Unveiling the Secrets of Particle Size in Aerobic Granules: Impacts on Emerging Contaminants Removal, Stability, and Sustainability: A Review

Abstract

Aerobic granular sludge (AGS) has attracted considerable attention in the field of wastewater treatment due to its numerous advantages. This paper presents a comprehensive review of the key factors influencing AGS particle size, highlighting the varying degrees of impact exerted by different factors. Particle size is a critical determinant in several aspects, including the removal efficiency of emerging contaminants, the energy consumption associated with the long-term stable operation of the system, and greenhouse gas (GHG) emissions. Smaller particles enhance the removal efficiency of emerging contaminants due to their larger specific surface area and increased number of reaction sites. In contrast, larger particles often lack internal structural mechanisms, which can facilitate the growth of filamentous bacteria, thereby undermining granule stability. Moreover, smaller AGS particles are linked to decreased simultaneous nitrification and denitrification (SND) efficiency, leading to increased GHG emissions. Consequently, the optimal size range for AGS is generally between 1.0 and 2.0 mm.

1. Introduction

In recent years, aerobic granular sludge (AGS) has emerged as an innovative wastewater treatment technology characterized by its exceptional settling properties, high biomass concentration, and ability to treat elevated concentrations of organic wastewater under conditions of high volumetric loading [1]. During the second seminar on AGS in 2006, the minimum particle size of AGS was generally established at 0.2 mm; however, this standard is subject to modification based on specific conditions. Consequently, particle size alone is not a definitive criterion for determining AGS formation [2]. Within cultivation systems, the particle size of AGS is typically closely associated with its performance. As microorganisms proliferate, the particle size increases, with larger particles generally demonstrating improved settleability. Nevertheless, excessive growth of filamentous bacteria can eventually result in granule disintegration [3]. In contrast, smaller particles are susceptible to flotation, which reduces settleability and negatively impacts the stable operation of the system.

The surface of AGS is distinguished by its high porosity, which enhances the diffusion of dissolved oxygen and accelerates the transfer of metabolic products. Additionally, the presence of aerobic bacteria on the outer layer and anaerobic denitrifying bacteria within the granule’s interior facilitates simultaneous nitrification and denitrification processes, thereby effectively collaborating to remove various pollutants from wastewater [4]. AGS has demonstrated exceptional efficacy in the removal of conventional pollutants, including organic matter, nitrogen, and phosphorus. With the ongoing enhancement of wastewater treatment standards and the increased focus on emerging contaminants, researchers have increasingly explored the potential application of AGS technology in this area. The granules’ particle size is critical in determining the removal efficiency of emerging contaminants during AGS treatment. The superior settleability and larger specific surface area of the granules promote efficient sludge–water separation and the adsorption and degradation of emerging contaminants, thereby enhancing their treatment capacity. As a result, AGS technology not only effectively mitigates traditional pollutants but also exhibits significant potential for managing emerging contaminants.

In the current context of sustainable development, optimizing wastewater treatment technologies is essential for reducing environmental impacts and resource consumption. The influence of granule size on the regulation of greenhouse gas (GHG) emissions from AGS systems is critically significant. Modifying granule size can markedly affect the settleability and biodegradation rates of sludge, which are directly associated with the production and release of GHGs, including carbon dioxide (CO₂) and methane (CH₄) [5]. Typically, larger granule sizes enhance settling performance, facilitating the separation of sludge from the mixed liquor. This rapid settling allows for more efficient solid–liquid separation during each operational cycle, thereby improving sludge discharge efficiency and decreasing the system’s dependence on sludge retention. As a result, the sludge retention time (SRT) is reduced. A shortened SRT limits the prolonged residence of sludge within the system, suppressing the formation of anaerobic zones and effectively mitigating the generation and release of greenhouse gases, such as methane, in wastewater treatment processes [6].

Although the application of AGS in wastewater treatment has shown promising results, there is a notable lack of systematic analyses and scholarly articles addressing the size of AGS within the current body of literature. Therefore, further investigation in this area is necessary. This article primarily synthesizes research studies from the past two decades related to AGS particle size. It explores the factors influencing the size of aerobic sludge particles and assesses the impact of particle size on the removal of emerging contaminants, granule stability, and environmental sustainability. The aim of this work is to provide theoretical and technical support for advancing research and application of aerobic granular sludge technology.

2. Formation Mechanism of AGS

The formation process of AGS is highly complex, involving physical, chemical, and biological dimensions. Numerous researchers have proposed various hypotheses to explain the formation of AGS, which primarily include the following.

2.1. Microbial Auto-Aggregation Hypothesis

As demonstrated in Figure 1, microorganisms can spontaneously aggregate under specific environmental conditions to form a symbiotic entity characterized by a compact structure and enhanced activity. This is achieved through the modulation of their metabolic processes and surface properties [7]. Adav et al. [8] successfully cultivated the Acinetobacter calcoaceticus strain within a sequencing batch reactor (SBR) system, resulting in the development of granular sludge with an average diameter of approximately 2.3 mm after a 7-week cultivation period. Similarly, Ivanov et al. [9] documented the formation of aerobic granules with an average diameter of 446 µm within just 8 days by combining flocculant strains with activated sludge as an inoculum. While this hypothesis provides a theoretical framework for the development of granular sludge, it does not fundamentally elucidate the underlying mechanisms driving microbial flocculation.

2.2. Extracellular Polymeric Substances Hypothesis

As illustrated in Figure 2, the EPS produced by microorganisms, which is mainly composed of polysaccharides, proteins, and nucleic acids, has adhesive properties that promote interactions between cells, facilitating the aggregation and sedimentation of particles. The continuous growth and reproduction of microorganisms contributes to an increase in the volume of sludge particles, while hydrodynamic shear forces aid in the development of AGS with a smooth morphology [10,11]. Deng et al. [12] successfully cultivated granular sludge with diameters ranging from 0.8 to 1.1 mm after 35 days of cultivation. Their monitoring of EPS variations during the granulation process revealed that EPS significantly influences the formation of AGS. Furthermore, Liu et al. [13] applied a magnetic field of 50 mT in an SBR, which further stimulated EPS secretion and reduced the granulation time by 15 days. The enhanced secretion of EPS can expedite the formation of granular sludge. In contrast, insufficient EPS content can adversely affect granule formation, leading to cell lysis and decay, thereby compromising the structural integrity of the AGS [14].

2.3. Filamentous Bacteria Framework Hypothesis

As illustrated in Figure 3, filamentous bacteria predominate within the microbial community, promoting the attachment and proliferation of microorganisms through their expansion and the establishment of network structures. This interwoven network of filamentous bacteria serves as the foundational support for the granule structure, providing a stable three-dimensional framework for the granules [15]. Du et al. [16] successfully cultivated AGS with a settling velocity of 45 m/h and particle sizes between 2 and 4 mm within a mere 15 days by enhancing the growth of filamentous bacteria in an SBR, thereby demonstrating that filamentous bacteria are instrumental in the formation of granular sludge. Similarly, Han et al. [17] achieved rapid granulation of AGS by introducing filamentous bacteria, thereby reducing the granulation period by 30 days. However, excessive proliferation of filamentous bacteria can compromise the structural integrity of the granular sludge and may even result in the disintegration of the granules. Therefore, maintaining filamentous bacteria at a controlled level is advantageous for preserving the structural stability of AGS [18].

2.4. Nucleation Hypothesis

As shown in Figure 4, the hypothesis posits that the formation and stability of AGS are contingent upon specific microorganisms or substances within the microbial community that act as crystal nuclei. These nuclei are instrumental in granule aggregation and maturation by promoting microbial accumulation and attachment on their surfaces [19]. The core of AGS may comprise inoculated mature granular sludge, inorganic materials, and inert organic matter [20]. In their study, Verawaty et al. [21] utilized artificially crushed granular sludge as induction nuclei. The findings indicated that these sludge fragments provided attachment sites for flocculent sludge within the system, thereby functioning as crystal nuclei and expediting the granulation process. Kong et al. [22] observed that the strong reducing properties of zero-valent iron (ZVI) not only shortened the granulation period by 21 days and enhanced EPS secretion but also improved the composition of the microbial community. Additionally, other researchers have successfully cultivated nitrogen-removing AGS by incorporating spherical inert carriers into reactors, further substantiating the crystal nucleus hypothesis [23].

2.5. Metal Cation Hypothesis

As depicted in Figure 5, the hypothesis suggests that metal cations, such as Ca²⁺, Fe²⁺, and Mg²⁺, interact with negatively charged groups within EPS, thereby neutralizing surface charges and diminishing electrostatic repulsion among microbial cells. This interaction compresses the electrical double layer, enhancing van der Waals forces and subsequently facilitating sludge granulation [24,25]. Jiang et al. [26], through comparative experiments, demonstrated that the addition of Ca²⁺ at a concentration of 100 mg/L significantly expedited the granulation process of AGS by 16 days relative to the control group without Ca²⁺, thereby affirming the positive influence of Ca²⁺ on sludge granulation. Nevertheless, excessive concentrations of inorganic substances can hinder oxygen and substrate transfer, disrupting the internal micro-environmental equilibrium of granules and resulting in the formation of “dead zones” within the core region. Additionally, Agridiotis et al. [27] found that adding 30 mg/L of Fe²⁺ facilitates the transformation of flocculent sludge into denser structures, thereby creating favorable conditions for granulation and significantly enhancing the system’s efficiency in removing organic matter and total phosphorus [28].

2.6. Selective Pressure-Driven Hypothesis

The settling velocity of sludge is utilized as a selective pressure mechanism. By adjusting the reactor’s settling time, sludge with suboptimal settling characteristics is removed from the reactor, thereby preserving sludge with superior settleability and promoting the granulation process [29]. As demonstrated in Figure 6, using the continuous-flow reactor as an example, in addition to settling velocity, selective pressures such as granular size and density are crucial in the selection of sludge for AGS cultivation [30,31,32]. Xu et al. [33] developed an innovative cultivation mode for AGS by constructing an anoxic zone using sponge fillers. Subsequently, through intermittent squeezing and periodic aeration with large air bubbles, the aggregates were transferred into the aeration zone. The combination of hydraulic shear force and the “two-zone clarification” selection pressure facilitated the granulation of aerobic sludge, resulting in an average particle size of 0.36 mm. Liu et al. [34] developed an innovative screening system based on particle size, employing a mesh range of 0.1–1.0 mm to fractionate mixed sludge. This system selectively retained larger granules while allowing the discharge of flocculent sludge, thereby improving granulation efficiency and successfully cultivating AGS within 14 days. Furthermore, Liu and Xu [35,36] independently incorporated hydrocyclones into anoxic–oxic (AO) processes and SBR, respectively. Although these integrations retained high-density sludge to enhance microbial activity, the application of shear forces and centrifugation resulted in reduced particle size and diminished settleability.

3. Investigation of Factors Influencing Particle Size of AGS

The granulation process of AGS is subject to a variety of physical, chemical, and microbial influences. This paper provides a comprehensive overview of the primary regulatory factors impacting AGS particle size, as shown in

Table 1. Influencing factors of AGS particle size.

Factors	Effect on AGS Particle Size
DO	DO concentration of 2–6 mg/L helps form larger sludge particles and maintain stability.
Temperature	A temperature range of 20 °C to 30 °C effectively promotes larger AGS formation.
pH	Keeping the pH in the range of 6.5 to 7.5 helps maintain particle uniformity and moderation.
Seed sludge type	Inoculating mature AGS and specific microbial strains accelerates the granulation process.
SLR	SBRs readily form large, uniform AGS granules at elevated SLR, while CFRs under identical SLR yield significantly smaller granules.
EPS	Increased EPS content promotes sludge particle agglomeration and the formation of larger AGS particles.
MLSS and SRT	Higher sludge concentration and a sludge age of 10 to 20 days promote stable AGS formation with larger particle size.
Shear force	The lower shear force (0.8–1.2 N/m2) contributes to the formation of AGS and the increase of particle size.
Wastewater concentration	At high organic, nitrogen and phosphorus concentrations, SBR-cultivated AGS attains larger particle sizes, whereas under low concentrations, CFR maintains AGS at a moderate, stable size.
External additive	The addition of PAC, MMF, and biochar can affect the particle size of AGS.
Reactor type	The particle size of AGS cultivated in CFRs is often smaller than that cultured in SBRs.

Note: PAC represents polyaluminum chloride. MMF represents modified microbial flocculants.

.

3.1. DO Concentration

The concentration of DO plays a crucial role in the formation and development of AGS, influencing granule size in both sequencing batch reactors (SBRs) and continuous flow reactors (CFRs). Research conducted by Kent et al. [37] suggests that successful aerobic granulation is feasible only when the DO concentration surpasses 0.1 mg/L. Nonetheless, numerous studies have demonstrated that low DO concentrations inhibit microbial metabolic activity and negatively impact granular sludge formation [38,39]. Di Bella et al. [40] observed that in SBRs maintained at a DO concentration of 7–8 mg/L, the average particle size of the resulting granular sludge exceeded 1.5 mm. This finding indicates that elevated DO concentrations enhance microbial growth and metabolism, thus promoting the formation and development of granular sludge. Conversely, Mady et al. [41] reported that DO levels exceeding 6.5 mg/L have a detrimental effect on EPS secretion, leading to reduced settleability. Consequently, excessively high DO concentrations can result in a loose and unstable internal granule structure, thereby limiting further increases in granule size.

Overall, under aerobic conditions, maintaining the DO concentration within the range of 2–6 mg/L promotes the formation of larger sludge particle sizes, thereby enhancing treatment efficacy.

3.2. Temperature

Temperature is a pivotal determinant of microbial metabolism and growth. In both SBRs and CFRs, temperature fluctuations exert similar effects on the particle size of AGS. A comprehensive review of the literature reveals extensive research on the impact of temperature on aerobic granulation [42]. Sun et al. [43] reported rapid AGS granulation in plug flow bioreactors within a temperature range of 10–22.5 °C, resulting in particle diameters between 0.2 and 3.4 mm. Similarly, Xu et al. [32] achieved rapid granulation in a CFR system maintained at 15–25 °C, producing AGS with an average particle size of 0.58 mm. Song et al. [44] explored the influence of temperature (25–35 °C) on AGS formation characteristics and the dynamic composition of the microbial community in an SBR. Their study demonstrated that mature AGS cultured at 30 °C exhibited high biological activity, a compact structure, large granule size, and stability, which enhanced pollutant removal. These findings are consistent with those of Araujo et al. [45].

Generally, the optimal temperature range for microbial activity and growth is between 20 °C and 30 °C. Within this range, microorganisms exhibit optimal activity and growth rates, which effectively facilitate the increase in particle size.

3.3. pH

The impact of sewage pH on AGS is predominantly observed in the activity and metabolic processes of microorganisms. In SBRs and CFRs, fluctuations in pH similarly influence the development of granular sludge and result in variations in particle size. Yang et al. [46] observed that in SBR reactors operating under acidic pH conditions, the granular sludge was predominantly composed of fungi. These granules were characterized by a large size and a loose structure, rendering them susceptible to fragmentation due to aeration. Conversely, under alkaline conditions, with a pH of 8.1 [47], the granular sludge was primarily constituted of bacteria. These granules were smaller in size but exhibited a dense structure. Consequently, a neutral pH environment is most favorable for the formation of granular sludge, aligning with findings under continuous flow conditions [48,49]. Furthermore, Sun et al. [43] reported that the particle size of AGS cultured within a pH range of 7.1 to 7.5 can reach up to 3.4 mm.

Generally, maintaining the pH within the range of 6.5 to 7.5 is advantageous for the growth and metabolism of most microorganisms, resulting in the formation of larger granule sizes.

3.4. Seed Sludge Type

In general, the various types of seed sludge employed in CFRs and SBRs exert distinct effects on the particle size of AGS.

Wang et al. [50] conducted an experiment in which mature AGS and activated sludge were separately inoculated into an SBR for cultivation. Their findings demonstrated that the reactor inoculated with AGS experienced a more rapid increase in particle size during the initial start-up phase. By the 14th day, granular sludge with a particle size ranging from 3 to 4 mm and exhibiting a regular morphology was observed. Furthermore, Lei et al. [51] cultivated AGS using anaerobic granular sludge with an initial particle size of 1.62 mm and reported that the majority of the particles remained intact, with only a minor fraction disintegrating into smaller granules. Moreover, AGS can be developed through the inoculation of specific microbial strains. In a study by Zhang et al. [52], the strains Propioniferax-like PG-02 and Comamonas sp. PG-08 were inoculated, resulting in the successful cultivation of AGS with diameters ranging from 0.2 to 0.7 mm within a period of 7 days for the purpose of phenol degradation, owing to their robust aggregation capabilities. Additionally, the dewatering performance of sludge can be enhanced through the synergistic conditioning with tannic acid (TA) and ammonium sulfate ((NH₄)₂SO₄) [53]. The high-density sludge solids in the dewatered sludge can serve as a framework for AGS formation. Abirami et al. [54] inoculated dewatered sludge to cultivate AGS, and with the addition of zeolite, aerobic granulation was achieved within just 7 days.

In conclusion, the selection of appropriate inoculants, such as mature aerobic or anaerobic granular sludge and specific microbial strains, is essential for accelerating the granulation process and facilitating the formation of larger-sized sludge particles.

3.5. Sludge Loading Rate (SLR)

The sludge loading rate is a critical operational parameter that significantly influences the particle size of AGS. It is important to note that the impact of SLR on particle size varies between SBRs and CFRs.

Jafari et al. [55] demonstrated that decreasing the SLR in an SBR from 0.42 kg COD/(kg MLSS·d) to 0.27 kg COD/(kg MLSS·d) resulted in a reduction of the average granule size to 1.11 mm. This reduction was attributed to substrate limitation at the lower loading rate, which inhibited microbial growth activity and EPS synthesis. Conversely, increasing the SLR generally enhances microbial metabolic activity and growth rate, thereby facilitating granule enlargement. Liu et al. [56] investigated AGS at loading rates ranging from 0.43 to 0.5 kg COD/(kg MLSS·d) and observed a consistent increase in granule size within this range, corroborating the findings of Yue [57]. However, excessively high loading rates (>2 kg COD/(kg MLSS·d)) may lead to granule disintegration due to anaerobic conditions in the inner layers and mass transfer limitations. Conversely, excessively low loading rates (<0.2 kg COD/(kg MLSS·d)) can result in microbial starvation and death due to inadequate nutrient penetration, leading to the formation of loose granule structures [58,59]. Higher loading rates have been observed to stimulate EPS secretion, which provides structural support and carbon sources for granule expansion [60]. However, some studies have reported contradictory findings, suggesting that increased loading rates may instead reduce EPS production, leading to smaller granule sizes [61]. These observations have been consistently corroborated, indicating that EPS synthesis under varying loading conditions may be governed by complex, multifactorial mechanisms that warrant further investigation.

Overall, due to the cyclic operation and strong selective pressure characteristic of SBR systems, maintaining a higher SLR generally promotes the formation of larger and more uniform AGS particles. In contrast, CFR systems tend to produce smaller and less uniformly distributed granules. This discrepancy may be attributed to the continuous inflow/outflow regime in CFRs, which often results in substrate overexposure and intensified hydrodynamic shear. These conditions can promote excessive microbial proliferation and granule disintegration, ultimately hindering stable granule development.

3.6. Wastewater Concentration

Variations in wastewater concentration exert a significant impact on sludge settleability and particle size distribution. Under conditions of high organic and nutrient loading (COD > 600 mg/L, TN ≈ 80 mg/L, TP ≈ 10 mg/L), abundant substrates promote rapid microbial growth and EPS secretion [62]. This leads to the formation of dense granular structures characterized by accelerated particle size growth. In SBR systems, this enhanced growth effect results in the development of larger granules with average diameters ranging from 2.0 to 3.0 mm, exhibiting excellent settling properties (SVI₃₀ < 40 mL/g). In contrast, CFR systems are subject to continuous particle attrition due to hydraulic shear forces and re-suspension effects, which result in smaller average particle sizes and a broader size distribution [63]. Consequently, SBRs demonstrate a superior capacity for developing larger AGS under high-load conditions. Conversely, under low-strength wastewater (COD < 200 mg/L, TN ≈ 15 mg/L, TP ≈ 2 mg/L), nutrient limitations impede microbial growth and EPS production. In SBRs, deficiencies in carbon, nitrogen, and phosphorus restrict granule development, leading to a decrease in average particle size to 0.4-0.8 mm and significantly compromised settleability (SVI₃₀ > 80 mL/g). CFR systems sustain relatively larger granules (0.8–1.0 mm) through the ongoing supply of substrate; however, the stability of particle size remains limited [64].

In summary, under conditions of high organic and nutrient concentrations, AGS cultivated in SBRs demonstrates significantly larger particle sizes compared to CFRs. Conversely, under low organic loading conditions, AGS in SBRs exhibits markedly reduced average particle diameter and deteriorated settling performance due to nutrient limitations, while CFR systems maintain relatively larger granule sizes through continuous, albeit minimal, substrate supply.

3.7. Shear Force

Shear force is a critical determinant of the particle size of AGS, primarily influenced by aeration intensity, mechanical agitation and other operational parameters. Research conducted by Tay et al. [65] explored the effects of varying aeration rates (0.2–0.6 vvm) on AGS formation, revealing that lower aeration rates result in the development of larger granules. This phenomenon occurs because reduced shear forces diminish hydraulic shear stress on sludge particles, thereby promoting an increase in granule size. However, granules formed under these conditions often possess loose structures, are susceptible to disintegration, and exhibit limited impact resistance, corroborating the findings of Xu et al. [66]. As shear forces escalate, the granular structure becomes more compact, yet the particle size correspondingly decreases. Although elevated shear forces can effectively remove loosely attached microorganisms from the surfaces of granules and inhibit filamentous bacteria growth, they concurrently impede the formation of larger granules. Consequently, moderate shear forces (0.8–1.2 N/m²) significantly contribute to granule densification by modulating EPS secretion levels and influencing microbial aggregation behavior, thereby enhancing structural compactness and stability [37,67]. The impact of shear force is equally crucial in the development of AGS within CFR. Ji et al. [68] demonstrated that stable granular sludge formation is achievable even when the SGV is reduced to 0.16 vvm, suggesting that under low shear conditions, spontaneous microbial aggregation and the shear induced by rising bubbles can still drive the granulation process. Winkler et al. [69] found through their research that AGS can remain stable in full-scale reactors under low SGV. In CFR systems, shear forces are generated not only by hydraulic flow but also through inter-particle collisions and disturbances caused by rising bubbles. These mechanical disturbances facilitate microbial redistribution and EPS renewal, thereby promoting structural stability. While higher shear forces can enhance granule density and surface hydrophobicity, thereby improving microbial aggregation capacity, excessive shear may also damage granular integrity, leading to granule breakage [70].

In conclusion, the regulation of shear forces plays a pivotal role in influencing the particle size and stability of AGS in both SBR and CFR systems. Lower shear forces facilitate granule growth and larger particle formation but result in loose structures and poor settleability, whereas excessively high shear forces may lead to granule fragmentation and dispersion.

3.8. MLSS and SRT

The concentration of MLSS and the SRT exert a significant influence on the particle size of AGS, although they are not the definitive factors for granulation [71].

In most experimental studies, the MLSS concentration typically ranges from 4 to 6 g/L, regardless of whether CFRs or SBRs are utilized [72]. Lin et al. [73] reported that in an SBR reactor, the average particle size was larger at a higher MLSS concentration of 6.7 g/L compared to a lower concentration of 4.64 g/L. However, excessively high MLSS levels can result in a decrease in granular size. Sun et al. [74] adjusted the maximum MLSS value in the CFR to 16 g/L, which led to a reduction in granule diameter to 2 mm. This finding is consistent with the study by Chen et al. [75]. This advantage is primarily attributed to the increased MLSS, which enhances the likelihood of collisions and aggregation among granules and also increases the number of microorganisms within the granules, thereby facilitating more rapid granule growth [76].

Luo et al. [77] demonstrated that within an SBR system, maintaining the SRT at 4–5 days resulted in an increase in particle size. Conversely, Chen et al. [78] reported that extending the SRT to 15–20 days increased the median particle size to 0.27 mm and 0.257 mm, respectively, highlighting the critical importance of optimizing SRT to enhance particle size. Under continuous flow conditions, Yu et al. [79] found that in CFRs, an excessively prolonged SRT can negatively affect the settleability of granules, leading to partial sludge loss and a gradual reduction in particle size. In the later stages of operation, regular sludge discharge improved settling performance and stabilized particle size at approximately 0.6 mm. An extended SRT may impair the settling properties of granular sludge and encourage the overgrowth of filamentous bacteria, thereby disrupting the granules.

Overall, higher MLSS concentrations can increase the likelihood of collisions and aggregation among granules, thus promoting granule growth. However, excessively high MLSS concentrations may result in mass transfer limitations and a loose granule structure. Meanwhile, maintaining an SRT between 10 and 20 days optimizes microbial growth and settling characteristics, resulting in larger and more stable granular sludge.

3.9. EPS

The content of EPS is a crucial determinant of the particle size and structural stability of AGS. The impact of EPS content on AGS particle size is consistent across both SBRs and CFRs [80,81].

Research by Kent et al. [37] indicates that an increase in EPS leads to a higher concentration of polysaccharides (PS) on granule surfaces [82]. Due to their adhesive properties, PS polymers facilitate the connection of smaller granules, thereby enhancing the aggregation of microorganisms and promoting the formation of larger granules within the biopolymer matrix. In experiments conducted by Corsino et al. [83] under continuous-flow conditions, a reduction in EPS content from 500 mg/g volatile suspended solids (VSS) to 200 mg/g VSS resulted in a decrease in the particle diameter of granular sludge from 2 mm to 1 mm, along with a marked weakening of the granules’ structural integrity. These findings suggest that a decrease in EPS content can reduce the adhesion and cohesion between granules, leading to smaller particle sizes. Wang et al. [24] observed within the SBR system that during the granulation process of AGS, the tightly bound extracellular polymeric substances (TB-EPS) within the EPS matrix exhibit significant adhesive and adsorptive characteristics. Notably, the concentration of TB-EPS surpasses that of other EPS forms, thereby facilitating an increase in particle size [84].

In general, an augmentation in EPS content enhances microbial aggregation, which subsequently promotes the agglomeration of sludge granules, resulting in an increase in the particle size of the granular sludge.

3.10. Microbial Community

Recent studies suggest that microbial communities in CFRs and SBRs have similar impacts on the particle size of AGS. At the phylum level, Proteobacteria is identified as the dominant group. Within this phylum, the increased presence of key genera such as Lysobacter and Hyphomicrobium significantly enhances EPS secretion and improves the hydrophobicity of the granules, thereby promoting the rapid formation and stability of AGS in the reactor [85]. Zhou et al. [86] reported that in AGS with larger particles (>3 mm), the genera Comamonas and Acinetobacter, both obligate anoxic denitrifiers, were more abundant. This observation led to the hypothesis that the increased abundance of these genera supports granule growth. Furthermore, an increase in the abundance of Thauera and Zoogloea was also found to influence EPS secretion [87].

In addition to Proteobacteria, the enrichment of glycogen-accumulating organisms (GAOs), predominantly from the genera Candidatus Competibacter and Amaricoccus, facilitates EPS secretion, thereby contributing to increased particle size [88]. Consequently, it can be hypothesized that microorganisms possessing EPS secretion capabilities may aid in the aggregation of diverse microbial types and simultaneously enhance the particle size [89].

3.11. External Additive

The nature and application of external additives play a crucial role in influencing the size and performance of AGS. Recent studies have identified polyaluminum chloride (PAC), biological flocculants, and biochar as the most commonly employed external additives [90,91]. Zhang et al. [92] investigated the modifications in granules within an aerobic granular membrane bioreactor (AGMBR) and reported an improvement in zeta potential from −5.38 mV to −0.395 mV, alongside a fourfold increase in particle size compared to systems without flocculant, attributable to the synergistic effects of PAC and modified microbial flocculants (MMF). Furthermore, the incorporation of biochar under both SBR and continuous flow conditions enhances the granulation process. This enhancement is attributed not only to biochar’s high specific surface area, pore structure, and hydrophilicity but also to the potential stimulation of EPS production by Al³⁺ and Mg²⁺ ions present on the biochar surface, leading to a significant increase in the proportion of particles exceeding 0.2 mm in size [93,94].

In conclusion, research suggests that the strategic addition of external additives in AGS systems can facilitate the rapid aggregation of microbial particles and other particulate matter into larger particles, thereby increasing the diameter and density of AGS.

3.12. Reactor Configuration

Different reactor configurations have distinct impacts on the formation and development of AGS, influencing both its size and performance. A significant body of research has primarily concentrated on SBRs and CFRs [95,96]. The intermittent operational characteristics of SBRs allow for improved control over feeding conditions, selective pressure, and hydraulic shear forces, which are conducive to the formation of AGS with larger particle sizes [78]. Conversely, CFRs, due to their continuous flow nature, face challenges in regulating these conditions, which adversely affects the development of AGS particle size. This issue can be partly attributed to a reduced diffusion driving force for nutrients, which impedes the formation of larger granules. Moreover, the continuous feeding regime in CFRs may decrease the settling velocity, resulting in the development of smaller and less compact granule structures. Nevertheless, although granule size in CFRs is typically smaller, aerobic granulation can be enhanced by integrating a three-phase separator or implementing a dual-zone sedimentation tank [97,98].

Overall, AGS developed within CFRs typically exhibits a smaller particle size compared to that cultivated in SBRs, but aerobic granulation can be enhanced through the integration of a three-phase separator or the implementation of a dual-zone sedimentation tank (Figure 6).

4. Impact of AGS Size on Removal of Emerging Contaminants

AGS technology has demonstrated remarkable efficacy in the removal of organic matter and essential nutrients, including nitrogen and phosphorus, from wastewater. In terms of total phosphorus (TP) removal, the AGS system primarily achieves efficient phosphorus elimination through the synergistic mechanisms of enhanced biological phosphorus removal (EBPR) and physicochemical adsorption. When the AGS particle size is within the range of 0.5–0.9 mm, stable anaerobic/aerobic microenvironments can develop within the granules. This facilitates phosphorus release by polyphosphate-accumulating organisms (PAOs) during the anaerobic phase and phosphorus uptake during the aerobic phase. Furthermore, this optimal particle size provides a sufficient specific surface area for enhanced mass transfer while maintaining excellent settleability [99]. Additionally, the TB-EPS secreted by AGS not only adsorbs phosphate but also form precipitates with metal ions, thereby further enhancing phosphorus removal.

The impact of AGS particle size on TP removal is predominantly determined by the equilibrium between mass transfer efficiency and microbial activity. Granules that are excessively small (<0.2 mm), although characterized by a high specific surface area and rapid mass transfer, are susceptible to washout and structural instability [100]. In contrast, oversized granules (>1.5 mm) experience restricted diffusion of dissolved oxygen and substrates, resulting in diminished PAO activity within the inner layers [101]. Research indicates that by modulating reactor shear force and organic loading to maintain AGS particle size within the 0.5–0.9 mm range, enhanced TP removal rates can be achieved. This optimal size range also promotes the enrichment of key functional microbial communities, such as Proteobacteria, which possess phosphorus-metabolizing capabilities, thereby further augmenting the system’s phosphorus removal efficiency.

The AGS technology demonstrates not only remarkable treatment efficiency but also outstanding operational stability. This technology has successfully transitioned from laboratory research and development to large-scale application, leading to significant advancements in the wastewater treatment sector. However, as the diversity of pollutants continues to expand, traditional treatment methods are increasingly challenged by the need to address emerging contaminants. Consequently, exploring the potential of AGS technology for the removal of these emerging contaminants has become a critical area of focus to enhance the overall effectiveness of wastewater treatment processes. Emerging contaminants primarily include persistent organic pollutants (POPs), antibiotics, microplastics, and endocrine-disrupting chemicals (EDCs). The inherent structural advantages of AGS facilitate the removal of these contaminants through mechanisms of biodegradation and bioadsorption [102]. For analytical purposes,

Table 2. Relationship between size of aerobic granular sludge (AGS) and removal of emerging contaminants.

Emerging Contaminants	Granulation Strategy	Removal Efficiency	Relationship Between Particle Size and Pollutant Removal	Removal Method	Reference
Microplastics	Inoculate aerobic granular sludge and introduce TMPs	SLR = 0.24 kg COD/(kg MLSS·d), TMPs > 52.6%, COD > 80%, NH4+-N > 96%, TP = 60–75%, TMPs > 52.6%	With increasing TMPs, the proportion of small particles (<40 μm) decreased, while the proportion of larger particles (90–355 μm) increased. However, larger granules (>710 μm) nearly disappeared.	Biodegradation, adsorption, and chemical transformation.	[100]
	Inoculate active sludge and introduce MPs or NPs	SLR = 0.36 kg COD/(kg MLSS·d), MPs = 95%, NPs = 98.9%, COD > 91.3%, PO43−-P = 68.4–73.6%	Exposure to MPs and NPs reduced the AGS particle size to 3.25 mm and 3.02 mm, respectively, impairing TN removal efficiency.	Resisting the stress of microplastics by enhancing EPS secretion and leveraging sludge adsorption.	[103]
	Inoculate mature ABGS	SLR = 0.13 kg COD/(kg MLSS·d), MPs > 96%, COD = 90.6–92.1%, TP = 92.8–95.7%, TN = 98.8–99.6%	AGS (0.67 mm) may experience structural disruption due to microplastic attachment and excessive filamentous bacterial growth, impairing pollutant removal efficiency.	Resisting the stress of microplastics by enhancing EPS secretion and leveraging sludge adsorption.	[104]
Persistent Organic Pollutants (POPs)	Internal settling tank	Under an SLR of 0.13 kg COD/(kg MLSS·d), the PFOS removal efficiencies reached 89.23 ± 0.92%, 71.92 ± 1.45%, and 48.15 ± 1.90% at concentrations of 0.1, 0.5, and 5.0 mg/L, respectively.	The average particle size of AGS was 1.3 mm. Smaller particles, due to their larger specific surface area, could provide more adsorption sites, and thus exhibit greater pollutant removal capacity.	AGS are mainly removed by adsorption methods such as electrostatic, hydrophobic and ion bridging.	[105]
Endocrine Disruptors Chemicals	Alternating anaerobic and aerobic conditions with SRT of 15 days	SLR = 0.2 kg COD/(kg MLSS·d), COD = 94%, NH4+-N = 96%, PO43 – P = 90%, E2 = 99%, EE2 = 93%	Smaller granules enhance rapid adsorption and degradation due to higher mass transfer efficiency, while larger ones provide stronger adsorption but slower degradation.	Biodegradation, adsorption, and the role of microbial communities.	[106]
Endocrine Disruptors Chemicals	Inoculate high-salt-adapted sludge	SLR = 0.22 kg COD/(kg MLSS·d), E2 = 100%, EE2 = 47%, BPA = 95%	Intermediate-sized granules (0.32–0.52 mm) achieve higher adsorption and degradation efficiencies, leading to more effective pollutant removal.	Biodegradation, adsorption, and the role of microbial communities.	[107]
Antibiotics	Internal settling tank	Under an SLR of 0.114 kg COD/(kg MLSS·d), the removal rate of COD decreased to 82.42% and the removal rate of SMX decreased. The coexistence of PS and SMX reduces the wastewater treatment performance of SBR.	The coexistence of PS and SMX gradually reduces the particle size (from 1.86 to 1.43 mm), reduces the sedimentation, and reduces the removal efficiency of pollutants.	The AGS system responds to adverse environments by increasing the secretion of EPS.	[108]
	Submersible MFCS are coupled to AGS.	SLR = 0.24 kg COD/(kg MLSS·d), COD = 92.1%, CIP (average removal efficiency) = 83.2%	AGS of 1.5–2 mm was conducive to the enrichment of antibiotic resistance genes and the removal rate of CIP was effectively improved.	The continuous electrical stimulation promotes microbial activity and secretes more EPS to resist CIP stress.	[109]

Note: E2 represents 17b-estradiol. EE2 represents 17a-ethinylestradiol. BPA represents bisphenol-A. TMPs represents tire microplastics. MPs represents microplastics. NPs represents nanoplastics. MFCS represents microbial fuel cells.

provides a summary of the correlation between the removal efficiency of various emerging contaminants and the particle size.

4.1. Microplastics

Microplastics, recognized as an emerging pollutant of considerable concern, are frequently detected in aquatic environments and pose significant threats to ecological safety [110]. Research demonstrates that microplastics at concentrations of 20 mg/L markedly inhibit biomass growth and sludge settleability, with a notable propensity to adhere to the surface of granular sludge. This adherence influences the electrostatic and hydrophobic interactions on the sludge surface, resulting in pore blockages and hindering the transfer of substrates to the interior [111]. Moreover, the biofilm that forms on microplastic surfaces can effectively prevent direct contact between microplastics and microorganisms, thereby mitigating their toxic effects. Additionally, microplastics can stimulate AGS to secrete increased quantities of protective proteins for microorganisms. However, excessively high concentrations of microplastics can induce the overproduction of reactive oxygen species, leading to the deterioration of sludge settling performance, reduced sludge concentration, compromised cell integrity, and diminished microbial activity [112].

Zheng et al. [113] have demonstrated that within SBRs, polyethylene microplastics (PE-MPs) compromise cellular integrity by increasing levels of reactive oxygen species and lactate dehydrogenase. In contrast, Zhang et al. [103] observed that at low concentrations (1 mg/L) of microplastics and nanoplastics, there was no significant impact on the properties and size of the particles, with removal efficiencies for microplastics and nanoplastics reaching 95% and 98.9%, respectively. However, as concentrations increased to 20 and 100 mg/L, the removal rates of total nitrogen (TN) significantly decreased, and the particle size of AGS also began to diminish. In practical applications, AGS typically achieves a removal rate of 60–90% for microplastics [104], primarily through the combined mechanisms of surface adsorption, capture by EPS, and retention by biofilms. Under stable operating conditions, the secretion of EPS and the self-regulatory capacity of the granule structure are critical for the retention and fixation of microplastics. At low concentrations (20 ng/L), an increase in PE-MPs concentration induces cells to secrete EPS, which encapsulate the sludge surface to mitigate toxicity [114]. This process results in the formation of larger and less dense granules, thereby enhancing the removal efficiency of tire microplastics (TMPs) in systems where biodegradation and adsorption are predominant [115]. However, as the concentration of PE-MPs continues to rise, the excessive secretion of loosely bound EPS (LB-EPS) leads to a progressively loose structure, increased volume, obstruction of sludge pores, and inhibited nutrient transport. In contrast, the augmentation of TB-EPS is beneficial for capturing small-sized plastic granules, thus preventing their infiltration into cells [116].

Song et al. [117] investigated the effects of varying concentrations of polylactic acid microplastics (DMP-PLAs) on particle size and granule removal performance. Their findings indicate that AGS exposed to lower concentrations of DMP-PLAs exhibited larger particle sizes without a notable change in removal efficiency. In contrast, at elevated concentrations (200 ng/L), the increase in particle size resulted in a loosening of the granule structure and a marked reduction in the removal efficiency of both polylactic acid microplastics and organic pollutants. In response to the toxicity, AGS secreted substantial amounts of EPS, which contributed to an increase in granule size. However, excessively large particle sizes were found to negatively impact pollutant removal efficiency.

4.2. Antibiotics

Under typical conditions, both the human body and animals can metabolize less than 50% of administered antibiotics, with the remainder being excreted into the environment either as metabolites or in their unaltered form. Due to their persistent nature, these antibiotics can pose considerable risks to ecosystems, even at low concentrations. Wang et al. [118] investigated the use of AGS for the treatment of piggery wastewater containing kanamycin, tetracycline, ciprofloxacin, ampicillin, and erythromycin. The study reported a total antibiotic removal rate of 88.4 ± 4.5%, with 62.5% attributed to biodegradation and 32.3% to adsorption. Mature AGS, characterized by an average granule size of 0.9 mm, exhibited high efficiency in removing antibiotics from piggery wastewater. Furthermore, research by [119] demonstrated that adjusting the influent carbon-to-nitrogen (C/N) ratio can influence the microbial community composition, promoting the enrichment of functional microorganisms within the AGS. This microbial enrichment facilitated the degradation of recalcitrant organic matter and enhanced EPS secretion, ultimately improving the removal rates of erythromycin and sulfamethoxazole (SMX) through bioadsorption and biodegradation processes. As the particle size of AGS increased, an enrichment of antibiotic resistance genes was observed [108]. Cheng et al. [120] investigated the characteristics of enhanced biological phosphorus removal–aerobic granular sludge (EBPR-AGS) systems under various antibiotic stress conditions. Initially, the particle size increased slowly as microorganisms mitigated antibiotic toxicity through polysaccharide secretion. After 30 days, there was a gradual increase in particle size, with microorganisms enhancing the granulation structure and stability through the production of LB-PN. The granulation process was completed within 40 days, resulting in a stable particle structure with a size of 0.52 mm. With the exception of ofloxacin, the average removal rates of tetracycline, sulfamethoxazole, and roxithromycin were significantly higher. Consequently, it can be inferred that granular sludge with a medium size (0.5–1 mm) demonstrates optimal performance in antibiotic removal.

Additionally, numerous researchers have advanced the removal of emerging contaminants by integrating various treatment methodologies. For example, Li et al. [121] explored the degradation process and the mechanism of toxicity alteration of sulfamethazine by coupling a micro-electric field with the AGS system. Through electrical stimulation, they modified the permeability of microbial cell membranes, thereby promoting the secretion of EPS and enhancing resistance to toxicity. The average granule size achieved was 3.4 mm. Similarly, some studies [109] have combined submerged microbial fuel cells (MFCs) with the AGS process to mitigate the toxicity of ciprofloxacin (CIP). The integration of MFCs enhanced the contact area and settling performance of the sludge, while also facilitating the enrichment of antibiotic resistance genes. As the particle size of the AGS-MFCs increased to 1.86 mm, the system effectively reduced the toxic effects of harmful substances.

4.3. Endocrine-Disrupting Chemicals

Endocrine Disrupting Chemicals (EDCs) are exogenous substances that can interfere with the endocrine functions of living organisms. In the context of wastewater treatment, the granular sludge size characteristics of granular sludge play a crucial role in determining the removal efficiency of these compounds. Castellanos et al. [106] found that the removal of 17β-estradiol (E2) and 17α-ethinylestradiol (EE2) by AGS is mainly achieved through biodegradation. Smaller granules, with their larger specific surface area, may provide more adsorption sites, thereby enhancing the adsorption capacity for EDCs. On the one hand, E2 and EE2 are degraded through microbial metabolism; on the other hand, AGS can significantly reduce the estrogenic activity of these compounds. Moreover, smaller granular sludge has higher mass transfer efficiency, allowing the adsorbed EDCs to be more rapidly transferred to the microbial community inside the granules for degradation. EDCs are rapidly adsorbed during the anaerobic phase and subsequently degraded during the aerobic phase. Ely et al. [107] investigated the removal performance of EDCs (E2, EE2, and bisphenol-A(BPA)) during the treatment of saline wastewater using AGS-SBR. During the initial phase of the experiment, the granule size predominantly ranged from 0.15 to 1.5 mm. The rapid removal of E2 suggests that the high mass transfer and adsorption capacity of small-sized granular sludge may have played a key role in E2 removal. After bioaugmentation, the mass transfer and degradation efficiency of small-sized granular sludge were further enhanced, leading to an increase in BPA removal efficiency. In the second phase, larger granules (D > 1.5 mm) were predominant. Initially, EE2 was adsorbed onto the granular sludge, but it was subsequently released back into the liquid phase. This indicates that while the larger granules have a strong adsorption capacity, their mass transfer efficiency is relatively low, resulting in incomplete degradation of EE2. It can be inferred that medium AGS may be more suitable for rapid mass transfer and degradation of EDCs, especially after bioaugmentation, which significantly improves its removal efficiency. In contrast, larger AGS has stronger adsorption capacity but lower mass transfer efficiency, resulting in slower degradation of EDCs. Therefore, granular sludge of intermediate size may achieve a balance between adsorption and degradation of EDCs, thereby realizing more efficient removal.

4.4. Persistent Organic Pollutants

In recent years, scholars have increasingly acknowledged that compounds exhibiting the characteristics of POPs extend beyond traditional POPs to include dyes, perfluoroalkyl substances (PFASs), and other organic compounds.

In the context of AGS treatment, particle size is intricately associated with the degradation capacity of pollutants. Zhu et al. [122] successfully cultivated AGS from anaerobic granules in an SBR, achieving complete mineralization of azo dyes. When the granular sludge particle size ranged from 0.8 to 1 mm, there was an increase in internal biomass, a more complex community structure, and a larger specific surface area. This facilitated the coexistence of anaerobic decolorizing bacteria and aerobic degrading bacteria, significantly enhancing the degradation efficiency of azo dyes. Due to the exceptionally high stability of the carbon–fluorine (C-F) bonds (513 ± 10 kJ/mol) in PFAS [123], these compounds are challenging to remove through biological degradation pathways. Consequently, in wastewater treatment plants, PFAS are primarily eliminated through adsorption by sludge.

Research on the elimination of POPs, such as perfluorooctane sulfonic acid (PFOS), in SBR systems has shown that medium-sized granules, owing to their enhanced surface area, provide more adsorption sites and exhibit a greater capacity for removing emerging contaminants [105]. Additionally, the efficiency of PFOS removal decreases as influent concentrations increase, likely due to the particle size distribution and its related adsorption properties. Larger particles tend to settle more quickly, thus reducing the contact time with pollutants and subsequently lowering removal efficiency. Comparatively, the use of appropriately fine granules presents advantages in terms of effective contact and reaction time, thereby enhancing the biological adsorption and removal of emerging contaminants.

In conclusion, the particle size of AGS is a critical determinant in the removal efficiency of emerging contaminants. Medium-sized particles are particularly advantageous for the adsorption and elimination of pollutants due to their larger specific surface area and increased number of reaction sites, while not contributing to internal mass transfer resistance associated with excessive size. The interaction between antibiotic types and granule size is intricate and variable; for example, larger AGS particles are correlated with decreased removal efficiency of roxithromycin (ROX). In the realm of microplastic treatment, an increase in pollutant concentration prompts elevated EPS secretion, leading to an enlargement of AGS particle size, which may result in pore blockage. Currently, no definitive conclusions have been drawn regarding the relationship between particle size and the removal efficiency of emerging contaminants.

5. Influence of AGS Size on Bulking Prevention and Long-Term Stability

5.1. Expansion Causes

During the operation of AGS systems, challenges such as sludge swelling and granule collapse are frequently encountered. These issues are primarily attributed to the overgrowth of filamentous bacteria and the presence of excessive nutrients in the water. Recent research has increasingly focused on elucidating the role of AGS size in preventing sludge bulking and managing granule collapse, underscoring its importance in maintaining the long-term stability of the system. In laboratory-scale AGS systems, rapid increases in particle size render the granules particularly susceptible to bulking, leading to reduced pollutant removal efficiency and poor settleability. However, in full-scale wastewater treatment plants employing AGS technology, the influence of particle size on sludge bulking may be modulated by additional factors, such as hydraulic conditions and influent characteristics.

5.2. Measures to Prevent Expansion

According to Qiu et al. [124], the integration of a multi-ionic matrix with a bio-carrier can facilitate the rapid formation and long-term stability of AGS in laboratory-scale reactors. The granular sludge exhibited an average particle size of approximately 0.8 mm, indicating that, under optimal conditions, the particle size can be consistently maintained to prevent excessive bulking. Zhou et al. [86] conducted an investigation into the structural integrity of granules by examining particle size using a peroxy-sensitive fluorescence probe and hydraulic shear experiments in laboratory settings. Their findings indicated that excessively large particle sizes (>3 mm) tend to result in deficiencies within the internal matrix of the granules, promote the overgrowth of filamentous bacteria, and lead to instability in the granule structure. In contrast, smaller particle sizes (<1.8 mm) facilitate complete penetration of DO, which reduces the efficacy of simultaneous nitrification and denitrification (SND) and negatively impacts removal efficiency. Consequently, it can be inferred that maintaining an aerobic granular size within the range of 1.8–3 mm is essential for controlling sludge bulking in laboratory-scale systems.

In full-scale wastewater treatment plants, the inhibition of filamentous bacteria and the enrichment of slow-growing microorganisms, as demonstrated by De Vleeschauwer et al. [125], contribute to the control of sludge bulking and the promotion of aerobic granulation. Following the addition of enhanced biological phosphorus removal sludge, Ni et al. [101] observed an increase in sludge particle size to 0.52 mm in laboratory-scale experiments, accompanied by a decrease in settleability from an initial 400 mL/g to 93.0 mL/g in terms of SVI₃₀, thereby indicating effective control of the bulking issue. In practical applications, the increase in AGS particle size is closely associated with improved settleability, which is essential for inhibiting the overgrowth of filamentous bacteria, thereby preventing sludge bulking and granule disintegration. This approach also provides a viable strategy for further optimizing the activated sludge system. Typically, the AGS within the reactor comprises granules of varying sizes, with the size distribution evolving throughout the operational process, as noted by Qiu et al. [124]. Smaller granules generally exhibit lower settleability, whereas larger granules are more susceptible to core dissolution, rendering them prone to disintegration.

5.3. Instability Causes

For a detailed examination of the relationship between the particle size of AGS and the long-term stability of the system, please consult

Table 3. Relationship between size of AGS and system stability.

Reactor Type	Working Volume	Granular Size	Granulation Strategy	Treatment Performances	Relationship Between Particle Size and Stability	Reference
SBR	3 L	D(average) = 0.404–0.52 mm (90 days)	The activated sludge with EBPR activity is added and the precipitation selective pressure is applied	Under a sludge loading rate of 0.3 kg COD/(kg MLSS·d), PO43−-P stabilized below 0.5 mg/L while TN removal efficiency reached 71.7 ± 0.4%.	The small size (0.404-0.52 mm) and compact structure of AGS is conducive to improving particle stability and simultaneous nitrification and denitrification efficiency.	[101]
SBR	5 L	D = 0.7–2.8 mm	Internal settling tank	Under an SLR of 0.413-0.484 kg COD/(kg MLSS·d), the TN removal efficiency (83.57 ± 4.38%) and SND removal efficiency (87.17 ± 3.04%) were the highest in 87.51% of the particles.	The AGS particle size range of 1.8–3 mm is the best particle size, which is conducive to structural stability and synchronous nitrification and denitrification rate.	[112]
MBR		D(average) = 0.8 mm (125 days)	Addition of biological carrier and multi-ion matrices	COD removal efficiency is high and stable, and the TN removal efficiency fluctuates greatly.	When the particle size of AGS was about 0.8 mm, the granular sludge remained stable without excessive expansion.	[124]
SBR	10 L	D = 0.7–1.9 mm	Novel funnel-shaped internals	Under an SRT of 0.38 kg COD/(kg MLSS·d), the removal efficiency of COD is 95.8 ± 3.7%, the removal efficiencies of NH4+-N and TN are 98.1 ± 0.9% and 81.6 ± 2.1%, respectively.	The optimal stability of AGS is found within the particle size range of 0.7 mm to 1.9 mm, with the highest nitrogen removal efficiency observed at a diameter of 1.3 mm.	[126]
CAGR	24.2 L	D(average) = 1.8 mm (65 days)	Different selection pressures are generated by adjusting the depth of the movable baffle	At 67 days, under an SRT of 0.8 kg COD/(kg MLSS·d), the removal efficiencies of COD, TIN and TP are 93.3%, 86.7% and 90%, respectively.	The size range of 1-1.43 mm is conducive to the stability of the AGS, and the large particles (d > 1.43 mm) are easy to destabilize.	[130]
SBR	125 L	D = 2–3 mm	Size selection pressure and the control of SRT	Under an SRT of 0.36 kg COD/(kg MLSS·d), effluent COD and TP concentrations are below 70 mg/L and 0.7 mg/L, respectively. Effluent TN concentration is between 30–40 mg/L.	The stability of the particle size range of 2–3 mm AGS is the best.	[131]
SBR	5 L	D = 0.164–1.55 mm	Inoculate mature aerobic granular sludge	Under an SLR of 0.114 kg COD/(kg MLSS·d), the removal efficiencies of TN and TP were 86% and 30%, respectively.	When the particle size is in balance with the critical size, it is beneficial to the stability of AGS structure.	[132]
SBR	3.3 L	D(average) = 0.725 mm	Internal settling tank	Under a sludge loading rate of 0.625 kg COD/(kg MLSS·d), the system demonstrated removal efficiencies of 86% for COD and 30% for TN, with the effluent COD concentration stabilized at 18.9 ± 6.6 mg/L.	When the size of AGS ranges from 0.4–0.8 mm, the density and hydrophobicity of AGS particles are high and stable.	[133]
EBPR anaerobic/aerobic granular sludge reactor	First phase = 11.1 L Second phase = 6 L	D = 0.3 mm (100 days)	Dynamic control of anaerobic and aerobic	Under an SLR of 0.4 kg COD/(kg MLSS·d), the effluent COD and PO4-P concentrations averaged 58 ± 27 mg/L and 0.53 ± 0.77 mg/L, respectively, while TN remained between 30–40 mg/L.	Medium aerobic granular sludge is beneficial to inhibit filamentous bacteria and accumulate slow-growing microorganisms, and increase structural stability.	[134]

Note: CAGR represents cyclic aerobic granular reactor.

. In laboratory-scale AGS systems, excessively large granule diameters can cause blockages in the sludge bed, increase sludge loss rates, impair water flow, and create anoxic conditions, among other issues, thereby adversely affecting treatment efficiency and system stability [126]. The significant mass transfer distance within the matrix is likely to enhance the concentration gradient inside the granules. A large particle size (D > 3 mm) can result in an inadequate internal matrix, which promotes the excessive growth of filamentous bacteria and may lead to sludge bulking or collapse. Zhou et al. [91] reported that when the particle size exceeds 4 mm in laboratory-scale experiments, the granules become increasingly loose and develop a cavity structure, eventually disintegrating into smaller flocs or undergoing internal collapse, resulting in a hollow and loose form. The AGS is prone to disintegration under external shear forces. Furthermore, an increase in particle size is associated with a decrease in both the abundance and diversity of bacteria within the AGS. Moreover, the particle size plays a crucial role in determining the mass transfer resistance and oxygen transfer efficiency within the matrix, with the growth of filamentous bacteria contributing to a reduction in granule stability [127]. In addition, nutrient limitations in AGS with larger particle sizes significantly diminish the metabolic activity of functional microorganisms. This reduction in metabolic activity facilitates the development of cavity structures within the aerobic granules, thereby compromising their stability and increasing their susceptibility to dissolution under hydraulic shear forces [128,129]. Bei et al. [130] observed that both excessively high and low organic loads can modify the internal mass transfer resistance of AGS, impacting particle size and ultimately leading to nutrient deficiencies for the microorganisms, which results in the disintegration of granular sludge in laboratory-scale reactors. While granulation persists with reduced particle size, once it exceeds a certain threshold, disintegration becomes more likely due to increased resistance to mass transfer.

Among the various characteristics of granules, particle size serves as a direct indicator of the stability of AGS. The most pronounced alteration during AGS disintegration is a reduction in particle size. Previous studies have demonstrated that AGS stability is influenced by particle size distribution within the range of 0.2 to 7 mm [135]. Shameem et al. [136] suggest that the stability coefficient of aerobic granules is linked to variations in particle size, with a smaller coefficient indicating enhanced stability. This finding suggests that smaller granules exhibit greater stability under shear stress in laboratory-scale systems. Moreover, a gradual increase in particle size within the range of 1.43 to 2 mm has been shown to be beneficial for system stability [130]. Bei et al. [131] demonstrated that SBRs cultivating AGS, particles within the size range of 2–3 mm exhibited the highest granulation rate and the lowest specific oxygen uptake rate (SOUR) following ultrasonic fragmentation, thereby indicating superior granule stability compared to granules of other sizes. Increasing the proportion of 2–3 mm granules within the system promotes the formation of regular and compact granules during extended reactor operation. This is advantageous for maintaining granule stability and improving pollutant removal efficiency. Additionally, the concentration of granules was observed to have minimal impact on ultrasonic disruption, further supporting the stability of 2–3 mm granules. When granule sizes are less than 3 mm, an increase in particle size correlates with increased mass transfer resistance and a decrease in the SOUR of heterotrophic bacteria (SOUR_H). Conversely, particles exceeding 3 mm in size can more effectively enhance bacterial growth rates. In contrast, particles with diameters between 2–3 mm exhibit a relatively lower bacterial growth rate, rendering them less efficient in pollutant treatment compared to larger particles.

5.4. Granule Size Optimization for Stability

To address this issue, researchers have developed various strategies to optimize granule size distribution. In laboratory-scale experiments, Zhou et al. [86] demonstrated that adjusting the influent inlet height to maintain the influent within the sludge bed layer with the optimal particle size enhances the internal storage of carbon sources, thereby promoting the growth advantage of the granules. Their findings indicated that 87.51% of the granules fell within the optimal particle size range, there was an increase in the abundance of SND bacteria, and the TN removal efficiency was significantly improved. Additionally, Wan et al. [67] utilized a reactor with an innovative funnel-shaped internal structure designed to inhibit the excessive growth of large granules and selectively modulate the hydrodynamic shear force acting on the granules to optimize their internal size distribution in laboratory-scale systems. By increasing the total shear rate applied to large granules, their wear was enhanced, whereas reducing the total shear rate applied to small granules facilitated their growth. In laboratory-scale AGS systems, Verawaty et al. [99] identified a critical granule size at which the rates of granule fragmentation or wear are balanced by their growth rate, thereby ensuring stable system operation. Typically, when the particle size distribution within the AGS reactor aligns with this critical size, the system demonstrates enhanced stability. Their findings indicated that granules within the reactor tended to stabilize around a critical size of approximately 0.6–0.8 mm. This suggests that aerobic granular reactors under steady-state conditions may exhibit a granule size distribution close to this critical size. However, it is crucial to acknowledge that the critical granule size required for maintaining structural stability in AGS systems can vary depending on specific operational conditions. In full-scale wastewater treatment plants, maintaining a smaller granule diameter is a widely adopted strategy to ensure long-term granule stability. Li et al. [132] investigated the characteristics of AGS derived from a conventional plug-flow aeration tank at a municipal sewage treatment plant and observed that the granular sludge predominantly ranged from 0.2 to 0.8 mm in diameter. Consequently, controlling the particle size of AGS is essential for maintaining its stability in full-scale applications.

In conclusion, optimizing the particle size of AGS not only reduces sludge bulking but also enhances the overall efficiency of wastewater treatment processes. By effectively managing particle size, both the stability and removal efficiency of the system can be significantly improved in laboratory-scale and full-scale applications. Aerobic granules within the size range of 1.8–3 mm are particularly effective in inhibiting the excessive growth of filamentous bacteria, thereby preventing sludge bulking and granule collapse in laboratory-scale systems. Additionally, smaller to medium-sized granules are better suited to maintaining a balance between granulation and disintegration. However, excessively small granule diameters may lead to increased sludge loss, resulting in higher concentrations of suspended solids in the effluent. Therefore, particles with diameters ranging from 2–3 mm are more likely to sustain system stability in full-scale wastewater treatment plants [137].

6. Impact of AGS Particle Size on Environmental Sustainability

6.1. Energy Consumption of AGS

Currently, wastewater treatment processes primarily focus on managing pollutant emissions, often neglecting the energy potential present in wastewater and sludge. Consequently, the wastewater treatment sector has been characterized as “high energy-consuming.” Unlike the conventional activated sludge (CAS) process, AGS technology significantly reduces energy consumption. According to recent studies [138], AGS systems consume 58–63% less energy compared to the average energy consumption of CAS plants in the Netherlands.

Within AGS systems, the aeration process is the most energy-demanding phase. Increased aeration rates generate substantial hydraulic shear forces, which enhance friction between granules and subsequently decrease particle size [139]. This observation suggests that a reduction in particle size may be associated with increased energy consumption within the system. Simultaneously, high hydraulic shear forces can inhibit excessive particle growth, as smaller granules are more susceptible to these forces, leading to granule breakage and dispersion. This requires increased energy expenditure for the reaggregation and stabilization of granules. Additionally, a decrease in granule size significantly reduces the efficiency of SND, leading to heightened energy consumption [86]. Conversely, smaller granules possess a larger specific surface area, facilitating enhanced oxygen transfer and pollutant contact, which can improve treatment efficiency and potentially offset some energy consumption.

Larger granules demonstrate superior settling performance, facilitating rapid sedimentation and reducing the quantity of suspended granules, thereby decreasing energy consumption in separation and treatment processes. However, larger granules may experience internal mass transfer and diffusion resistance, which can impede the transmission efficiency of oxygen and nutrients, consequently diminishing the activity of microorganisms within the particles. As a result, an increased aeration rate is necessary to sustain the activity of the entire granule [140]. Additionally, the formation of larger granules may demand higher energy consumption to overcome inter-granule friction and hydraulic shear forces. Therefore, the optimal particle size range for AGS is typically between 1.0 and 2.0 mm. Within this range, the settleability, mass transfer efficiency, and stability of the granules are balanced effectively, thereby minimizing system energy consumption while ensuring treatment efficiency.

6.2. GHG Emissions

Greenhouse gas (GHG) emissions from wastewater treatment plants (WWTPs) are recognized as significant contributors to global warming, primarily due to microbial metabolic processes that transform carbon and nitrogen elements into methane (CH₄), nitrous oxide (N₂O), and carbon dioxide (CO₂) [5]. Studies have demonstrated that the impact of a single N₂O molecule on the greenhouse effect is approximately 300 times greater than that of a CO₂ molecule [141,142]. AGS, characterized by its unique multilayer microbial structure, predominantly generates N₂O through mechanisms such as nitrification and denitrification, hydroxylamine oxidation, heterotrophic denitrification, and abiotic/hybrid processes, subsequently releasing it during aeration [143,144,145]. The N₂O within AGS is more effectively retained and diffused deep within the granules, resulting in greater consumption and reduced transfer compared to activated sludge. Table 4 presents the N₂O emission data for AGS as reported in the current literature.

In SBRs, Gao et al. [4] experimentally quantified that N₂O emissions account for approximately 2.72% of the ammonia oxidation processes. Both ammonia-oxidizing bacteria (AOB) and heterotrophic denitrifying bacteria contribute to N₂O emissions, with heterotrophic denitrification significantly enhancing N₂O production. Conversely, CFRs demonstrate markedly lower N₂O emissions compared to SBRs, as documented by Sun et al. [146]. Quoc et al. [147] observed that, under aerobic conditions, smaller granules (0.212–1 mm) emit significantly more N₂O than larger AGS (1–2 mm). The rate of N₂O emissions is positively correlated with both DO levels and nitrification rates. Additionally, smaller granule sizes are associated with the formation of thinner biofilms. Eldyasti et al. [148] noted that as granule size decreases, there is a corresponding reduction in biofilm thickness, leading to an exponential increase in N₂O emissions. This phenomenon can be attributed to the combined effects of microbial composition and the diffusion rate of N₂O within the biofilm. The infiltration of oxygen into the anoxic zones of the granules may hinder the reduction of N₂O, resulting in incomplete denitrification and, consequently, elevated N₂O emissions.

In contrast, larger granular sludge sizes may contribute to a reduction in greenhouse gas emissions due to their diminished specific surface areas and endogenous respiration rates. These larger granules exhibit a more complex internal pore structure, which can potentially extend the retention time of N₂O within the biofilm [149], thus decreasing the N₂O emission rate by enhancing the contact time for microbial N₂O reduction. Furthermore, excessively large granule sizes may hinder DO transfer, leading to a reduction in DO concentration. This reduction can shift the process towards denitrification, thereby promoting the formation of anoxic zones within the aerobic granules. Specifically, denitrification processes are potentially enhanced under conditions of reduced DO concentrations, which are considered a source of N₂O emissions [150]. Larger granules have been observed to significantly increase N₂O production via the AOB denitrification pathway, whereas smaller granules tend to decrease N₂O production by favoring the hydroxylamine (NH₂OH) pathway. Consequently, the cultivation of sludge granules of optimal size not only improves nitrogen removal efficiency but also prolongs the residence time of N₂O within the granules. This extended residence time facilitates a higher conversion rate of N₂O, thereby reducing its production and subsequent emissions [144].

In addition to the influence of N₂O, CO₂ and CH₄ are also significant GHGs. The emission of CO₂ is primarily linked to the removal of organic matter and nutrients, rather than the endogenous respiration of microorganisms [151]. Larger granules enhance the degradation of organic matter, reduce the SRT, and consequently decrease the long-term accumulation of CO₂. Conversely, excessively small granule sizes may lead to uneven mass transfer, causing internal microorganisms to shift to anaerobic metabolism due to insufficient oxygen, thereby increasing CO₂ emissions [152]. CH₄ emissions predominantly arise from anaerobic conditions [153]. Reduced sludge particle sizes, particularly under extended retention times or suboptimal treatment conditions, can promote the development of anaerobic environments [154], wherein microorganisms convert organic matter into CH₄. Dababat et al. [155] reported that the biochemical methane potential (BMP) of granular sludge with an average particle size greater than 0.25 mm was markedly lower than that of smaller granules, indicating that smaller granules generated considerably more CH₄, in agreement with the findings of [156]. Additionally, excessively small granule sizes may indirectly facilitate increased CH₄ production. Variations in granule size can influence the composition of the microbial community; smaller granules may promote the proliferation of anaerobic bacteria, thereby enhancing methane production. In contrast, larger granules, due to their improved settleability and aeration properties, mitigate anaerobic conditions, thereby reducing CH₄ emissions.

In recent years, the microalgal-bacterial granular sludge (MBGS) process has garnered significant attention due to its low energy consumption, high efficiency, and reduced GHG emissions. The MBGS system inherently maintains the balance of oxygen and carbon dioxide during diurnal cycles without the necessity for external aeration, thereby facilitating efficient pollutant removal and mitigating the energy demands typically associated with aeration. Additionally, microalgae within the system sequester the carbon dioxide generated by bacterial activity, thereby curbing GHG emissions. The resultant biomass can be repurposed for resource and energy recovery [157]. When compared to the CAS process, the MBGS process achieves an approximate 63% reduction in carbon emissions, presenting a novel methodology for the concurrent purification of wastewater and recovery of energy and resources. This approach offers a promising pathway for the advancement of municipal wastewater treatment systems that prioritize environmental sustainability.

The research findings indicate that MBGS demonstrates a significantly lower CO₂ emission rate (5.8%) in comparison to AGS (44.4%) [158]. This reduction can be attributed to the abundant microbial community within MBGS and its smaller average granule size compared to conventional AGS. The reduced granule size enhances surface area, potentially improving the efficiency of photosynthesis during the treatment process. Photochemical reactions are instrumental in mitigating CO₂ emissions. Under aerobic conditions, microorganisms can effectively metabolize organic matter without producing CH₄, given that the conditions are optimal [159]. However, excessive concentrations of organic matter or suboptimal operating conditions may lead to anaerobic fermentation, resulting in CH₄ production. Therefore, future research should focus on developing strategies to minimize greenhouse gas emissions across various wastewater treatment processes to achieve sustainability in wastewater management [160].

In conclusion, granule size significantly impacts energy consumption and GHG emissions, with smaller particle sizes generally leading to increased energy use and GHG emissions. This effect arises because sludge with smaller granule sizes requires more frequent agitation and mixing to ensure sufficient oxygen transfer, which is critical for maintaining microbial growth and metabolism. Additionally, sludge composed of smaller granules poses challenges for sedimentation and separation processes, as the granule structure impedes the formation of anaerobic or anoxic zones, thereby affecting the stability and treatment efficacy of the sludge and indirectly increasing energy consumption [6]. Furthermore, smaller particles have a larger specific surface area, which may enhance microbial activity and consequently elevate GHG production. Moreover, sludge with reduced particle sizes typically exhibits a higher endogenous respiration rate, accelerating the decomposition of organic matter within the sludge and further intensifying GHG emissions. Therefore, precise regulation of the particle size of AGS is crucial for optimizing GHG emissions and mitigating environmental impact. To manage GHG emissions within wastewater treatment systems, the granule size can be modulated by adjusting system parameters to minimize GHG production.

Table 4. Relationship between AGS particle size and N₂O emission.

Reactor Type	Working Volume	Granular Size	Influent COD and TN	DO and OLR	pH and Temperature	N2O Emission	Relationship	Reference
SBR		D = 0.32 mm	COD = 500 mg/L TN = 300 mg/L	DO = 0.35 mg/L	T = 20–32 °C pH = 6.5–8.0 OLR = 1.6 kg/(m3·day)	The removal rate of TN was 53%, and the N2O emission of AGS was the lowest at 22.3°C, pH 7.1, and aeration rate 0.20 m3/h.	When the particle size of AGS is small, the emission of N2O is higher, and with the gradual increase of particle size, the emission of N2O is gradually reduced.	[11]
SBR	3 L	D = 0.212–2 mm	COD = 258 mg/L TN = 73.7 mg/L	DO = 1–4 mg/L	pH = 7.5	The N2O emission rate was significantly higher in the 0.212–1 mm range, being 1.5–17 times greater than in particles larger than 1 mm.		[147]
DFBBRS	608 mL	D(average) = 0.6–0.85 mm	COD = 144–182 mg/L TN = 29.5–53.5 mg/L	OLR = 5.4–7.13 kg/(m3·day)	pH = 7.3–7.72 T = 17–23 °C	TN effluent concentration of 6.2 ± 1 mg/L, N2O conversion rate of 78.53%.		[148]
SBR	1250 m3	D = 0.212–2 mm	COD = 531 mg/L TN = 54 mg/L	OLR = 5-10 kg/(m3·day)	T = 9.7–20.6 °C	Daily averaged N2O emission factor ranged from 0.02% to 1.58%.		[150]
SBR	98 L	D = 0.256-0.427 mm	COD = 224 ± 38 mg/L TN = 60 ± 2 mg/L		T = 21–26 °C pH = 7.0 ± 0.5	The removal efficiency of TN is 36–44%. The conversion rate of nitrogen into N2O in influent water is 0.47–5.28%.		[161]
SBAR	6 L	D = 0.8–1 mm	COD = 300 ± 50 mg/L TN = 45 ± 5 mg/L		T = 28 ± 1 °C pH = 7.5 ± 0.2	In SBARs during stable operation, total N2O emissions from fully aerobic SND were three times higher than from anoxic-aerobic SND.		[162]
SBR	98 L	D = 0.2–0.4 mm	COD = 174 mg/L TN = 54 mg/L	DO = 7.2–8.5 mg/L	T = 23 ± 2 °C pH = 7.0 ± 0.5 OLR = 1.28 kg/(m3·day)	When COD/N = 1.55 mg/L, the yield of N2O is 1.08. In the anoxic phase, the yield of N2O was 2.06 mg/L.		[163]
SBR	3.2 L	D = 0.212–2 mm	COD = 160 mg/L TN = 40 mg/L	DO = 1–4 mg/L	T = 31 ± 1 °C pH = 7.3–7.8 OLR = 0.08–0.32 kg/(m3·day)	When DO concentration is 2 mg/L, the maximum N2O yield was 127.6 mg/m3, and the removal efficiency of TN was 61.68%.		[164]

Note: DFBBR(s) represents denitrifying fluidized bed bioreactor(s).

Table 4. Relationship between AGS particle size and N₂O emission.

Reactor Type	Working Volume	Granular Size	Influent COD and TN	DO and OLR	pH and Temperature	N2O Emission	Relationship	Reference
SBR		D = 0.32 mm	COD = 500 mg/L TN = 300 mg/L	DO = 0.35 mg/L	T = 20–32 °C pH = 6.5–8.0 OLR = 1.6 kg/(m3·day)	The removal rate of TN was 53%, and the N2O emission of AGS was the lowest at 22.3°C, pH 7.1, and aeration rate 0.20 m3/h.	When the particle size of AGS is small, the emission of N2O is higher, and with the gradual increase of particle size, the emission of N2O is gradually reduced.	[11]
SBR	3 L	D = 0.212–2 mm	COD = 258 mg/L TN = 73.7 mg/L	DO = 1–4 mg/L	pH = 7.5	The N2O emission rate was significantly higher in the 0.212–1 mm range, being 1.5–17 times greater than in particles larger than 1 mm.		[147]
DFBBRS	608 mL	D(average) = 0.6–0.85 mm	COD = 144–182 mg/L TN = 29.5–53.5 mg/L	OLR = 5.4–7.13 kg/(m3·day)	pH = 7.3–7.72 T = 17–23 °C	TN effluent concentration of 6.2 ± 1 mg/L, N2O conversion rate of 78.53%.		[148]
SBR	1250 m3	D = 0.212–2 mm	COD = 531 mg/L TN = 54 mg/L	OLR = 5-10 kg/(m3·day)	T = 9.7–20.6 °C	Daily averaged N2O emission factor ranged from 0.02% to 1.58%.		[150]
SBR	98 L	D = 0.256-0.427 mm	COD = 224 ± 38 mg/L TN = 60 ± 2 mg/L		T = 21–26 °C pH = 7.0 ± 0.5	The removal efficiency of TN is 36–44%. The conversion rate of nitrogen into N2O in influent water is 0.47–5.28%.		[161]
SBAR	6 L	D = 0.8–1 mm	COD = 300 ± 50 mg/L TN = 45 ± 5 mg/L		T = 28 ± 1 °C pH = 7.5 ± 0.2	In SBARs during stable operation, total N2O emissions from fully aerobic SND were three times higher than from anoxic-aerobic SND.		[162]
SBR	98 L	D = 0.2–0.4 mm	COD = 174 mg/L TN = 54 mg/L	DO = 7.2–8.5 mg/L	T = 23 ± 2 °C pH = 7.0 ± 0.5 OLR = 1.28 kg/(m3·day)	When COD/N = 1.55 mg/L, the yield of N2O is 1.08. In the anoxic phase, the yield of N2O was 2.06 mg/L.		[163]
SBR	3.2 L	D = 0.212–2 mm	COD = 160 mg/L TN = 40 mg/L	DO = 1–4 mg/L	T = 31 ± 1 °C pH = 7.3–7.8 OLR = 0.08–0.32 kg/(m3·day)	When DO concentration is 2 mg/L, the maximum N2O yield was 127.6 mg/m3, and the removal efficiency of TN was 61.68%.		[164]

Note: DFBBR(s) represents denitrifying fluidized bed bioreactor(s).

7. Conclusions and Prospect

This review provides a comprehensive overview of the key characteristics of AGS particle size and its pivotal role in wastewater treatment. It examines the factors affecting particle size in SBRs and CFRs to optimize pollutant removal. The size of AGS is critical for the effective elimination of emerging contaminants, the maintenance of granule stability, and the reduction of environmental impact. Medium-sized granules are particularly effective in enhancing pollutant removal due to their larger surface area, although they also contribute to increased microbial activity and GHG emissions. By adjusting system parameters, it is possible to optimize granule size to reduce GHG emissions and improve the sustainability of AGS. Although research has investigated the effects of particle size on wastewater treatment, determining the optimal granule size for practical applications remains a crucial objective. Future research should focus on understanding how various environmental factors influence AGS growth and performance to better manage its size.

(1)

Investigate the ideal particle size range: Research suggests that AGS particles between 0.3 and 1.0 mm enhance wastewater treatment, but understanding is limited and findings often conflict with practical results. Future studies should explore optimal AGS sizes under varying conditions to optimize particle size and maintain granule stability.

(2)

Enhancing the removal efficiency of emerging contaminants: Future research will concentrate on examining the influence of various AGS particle sizes on the removal efficiency of emerging contaminants. By accurately identifying the optimal particle size range, it is anticipated that the removal efficiency of these contaminants can be substantially enhanced.

(3)

Evaluate the impact on environmental sustainability: The integration of sustainability indicators into the analysis of AGS systems offers a framework for investigating strategies to decrease energy consumption, reduce greenhouse gas emissions, including N₂O and CO₂, and mitigate the environmental impact on surrounding ecosystems by optimizing granule size and operational parameters.