effective in treating a wide range of wastewater and generating biogas, thereby reducing fossil energy consumption and carbon emissions, this technology may encounter challenges such as process instability and acidification (Shamurad et al., 2020).

FIGURE 2

Anaerobic digesters can be classified into dry and wet types based on total solids (TS) content (Van et al., 2020). Dry digesters are appropriate for feedstocks with TS ≥20% and are further categorized into horizontal plug-flow, vertical plug-flow, and no-flow (batch) types. In contrast, wet digesters are appropriate for feedstocks with TS ≤15% and are commonly used in wastewater treatment. Table 3 shows a comparison between single-stage anaerobic digestion and multistage anaerobic digestion. However, acidification during single-stage digestion poses challenges that may impact its operational efficiency and stability. Multistage anaerobic digestion technology enhances solid removal and methane production during wastewater treatment by physically separating the digestion steps to optimize the reaction conditions at each stage. Multistage digestion systems typically include two- or three-stages. Two-stage systems carry out hydrolysis and methanogenesis in two separate reactors, whereas three-stage systems perform hydrolysis, acid/acetone production and methanogenesis in distinct reactors (Van et al., 2020). A two-stage system optimizes hydrolysis and acid production separately from methanogenesis, speeding up the overall digestion rate and offering numerous advantages. In a two-stage anaerobic digestion system, methanogenic bacteria thrive in an optimal environment, inhibiting the growth of other microorganisms and making the system more stable and adaptable to changes in wastewater composition. Additionally, the gas generated during hydrolysis/acidification can be directly removed from the gas collection system without further treatment, significantly reducing methane enrichment costs. Most studies indicate that two-stage systems provide superior treatment compared to single-stage systems. However, when the feedstock solids content is low (TS <3%), a two-stage system is not significantly more efficient than a single-stage system. Therefore, single-stage anaerobic digestion is more suitable for simple wastewater treatment scenarios, while multistage anaerobic digestion is ideal for treating complex wastewaters and is more effective in resource recovery. These technologies have been widely implemented in wastewater treatment plants globally, demonstrating positive environmental and economic benefits.

3.3 Technical advantages and limitations

Nevertheless, anaerobic digestion (AD) technology encounters several challenges in practical applications. First, the start-up process is lengthy due to the slow growth rate of anaerobic microorganisms; the acceleration of the start-up speed is contingent upon the sludge inoculum in the system reaching a specific concentration. Second, the operation and management of the AD system are more complex, and maintaining the microbial balance of the system is challenging because of the varying characteristics and physiological traits of the different bacteria present in the reaction system. An excessive organic matter loading rate can inhibit the activity of methanogenic bacteria and disrupt the microbial equilibrium, potentially leading to system failure. In addition, sludge AD technology faces other issues, such as intricate processes, high equipment investment costs, and substantial operational and management requirements. Therefore, optimizing and enhancing AD technology is an urgent matter that must be addressed, and these challenges can be tackled through technological innovations and process optimization (Chen W. et al., 2023).

4 Anaerobic technology for resource utilization in wastewater treatment

Methane can be produced and utilized in a number of ways and optimized, and optimizing the design of anaerobic digestion can significantly improve methane production and purity. Meanwhile, as human activities and resource consumption increase, nutrient recycling is essential for sustainable development and environmental protection. In wastewater treatment, nitrogen and phosphorus are the main causes of eutrophication in water bodies, and anaerobic biotechnology can both reduce pollution and recycle resources.

4.1 Methane generation

Figure 3 Breakdown of conventional methane generation methods (right) and front-end novel methane generation methods (left), which provides a more intuitive view of the forward-looking nature of the industry involved. Methane generation is one of the core processes of anaerobic digestion, and by optimizing the design of this process, both the yield and purity of methane can be significantly enhanced. The collected biogas is purified and treated for both power generation and transportation fuel as an alternative to natural gas, resulting in more efficient resources utilization. It is important to note that endogenous cross-linked EPS structures inhibit hydrolysis and electron transfer during methanogenesis. Recent studies have shown that humification of EPS and protein-polyphenol interactions can greatly hinder substrate accessibility and microbial activity (Tang et al., 2020; Tang et al., 2018; Tang et al., 2023). The current focus on biomethane ignores two key constraints: 1. Low energy recovery efficiency: Typically only 10%–14% (0.2 kWh/m³) of the influent chemical energy is converted to electricity through a cogeneration system (Zeng et al., 2022). 2. Purification challenges: Fluctuating biogas composition complicates commercial upgrading. Whereas in methane production, valorizing liquid/solid residues as sustainable fertilizers or platform bioproducts (e.g., plant growth biostimulants, aromatic amino acids) can yield higher economic and environmental returns (Tang et al., 2022a; Tang et al., 2022b). To maximize energy use and enhance methane production, anaerobic digestion can be applied to a single material (monodigestion) or a mixture of multiple materials (mixed digestion or co-digestion). Anaerobic co-digestion enhances the digestion process and energy production by increasing the nutrients available to microorganisms and increasing the organic load while reducing the effects of inhibitory chemical toxicity through co-substrate dilution. In small biogas production facilities, animal manure is usually treated by single digestion, but co-digestion is commonly used in larger facilities that treat biowaste from different sources (e.g., farm, residential, and industrial). Studies have shown that co-digestion improves the stability of digestion when different feedstocks are digested simultaneously in the same reactor. Traditionally, anaerobic digestion techniques have been applied mostly to single feed materials, but in recent years, a growing body of research has shown that the co-digestion of multiple substrates makes the anaerobic digestion process more stable. Compared with single-substrate digestion, the co-digestion of multiple substrates significantly increases the biogas production potential (Lee et al., 2020). A summary of experimental parameters and research progress on the methane production performance of anaerobic digestion of various organic wastes is presented in Table 4. On the other hand, Lu et al. (2021) conducted a 25-day experiment at a constant temperature of 35°C ± 1°C to study the enhancement of biomethane production from waste activated sludge (WAS) in anaerobic digestion. They reported that the biochemical methane potential increased by 28.1% with the addition of 0.3 g/L choline. This finding suggests that the use of choline can significantly increase methane production and provides a new pathway to optimize the energy-based treatment of waste. Darja and Goršek (2020) studied the anaerobic fermentation of mango grass and chicken manure mixtures (CMSs) under different temperature and mass ratio conditions. Their results revealed that the methane production of CMS was closely related to the fermentation temperature and mass ratio of the mixture, suggesting that the optimal design of the temperature and mass ratio in anaerobic fermentation has a significant effect on enhancing methane production. These studies provide different paths and ideas to further enhance the production efficiency of renewable energy.

FIGURE 3

4.2 Recovery and removal of nutrients from wastewater

Nutrients such as nitrogen and phosphorus contribute to eutrophication issues in aquatic ecosystems (Xie et al., 2024; Zhang et al., 2020). Anaerobic biotechnology not only addresses the issue of nitrogen and phosphorus pollution in wastewater but also recycles these nutrients through specialized techniques (Xie et al., 2025; He et al., 2017; Zhao et al., 2023). These recovered nutrients can be utilized in agricultural production as fertilizers or transformed into high-value products through additional processing, thereby maximizing the resource potential of Fang et al. (2016), Li JY. et al. (2024). As human activities and resource consumption continue to rise, nutrient recycling has become increasingly vital.

Table 5 summarizes the advancements in anaerobic biotechnology for nutrient recovery from wastewater using various raw materials. Xu et al. (2021) investigated anaerobic treatment with activated sludge, which lasted for 430 days at 31°C. This process achieved a phosphorus removal rate of 73.1% ± 6.6% of influent water and an impressive nitrogen removal efficiency of 87.8% ± 1.7%, highlighting the advantages of anaerobic processes in nutrient removal. Li JY. et al. (2024) investigated the treatment of anaerobic ammonia oxidation sludge using synthetic digestate wastewater, with the pH adjusted to 7.9 ± 0.2, and successfully achieved simultaneous nitrogen removal and phosphorus recovery, with 88.5% nitrogen removal and 83.8% phosphorus removal after 216 days of experimentation, thus demonstrating the benefits of combining anaerobic ammonia oxidation with magnesium phosphate crystallization.

4.3 Sludge treatment and resource utilization

Global sewage sludge production is rapidly growing due to the increasing demand for wastewater treatment. Effective sludge management is therefore essential for the economic and environmental sustainability of wastewater treatment plants. Recovering nutrients from sludge has been identified as a critical step in transitioning from a linear economy to a circular economy by converting sludge into an economically sustainable source of materials (Wang et al., 2017). Typically, sludge is treated through anaerobic digestion, which facilitates energy recovery through the production of biogas (Costanzo et al., 2021). Anaerobic digestion not only effectively degrades pollutants but also stabilizes and reduces sludge volume, thereby recycling resources and energy. Studies have shown that sludge has a high organic matter content, usually ranging from 35% to 52.7%, with a C/N ratio of 11.8, indicating that organic matter mineralization begins shortly after sludge digestion, making nitrogen nutrients more readily available to crops (Boumaleka et al., 2019). For example, anaerobic digestion removed 41.38, 62.26, and 68.68% of heavy metals, nutrients, persistent organic pollutants (POPs), such as polycyclic aromatic hydrocarbons (PAHs). and antibiotics and disinfection byproducts, respectively, from sludge in a sludge treatment plant in Anhui Province, China (Guo et al., 2021). The results of these studies indicate that anaerobic digestion not only enables resource recovery but also effectively reduces the emission of hazardous substances and contributes to the environmental sustainability of the wastewater treatment process (Liu et al., 2018). Table 6 summarizes the research advances in anaerobic digestion technology for sludge treatment and resource recovery. Garlicka et al. (2020) investigated the treatment of thickened primary sludge (TPS) at 36°C under different energy density conditions and reported that the energy input significantly affects methane production during anaerobic digestion. Nites et al. (2024) investigated the addition of various alkaline materials (NaOH, Ca(OH)₂, etc.) to the digestion of waste activated sludge over a period of 60 days and reported that the optimization of input variables such as pH had a significant effect on enhancing methane production.

5 Utilization of renewable energy sources

5.1 Biogas to energy

Anaerobic digestion of wastewater not only produces sludge but also releases significant amounts of methane gas. Due to the gradual depletion of conventional methane-generating resources, the development of nonconventional methane gas-containing fuel resources is particularly crucial. The treated biogas can be utilized for power generation or as transportation fuel, which is environmentally friendly and can enhance its economic value by utilizing the continuously generated biowaste. Furthermore, by reducing carbon emissions, biogas utilization contributes to a cleaner environment (Estrada-Arriaga et al., 2020). This use of renewable energy not only reduces the dependence on fossil fuels but also achieves energy self-sufficiency in wastewater treatment plants.

5.2 Combination of other renewable energy sources

The synergistic application of anaerobic technology in combination with other renewable energy technologies, such as solar energy and anaerobic biotechnology, can significantly improve the efficiency of energy utilization in the wastewater treatment process and provide additional renewable energy options for wastewater treatment plants (Table 7). Solar energy can assist anaerobic digestion bacteria in achieving optimal growth temperatures, thereby promoting their reproduction and improving the efficiency of both anaerobic digestion and wastewater treatment. Furthermore, the byproducts of the anaerobic digestion process can be utilized for hydrogen production, broadening the scope of renewable energy applications. The roller pond reactor (RPR) developed by Monteiro et al. (2024) employs solar energy in combination with thermal treatment to enhance the anaerobic digestion of residual sludge for efficient methane production without chemical residues. The combined vertical flow artificial wetland and solar photofenton system investigated by Cavalheri et al. (2023) targets the removal of organic matters and nutrients from wastewater, particularly emerging pollutants. Di et al. (2019) studied an integrated system to produce electrical and thermal energy based on geothermal energy, showcasing the diverse applications of renewable energy in wastewater treatment.

5.3 Energy management and optimization

To achieve energy self-sufficiency in wastewater treatment processes, it is essential to effectively manage and optimize the energy system. By implementing energy integration and optimization strategies, energy efficiency can be maximized, and operating costs can be reduced. Table 8 summarizes research progress in optimizing wastewater treatment for efficiency and resource efficiency. Ferrentino et al. (Si et al., 2022) conducted a study on the integration of water and sludge lines in a wastewater treatment plant. This integration optimized activated sludge recirculation, improved solids removal and biogas production, and enhanced the overall system performance by integrating multiple treatment processes. On the other hand, Sanaye et al. (2023) integrated a sewage sludge anaerobic digester with a solid oxide fuel cell, resulting in a 17.86% increase in the annual yield through electrochemical reactions while reducing energy loss by 7.75%. Cengiz et al. (2024) investigated co-digestion in an anaerobic membrane bioreactor, which achieved solid-liquid separation through a membrane module, facilitated the recovery of biomethane, and improved system stability, allowing the wastewater treatment plant to achieve energy neutrality.

6 Case study

As an efficient wastewater treatment technology, anaerobic digestion has gained widespread adoption globally in recent years. Numerous successful projects have contributed valuable experience and data to its advancement. Table 9 summarizes several typical cases showing the results of different types of anaerobic digestion technologies in wastewater treatment, offering important references and insights for researchers and engineers in related fields.

Anaerobic co-digestion of water and sewage sludge at the Płaszów-Kraków municipal wastewater treatment plant has significantly increased biogas production by approximately 20% (Górka et al., 2022). This result not only enhances energy recovery efficiency but also demonstrates the substantial potential of anaerobic digestion technology for resource recovery. Through the rationalization of different types of sludge, this project has achieved energy self-sufficiency in wastewater treatment and provides a viable implementation option for other wastewater treatment plants.

On the other hand, the Tehran Wastewater Treatment Plant (TWTP) has successfully enhanced sludge hydrolysis rate through ozonation pretreatment. This method effectively disrupts bacterial cell walls, thereby enhancing the efficiency of anaerobic digestion (Hodaei et al., 2021). As an innovative technology, ozonation pretreatment offers significant advantages in enhancing sludge digestibility, which further promotes the advancement of wastewater treatment technology. The implementation of this technology not only enhances sludge treatment but also alleviates the burden of subsequent treatment stages.

The use of intermittent aeration (IA) with an oxidative settling anaerobic (OSA) process at the Corleone Wastewater Treatment Plant (CWTP) demonstrated superior performance in terms of pollutant removal and sludge yield compared to the conventional activated sludge process (Mannina et al., 2024a). Notably, this process has achieved significant results in phosphorus removal. This case highlights the importance of technological innovation in enhancing the efficiency and effectiveness of wastewater treatment and offers guidance for future process optimization.

At the Juan Diaz treatment plant, the co-digestion of secondary sludges resulted in increased organic and nutrient loading, which significantly enhanced biogas production (Moretto et al., 2020). This finding suggests that a moderate increase in organic loading contributes to the efficiency of sludge treatment and promotes energy recovery and utilization. This approach not only enhances treatment effectiveness but also offers innovative strategies for achieving sustainable development goals.

The integrated anaerobic co-digestion system implemented at the Treviso Wastewater Treatment Plant (WWTP) not only achieves efficient energy recovery but also promotes the valorization of waste without the need for significant investment (Moretto et al., 2020). The principle is that ozone, as a strong oxidizing agent, can increase the degradation rate of complex organic matter and help to produce biogas through ozone pretreatment, and the effect of ozone on biogas production and other anaerobic digestion parameters of waste activated sludge depends largely on the ozone dosage and sludge residence time. This project highlights the dual benefits of economic and environmental advantages, demonstrating the significance of integrated systems in modern wastewater treatment. By strategically allocating resources and optimizing the process, the plant succeeded in developing synergies between wastewater treatment and energy recovery.

In addition, the Foligno Municipal Wastewater Treatment Plant has significantly increased biogas production, and the amount of energy recovered through the anaerobic co-digestion process (Salehiyoun et al., 2020). This result further demonstrates the importance of anaerobic co-digestion in energy recovery. By effectively combining various processes, these projects not only enhance energy recovery efficiency but also have significant potential for reducing pollutant emissions and lowering operating costs.

Overall, these success stories demonstrate the wide range of applications and significant benefits of anaerobic digestion technology in wastewater treatment worldwide. This approach not only enhances biogas production and energy recovery efficiency but also offers limitless opportunities for promoting environmental protection and sustainable resource utilization. In the future, with ongoing advancements and innovations in this technology, anaerobic digestion is expected to play a crucial role in a wider range of fields, contributing to the achievement of green and sustainable development.

7 Challenges

7.1 Technical challenges

Biogas production from sewage sludge faces many research challenges. Different types of digesters, such as solid anaerobic digesters, semicontinuous solid anaerobic digesters, sewage sludge hydrothermal treatment, thermophilic digesters, two-stage anaerobic digesters (acid digesters and methanogenic digesters), double-digesting and thermophase anaerobic digesters, and bioelectrochemical anaerobic digesters, vary in their operating parameters, sizes, and sources of sludge (e.g., sewage treatment plants) (Khawer et al., 2022). Although it is becoming increasingly difficult for conventional wastewater treatment technologies to meet current demands, anaerobic biotechnology demonstrates significant potential for wastewater treatment and resource utilization. However, process optimization and stability are still technical challenges that need to be addressed. To overcome these challenges, traditional anaerobic digestion technologies need to be improved by integrating them with emerging technologies. In addition, improving resource recovery efficiency and developing efficient recycling technologies are urgent issues that must be also addressed. Currently, there are a variety of emerging technologies that not only enhance wastewater treatment and the aquatic environment but also enable more efficient utilization of the resulting sludge. These technologies achieve high resource recovery rates and play a crucial role in advancing a circular economy. For instance, microbial fuel cells (MFCs) are recognized as one of the sustainable solutions to address the excess sludge and water-energy crisis (Nikhil et al., 2018).

7.2 Economic and policy challenges

Many wastewater treatment plants still face challenges due to the complexity and high costs associated with their processes (Zhang, 2025). Despite the significant number of wastewater treatment plants in China, substantial amounts of untreated wastewater are still being discharged into rivers, and the volume of sewage continues to rise, leading to serious environmental issues. Polluted water bodies can pose a potential threat to human health, making public health issues a concern (Sanaye et al., 2023). According to a statement by the State Council, addressing water pollution and other environmental issues has been recognized as a priority, and the government is encouraging cities to build advanced wastewater treatment facilities. In 2015, the Chinese government released a new national strategy aimed at combating water pollution (Chang et al., 2018). Future experiments and practices should adopt sustainable programs that integrate multiple aspects of technology, environmental protection, health, legal considerations, economic and social aspects. The implementation of anaerobic technologies typically requires a high initial investment, while the lack and high cost of methane-fueled power generation technologies and insufficient funding can be major barriers to project advancement (Chernicharo et al., 2015). In addition, the policy and regulatory environments of different countries and regions may also affect the promotion and application of anaerobic technology. Therefore, securing adequate economic and policy support will be the key to the future development of this technology.

7.3 Social and environmental impacts

The promotion of anaerobic wastewater technology involves not only breakthroughs at the technical level but also considerations of social acceptance and public awareness (Xue et al., 2019). Currently, many people have a limited understanding of anaerobic biotechnology and may even have misconceptions. Therefore, enhancing the public’s environmental awareness and comprehension of this technology is essential for fostering social acceptance. This process can be achieved by strengthening public education and the popularization of science. For instance, the government, scientific research institutions and environmental organizations can collaborate to conduct outreach activities to make the public aware of the significant potential and positive contributions of anaerobic biotechnology in waste treatment, energy recovery. and environmental protection. Moreover, policymakers and technology advocates should ensure that a comprehensive scientific assessment of the environmental impacts of anaerobic biotechnology is carried out prior to its large-scale implementation to mitigate potential negative impacts. This approach will not only help ensure the sustainable development of technology but also enhance public trust and support. Overall, social acceptance is closely related to the successful implementation of technology; thus, increasing public awareness and scientific assessment of environmental impacts are indispensable links in the promotion of anaerobic biotechnology.

8 Conclusion

“Energy self-sufficiency” in the wastewater treatment field usually means that the wastewater treatment plant itself meets all or most of its operational energy needs through energy recovery (e.g., biogas from anaerobic digestion to generate electricity) without the need for external inputs. The key ways to achieve energy self-sufficiency include: improving organic matter removal and methane production, enhancing biogas collection and utilization efficiency, optimizing energy-intensive unit processes (e.g., aeration), adopting high-efficiency equipment and energy management strategies, and may incorporate the comprehensive use of other renewable energy sources such as solar energy and heat pumps. Some advanced sewage plants have basically realized energy self-sufficiency through biogas power generation and waste heat recovery, providing a demonstration of green and low-carbon transformation for the industry. The full realization of energy self-sufficiency requires synergistic innovation in process, equipment, management and other aspects.

In the context of “carbon neutrality”, the application of anaerobic biotechnology in wastewater treatment and resource utilization demonstrates significant potential. Studies have shown that anaerobic digestion can effectively reduce carbon dioxide emissions from wastewater treatment while converting organic matter into methane, a renewable energy source. This technology not only helps reduce greenhouse gas emissions but also facilitates resource recycling, aligning with the principles of a circular economy. While traditional wastewater treatment methods involve high energy consumption and significant carbon emissions, anaerobic technology, which uses the metabolic activities of anaerobic microbial populations, not only reduces the release of carbon dioxide but also provides energy for wastewater treatment plants by generating methane. Anaerobic digestion technology is increasingly being studied, including the comparison of single-stage and multistage digestion, the optimization of different reactors, and the recovery of nutrients such as nitrogen and phosphorus. These studies highlight the ability of multistage anaerobic digestion to optimize treatment conditions at different stages, thereby improving overall efficiency. Case studies have shown that the use of anaerobic co-digestion in several wastewater treatment plants has significantly increased biogas production and improved energy recovery efficiency. Combining other renewable energy technologies, such as solar energy, with anaerobic biotechnology has also further improved energy efficiency. Overall, anaerobic biotechnology provides an important solution for achieving a low-carbon transition in wastewater treatment, with multiple benefits in terms of resource utilization, energy recovery and environmental protection.

The importance of anaerobic biotechnology in the field of wastewater treatment is becoming increasingly apparent with the advancement in technology. Due to its efficient performance in terms of carbon reduction and resource recovery, it holds considerable potential for various applications. In the future, the development of anaerobic biotechnology will focus on technological innovation and diverse implementations. Moreover, integrating anaerobic biotechnology with other technologies will also provide additional opportunities to achieve the carbon neutrality goal of wastewater treatment. Among the available high-rate anaerobic technologies, the front-end technology is mainly the membrane bioreactor. Since the performance of membranes cannot be scaled up directly from the laboratory to actual plants, further research is needed to facilitate the implementation of this technology in large-scale wastewater treatment plants.

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