Temperature and salt effects on settling velocity in granular sludge technology

Effects of the Hydroxy-Regulated Li-Montmorillonite Atomic Interface Arrangement on Li Cycle of Marine Sedimentation and pH

The reaction of ubiquitous clay is related to the global cycle of the key metals, but the relationship between the Li occurrence interface and the sedimentation in the Li cycle remains unclear. We investigated the atomic interface arrangement of Li-montmorillonite (Li-Mt) during low-temperature water-rock reactions and Li migration. The results show that, in Cl-rich systems, deprotonation and exposure of Na adsorption sites cause Li enrichment and O pairing, which lead to the weakening of the shielding effect of Mt on anions and the formation of a Mt-Li-Cl atomically interfacial arrangement. Only up to 20.3% of the Li is contained in the atomic interface of Li-Mt. In F-rich system, the dehydroxylation of F paired with Al in octahedral sites causes Li accumulation via local crystallization of LiF, and co-complexation of F and Li forms a Mt(Al)-F-Li atomic interface, in which up to 46.8% of the Li is enriched by the Mt. The participation of F and Cl in the complexation intensifies lattice collapse of the Li-Mt edge. The sedimentation velocity decreases with the smaller particle size affected by the Li loading. Lithium leached from igneous rocks serves as the marine Li source, which contributes up to 99.8% and 99.5% of the Li in Cl- and F-rich systems, respectively. The response of Mt(OH) to Li migration with a time accumulating effect may make an important regulatory of oceanic pH by either acidification or alkalization.

Stress response and signalling of a low-temperature bioaugmentation system in decentralized wastewater treatment: Degradation characteristics, community structure, and bioaugmented mechanisms

Low temperatures present challenges for stable wastewater treatment operations in cold regions. Low-temperature effective microorganisms (LTEM) were added as a bioaugmentation strategy at a decentralized treatment facility to improve performance. The effects of a low-temperature bioaugmentation system (LTBS) with LTEM at low temperatures (4 °C) on organic pollutant performance, microbial community changes, and the metabolic pathways of functional genes and functional enzymes were studied. To explore the bioaugmentation mechanism of LTBS based on stress response and signalling. The results showed that the start-up time of the LTBS (S2) with LTEM was shorter (8 days) and that it removed COD and NH₄⁺-N at higher rates (87 % and 72 %, respectively) at 4 °C. LTEM effectively degraded complex macromolecular organics into small molecular organics, and decomposing sludge flocs and the changing the extracellular polymeric substances (EPS) structure removed more organics and nitrogen. LTEM and local microbial communities (nitrifying and denitrifying bacteria) improved the ability of organic matter degradation and denitrification of the LTBS and formed a core microbial community dominated by LTEM (Bacillus and Pseudomonas). Finally, based on the functional enzymes and metabolic pathways of the LTBS, a low-temperature strengthening mechanism consisting of 6 cold stress responses and signal pathways under low temperatures was formed. This study demonstrated that the LTEM-dominated LTBS could provide an engineering alternative for future decentralized wastewater treatment in cold regions.

Low-temperature-resistance granulation of activated sludge and the microbial responses to the granular structural stabilization

Completely loss of granular structural stability and reliable start-up of aerobic granular sludge (AGS) system are considered as the biggest challenges for its engineering application under seasonal temperature variation, especially extremely low temperatures. In this study, two identical sequencing batch reactors (SBR) were successfully start-up at 10 °C (R1) and 25 °C (R2), respectively, and then operated under a strategy of stepwise change of temperatures to investigate the stability of the granular sludge by examining its microbial characteristics, bis (3'-5')-cyclic dimeric guanosine monophosphate (c-di-GMP), extracellular polymeric substance (EPS) and sludge physiochemical properties. The results showed that AGS formed under the low temperature preferentially secrete EPS and c-di-GMP for stable granulation and improvement of its resistance to temperature changes. Meanwhile, R1 successfully obtained aerobic granulation with high biomass concentration and superior settleability, as well as high pollutant removal performance. In comparison, R2 took a longer time for granulation and was subjected to serious disintegration of AGS. The matrix structure partially formed by filamentous bacteria during the start-up stage in R1 was one of major reasons for its own superiority beyond R2 in granulation. Slow-growing organisms such as autotrophic nitrifying and Anammox bacteria, phosphorus accumulation organisms, EPS-producing genera, and c-di-GMP pathway-dependent genera, were exclusively enriched in the R1 and resulted in higher pollutants removal efficiencies and stable structure, whereas Sphaerotilus dominated in R2 that related closely with its unstable performance. Therefore, the strategy based on the stepwise change of temperatures from extremely low temperatures may be one feasible way for the sustainable application of AGS system, which is of significance to address the challenging problems of AGS applications.

Formation, application, and storage-reactivation of aerobic granular sludge: A review

It was an important discovery in wastewater treatment that the microorganisms in the traditional activated sludge can form aerobic granular sludge (AGS) by self-aggregation under appropriate water quality and operation conditions. With a typical three-dimensional spherical structure, AGS has high sludge-water separation efficiency, great treatment capacity, and strong tolerance to toxic and harmful substances, so it has been considered to be one of the most promising wastewater treatment technologies. This paper comprehensively reviewed AGS from multiple perspectives over the past two decades, including the culture conditions, granulation mechanisms, metabolic and structural stability, storage, and its diverse applications. Some important issues, such as the reproducibility of culture conditions and the structural and functional stability during application and storage, were also summarized, and the research prospects were put forward. The aggregation behavior of microorganisms in AGS was explained from the perspectives of physiology and ecology of complex populations. The storage of AGS is considered to have large commercial potential value with the increase of large-scale applications. The purpose of this paper is to provide a reference for the systematic and in-depth study on the sludge aerobic granulation process.

Start-up of an aerobic granular sludge system from stored granules: Evaluating the impact of storage period on biomass activity and stability and the effect of temperature on nitrification and phosphorus removal rates

Aerobic granular sludge (AGS) has been considered a breakthrough within the wastewater treatment sector. However, the long reactor start-up for the formation of granules is challenging and may hinder the spread of this technology. To circumvent this obstacle, inoculation of the reactors with pre-formed granules from existing plants is an interesting approach. In this context, issues related to biomass storage becomes very relevant. In this study, reactivation of aerobic granular biomass after storage was evaluated in a sequencing batch reactor (SBR) designed for achieving simultaneous organic matter, nitrogen and phosphorus removal. Two different scenarios, short (40 days) and long (180 days) storage periods, were assessed, and their influence on the granules physical properties and bioactivity was addressed. The results revealed that the granules stored for a shorter period showed higher resistance to breakage and underwent smooth color changes. On the other hand, the biomass stored for a longer period acquired a dark color and was more susceptible to disruption during reactivation. The granules stored for 6 months become swollen and exhibited an irregular morphology and fluffy structure within the first days of reactivation. Consequently, their settling properties were adversely affected, and some parameters such as the food-to-microorganism ratio had to be adjusted to prevent granules disintegration. Regarding the bioactivity of important microbial functional groups, COD removal was rapidly restored within a few days of SBR operation with the biomass stored for a shorter period. However, it took longer for the biomass stored for 180 days to reach the same performance observed for the granules stored for 40 days. A similar trend is valid for nitrification. In the experiments with sludge stored for a longer time, it took almost twice as long to reach effluent ammonium concentrations lower than 1 mg NH₄⁺-N L^-1 compared to the test using biomass stored for 40 days. Phosphate removal was strongly affected by biomass storage, especially after 180 days of inactivity, a condition found to be detrimental for polyphosphate-accumulating organisms. Finally, cycle tests were also conducted to assess substrate conversion rates for comparison between different trials and evaluate the influence of temperature (10-35 °C) on nitrification and phosphate removal rates.

Anaerobic hydrolysis of complex substrates in full-scale aerobic granular sludge: enzymatic activity determined in different sludge fractions

Abstract

Complex substrates, like proteins, carbohydrates, and lipids, are major components of domestic wastewater, and yet their degradation in biofilm-based wastewater treatment technologies, such as aerobic granular sludge (AGS), is not well understood. Hydrolysis is considered the rate-limiting step in the bioconversion of complex substrates, and as such, it will impact the utilization of a large wastewater COD (chemical oxygen demand) fraction by the biofilms or granules. To study the hydrolysis of complex substrates within these types of biomass, this paper investigates the anaerobic activity of major hydrolytic enzymes in the different sludge fractions of a full-scale AGS reactor. Chromogenic substrates were used under fully mixed anaerobic conditions to determine lipase, protease, α-glucosidase, and β-glucosidase activities in large granules (>1 mm in diameter), small granules (0.2–1 mm), flocculent sludge (0.045–0.2 mm), and bulk liquid. Furthermore, composition and hydrolytic activity of influent wastewater samples were determined. Our results showed an overcapacity of the sludge to hydrolyze wastewater soluble and colloidal polymeric substrates. The highest specific hydrolytic activity was associated with the flocculent sludge fraction (1.5–7.5 times that of large and smaller granules), in agreement with its large available surface area. However, the biomass in the full-scale reactor consisted of 84% large granules, making the large granules account for 55–68% of the total hydrolytic activity potential in the reactor. These observations shine a new light on the contribution of large granules to the conversion of polymeric COD and suggest that large granules can hydrolyze a significant amount of this influent fraction. The anaerobic removal of polymeric soluble and colloidal substrates could clarify the stable granule formation that is observed in full-scale installations, even when those are fed with complex wastewaters.

Key points

• Large and small granules contain >70% of the hydrolysis potential in an AGS reactor.

• Flocculent sludge has high hydrolytic activity but constitutes <10% VS in AGS.

• AGS has an overcapacity to hydrolyze complex substrates in domestic wastewater.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s00253-021-11443-3.

A settling model for full-scale aerobic granular sludge

The settling behavior of aerobic granular sludge (AGS) in full-scale reactors is different from the settling of normal activated sludge. Current activated sludge models lack the features to describe the segregation of granules based on size during the settling process. This segregation plays an important role in the granulation process and therefore a better understanding of the settling is essential. The goal of this study was to model and evaluate the segregation of different granule sizes during settling and feeding in full-scale aerobic granular sludge reactors. Hereto the Patwardhan and Tien model was used. This model is an implementation of the Richardson and Zaki model, allowing for multiple classes of particles. To create the granular settling model, the most relevant parameters were identified using aerobic granular sludge from different full-scale Nereda® reactors. The settling properties of individual granules were measured as was the bulk behavior of granular sludge beds with uniform granular sludge particles. The obtained parameters were combined in a model containing multiple granule classes, which then was validated for granular sludge settling in a full-scale Nereda® reactor. In practice a hydraulic selection pressure is used to select for granular sludge. Under the same hydraulic selection pressure the model predicted that different stable granular size distributions can occur. This indicates that granular size distribution control would need a different mechanism then the hydraulic selection pressure alone. This model can be used to better understand and optimize operational parameters of AGS reactors that depend on granular sludge size, like biological nutrient removal. Furthermore insights from this model can also be used in the development of continuously fed AGS systems.

Enhanced aerobic granulation at low temperature by stepwise increasing of salinity

High salinity and low temperature are generally considered to have negative effects on the formation, stability and performance of aerobic granular sludge (AGS). This study investigated whether and how salinity acclimation strategies can enhance aerobic granulation at low temperature (12 °C) in three sequencing batch reactors (SBRs). Stepwise increased concentrations of NaCl (2-10 and 4-20 g/L) were added to the influent of R1 and R2 with steps of 1 and 2 g/L per week respectively, while R0 was set as a control (salt-free). The granulation processes in R1 and R2 were rapidly started up within 9 days, and were completed within 21 and 18 days, respectively. By contrast, R0 took 25 days and 49 days to start and complete granulation. The salinity acclimation strategies improved sludge hydrophobicity, reduced repulsion barrier between cells, and stimulated EPS production during granulation processes, which simultaneously promoted the formation of AGS. When the influent salinity reached 14 g/L on day 35, granule hydrophobicity, density and size in R2 sharply decreased and granules began to disintegrate afterwards. When operated under salt-free condition, sludge bulking occurred in R0 since day 60. The treatment performance was thus impaired in these two reactors, especially in R2 with significant biomass loss. Conversely, the AGS developed in R1 maintained stable structure with high biomass concentration (8.0 gSS/L) and excellent treatment performance for COD (90%), ammonium (95%) and total nitrogen (70%). Genera Thauera, Azoarcus, and Nitrosomonas were more enriched, while Flavobacterium and Meganema were more suppressed in R1, which would have contributed to granule stability and treatment performance. In conclusion, great care has to be taken for cultivating and operating AGS at low temperature for treating saline wastewater. Increasing salinity with a lower salt gradient provides a possibility for rapid granulation of AGS with excellent treatment performance under such conditions.

Fit-for-purpose treatment of produced water with iron and polymeric coagulant for reuse in hydraulic fracturing: Temperature effects on aggregation and high-rate sedimentation

Reusing produced water for hydraulic fracturing simultaneously satisfies challenges of fresh water sourcing and the installation/operation of an extensive disposal well infrastructure. Herein, we systematically and rigorously investigate produced water treatment for reuse during hydraulic fracturing. Highly saline and turbid produced water from the Permian Basin was treated by adding chlorine as an oxidant, FeCl₃ as the primary coagulant, and an anionic polymer to induce high rate sedimentation to generate "clean brine" by removing suspended solids and iron over a range of environmentally relevant temperatures. Mobile phone video capture, optical microscopy, and digital image/video analysis were employed to characterize floc morphology and measure its size and settling velocity. Conformational changes of the polymeric coagulant between 4 and 44 °C were inferred from viscosity and dynamic light scattering measurements providing clues to its performance characteristics. Floc settling velocities measured over the entire range of polymer dosages and temperatures were empirically modelled incorporating their fractal nature, average size, and the viscosity of the produced water using only a single fitting parameter. Juxtaposing the anionic polymer with the hydrolyzing metal-ion coagulant effectively destabilized the suspension and caused floc growth through a combination of enmeshment, adsorption and charge neutralization and inter-particle bridging as evidenced by Fourier transform infrared spectroscopy and thermogravimetric analysis. Very high turbidity (≥98%) and total iron (≥97%) removals were accomplished even with very short flocculation and sedimentation times of only 6 minutes each suggesting the feasibility of this approach to reuse produced water for hydraulic fracturing.

The key role of inoculated sludge in fast start-up of sequencing batch reactor for the domestication of aerobic granular sludge

Two types of inoculated sludges, granular sludge that had been stored at -20°C and activated sludge, were investigated for the domestication of aerobic granular sludges (AGSs) in sequencing batch reactors (SBRs). The results showed that using the stored granular sludge as inoculation sludge could effectively shorten the domestication time of AGS and yielded mature granular sludge after 22 days of operation. The AGS domesticated by stored granular sludge had better biomass and sedimentation properties; its MLSS and SVI reached 8.55 g/L and 35.27 mL/g, respectively. The removal efficiencies for chemical oxygen demand (COD), ammonium nitrogen (NH₄⁺-N) and total phosphorus (TP) reached 90.76%, 97.39% and 96.40%, respectively. By contrast, 54 days were needed to obtain mature granules using activated sludge. The microbial community structure was probed by using scanning electron microscopy (SEM) and high-throughput sequencing. The results showed that the diversity of the microbial community in mature granules was reduced when stored granular sludge rather than activated sludge was employed as inoculation sludge, and the dominant microbes were changed. The dominant species in mature granules domesticated using stored granular sludge were Zoogloea, Acidovorax and Tolumonas at the genus classification level, while the dominant species were Zoogloea and TM7-genera in granules developed from activated sludge.

Development of aerobic granular sludge under tropical climate conditions: The key role of inoculum adaptation under reduced sludge washout for stable granulation

Aerobic granular sludge (AGS) is a promising technology for wastewater treatment. However, the success of the process depends on the formation of stable granular biomass, which is associated with the microbiological aspects of the sludge and reactor operating conditions. In this study, the development of AGS from a poor nitrifying flocculent sludge obtained in a sewage treatment plant designed only for organic matter removal was assessed in a sequencing batch reactor (SBR) under tropical climate conditions (temperatures of 28 ± 4 °C). The results showed that, despite the alternating anaerobic-aerobic conditions during the granules selection phase under high sludge washout rates (low settling time), readily biodegradable organic matter was mainly removed aerobically. The formed granules were unstable, exhibiting a substantial amount of filaments and pasty consistency. The biomass characteristics (e.g., sludge volume index, density, diameter and settling velocity) were negatively impacted as complete granulation was reached, while biomass loss and degranulation became inevitable. Poor nitrification and no enhanced biological phosphate removal (EBPR) were observed. Implementation of a new operational strategy incorporating an adaptation of the seed sludge under reduced washout conditions (high settling time) prior to the granules selection stage enabled most of the influent organics to be removed anaerobically. Besides allowing a feast-famine regime to be established in the reactor, the sludge acclimation phase favoured the development of slow-growing organisms and suppressed the appearance of filamentous-like structures. Fast-settling granules with regular shape remained stable in the long-term, while high ammonium (>95%) and total nitrogen removal (>90%) was obtained. However, EBPR activity was very unstable, most likely due to the high temperatures. The findings of this study are important for the spreading of the AGS technology worldwide, especially in developing countries where the conditions are different in all aspects.

A new process for simultaneous nitrogen removal and phosphorus recovery using an anammox expanded bed reactor

Phosphorus recovery from wastewater is an important approach for sustainable phosphorus use. In this work, a process combining anammox and hydroxyapatite (HAP) precipitation in an expanded bed reactor for simultaneous nitrogen removal and phosphorus recovery was developed by applying specific Ca/P ratio and pH control. A high phosphorus removal rate (0.14 ± 0.01 kg-P/m³/d) was obtained while a stable nitrogen removal efficiency (87.4 ± 2.9%) maintained with an effluent recirculation system applied. Average 13.4% phosphorus (30.7% in P₂O₅) accumulation in the dry sludge and a Ca/P ratio very close to HAP was observed by quantitative elemental analysis. In this work, different analysis revealed the two layers structure with anammox biofilm attached to inorganic core of the granules. Different spectral analysis determined the major phase of the inorganic content as hydroxyapatite. With proper Ca/P ratio and pH control, anammox expanded bed reactor was transformed into an efficient process to simultaneously remove nitrogen and recover phosphorus.

A seasonal observation on the distribution of engineered nanoparticles in municipal wastewater treatment systems exemplified by TiO2 and ZnO

The present research attempted to assess the seasonal variation of engineered nanoparticles (ENPs) in a major municipal wastewater treatment system. A monthly survey over a 12-month period was conducted to monitor the concentration of TiO₂ and ZnO nanoparticles throughout the treatment process. Results showed inflow concentrations in the range of 21.6±5.0-391.0±43.0μg/L and 20.0±12.0-212.0±53.0μg/L for TiO₂ and ZnO, respectively. Seasonal pattern of the inflow ENPs concentration showed elevated value in the summer and winter periods for both TiO₂ and ZnO. Based on the concentration profile, the hydraulic flow rate, and the concentration of mixed liquid suspended solid (MLSS), the daily mass loading (DML) or mass flow rate of nanoparticles and the mass ratio of engineered nanoparticle to MLSS were calculated. DML provided a real-time estimate of temporal distribution of ENPs in the treatment processes. Results indicated a daily mass loading of 50.1±12.7 and 44.7±14.1kg/day (yearly average) for TiO₂ and ZnO, respectively. The amount of ENPs captured by sludge particulates were, yearly average, of 7.1kg-ZnO/d and 39.8kg-TiO₂/d, and 8.9kg-ZnO/d and 25.1kg-TiO₂/d, by the primary and the secondary sludge particulates, respectively. ENPs to MLSS mass ratio also showed a seasonal patter similar to the inflow ENPs concentration, where summer and winter periods showed elevated values. Additionally, loss of ENPs throughout the treatment plant that was not accounted for, also can be estimated from the daily mass loading rate and the mass ratio of ENPs to MLSS. Based on the seasonal distribution of ENPs in wastewater treatment systems, especially the daily mass loading rate, it is possible to estimate the uses of nanoparticle-related commercial and personal care products in the urban areas and enable decision-making on the strategy of sludge disposal management.

An anoxic-aerobic system for simultaneous biodegradation of phenol and ammonia in a sequencing batch reactor

A laboratory-scale sequencing batch reactor (SBR) was investigated to treat artificial pretreated coal gasification wastewater that was mainly contained of ammonia and phenol. The efficiency of SBR fed with increasing phenol concentrations (from 150 to 300 mg l^-1) and the relationship among phenol, nitrogen removal, and the microbial community structure were evaluated. When the phenol feeding concentration was increased to about 300 mg l^-1, the removal efficiency was above 99.0%, demonstrating the robustness of phenol removal capacity. The study showed that most phenol was degraded in anoxic stage. The average removal efficiencies of ammonia and total nitrogen were 98.4 and 81.9%, respectively, with average NH₄⁺-N concentration of 107.5 mg l^-1 and COD/N 7.5. Low temperature caused sludge loss that led to the decreased performance. Increasing the temperature could not recover the performance effectively. The data from bacterial analysis revealed that Delftia, Hydrogenophaga, and unclassified Xanthomonadaceae played a significant role in phenol degradation before the temperature increase, while uncultured Syntrophococcus sp. and unclassified Rhodocyclaceae were responsible for phenol degradation after the temperature increase. These results imply that the SBR holds potential for the simultaneous removal of phenolic compounds and nitrogen through aerobic ammonia oxidation and anoxic denitrification with phenol as the co-organic carbon source.

Influence of an aniline supplement on the stability of aerobic granular sludge

In order to evaluate the stability of aerobic granules in a toxic environment, this study discussed the influence of an aniline supplement on the properties and microbial community of aerobic granules. In the early stages of sequencing batch reactor (SBR) operation, an aniline supplement slightly affected the properties of the aerobic granules (strength, growth rate, SVI and so on). This effect was thereafter removed because of a change in the microbial community and the structure of aerobic granules: with the present of aniline, microbes with biodegradation ability appeared and gathered in the aerobic granules and the aerobic granules densified and settled faster as their SVI decreased to 35 mL/g and settling velocity increased to 41.56 m/h. When a synthetic waste water containing acetate as carbon source was used as influent, aniline (10-500 mg/L) could be degraded in 6 h, at a rate as high as 37.5 mg aniline/(L·h), with a removal rate in excess of 90%, while the effluent COD fell below 100 mg/L from the initial about 2000 mg/L. The aerobic granules cultured by acetate were compact, stable and resistant to aniline.

Aerobic granules cultivated and operated in continuous-flow bioreactor under particle-size selective pressure

A novel method based on the selective pressure of particle size (particle-size cultivation method, PSCM) was developed for the cultivation and operation of aerobic granular sludge in a continuous-flow reactor, and compared with the conventional method based on the selective pressure of settling velocity (settling-velocity cultivation method, SVCM). Results indicated that aerobic granules could be cultivated in continuous operation mode by this developed method within 14days. Although in the granulation process, under particle-size selective pressure, mixed liquor suspended solids (MLSS) in the reactor fluctuated greatly and filamentous bacteria dominated the sludge system during the initial operation days, no obvious difference in profile was found between the aerobic granules cultivated by PSCM and SVCM. Moreover, aerobic granules cultivated by PSCM presented larger diameter, lower water content and higher specific rates of nitrification, denitrification and phosphorus removal, but lower settling velocity. Under long term operation of more than 30days, aerobic granules in the continuous-flow reactor could remain stable and obtain good chemical oxygen demand (COD), NH4(+)-N, total nitrogen (TN) and total phosphorus (TP) removal. The results indicate that PSCM was dependent on the cultivation and maintenance of the stability of aerobic granules in continuous-flow bioreactors.

Characteristics, process parameters, and inner components of anaerobic bioreactors

The anaerobic bioreactor applies the principles of biotechnology and microbiology, and nowadays it has been used widely in the wastewater treatment plants due to their high efficiency, low energy use, and green energy generation. Advantages and disadvantages of anaerobic process were shown, and three main characteristics of anaerobic bioreactor (AB), namely, inhomogeneous system, time instability, and space instability were also discussed in this work. For high efficiency of wastewater treatment, the process parameters of anaerobic digestion, such as temperature, pH, Hydraulic retention time (HRT), Organic Loading Rate (OLR), and sludge retention time (SRT) were introduced to take into account the optimum conditions for living, growth, and multiplication of bacteria. The inner components, which can improve SRT, and even enhance mass transfer, were also explained and have been divided into transverse inner components, longitudinal inner components, and biofilm-packing material. At last, the newly developed special inner components were discussed and found more efficient and productive.

Long-term impact of anaerobic reaction time on the performance and granular characteristics of granular denitrifying biological phosphorus removal systems

Removal of nitrogen and phosphorus (P) from wastewater is successfully and widely practiced in systems employing both granular sludge technology and enhanced biological P removal (EBPR) processes; however, the key parameter, anaerobic reaction time (AnRT), has not been thoroughly investigated. Successful EBPR is highly dependent on an appropriate AnRT, which induces carbon and polyphosphate metabolism by phosphorus accumulating organisms (PAOs). Therefore, the long-term impact of AnRT on denitrifying P removal performance and granular characteristics was investigated in three identical granular sludge sequencing batch reactors with AnRTs of 90 (R1), 120 (R2) and 150 min (R3). The microbial community structures and anaerobic stoichiometric parameters related to various AnRTs were monitored over time. Free nitrite acid (FNA) accumulation (e.g., 0.0008-0.0016 mg HNO2-N/L) occurred frequently owing to incomplete denitrification in the adaptation period, especially in R3, which influenced the anaerobic/anoxic intracellular intermediate metabolites and activities of intracellular enzymes negatively, resulting in lower levels of poly-P and reduced activity of polyphosphate kinase. As a result, the Accumulibacter-PAOs population decreased from 51 ± 2.5% to 43 ± 2.1% when AnRT was extended from 90 to 150 min, leading to decreased denitrifying P removal performance. Additionally, frequent exposure of microorganisms to the FNA accumulation and anaerobic endogenous conditions in excess AnRT cases (e.g., 150 min) stimulated increased extracellular polymeric substances (EPS) production by microorganisms, resulting in enhanced granular formation and larger granules (size of 0.6-1.2 mm), but decreasing anaerobic PHA synthesis and glycogen hydrolysis. Phosphorus removal capacity was mediated to some extent by EPS adsorption in granular sludge systems that possessed more EPS, longer AnRT and relatively higher GAOs.