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Review

Algal–Bacterial Symbiotic Granular Sludge Technology in Wastewater Treatment: A Review on Advances and Future Prospects

by
Shengnan Chen
,
Jiashuo Wang
,
Xin Feng
and
Fangchao Zhao
*
School of Environmental and Municipal Engineering, Huangdao Campus, Qingdao University of Technology, Qingdao 266525, China
*
Author to whom correspondence should be addressed.
Water 2025, 17(11), 1647; https://doi.org/10.3390/w17111647
Submission received: 24 February 2025 / Revised: 17 May 2025 / Accepted: 23 May 2025 / Published: 29 May 2025
(This article belongs to the Special Issue Algae-Based Technology for Wastewater Treatment)
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Abstract

This review systematically examines the critical mechanisms and process optimization strategies of algal–bacterial granular sludge (ABGS) technology in wastewater treatment. The key findings highlight the following: (1) enhanced pollutant removal—ABGS achieves >90% COD removal, >80% total nitrogen elimination via nitrification–denitrification coupling, and 70–95% phosphorus uptake through polyphosphate-accumulating organisms (PAOs), with simultaneous adsorption of heavy metals (e.g., Cu2+, Pb2+) via EPS binding; (2) energy-saving advantages—microalgal oxygen production reduces aeration energy consumption by 30–50% compared to conventional activated sludge, while the granular stability maintains >85% biomass retention under hydraulic shocks; (3) AI-driven optimization—machine learning models enable real-time prediction of nutrient removal efficiency (±5% error) by correlating microbial composition (e.g., Nitrosomonas abundance) with operational parameters (DO: 2–4 mg/L, pH: 7.5–8.5). This review further identifies EPS-mediated microbial co-aggregation and Chlorella–Pseudomonas cross-feeding as pivotal for system resilience. These advances position ABGS as a sustainable solution for low-carbon wastewater treatment, although challenges persist in scaling photobioreactors and maintaining symbiosis under fluctuating industrial loads.

1. Introduction

With the acceleration of urbanization and industrialization, global environmental ecology is facing significant challenges, among which one of the most significant is the inadequacy of wastewater treatment systems. Although the application scope of the traditional activated sludge process has significantly expanded, its limitations in efficiency and cost mean that it cannot be the only solution. In recent years, algal–bacterial symbiotic granular sludge technology has attracted widespread attention from environmental engineers and scholars. As a cost-effective wastewater management solution, this technology has shown promising application prospects [1,2,3,4,5,6,7,8,9,10,11].
The alga–bacterial granular sludge system is an emerging ecological engineering innovation that utilizes the synergistic effect between microalgae and bacteria to achieve efficient wastewater treatment in self-sufficient units. Compared with traditional processing methods, this technology has significant advantages in nutrient removal, energy consumption reduction, and biomass deposition characteristics [3]. The photosynthesis of bacteria and algae creates an oxygen-rich environment. The carbon dioxide (CO2) produced by bacteria can be utilized by microalgae through photosynthesis, forming a synergistic carbon exchange in algal–bacterial symbiotic systems, creating an ideal environment for algae growth, thus achieving mutual benefit and synergy between the two [4]. Although algae are aerobic organisms (relying on oxygen for respiration), their growth primarily depends on photosynthesis, which requires CO2 as a carbon source. The solubility of CO2 in natural water bodies is relatively low, leading to insufficient availability of free CO2. Supplementing CO2 can help maintain the concentration of dissolved CO2 and thereby enhance the supply of carbon sources more effectively.
The performance of algal–bacterial granular sludge is significantly influenced by multiple factors, such as microbial community composition, structure, light intensity, nutrient load, and hydraulic retention time [5]. A deep understanding of these key parameters is crucial for optimizing the cultivation process and improving system performance [6]. Existing research indicates that precise control of the aforementioned factors can significantly improve system efficiency. This sludge has shown outstanding potential in the field of bioremediation, and it is capable of efficiently removing multiple pollutants from wastewater, especially with its significant advantage in simultaneously removing organic pollutants, nitrogen compounds, and phosphorus [7]. By utilizing the photosynthetic oxygen release mechanism of algae, this technology not only significantly optimizes the efficiency of aeration energy utilization, but also greatly enhances the ecological sustainability of biological treatment systems [8].
The algal–bacterial granular sludge system is not only highly compatible with the urgent demand for resource recovery and a circular economy in the current wastewater treatment field, but also in line with the growth mechanism of microorganisms, and it has significant environmental friendliness. The algal biomass generated by this technology can be further converted into biofuels, ecological fertilizers, or diversified biomaterials [9]. This significantly enhances the sustainability and economic value of the technology itself.
Research has shown that although this technology does demonstrate certain application prospects, there are still key technical bottlenecks in the large-scale implementation stage. The current main research focus is on the following factors: (1) solving the restrictive problem of light penetration in large reactors [12], (2) optimizing the particle structure in anaerobic IC reactors to improve their adaptability to external disturbances, and (3) developing advanced control mechanisms that ensure long-term operational stability [1].
This review aims to comprehensively examine the latest research progress in the field of algal–bacterial granular sludge cultivation and its potential applications in wastewater treatment. Through in-depth exploration of the basic mechanism of granular sludge formation, the evolution of granulation technology, and related efficiency and practicality, this article will systematically analyze the status of the current research. The focus will be on the key obstacles to technological development and future research directions, in order to provide comprehensive insights for the field of environmental biotechnology. This study will systematically integrate the existing literature, provide valuable theoretical guidance for researchers and practitioners in this field, and elucidate innovative strategies to enhance the treatment performance of algal–bacterial systems.

2. Formation Mechanism of Algal–Bacterial Granular Sludge

2.1. Characteristics of Microbial Community Composition

The algal–bacterial microbial community in granular sludge exhibits highly complex and evolving characteristics, comprising phototactic microalgae, heterotrophic bacteria, and diverse microbial species [13]. With the help of modern molecular biology techniques, researchers have been able to comprehensively analyze the micro-ecological structure of this community, demonstrating its inherent complexity and biodiversity [14]. The schematic in Figure 1 illustrates the stratified spatial distribution and metabolic synergy between photosynthetic microalgae and heterotrophic bacteria in algal–bacterial granular sludge systems.
Chlorella, Scenedesmus, and Chlamydomonas are dominant microalgae species in biological systems [15,16]. They have excellent environmental adaptability and play a key role in the process of oxygen generation during photosynthesis. The genus Chlorella is particularly widely distributed, known for its significant cold resistance and efficient reproductive mechanism [17]. In algal–bacterial granular sludge systems, Chlorella enhances granulation by producing oxygen via photosynthesis to support bacterial metabolism and secreting extracellular polymeric substances (EPSs) that stabilize the granular structure, while synergistically removing nutrients and organic pollutants through microbial cross-feeding.
Algal granular sludge exhibits significant diversity in microbial communities, integrating specialized aerobic and anaerobic functional bacteria [18]. Heterotrophic bacteria play a central role in the degradation of organic matter, and they are mainly distributed in the oxygen-rich areas around particles [19]. Through systematic research, important taxonomic units such as Pseudomonas, Bacillus, and Nitrosomonas have been identified, which significantly enhance the purification efficiency of algal–bacterial composite sludge [20].
The internal micro-ecosystem can be divided into multiple functional areas, which perform specific biological processes and are crucial for maintaining the dynamic balance of nutrients and microbial symbiosis within granular sludge [21,22]. Through advanced resonance microscopy techniques, the microbial community structure exhibits distinct spatial stratification characteristics, with the outer layer typically enriched in photosynthetic microorganisms and the inner layer containing a more diverse bacterial community and complex oxygen gradient [23]. As shown in Figure 2, CLSM analysis reveals the spatial arrangement of algae and bacteria within mature algal–bacterial granules. As illustrated in Figure 3, the temporal dynamics of core microbial communities during the development of algal–bacterial granular sludge reveal a succession from filamentous fungi-dominated consortia to functional bacteria-enriched biofilms. As we expected, Scenedesmus and Chlorella remained the dominant in the phototroph community of the granular consortia, whereas the obvious accumulation of harmful cyanobacteria, which would have affected the ecological safety of this technology by the production of cyanotoxin, was not observed.

2.2. Synergistic Symbiosis Mechanism Between Algae and Bacteria

In the microbial community of granular sludge, a precise ecological interaction network is formed between algae and bacteria, and this interdependent pattern is the fundamental mechanism for maintaining the ecological balance of the system [24]. The oxygen produced by photosynthesis in algae provides a redox environment for bacteria. Bacteria enhance algal growth and development by releasing carbon dioxide and essential metabolites such as acetate, vitamin B12, and glycerol, which drive photosynthesis and biomass synthesis in algal–bacterial symbiotic systems [25]. The metabolic interaction network is shown in Figure 4.
The material exchange in microbial symbiotic systems presents multidimensional complexity. Bacteria provide essential nutrients for algae growth [27]. Meanwhile, algae support bacterial metabolic activity by secreting soluble organic carbon compounds [28]. The microstructure of granular sludge, especially the outer photosynthetic membrane, not only maintains an oxygen concentration gradient, but also creates specific ecological niches for different bacterial groups [29]. We have established a precise ecological balance mechanism [30].

2.3. Key Factors Affecting the Granulation Process of Algal–Bacterial Granular Sludge

In the study of algal–bacterial symbiotic sludge granulation, fluid dynamic shear stress is a key parameter affecting particle formation and stability [31]. Moderate shear force promotes microbial cell aggregation and alters the physical structure of sludge particles. Specific levels of shear stress can stimulate microorganisms to secrete extracellular polymeric substances (EPSs) [32]. This substance plays a crucial role as a biological adhesive in the formation and stabilization of sludge particles [21]. By precisely regulating shear stress, researchers can gain a deeper understanding and control of the granulation mechanism of algae bacteria symbiotic sludge, thereby optimizing the performance of sewage treatment systems. Different intensities of shear force significantly affect microbial community aggregation behavior and particle morphology characteristics [33], as well as the coagulation process of the overall biological community.
The granulation process is significantly affected by light intensity, and the optimal light range is 150–200 μmol/m2/s [34]. Within this range, light can effectively promote granulation, while excessive light may damage the structural stability of algal–bacterial granular sludge [35]. The cycle operation time is another key factor, and optimizing the settling time can increase the selective pressure [36], which is beneficial for screening aggregates with rapid settling and eliminating flocs with poor settling performance [37].
Volume load is a key factor affecting the formation and stability of algal–bacterial granular sludge [38]. Different organic loading rates significantly affect microbial growth kinetics and the formation of EPSs. The organic load rate is a crucial indicator in sewage treatment and environmental engineering, reflecting the amount of biological oxygen demand applied per unit treatment area per day. Within a specific load rate range, microorganisms can form tightly structured and stable biological particles. When the organic loading rate exceeds the optimal range, the internal structure and aggregation of particles will rapidly degrade, ultimately leading to particle decomposition and failure [39]. Temperature and pH are key environmental parameters in the granulation process [40]. They directly affect the physiological and metabolic activity of microorganisms and the physicochemical properties of EPSs. Temperature changes can alter the growth rates and metabolic pathways of microorganisms, and affect the molecular structure and aggregation of EPSs [41]. The pH value affects particle formation and stability by regulating the permeability of microbial cell membranes and enzyme activity [42]. Therefore, precise control of temperature and pH values is crucial for maintaining an efficient and stable granular sludge system [43].

2.4. Stability Mechanism of Granular Sludge

The stability mechanism of algal–bacterial granular sludge is a complex systematic process involving deep coupling and synergistic effects of multiple biological, physical, and chemical parameters [44]. These key parameters not only determine the overall structure of the system, but also affect its functional capacity and adaptability. In this mechanism, EPSs play a crucial role. By constructing a highly organized three-dimensional network structure [45], EPSs achieve the precise positioning of and spatial constraints on microbial communities, and form a protective barrier to effectively resist adverse external environmental pressures, including physical shear forces, osmotic pressure changes, and chemical toxicity, thereby maintaining the structural integrity and functional stability of granular sludge [46].
The ecological interaction system between algae and bacteria is a complex metabolic dependency network. Algae produce oxygen through photosynthesis, creating an oxygen-rich microenvironment that provides high-quality living conditions for aerobic bacteria and promotes their metabolic activity [47]. Bacteria continuously supply key nutrients such as carbon dioxide, nitrogen, and phosphorus to algae by decomposing organic and inorganic substances, supporting their growth and reproduction [48]. This mutually beneficial symbiotic relationship moves beyond simple nutrient exchange and forms a dynamically balanced and highly organized micro ecosystem. Through continuous material cycling and energy transfer, the system exhibits significant ecological stability and adaptability, effectively resisting external environmental disturbances [49].
The multi-layered structure of algal–bacterial granular sludge is a key factor in enhancing physical stability [50]. For example, the special structure of the outer layer of algal cells can resist external mechanical pressure while ensuring moderate penetration of light, ensuring the progress of internal photosynthesis [51]. By finely dividing the organizational structure, the system creates specialized functional areas for various metabolic activities, significantly improving overall stability [52]. Due to this complex hierarchical structure, mature particles can maintain structural integrity under different environmental conditions [46].
The dynamic balance between internal growth and decay is the key mechanism for maintaining particle stability in the formation and maintenance of algal–bacterial granular sludge [53]. The continuous renewal and metabolic turnover of microbial populations and extracellular polymers not only maintain the integrity of the particle structure, but also effectively regulate the internal ecological balance of particles. This dynamic balance mechanism effectively prevents excessive accumulation of inert substances, maintaining particle vitality and functionality. By carefully designing operating conditions such as optimizing settling time and hydraulic retention time, selective pressure can be applied to the particle formation process [38]. The selective mechanism of screening and retaining efficient and stable biological particles, while eliminating loosely structured and metabolically inefficient aggregates [38], ensures the overall performance and stability of the biological particle system, thereby promoting the sustained and effective operation of the biological treatment process.
At the molecular level, complex signaling networks and quorum sensing mechanisms exist within microbial communities [54], coordinating the interactions between systems. This communication mode enables microbial communities to systematically respond to environmental changes and effectively regulate the growth dynamics of microorganisms within particles [55]. In practical operation modes, this mechanism provides key theoretical references [56].

3. Cultivation Methods and Optimization Strategies

3.1. Comparison of Different Types of Cultivation Reactors

Researchers point out the key role of reactor configuration in the cultivation of algal–bacterial granular sludge [57]. Through systematic improvement and design, diverse reactor types have been introduced to accelerate the formation of granular sludge and enhance its structural stability [58]. Among them, the sequencing batch reactor (SBR), as the most representative type of reactor, exhibits superior operational performance and selective pressure characteristics in the granulation process [59].
Recently, researchers and engineers have actively developed innovative photobioreactors to optimize the interaction between photons and biological systems [60]. The column-type photocatalysis sequence batch reactor (PSBR) exhibits significant performance in enhancing the formation of granular sludge, with its core innovations being the significant improvement in optical transparency and the unique reactor mixing mode [61]. The column-type photocatalytic sequencing batch reactor is an advanced water treatment device that combines photocatalytic oxidation technology with a sequencing batch operation mode. The design concept of this photobiological reaction system focuses on two key dimensions: highly sustainable technical features and precise optimization of hydraulic parameters [16]. Through these innovative methods, researchers are committed to maximizing the efficiency of photosynthesis and providing revolutionary breakthroughs in the fields of biotechnology and environmental engineering. This not only significantly improves the energy conversion efficiency of photobiological processes, but also provides more advanced technological solutions for future renewable energy and environmental governance.
According to Table 1, it can be concluded the SBR demonstrates consistent removal efficiencies of 85 ± 5% for COD, 75 ± 5% for TN, and 70 ± 5% for TP in highly variable environments. However, it exhibits elevated operational costs of 18–22% compared to conventional systems due to low photon conversion efficiency (<15%) and high energy intensity (≥1.2 kW·h/m3). Flat-panel photobioreactors (PBRs) enhance bio-interface functionality by maximizing surface area and optimizing hydrodynamics, achieving a photosynthetic efficiency of 28–32%, surpassing the SBR’s removal rates by 5–8 percentage points. Nevertheless, turbulent flow regimes increase capital costs by 42.7%, with frequent maintenance needs impeding large-scale deployment. Membrane-integrated photobioreactors (MPBRs) achieve exceptional treatment performance through selective molecular sieving. Accelerated membrane fouling kinetics due to transmembrane pressure fluctuations lead to high costs for membrane replacement. The columnar photo-sequencing batch reactor (PSBR) tackles these issues by employing axial photonic engineering and enhancing sedimentation through gravity. These innovations reduce recirculation energy demand to 0.85 kW·h/m3 while maintaining 90 ± 5% COD removal, leading to a 12.4% cost reduction compared to SBR, despite facing 25–30% scale-up penalties due to modularity constraints.
Among different types of reactors, each exhibits unique performance characteristics during particle cultivation. Membrane photobioreactors have attracted much attention due to their excellent effluent quality [62]. Especially in dealing with pollutants that are difficult to effectively degrade using traditional methods, they have significant advantages. These reactors can significantly improve the efficiency and purification level of wastewater treatment through advanced membrane separation technology and photocatalytic process. However, their complex process flow and high operating costs remain key obstacles, limiting their widespread application [63]. At the same time, due to specific process limitations, the application scope of continuous flow systems is relatively narrow; they are mainly suitable for specific application scenarios in which sewage treatment quality is the primary goal and granulation cultivation is a secondary factor to consider [64]. However, in certain specific fields, continuous flow systems still demonstrate their unique technological advantages and processing potential [65].

3.2. Optimization of Key Operating Parameters

According to the data in Table 2, it can be concluded that Under optimized operating conditions, the algal–bacterial granular sludge system exhibits significant growth kinetics and excellent stability. Research reveals that the growth density of microalgae is closely related to light intensity and photoperiod [66]. It has a critical impact on the overall performance of the system. Research has shown that maintaining the light intensity within the range of 150–400 μmol/m2/s and adopting a 12/12 h light–dark cycle mode can effectively promote particle formation and maintain stability [67].
In microbial treatment systems, the organic load-to-nutrient ratio is crucial for system performance [70]. The Chemical Oxygen Demand (COD) load, as a key indicator for measuring organic matter load, directly affects the growth dynamics and metabolic activity of microorganisms [71]. The extreme value of COD load may lead to system instability, thereby reducing processing efficiency [72]. The nitrogen/phosphorus ratio (N/P) is an important nutrient balance parameter, and its reasonable ratio can ensure that microorganisms obtain balanced nutrients, promoting their metabolism and growth [73]. An improper nitrogen/phosphorus ratio may cause changes in the microbial community structure, thereby affecting the treatment performance of the system. The hydraulic retention time (HRT) is a key influencing factor in biological treatment systems, and a retention time range of 12–48 h provides sufficient degradation and transformation conditions for microorganisms. The length of the residence time is directly related to the microbial removal efficiency and system operating costs, and multiple factors such as wastewater characteristics, treatment objectives, and system design need to be comprehensively considered [69].
In microbial ecosystems, temperature and pH constitute the core environmental parameters that affect metabolic processes and community structure. Research has shown that the physiological activities of algae and bacteria perform best within a specific temperature range (20–30 °C) and pH range (7.0–8.5) [40]. The system performance is affected by the composite effect of multiple parameters, and its interaction needs to be comprehensively considered [55]. Modern research suggests adopting real-time monitoring and dynamic optimization strategies to ensure long-term stability and operational efficiency of the system [74].

3.3. Challenges and Solutions in the Cultivation Process

The evolution and integration of algae–bacteria is a key mechanism for the formation of granular sludge, which not only reflects the complexity of the ecosystem, but also reveals the inherent laws of the collaborative development of microbial communities [75]. In practical applications, this method faces many environmental uncertainty challenges, such as sudden temperature changes, sudden changes in light intensity, and dynamic transitions in ecosystems, all of which may significantly affect the stability of algal–bacterial systems [76]. To cope with such uncertainty, researchers have proposed intelligent and adaptive monitoring and regulation strategies [77]. By dynamically monitoring the microbial community in real-time, the system can quickly respond to environmental changes, adjust community structure, and function in a timely manner, thereby enhancing its resilience and adaptability.
The limitation of light infiltration within the thick particle system poses significant technical challenges in algal–bacterial granular sludge. Due to the difficulty of light penetrating the thick layer of algal particles, the deep algae cells have insufficient illumination, which severely limits the efficiency of photosynthesis and biomass production [78]. To overcome this bottleneck, researchers have proposed innovative solutions, including integrating self-luminous systems in bioreactors [79]. Actively adjusting the light distribution through internal light sources ensures the uniform photosynthesis of algae particles at different depths; Another strategy is to use pulse lighting technology, which significantly improves light utilization and effectively reduces energy consumption through intermittent and precise control of lighting [80]. These technological routes aim to break through traditional light limitations, optimize the growth environment of microalgae, and ultimately improve the overall performance of bioreactors.
The large-scale reproduction of filamentous organisms can have a negative impact on system performance, mainly manifested as potential variations in particle structure, thereby reducing system operating efficiency [81]. In response to such challenges, researchers can effectively alleviate them by optimizing hydraulic selection pressure and implementing precise control strategies, such as dynamically adjusting time and height parameters. In addition, regular monitoring and balancing of nutrient and organic matter concentrations are crucial.
The conversion of experimental results to engineering scale faces many technical obstacles, especially in maintaining the same precise conditions as small-scale experiments in large reactors, which is extremely difficult [82]. In response to this challenge, future research will focus on modular design strategies and advanced mixing methods to enhance the scalability and performance consistency of the system [83]. By introducing advanced control equipment and automation systems [84], researchers have significantly improved work stability in large-scale application environments.
In the initial stage of the granulation process, the significant loss of activated sludge poses a key challenge for system startup [85]. By optimizing the settlement strategy and introducing specialized carrier materials [86], researchers have successfully alleviated the issue of biomass loss. In addition, improving inoculation methods and adjusting hydraulic conditions significantly enhanced the recovery efficiency of initial biomass.

3.4. Particle Size and Mass Transfer Diffusion Mechanism

The size of algal–fungal symbiotic granules (typically 0.5–5 mm) directly affects the internal mass transfer kinetics and metabolic activity of functional microorganisms. Studies have shown that when the granule diameter exceeds 1.5 mm, the diffusion paths for oxygen and substrates extend, leading to a pronounced oxygen concentration gradient (0.5–3 mg/L from the outer layer to the core) and nutrient-limited zones [87,88]. For example, in a 200 μm thick layer of microalgae, the oxygen produced by photosynthesis can only penetrate to a depth of about 500 μm within the granules, while deeper heterotrophic bacteria rely on external oxygen supply or endogenous respiration [87]. Although this stratified mass transfer characteristic is beneficial for simultaneous nitrification–denitrification (SND), it may also lead to excessive hydrolysis of extracellular polymers (EPSs) due to the anaerobic environment in the core region, thereby weakening granule stability [88].

4. Water Treatment Effect

4.1. Organic Matter Removal Performance

Research has shown that the algal–bacterial granular sludge system has significant advantages in organic matter removal, with its main functions including the removal of various organic pollutants. According to interdisciplinary research, the synergistic effect of different microbial communities significantly promotes the biodegradation and retention of simple and complex organic substances. Under optimal conditions, a mature granular sludge system can achieve a COD removal efficiency of over 90% [1,64,65,66,68,86,89,90,91,92,93,94,95,96,97,98].
It is worth noting that these ecosystems exhibit excellent performance in organic matter removal through biodegradation by heterotrophic bacteria, algal metabolism, and absorption by photosynthetic bacteria [90]. The three-dimensional structure of granular sludge creates progressive microbial sites, which facilitate diverse metabolic pathways and demonstrate more efficient organic matter removal capabilities than traditional activated sludge systems [91]. The oxygen aeration process provides conditions for aerobic degradation, further promoting the improvement of removal efficiency by photosynthetic organisms.
The inflow parameters of wastewater and the operating conditions of the system have a significant impact on the efficiency of pollutant removal. Research has revealed that the system can maintain stable performance within the organic load range of 2.0–5.0 kg COD/m3·d [71]. For organic compounds that are easily biodegradable, the removal efficiency is relatively high. However, more complex organic molecules may require an extended residence time to achieve effective treatment [92].

4.2. Nitrogen and Phosphorus Removal Efficiency

The algal–bacterial granular sludge system is an innovative biological treatment technology that achieves a nitrogen removal rate of over 80% through the synergistic effect of microbial communities [1,64,65,66,68,86,89,93,95,96,97,98,99,100]. In this system, algae and bacteria tightly combine to form a unique granular sludge structure. The interior of particles presents a diverse microenvironment, naturally forming anoxic, anaerobic, and aerobic zones, providing ideal living spaces for microorganisms with different metabolic functions. This partition structure enables key denitrification processes such as nitrification and denitrification to proceed simultaneously, significantly improving denitrification efficiency. Compared with traditional treatment processes, this system has stronger nitrogen removal ability and can effectively reduce the nitrogen concentration in the effluent, which is of great significance for improving water environment quality.
The algal–bacterial granular sludge system exhibits excellent phosphorus removal performance through complex biological and physicochemical interactions [93]. The system utilizes the mechanism of phosphate bioaccumulation and the direct absorption pathway of microalgae [94]. It effectively achieves efficient phosphorus removal and runs through the entire process of cell growth.
Research has shown that optimized algal–bacterial granular sludge systems can achieve a phosphorus removal efficiency of 75% to 95% [1,64,65,66,68,86,89,90,91,92,93,94,95,96,97,98,99,100]. The special layered structure of mature granular sludge promotes the synergistic growth of algal–bacterial and polyphosphate-accumulating, significantly improving system performance [97]. The oxygen fluctuations caused by the photosynthetic activity of algae within the particles create favorable alternating aerobic/anaerobic conditions for biological phosphorus removal [96]. In high-pH environments, especially when algae photosynthesis causes an increase in pH, the phosphorus removal mechanism is enhanced by the dissociation of phosphate from metal ions in the system [97]. This biochemical synergistic elimination method makes phosphorus removal more efficient and stable, and the phosphorus in biomass has significant potential for recovery and reuse, which is in line with the concept of circular economy development.

4.3. Treatment Effect of Special Pollutants

The algal–bacterial granular sludge system, as an innovative environmental remediation technology, has shown significant potential in the field of heavy metal pollution control. This system achieves efficient capture and accumulation of heavy metal ions through the synergistic mechanism of algae and bacteria [98]. The oxygen produced by algae during photosynthesis provides an optimized growth environment for bacteria, and the complex metabolic activities of bacteria significantly enhance their ability to adsorb heavy metals. The unique cell wall structure of algae provides a broad surface area for heavy metal removal, significantly improving adsorption efficiency [99]. Lead, cadmium, copper, and other heavy metal ions can be effectively removed, and their removal rates are significantly positively correlated with reaction time and sludge concentration. This system not only has high processing efficiency, but is also easy to operate, economical, and environmentally friendly. In the field of industrial wastewater treatment, this technology can effectively reduce the degree of heavy metal pollution and alleviate the negative impact on the ecological environment. By continuously optimizing the parameters and processes of the algal–bacterial granular sludge system, it is expected that the system will achieve more efficient heavy metal pollution control in the future, providing key technical support for ecological environment protection and sustainable development.
Existing research has confirmed that algal–bacterial granular sludge systems can effectively remove drug compounds and personal care products, with removal efficiencies generally ranging from 65% to 90% [100]. The microbial community in particles has complex metabolic mechanisms and can degrade complex organic molecules. During this process, algae play a crucial role, not only promoting the transformation of pollutants, but also providing oxygen sources for aerobic degradation processes [101].
The algal–bacterial granular sludge system demonstrates significant potential for treating industrial pollutants, with significant removal effects on textile dyes and phenolic compounds [102]. Research has shown that when the decolorization rate of synthetic dyes exceeds 80%, the system can maintain operational stability. The layered structure of its granular sludge creates a unique microenvironment, effectively promoting complex biodegradation pathways and significantly improving the ability to treat resistant compounds. This system meets the strict requirements for customized industrial wastewater treatment in terms of pollutant load adaptability while maintaining high efficiency [103].

4.4. System Stability and Impact Load Resistance

The algal–bacterial granular sludge system exhibits significant operational stability and load resistance, as evidenced by its dispersion characteristics under low-to-high-intensity disturbances. The outstanding functional efficiency of this system lies in its unique spherical particle structure and complex microbial community composition. The system’s resistance to load impact gradually increases, and the layered structure of particles provides an effective barrier for the internal microbial community [46]. The algal–bacterial granular sludge system exhibits notable resilience to fluctuations in ambient temperature, pH levels, and toxic substances. It sustains system stability through the self-regulating mechanism of bacteria–algae symbiosis [91].
Through algal photosynthesis, aeration can be significantly reduced, resulting in a 30–40% decrease in operating costs compared to activated sludge systems [104]. When fully implemented in the treatment plant, the system can achieve organic matter removal rates of 85–90% and nitrogen removal rates of 75–85%, while significantly reducing costs. This system is particularly adaptable to seasonal input changes and performs excellently in treating dispersed wastewater. It can coordinate well with algal growth dynamics and lighting conditions [105]. A comprehensive cost analysis shows that significant savings in energy use and sludge management costs make it economically feasible [106]. This technology can not only improve the existing processing level without the need to expand facilities, but also achieve resource reuse by recycling valuable biomass [107].
According to relevant research, the algal–bacterial granular sludge system has shown significant treatment effects in treating factory wastewater. This system can efficiently remove pollutants such as organic matter, nitrogen, and phosphorus from wastewater through the synergistic effect of algae and bacteria. It not only has high treatment efficiency, but also has the characteristics of low cost, low energy consumption, and low emissions [67]. The algal–bacterial granular sludge system fully utilizes the metabolic capacity of microorganisms to achieve deep purification of wastewater, which is of great significance for improving the quality of factory wastewater and providing a sustainable ecological treatment solution for industrial wastewater treatment.
The adoption of this technology in the pharmaceutical industry also looks promising, especially in the comprehensive application of treating antibiotic production wastewater. On average, it can remove more than 85% of the target substances that work with particles in a biologically active state [108].

5. Economic Evaluation and Prospects Analysis of Algae

The large-scale deployment of algal–bacterial granular sludge systems requires a comprehensive balance between technical feasibility and economic benefits [106]. The system evaluation for full-size devices shows that compared to traditional processing techniques, this system exhibits significant advantages in operational efficiency and cost control. The evaluation dimensions cover the energy potential, potential economic benefits, and operational and capital investment costs of resource recovery.
Economic analysis shows that the initial capital expenditure of algal–bacterial granular sludge systems is comparable to or slightly higher than that of traditional activated sludge systems. However, the significant savings in land acquisition costs, especially in urban environments, can reduce the required land area to 30–40% of the original system [87], substantially offsetting the initial investment increase. Although the initial investment is relatively high, introducing advanced monitoring systems can optimize operating costs in the long run [84]. Research on operating costs has confirmed a significant reduction in both energy consumption and chemical use. Compared to traditional treatment systems, energy demand can be reduced by about 40%, mainly due to the decrease in aeration requirements during algal photosynthesis [47]. At the same time, the use of chemical phosphorus removal agents has also been significantly reduced, further reducing the operating costs of the system.
By integrating resource recycling and the by-product value added, the economic feasibility of this technology has significantly improved. Research has confirmed that harvesting biomass can successfully extract biofuels, fertilizers, and high-value compounds such as proteins and pigments [107]. These additional sources of income contribute to improving the overall economic benefits of the technology. The life cycle cost assessment shows that from a long-term perspective, the technology has promising prospects, with an average investment payback period of 3 to 5 years, depending on multiple factors, such as local conditions and operational scale. In areas with sufficient sunlight, this technology is particularly economically beneficial, as it can maximize the utilization of solar energy resources [105]. In addition, the significant reduction in environmental pollution intensity translates into actual economic benefits for carbon reduction facilities, further enhancing their economic feasibility.
The technology of algal–bacterial granular sludge has become a cutting-edge research object of concern in academia and engineering fields, and it presents a wide range of potential values in laboratories and practical applications. The research trend of this technology reflects the deepening development of basic scientific exploration, and further clarification of its internal mechanism is needed through systematic research. With the help of advanced molecular sequencing technology, researchers can accurately analyze the complex dynamics of microbial communities and reveal their ecological functional relationships, thereby achieving the transformation of biological resources and economic value development.
With the increasing complexity of control systems, performance optimization systems based on machine learning and artificial intelligence are gradually reaching the forefront of research. These systems significantly enhance the stability and efficiency of the processing process by adjusting operating parameters in real time. More importantly, proactive prediction models enable researchers to estimate the potential performance of these systems in dynamic environments, thereby enabling the development of effective control strategies in advance [84].
Column-type PSBRs exhibit significant carbon footprint reduction potential through UV-LED optimization and solar integration. Key decarbonization strategies include renewable energy hybridization for PSBRs, sludge-to-energy conversion for SBRs, and bio-based material substitution in photobioreactors, which collectively have the potential to reduce sectoral emissions by 15–25% through optimized configurations.
The relevance of this technology in the field of wastewater treatment may have far exceeded the original standards. The academic community is actively researching its application in emerging pollutants, such as microplastics [109]. There are potential applications in the remediation of drug residues and persistent organic compounds. Customized functional materials for specific industrial pollutants have demonstrated significant scientific value. At present, research focuses on integrating this technology with advanced membrane separation systems and advanced oxidation technologies to optimize the overall performance of pollutant treatment [110].

6. Conclusions

This article provides a comprehensive review of the application of algal–bacterial symbiotic granular sludge technology in wastewater treatment, emphasizing its importance and potential in modern environmental engineering. With the acceleration of global urbanization and industrialization, traditional wastewater treatment methods are facing challenges of low efficiency and high costs. The symbiotic granular sludge technology of algae and bacteria, as an emerging ecological engineering innovation, utilizes the synergistic effect between microalgae and bacteria to demonstrate superior treatment efficiency and economy, becoming an effective solution to wastewater treatment problems.
Research has shown that the system has a removal rate of over 90% for organic matter, a total nitrogen removal rate of over 80%, and a phosphorus removal rate of 70–95%, and that it exhibits good treatment effects on heavy metals and other pollutants. This achievement not only proves the efficiency of the algal–bacterial symbiotic granular sludge system, but also provides strong support for its promotion in practical applications. In addition, the advantages of algal–bacterial symbiotic granular sludge technology in terms of economic cost and operational durability will make it highly competitive in the future wastewater treatment market.
In the study of microbial communities, this article emphasizes their complexity and synergistic effects under suitable environmental conditions. Optimizing cultivation methods and operating parameters is crucial for the reliability and performance of the system, providing direction for further improving the effectiveness of processing techniques. Through the application of artificial intelligence, the treatment efficiency of algal–bacterial symbiotic granular sludge technology is expected to be further improved, which opens up new possibilities for future research and applications.
Overall, the symbiotic granular sludge technology of algae and bacteria has shown great potential for application in the field of wastewater treatment. With in-depth research on its formation mechanism, cultivation strategy, and performance evaluation, this technology is expected to play a greater role in resource recycling and environmental protection. Future research should continue to focus on the dynamic changes of microbial communities and their impact on system performance, in order to achieve more efficient and sustainable wastewater treatment solutions.

Funding

This research was funded by Shandong province natural science foundation of China grant number ZR2023ME202.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Schematic diagram of the spatial distribution and metabolic interactions of microorganisms within algal and bacterial particles. (A,a,B,b) Schematic diagram of aerobic particles; (C,c,D,d,E,e,F,f) Schematic diagram of algal bacterial particles; (G,g) Schematic diagram of biofilm morphology and appearance on the reactor This figure shows the hierarchical structure, displaying the distribution of photosynthetic microalgae and heterotrophic bacteria, as well as their key metabolic interactions [9]. Morphological appearances of aerobic granules, algal–bacterial granules, and the biofilm attached to the reactor on days 1, 3, and 7, respectively.
Figure 1. Schematic diagram of the spatial distribution and metabolic interactions of microorganisms within algal and bacterial particles. (A,a,B,b) Schematic diagram of aerobic particles; (C,c,D,d,E,e,F,f) Schematic diagram of algal bacterial particles; (G,g) Schematic diagram of biofilm morphology and appearance on the reactor This figure shows the hierarchical structure, displaying the distribution of photosynthetic microalgae and heterotrophic bacteria, as well as their key metabolic interactions [9]. Morphological appearances of aerobic granules, algal–bacterial granules, and the biofilm attached to the reactor on days 1, 3, and 7, respectively.
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Figure 2. Digital and scanning electron microscopic (SEM) images of mature aerobic granule samples. (a) Digital image of the morphology of mature aerobic granules in PSBR. Scanning electron microscopic (SEM) images of (b) an entire aerobic granule in PSBR, (c–f) the outer surface of PSBR granule, (g,h) the outer surface of traditional SBR granule [1].
Figure 2. Digital and scanning electron microscopic (SEM) images of mature aerobic granule samples. (a) Digital image of the morphology of mature aerobic granules in PSBR. Scanning electron microscopic (SEM) images of (b) an entire aerobic granule in PSBR, (c–f) the outer surface of PSBR granule, (g,h) the outer surface of traditional SBR granule [1].
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Figure 3. The temporal dynamics of the main microbial communities during the development of algal–bacterial granular sludge. The graph displays the percentage distribution of various microbial populations at different time points, with each color denoting distinct microbial communities [17]. Group I (H/D = 4.18, diameter 8.5 cm, height 35.5 cm): AR-R1/2/3 with aeration rates of 2/4/6 L/min (superficial velocities 0.58/1.17/1.76 cm/s) under 6 h cycles (1/355/2/2 min for feeding/aeration/settling/decanting). Group II (H/D = 1.32, diameter 12.5 cm, height 16.5 cm): AS-R1/2/3 with cycle durations of 2/4/6 h, yielding aeration times of 115/235/355 min/cycle while maintaining fixed feeding/settling/decanting durations (1/2/2 min).
Figure 3. The temporal dynamics of the main microbial communities during the development of algal–bacterial granular sludge. The graph displays the percentage distribution of various microbial populations at different time points, with each color denoting distinct microbial communities [17]. Group I (H/D = 4.18, diameter 8.5 cm, height 35.5 cm): AR-R1/2/3 with aeration rates of 2/4/6 L/min (superficial velocities 0.58/1.17/1.76 cm/s) under 6 h cycles (1/355/2/2 min for feeding/aeration/settling/decanting). Group II (H/D = 1.32, diameter 12.5 cm, height 16.5 cm): AS-R1/2/3 with cycle durations of 2/4/6 h, yielding aeration times of 115/235/355 min/cycle while maintaining fixed feeding/settling/decanting durations (1/2/2 min).
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Figure 4. Metabolic interaction network illustrates the exchange of nutrients and metabolites between microalgae and bacteria in the granular sludge system [26].
Figure 4. Metabolic interaction network illustrates the exchange of nutrients and metabolites between microalgae and bacteria in the granular sludge system [26].
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Table 1. Reactor Performance Comparison for Algal–Bacterial Granular Sludge Systems: Cultivation Efficiency and Wastewater Treatment Applications.
Table 1. Reactor Performance Comparison for Algal–Bacterial Granular Sludge Systems: Cultivation Efficiency and Wastewater Treatment Applications.
Reactor TypeMain FeaturesAdvantageLimitationTreatment Efficiency
Column-type PSBR [61]-Vertical configuration
-External lighting
-Height-to-diameter ratio > 5		-Enhances transparency
-Efficient mixing
-A good settlement choice		-Difficulty in expanding scale
-Large-scale light attenuation		COD: 85–95%
Total nitrogen: 75–85%
Total phosphorus: 70–80%
Traditional SBR [59]-Loop operation
-Internal lighting
-Mechanical mixing		-High operational flexibility
-Easy to process control
-Mature technology		-Limited utilization of light
-High energy consumption		COD: 80–90%
TN: 70–80%
Total phosphorus: 65–75%
Flat-plate photobioreactor [44]-Large illumination area
-Shallow depth
-Continuous operation		-Maximum exposure
-High biomass productivity
-Uniform flow distribution		-High construction cost
-Maintenance difficulties
-Complex scaling up		COD: 75–85%
Total nitrogen: 65–75%
Total phosphorus: 60–70%
Membrane photobioreactor [62]-Membrane separation
-Hybrid power system
-Continuous operation		-High biomass retention rate
-Excellent effluent quality
-Compact footprint		-Severe membrane fouling
-High operating costs
-Complex operation		COD: 90–95%
TN: 80–90%
Total phosphorus: 75–85%
Table 2. Optimal Operating Parameters for Algal–Bacterial Granular Sludge Reactors: pH, HRT, and Biomass Control in Municipal Wastewater.
Table 2. Optimal Operating Parameters for Algal–Bacterial Granular Sludge Reactors: pH, HRT, and Biomass Control in Municipal Wastewater.
ParameterBest RangeImpact on the SystemPerformance Indicators
Light intensity [67]150–400 μmol/m2/s-Photosynthetic efficiency
-Biomass growth
-Particle stability		-O2 productivity
-Chlorophyll content
-EPS production
Nutrient load [68]Chemical oxygen demand: 400–800 mg/L
N/P ratio: 5:1–8:1		-Microbial growth
-Particle formation
-Treatment efficiency		-COD removal rate: >85%
-Nitrogen removal rate: >75%
-Phosphorus removal rate: >70%
HRT [69]12 to 48 h-Biomass retention
-Nutrient removal
-Particle maturity		-Settlement velocity
-Particle size
-Effluent quality
Temperature and pH value [40]Temperature: 20–30 °C
PH value: 7.0–8.5		-Metabolic activity
-Community structure
-System stability		-Specific growth rate
-Removal efficiency
-Particle strength
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Chen, S.; Wang, J.; Feng, X.; Zhao, F. Algal–Bacterial Symbiotic Granular Sludge Technology in Wastewater Treatment: A Review on Advances and Future Prospects. Water 2025, 17, 1647. https://doi.org/10.3390/w17111647

AMA Style

Chen S, Wang J, Feng X, Zhao F. Algal–Bacterial Symbiotic Granular Sludge Technology in Wastewater Treatment: A Review on Advances and Future Prospects. Water. 2025; 17(11):1647. https://doi.org/10.3390/w17111647

Chicago/Turabian Style

Chen, Shengnan, Jiashuo Wang, Xin Feng, and Fangchao Zhao. 2025. "Algal–Bacterial Symbiotic Granular Sludge Technology in Wastewater Treatment: A Review on Advances and Future Prospects" Water 17, no. 11: 1647. https://doi.org/10.3390/w17111647

APA Style

Chen, S., Wang, J., Feng, X., & Zhao, F. (2025). Algal–Bacterial Symbiotic Granular Sludge Technology in Wastewater Treatment: A Review on Advances and Future Prospects. Water, 17(11), 1647. https://doi.org/10.3390/w17111647

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