Stability Of Aerobic Granular Sludge Research Articles

Key function of Kouleothrix in stable formation of filamentous aerobic granular sludge at low superficial gas velocity with polymeric substrates

Forming and maintaining stable aerobic granular sludge (AGS) at a low superficial gas velocity (SGV) is challenging, particularly with polymeric substrates. This study cultivated filamentous aerobic granular sludge (FAGS) with filamentous Kouleothrix (Type 1851) at low SGV (0.15 cm/s) utilizing mixed acetate-soluble starch. Within approximately 260 days, notable increases in the relative abundance of Kouleothrix (from 4 % to 10 %) and Ca. Competibacter (from 1 % to 26 %) were observed through 16S rRNA gene analysis. Metagenomic analysis revealed increased expression of functional genes involved in volatile fatty acid (VFA) production (e.g., ackA and pta) and polyhydroxyalkanoate synthesis (e.g., phbB and phbC). Kouleothrix acted as a skeleton for bacterial attachment and was the key fermenting bacteria promoting granulation and maintaining granule stability. This study provides insight into the formation of FAGS with low-energy and non-VFA substrates.

Evaluation of the shear stability of aerobic granular sludge from a pilot-scale membrane bioreactor: Establishment of a quantitative method

This work established a quantitative method to access the shear stability of aerobic granular sludge (AGS) and validated its feasibility by using the mature AGS from a pilot-scale (50 tons/day) membrane bioreactor (MBR) for treating real municipal wastewater. The results showed that the changing rate (ΔS) of the peak area (S) of granule size distribution (GSD) exhibited an exponential relationship (R2≥0.76) with the shear time (y=a−b·cx), which was a suitable indicative index to reflect the shear stability of different AGS samples. The limiting granule size (LGS) was defined and proposed to characterize the equilibrium size for AGS after being sheared for a period of time, whose value in terms of Dv50 showed high correlation (R2=0.92) with the parameter a. The free Ca2+ (28.44–34.21 mg/L) in the influent specifically interacted with polysaccharides (PS) in the granule's extracellular polymeric substance (EPS) as a nucleation site, thereby inducing the formation of Ca precipitation to enhance its Young's modulus, while Ca2+ primarily interacted with PS in soluble metabolic product (SMP) during the initial granulation process. Furthermore, the Young's modulus significantly affected the parameter a related to shear stability (R2=0.99). Since the parameter a was more closely related (R2=1.00) to ΔS than that of the parameter b or c, the excellent correlation (R2=0.99) between the parameter a and the wet density further verified the feasibility of this method.

Effect of different feeding strategies on performance of aerobic granular sludge: From perspective of extracellular polymeric substances and microorganisms

Long-term structural stability of aerobic granular sludge (AGS) is a major challenge in AGS biotechnology. This work investigated variations in the sludge performance, structural stability, and intrinsic mechanisms of AGS under three feeding strategies (R1 direct aeration after fast feeding, R2 anaerobic stirring after fast feeding and R3 anaerobic plug-flow slow feeding). The AGS sequencing batch reactor used in this study was in stable operation with biomass of 6–10 gMLSS/L and a hydraulic retention time of 4 h. The results showed that the best sludge performance was demonstrated in R3, followed by R2 and R1. These differences were caused by extracellular polymeric substances (EPS) and microbial communities. The excellent sludge performance in R3 was attributed to the high hydrophobicity in the EPS as well as the gel properties due to the high hydrogen bonding content in the exopolysaccharide (PS). Similarly, the gel properties formed by the high hydrogen bonding content maintain the sludge stability in R1. Slow-growing dominant bacteria were enriched in the AGS of R2 and R3 with alternating anaerobic/aerobic operation modes, resulting in outstanding sludge performance. In contrast, high abundance of the filamentous bacteria Neomegalonema was enriched in the AGS of R1 without an anaerobic phase, resulting in relatively poor sludge stability. Variations in EPS characteristics and microbial community under the different feeding strategies were the main mechanisms responsible for the long-term operation of AGS. This study could provide a theoretical foundation and technical assistance for the sustainable operation of AGS reactors.

Effect of in situ ultrasonic wave and influent ammonia nitrogen fluctuation on stability of aerobic granular sludge

ABSTRACT This study elucidates the impact of fluctuating influent conditions and in situ ultrasonic wave exposure on the stability of aerobic granular sludge (AGS) in the treatment of simulated wastewater emanating from rare earth mining operations. During a stable influent period spanning from Day 1 to Day 95, the seed granules underwent an initial disintegration followed by a re-granulation phase. The secondary granulation was achieved on Day 80 and Day 40 for the ultrasonic reactor (R1) and the control reactor (R2), respectively. Notably, granules formed in R1 exhibited a more porous structure compared to those generated in R2. Subsequently, when the ammonia nitrogen in the influent oscillated between 100 and 500 mg/L during Days 96-140, both reactors yielded compact and densely structured granules. Nitrogen removal profiles were comparable between the two reactors: the removal efficiencies for ammonia nitrogen and total inorganic nitrogen escalated from 95% and 80%, respectively, during Days 1-95, to 95% and 90%, respectively, post-Day 140. A suite of performance metrics indicated that steady-state granules from R1 outperformed those from R2 across several parameters. Specifically, the nitrification/denitrification rates, and relative abundance of denitrifying bacteria were all higher in granules from R1. Conversely, the relative abundance of nitrifying bacteria was comparable between granules from both reactors. However, R1 granules demonstrated lower sludge concentration and smaller average particle size than their R2 counterparts. In conclusion, the AGS system demonstrated robust resilience to fluctuating ammonia nitrogen, and the application of ultrasonic waves significantly enhanced granular activity while achieving in situ sludge reduction.

Rapid static feeding combined with Fe2+ addition for improving the formation and stability of aerobic granular sludge in low-strength wastewater

Aerobic granular sludge (AGS) needs a long start-up time and always shows unstable performance when it is used to treat low-strength wastewater. In this study, a rapid static feeding combined with Fe2+ addition as a novel strategy was employed to improve the formation and stability of AGS in treating low-strength wastewater. Fe-AGS was formed within only 7 days and showed favorable pollutant removal capability and settling performance. The ammonia nitrogen (NH4+-N) and chemical oxygen demand (COD) concentration in the effluent were lower than 5 mg/L and 50 mg/L after day 23, respectively. The sludge volume index (SVI) and mixed liquid suspended solids (MLSS) was 37 mL/g and 2.15 g/L on day 50, respectively. Rapid static feeding can accelerate granules formation by promoting the growth of heterotrophic bacteria, but the granules are unstable due to filamentous bacteria overgrowth. Fe2+ addition can inhibit the growth of filamentous bacteria and promote the aggregation of functional bacteria (eg. Nitrosomonas, Nitrolancea, Paracoccus, Diaphorobacter) by enhancing the secretion of extracellular polymeric substances (EPS). This study provides a new way for AGS application in low-strength wastewater treatment.

Effects of feeding mode on the formation and stability of aerobic granular sludge under combined antibiotic stress

Aerobic granular sludge (AGS) is a promising technology for wastewater treatment due to its high biomass retention, nutrient removal, and resistance to toxic substances. However, the formation and stability of AGS are affected by various factors, such as feeding mode and antibiotic stress. This study investigated how three feeding modes, namely anaerobic plug-flow (APM), anaerobic mixed-flow (AMM), and aerobic feeding mode (AFM), influenced AGS formation and stability under exposure to a mixture of tetracycline, sulfamethoxazole, ofloxacin, and roxithromycin. The results showed that only APM and AMM achieved granulation within 40 d, while AFM failed to form AGS within 90 d due to the low shear force that led to a different composition and structure of extracellular polymeric substances and a high potential barrier calculated by extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory. The average granules diameters of APM (896.6 μm) were larger than that of AMM (519.4 μm), and the results of ultrasonic crushing experiment and XDLVO calculation showed that the structural stability of APM was slightly stronger. However, AMM outperformed APM and AFM in terms of total phosphorus removal, antibiotic removal, and long-term operational stability under low temperature conditions. Illumina MiSeq sequencing and qPCR results revealed that typical biofilm formers (Sediminibacterium and env.OPS_17) contributed to the stability of AGS and increased phosphorus accumulating organisms’ genes abundance ensured the high TP removal in AMM. This study offers valuable insights into the effects of diverse feeding modes on aerobic granulation and stability under combined antibiotic stress and suggests that AMM is a promising mode for enhancing AGS performance and resilience in wastewater treatment applications.

Long-term operational performance and membrane fouling mechanisms of AGMBR treating municipal wastewater under different superficial air velocities

The granulation and stability of aerobic granular sludge (AGS) as well as membrane fouling in AGS-membrane bioreactor (AGMBR) are significantly influenced by superficial air velocity (SAV). However, there is a lack of relevant research on this topic. Therefore, this study aims to investigate the specific impacts of SAV on AGMBR performance and membrane fouling. The results showed that the experimental SAVs (1.6, 2.1, and 2.6 cm/s) were found to provide sufficient dissolved oxygen for the removal of organics and ammonium nitrogen at efficiencies exceeding 94.50% and 99.40%, respectively. The larger SAV also extended the overall lifespan of the AGMBR, while altering both the content of foulants on fouled membranes and their associated microbial community structure. Moreover, the cake layer exhibiting the highest degree of porosity was observed at an SAV of 2.6 cm/s, which corresponded to the largest pore blocking fouling resistance (28.81% of the total fouling resistance) and the presence of fluorine on the fouled membrane. Additionally, the results from extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory were consistent with the increase in transmembrane pressure. Polysaccharides (PS) played a more significant role than proteins in membrane fouling, and the PS present in soluble microbial products exhibited a strong correlation with fouling resistance distribution. This study aims to optimize the treatment performance and minimize membrane fouling in AGMBR, thereby establishing a mutually beneficial scenario that is essential for the practical implementation of wastewater reuse in engineering.

Positive effects of lignocellulose on the formation and stability of aerobic granular sludge.

Lignocellulose is one of the major components of particulate organic matter in sewage, which has a significant influence on biological wastewater treatment process. However, the effect of lignocellulose on aerobic granular sludge (AGS) system is still unknown. In this study, two reactors were operated over 5 months to investigate the effect of lignocellulose on granulation process, structure stability and pollutants removal of AGS. The results indicated that lignocellulose not only promoted the secretion of tightly bound polysaccharide in extracellular polymeric substances, but also acted as skeletons within granules, thereby facilitating AGS formation, and enhancing structural strength. Lignocellulose imposed little effect on the removal efficiency of pollutants, with more than 95, 99, and 92% of COD, NH4+-N, and PO43--P were removed in both reactors. However, it did exhibit a noticeable influence on pollutants conversion processes. This might be due to that the presence of lignocellulose promoted the enrichment of functional microorganisms, including Candidatus_Accumulibacter, Candidatus_Competibacter, Nitrosomonas, and Nitrospira, etc. These findings might provide valuable insights into the control strategy of lignocellulose in practical AGS systems.

A powerful strategy to improve aerobic granular sludge stability towards organic shock loadings with varying carbon/nitrogen ratios: Weak magnetic field

The organic shock loadings with varying ratios of carbon to nitrogen (C/N) caused by fluctuations of wastewater flowrate and concentration inflict a great threat on the stability of aerobic granular sludge (AGS) in practical applications. Herein, the present work proposed an enhanced shock-loading-tolerance strategy for AGS by applying a weak magnetic field (WMF). Results indicated that aerobic granules exposed in WMF kept a significant propagation regardless of the various influent organic concentrations, while those in the control reactor exhibited inadaptability with decreasing biomass concentration and bioactivity at lean substrate conditions. The enhanced extracellular polymeric substances regulating process might be one of the inducement mechanisms to cope with organic shock loadings of WMF. In addition, the AGS system in WMF showed its strong resilience in terms of the ammonia and phosphate removal. Magnetic field could inhibit the overgrowth of filamentous bacteria, and the enhanced tolerance for organic shock loadings might attribute to the enrichment of functional microbes related to structural stability such as Firmicutes and Candidatus_Competibacter. Therefore, this work revealed the enhanced mechanisms of magnetic field for improving the AGS stability towards organic shock loadings, promoting the full-scale engineering applications of both AGS and magnetization technologies in wastewater treatment.

Effect of food-to-microorganisms ratio on aerobic granular sludge settleability: Microbial community, potential roles and sequential responses of extracellular proteins and polysaccharides

The food-to-microorganism ratio (F/M) is an important parameter in wastewater biotreatment that significantly affects the granulation and settleability of aerobic granular sludge (AGS). Hence, understanding the long-term effects and internal mechanisms of F/M on AGS settling performance is essential. This study investigated the relationship between F/M and the sludge volume index (SVI) within a range of 0.23–2.50 kgCOD/(kgMLVSS·d). Thiothrix and Candidatus_Competibacter were identified as two dominant bacterial genera influencing AGS settling performance. With F/M increased from 0.27 kgCOD/(kgMLVSS·d) to 1.53 kgCOD/(kgMLVSS·d), the abundance of Thiothrix significantly increased from 0.20% to 27.02%, and the hydrophobicity of extracellular proteins (PN) decreased, which collectively reduced AGS settling performance. However, under high-F/M conditions, the gel-like polysaccharides (PS) effectively retained the granular biomass by binding to the highly abundant Thiothrix (53.65%). The progressive increment in biomass led to a concomitant reduction in F/M, resulting in the recovery of AGS settleability. In addition, two-dimensional correlation infrared spectroscopy analysis revealed the preferential responses of PN and PS to the increase and decrease of F/M, and the content and characteristics of PN and PS played important roles in granular settling. The study provides insight into the microbial composition and the potential role of extracellular polymer substances in the AGS sedimentation behavior, offering valuable theoretical support for stable AGS operation.

Stability of aerobic granular sludge for treating inorganic wastewater with different nitrogen loading rates

ABSTRACT This paper investigated the effect of nitrogen loading rates (NLRs) on the stability of aerobic granular sludge (AGS) for treating simulated ionic rare earth mine wastewater with high ammonia nitrogen and extremely low organic content. Mature AGS from a sequencing batch reactor (SBR) was seeded into five identical SBRs (R1, R2, R3, R4 and R5). The five reactors were operated with different NLRs (0.2, 0.4, 0.8, 1.2 and 1.6 kg/m3·d). After 30 days of operation, R1, R2 and R5 were dominated by broken granules, while most of the granules in R3 and R4 still maintained a complete structure. The properties of granules from R1, R2, R3, R4 and R5 deteriorated to varying degrees, while the granules from R3 and R4 showed better stability than that from R1, R2 and R5. In R1, R2, R3 and R4, the steady-state ammonia nitrogen removal efficiencies were all greater than 90%, and the steady-state removal efficiencies of total inorganic nitrogen (TIN) were approximately 30%. In R5, the removal efficiencies of ammonia nitrogen and TIN were both approximately 70%. The dominant nitrifying and denitrifying bacterial genera of the granules from the five reactors were Nitrosomonas and Thauera, respectively, and their relative abundance was much higher in granules from R3 and R4. The results demonstrated that a relative equilibrium between the growth and metabolism of nitrifying/denitrifying bacteria was achieved when NLR was between 0.8 and 1.2 kg/m3·d, which could provide technical support for the stability maintenance of AGS in the treatment of ionic rare earth mine wastewater.

Stability improvement of aerobic granular sludge (AGS) based on Gibbs free energy change (∆G) of sludge-water interface

• Δ G sw a can normalize the stability of AGS . • Microbial and chemical properties were more important for Δ G sw a than physical ones. • Reasonable increase in OLR or HSF reduced Δ G sw a and enhanced stability of AGS. The stability of aerobic granular sludge (AGS) has been evaluated by lots of physicochemical and microbial characteristic indicators, but none of them could singly and comprehensively characterize the stability of AGS. Gibbs free energy change (∆G) could reflect the stable state of the whole system by the second law of thermodynamics, so ∆G of the sludge-water interface ( Δ G sw a ) was proposed to characterize the stability of AGS in this study. Organic loading rate (OLR) and hydraulic shear force (HSF) as the primary external energy inputs were selected at different levels in four reactors. Results showed that a reasonable increase in OLR or HSF could reduce Δ G sw a , and its influence on Δ G sw a had two paths: affecting the surface roughness of sludge directly (Path 1); on the other hand, EPS producers like denitrifiers were enriched, promoting the secretion of PN such as tryptophan and protein-like to affect Δ G sw a (Path 2). These indicators were correlated with Δ G sw a significantly, especially genus Paracoccus spp. (class Alphaproteobacteria) (-0.74, p <0.001) and Amide III (-0.81, p <0.001), indicating that Path 2 had a more significant influence on Δ G sw a . In addition, the absolute values of correlation between physical stability indicators and Δ G sw a were around 0.80 ( p <0.001). Therefore, Δ G sw a can be normalized to characterize the stability of AGS. This study establishes Δ G sw a as the normalized indicator for the stability of AGS, which provides specific theoretical guidance for operating parameter regulation in engineering application.

Rapid granulation of aerobic granular sludge in an integrated moving bed biofilm reactor-membrane bioreactor: Effects and mechanism of Streptomyces cellulosus microspheres

This work focuses on revealing the effects of functional microorganism on rapid granulation process of aerobic granular sludge (AGS) in an integrated moving bed biofilm reactor-membrane bioreactor (MBR), and tries to illustrate the related mechanism. The result verified that the functional microorganism, Streptomyces cellulosus microspheres (Scms), could promote the rapid granulation of AGS within only 4 days, and the average size of AGS kept at around 1850 μm on day 80 and showed no obvious reduction in the size even for operating over 3 months. Scms first reduced the negative surface charge on sludge particles, which was beneficial to the aggregation of floc sludge, and then caused the changes in the structure of AGS and the composition of microbial community within the system. Under these effects, the formed large and stable AGS promoted a higher and more stable efficiency in the removal of COD (> 93 %).

Understanding the N-acylated homoserine lactones(AHLs)-based quorum sensing for the stability of aerobic granular sludge in the aspect of substrate hydrolysis enhancement

Efficient substrate metabolism is the premise for stable operation of aerobic granular sludge and can be regulated by quorum sensing (QS). In this study, starch and acetate were selected to represent complex and simple substrates to provide comparable amount of metabolic energy for granules cultivation. Starch-fed granules were larger in size and contained higher EPS content than acetate-fed granules, though both granules exhibited similar substrate-degradation rates during sequencing batch reactor (SBR) cycle. Three N-acylhomoserine lactones (AHLs), including C8-HSL, 3OHC8-HSL and 3OHC12-HSL, were detected as dominant autoinducers in granules. They accumulated more in starch-fed granules than acetate-fed granules. The batch experiments were implemented to investigate QS regulation for granular stability in terms of substrate hydrolysis and transformation. The addition of three AHLs increased the activity of α-amylase, the main starch hydrolase, 4–6 times, significantly (p < 0.01) higher than the control treatment without AHLs amendment. While activity of dehydrogenase, the main simple substrate degradation enzyme, was increased only 1–2 times. Higher enzyme activity, especially α-amylase, significantly (p < 0.05) promoted the substrate-degradation rate (65 % than control group) and EPS yield in starch-fed system. Overall, QS can facilitate complex substrate uptake via hydrolysis enhancement and EPS secretion, which together promote sludge granulation and stability.

Effect of flow regime on mass transfer diffusion and stability of aerobic granular sludge (AGS) in view of interfacial thermodynamic

Aerobic granular sludge (AGS) technology has been widely studied as “The Next Generation Wastewater Treatment technology”. The effect of hydraulic conditions on the structural stability of AGS has been widely studied. However, the function of flow regime on the AGS stability, especially dissolved oxygen (DO) mass transfer, is still unknown. In this study, we used the Reynolds number (Re) to quantify the flow regime and selected different stages of AGS as experimental subjects. Results showed that the relatively suitable Re (Re = 150) could create lower DO mass transfer limitation (Lc = 27.4 μm) and increase protein (PN) contents and the abundance of hydrophobic functional groups in AGS. At this condition (Re = 150), the interfacial Gibbs free energy of sludge-water (ΔGLSa) was at a lower state (−129.75 ± 2.15 mJ·m−2), which favored the stability of AGS. Principal component analysis (PCA) and correlation analysis indicated that the response of ΔGLSa was affected by Lc, PN, and hydrophobic groups. In addition, results obtained for unstable AGS further verified that suitable Re regulates the structural stability of AGS. This study deepens the understanding of Re as an important hydraulic parameter for structural stability of AGS, which is also of great significance for energy saving of sequential batch reactors (SBRs) with agitation in practical engineering.

Stability of aerobic granular sludge for simultaneous nitrogen and Pb(II) removal from inorganic wastewater

ABSTRACT In this paper, we proposed a strategy for the establishment of an aerobic granular sludge (AGS) system for simultaneous nitrogen and Pb(II) removal from inorganic wastewater. AGS was stored in lead nitrate solution to select functional bacteria resistant to lead poison, and then an AGS system for ammonia nitrogen (180-270 mg/L) and Pb(II) (15-30 mg/L) removal was established based on carbon dosing and a two-stage oxic/anoxic operational mode. After storage for 40 days, the stability of AGS decreased because specific oxygen uptake rate, nitrification rate and abundance of Nitrosomonas decreased to different degrees compared with those before storage. During the first 70 days of the recovery process, AGS in R1 (the blank reactor) and R2 (the control reactor) both experienced a first breakage and then regranulation process. The main properties of AGS in reactors R1 and R2 tended to be stable after days 106 and 117, respectively, but the structure of steady-state AGS in R2 was more compact. The total inorganic nitrogen (TIN) in effluent from R1 and R2 basically remained below 25 mg/L after days 98 and 90, respectively. The Pb(II) concentration in effluent from R2 was always below 0.3 mg/L. On day 140, the relative abundance of Nitrosomonas in R2 (6.17%) was significantly lower than that in R1 (12.15%), whereas the relative abundance of denitrifying bacteria was significantly higher than that in R1 (62.44% and 46.79%). The system removed 1 kg of influent TIN only consuming approximately 1.85 kg of carbon source, demonstrating clear advantages in energy savings.

Enhancing nitrogen removal of aerobic granular sludge with optimization of dissolved oxygen spatial distribution

This study aims to investigate the impact of the aerator position on the dissolved oxygen spatial distribution and stability of aerobic granular sludge (AGS). Three SBRs with 2.5 mm mesh screen were established under condition of different aerator positions, the aerators of R1, R2 and R3 were set at the bottom, 1/6 and 1/3 height of reactors, respectively. The results revealed that R2 was more favorable to improve the sludge settling behavior and structural stability of AGS, resulting in 83.7 ± 1.4 % of total nitrogen (TN) removal. However, R3 led to an anoxic environment at the bottom of reactor, which prevented the formation of AGS and made a large amount of sludge lost. DO distribution indicated that oxygen content at the bottom of R2 was at the optimum range of 18.9 %–20.1 %, being beneficial to realize the simultaneous nitrification-denitrification. Therefore, the functional microbes of Zoogloea spp. (17.4 %), Paracoccus spp. (12.5 %), Thaurea spp. (13.6 %) and Rhodobacter spp. (10.8 %) were highly enriched in R2. Overall, raising the aerator position to 1/6 height of reactor led to 7.4 % increase of TN removal efficiency and 15.4 % decrease of energy consumption, which favors the application of AGS technology.

Formation and stability of aerobic granular sludge in a sequential batch reactor for the simultaneous removal of organic matter and nutrients from low-strength domestic wastewater

Simultaneous removal of organic matter, nitrogen, and phosphorus, via simultaneous nitrification and denitrification (SND) and enhanced biological phosphorus removal processes, was evaluated in a pilot-scale sequential batch reactor. The focus was on granule's morphology, stability, microbiological composition, and reactor performance while treating diluted domestic wastewater with total chemical oxygen demand (CODt) of ≈ 200 mg.L-1. The applied organic loading rate was 0.9 ± 0.3 kg CODt.m−3.d−1 in the experiment. Aerobic granular sludge developed gradually. After 87-day operation, granules (diameter ≥ 0.2 mm) were ≥ 50 % of the biomass, and after 168 days, complete granulation was obtained (≥ 80 % of biomass). In the third period (days 168–247, complete granulation), mixed liquor biomass reached a volatile suspended solids (VSS) concentration of 1.2 ± 0.3 g VSS.L−1, with the granules remaining stable until the experimental end. In this period, low effluent concentrations of COD, nitrogen (NH4+-N, NO2−-N and NO3−-N) and phosphate (PO43—P) were obtained (mg.L−1): 36 ± 11; 4 ± 5; 3 ± 3, 4 ± 5; and 0.9 ± 0.4, respectively. COD, NH4+-N, and PO43−-P removal efficiencies (%) were 80 ± 11; 83 ± 20; and 55 ± 24, respectively. Heterotrophic nitrification and SND were observed, resulting in a process efficiency of 31 % even with dissolved oxygen applied to saturation. The phosphate removal was mainly attributed to denitrifying phosphorus accumulating organisms. Pseudomonas, the dominant genus found, acted in nitrogen and phosphorus removal. Pseudoxanthomonas also assisted in phosphorus removal. Bacterial communities in the flocs (≈ 20 % of biomass) during the last period were similar to those in the granules; therefore, they constituted the basis for granule formation, directly contributed to the simultaneous good removal of organic matter and nutrients.

Efficient aerobic granular sludge production in simultaneous feeding and drawing sequencing batch reactors fed with low-strength municipal wastewater under high organic loading rate conditions

To establish rapid and efficient aerobic granular sludge (AGS) production for augmentation strategy to mainstream activated sludge process, AGS production was evaluated under high organic loading rate (OLR) conditions in a side-stream simultaneous feeding and drawing sequencing batch reactor (SFD-SBR) fed with low-strength municipal wastewater. Specifically, three SFD-SBRs were operated under different OLRs (≥ 1.3 kg-tCOD m−3 d−1) with a 100% volume exchange ratio (VER). Rapid granulation (SVI30 < 80 mL g−1 and granule fraction of ~ 70%) was achieved within 40 days in all reactors. However, an excessively high OLR (2.0 kg-tCOD m−3 d−1) tended to labilize the AGS settleability (SVI5 of ~ 120 mL g−1) despite the larger AGS size due to the outgrowth of fluffy flocs and filamentous bacteria. Furthermore, after granulation, application of a long settling time (10 min) to maintain a low sludge blanket level during the feeding period was key for stable and efficient AGS production (0.17 g-SS g-tCOD−1) under a high OLR of 1.6 kg-tCOD m−3 d−1 compared to AGS production with a short settling time (0.10 g-SS g-tCOD−1). Taken together, this study demonstrated that application of low selection pressure and moderate up-flow linear velocity enables stable and efficient AGS production in SFD-SBRs fed with low-strength municipal wastewater under high VER and OLR conditions.

Distinct responses of aerobic granular sludge sequencing batch reactors to nitrogen and phosphorus deficient conditions

The nutrients availability determines efficiency of biological treatment systems, along with the structure and metabolism of microbiota. Herein nutrients deficiencies on aerobic granular sludge were comparatively evaluated, treating wastewater with mass ratios of chemical oxygen demand : nitrogen : phosphorus being 200:20:4, 200:2:4, and 200:20:0.4 (deemed as nutrient-balanced, nitrogen-deficient, and phosphorus-deficient), respectively. Results revealed that both nitrogen and phosphorus deficiencies significantly raised the effluent qualities especially nitrogen removal. However, nitrogen deficiency aroused considerable growth of filamentous bacteria, while granules kept compact structure under phosphorus deficient condition. Extracellular polymeric substances (EPS) also varied in contents and structures in response to different wastewaters. Microbial community structure analysis demonstrated that nitrogen deficiency led to lower richness and higher diversity, while the reverse was observed under phosphorus deficient condition. Nitrogen deficiency mainly induced decrease of nitrifying bacteria, while similarly phosphorus deficiency led to loss of phosphorus accumulating organisms. Dramatic enrichment Candidatus_Competibacter and filamentous Thiothrix were found under nutrients deficiencies, in which the latter explained and indicated filamentous bulking potential especially under nitrogen limited condition. Bacterial metabolism patterns verified the functions of microbial community responding to nutrients via PICRUSt2 prediction mainly by up-regulating cell motility, and cellular processes and signaling. This study could aid understanding of long-term stability of aerobic granular sludge for low-strength wastewater treatment.