Effect of settling time on aerobic granulation in sequencing batch reactor

Efficient nitrogen removal and robustness enhancement of a two-stage partial nitrification-anammox (PN/A) process with low sludge concentration for mature landfill leachate

The biological treatment system for high-strength wastewater, particularly landfill leachate, typically requires high sludge concentration to maintain nitrogen removal performance. However, it frequently causes an excessive accumulation of toxins in microbial metabolism, resulting in unstable performance during long-term operation. An efficient two-stage partial nitrification/anammox (PN/A) mature landfill leachate process with low sludge concentration was constructed by settling time reduction and Ca2+ addition. The ammonia removal rate reached 46.7 mg N/(L·h) in PN-SBR. Nitrosomonas (2.0%) was the sole genus responsible for partial nitrification. The influent NO2--N/NH4+-N of A-SBR was kept at 1.39, leading to a dynamic equilibrium of anammox and denitrification. Ca. Brocadia recovered fastest (0.32% → 1.8%) among the detected AnAOB genera. The process achieved NRE of 95.0% with effluent TIN of 37.6 mg/L (<40 mg/L). This research offered recommendations for the favorable operation of the two-stage PN/A mature landfill leachate treatment system with low sludge concentration.

Biogas production and nutrients removal from slaughterhouse wastewater using integrated anaerobic and aerobic granular intermittent SBRs - Bioreactors stability and microbial dynamics

Slaughterhouse wastewater (SWW) was effectively treated in sequential anaerobic and aerobic granular intermittent sequencing batch reactors (ASBR+ISBR) for 665 days at different HRTs (48 h, 32 h, 24 h, and 12 h). The ASBR was stable at each HRT but performed relatively well at 12 h (OLR - 7.8-9.8 kg COD/m³-d) in terms of pollutants removal and biogas production than previously conducted research. The average biogas production was about 17.3 L/day having 70-76 % of CH₄ which could subsidize around 52 % of electricity demand while saving 103 US dollars/day if installed at full scale. In the case of post aerobic granular ISBR, carbon and nutrients removal (N&P) was achieved by enriching granules (1.7-2.2 mm) at low DO (0.5-0.8 mg/L) via the nitrite pathway. The ISBR was also well stable at 12 h HRT (average OLR of 2.1 kg COD/m³-d) and met the effluent discharge guidelines recommended by the Central Pollution Control Board of India. During steady-state conditions (12 h HRT), the average removal efficiencies for COD, TSS, O&G, TN, and PO₄-P were 98.8 %, 96.4 %, 98.7 %, 93.4 %, and 86.6 % respectively from combined ASBR and ISBR. The microbial analysis confirmed Euryarchaeota, Proteobacteria, Firmicutes, Chloroflexi, Bacteroidetes, Planctomycetes, and Synergistetes as the dominant phyla in ASBR. Methanosaeta (21.56 %) and Methanosarcina (6.48 %) were the prevailing methanogens for CH₄ production. The leading phyla observed in ISBR were Bacteroidetes, Proteobacteria, Firmicutes, Armatimonadetes, Verrucomicrobia, Chloroflexi, and Planctomycetes. Heterotrophic AOB (Thauera, Xanthomonadaceae, Pseudomonas, Sphingomonadaceae, and Rhodococcus) were mainly detected in the system for ammonia oxidation besides common autotrophic AOB. Similarly, a known PAO (Accumulibacter) was not identified but other PAO (Rhodocyclaceae, Dechloromonas, Pseudomonas, Flavobacteriaceae, and Sphingobacteriaceae) were prevalent inside aerobic granular ISBR that contributed to both carbon and nutrients removal. The results obtained would help implement the investigated reactor configurations at the pilot and full scale for SWW treatment.

Functional analysis of extracellular polymeric substances (EPS) during the granulation of aerobic sludge: Relationship among EPS, granulation and nutrients removal

Extracellular polymeric substances (EPS) with high molecular weights, secreted from microorganisms, play a critical functional role in the aerobic granular sludge (AGS). To investigate the level and function of EPS during the granulation of aerobic sludge and in the mature AGS, a sequencing batch reactor (SBR) was operated for 70 days. Aerobic granules with an average diameter of 0.25 mm were obtained with reducing settling time of sludge. Simultaneous removals of COD, nitrogen and phosphorus by the mature AGS exceeded 90, 95 and 95%, respectively. The EPS content increased significantly to above 333 mg/g MLVSS during the initial stage, and after that, it stabilized at about 240 mg/g MLVSS as the mature AGS formed, higher than that of the seed sludge (212 mg/g MLVSS). The increased EPS contents showed a negative correlation with SVI values, while a strong positive relationship with the formation of the AGS. The protein/polysaccharide (PN/PS) ratio in the EPS increased from 1.42 to 4.17, and TP/MLSS increased to about 6%, with the formation of AGS. The proportion of extracellular-P increased with the increase of EPS, and then maintained stable at about 20%, indicating EPS promoted the removal of phosphorus. Furthermore, the results from the Standards, Measurements and Testing (SMT) and X-Ray Diffraction (XRD) showed that phosphorus in the AGS mainly existed in the form of inorganic phosphorus (IP) and the proportion of Ca₅(PO₄)₃(OH) in IP was up to 92%. This investigation demonstrated that EPS had a positive relationship with the sludge granulation and nutrients removal.

Effects of wastewater type on stability and operating conditions control strategy in relation to the formation of aerobic granular sludge - a review

Currently, research trends on aerobic granular sludge (AGS) have integrated the operating conditions of extracellular polymeric substances (EPS) towards the stability of AGS systems in various types of wastewater with different physical and biochemical characteristics. More attention is given to the stability of the AGS system for real site applications. Although recent studies have reported comprehensively the mechanism of AGS formation and stability in relation to other intermolecular interactions such as microbial distribution, shock loading and toxicity, standard operating condition control strategies for different types of wastewater have not yet been discussed. Thus, the dimensional multi-layer structural model of AGS is discussed comprehensively in the first part of this review paper, focusing on diameter size, thickness variability of each layer and diffusion factor. This can assist in facilitating the interrelation between disposition and stability of AGS structure to correspond to the changes in wastewater types, which is the main objective and novelty of this review.

Potential of anammox process towards high-efficient nitrogen removal in wastewater treatment: Theoretical analysis and practical case with a SBR

Anammox process has been widely applied in the wastewater nitrogen removal for its high rate and low cost. However, few researches reported the process potential in treating low-strength nitrogen wastewater. In this study, a sequencing batch reactor (SBR) was taken to explore the feasibility of low-strength nitrogen wastewater treatment by anammox process in theory and practice. The practical operation indicated that the effluent with satisfactory quality (1.90 ± 0.70 mg-TN·L^-1) could be achieved, when the SBR was fed with low-strength nitrogen influent (6.20 ± 0.45 mg-NH₄⁺-N·L^-1 and 7.96 ± 0.59 mg-NO₂^--N·L^-1). The hydraulic retention time (HRT), nitrogen removal efficiency, nitrogen removal rate (NRR) and hydraulic loading rate of SBR were 5.42 h, 86.5%, 0.054 kg-N·m^-3·d^-1 and 4.43 m³·m^-3·d^-1 during the 79-day operation, respectively. The theoretical analysis revealed the potential of anammox SBR. When SBR is stably operated, the maximum NRR would be 0.062 kg-N·m^-3·d^-1 if the effluent nitrogen was required to be as low as 3 mg·L^-1. The NRR value is feasible for engineering. However, considering the lower specific substrates utilization rate in practice, the maximum stable NRR was calibrated and found inefficient afterwards. In order to improve the potential of anammox process, the reactors without back mixing and with periodic bioaugmentation should be taken in priority for the engineering applications. In particular, the bioaugmentation frequency and single addition amount were calculated as 7 d and 0.3 g-VSS·L^-1, respectively. The results may provide guidance for the development of high-efficient and stable nitrogen removal process under low-strength condition.

Unmasking photogranulation in decreasing glacial albedo and net autotrophic wastewater treatment

In both natural and built environments, microbes on occasions manifest in spherical aggregates instead of substratum‐affixed biofilms. These microbial aggregates are conventionally referred to as granules. Cryoconites are mineral rich granules that appear on glacier surfaces and are linked with expanding surface darkening, thus decreasing albedo, and enhanced melt. The oxygenic photogranules (OPGs) are organic rich granules that grow in wastewater, which enables wastewater treatment with photosynthetically produced oxygen and which presents potential for net autotrophic wastewater treatment in a compact system. Despite obvious differences inherent in the two, cryoconite and OPG pose striking resemblance. In both, the order Oscillatoriales in Cyanobacteria envelope inner materials and develop dense spheroidal aggregates. We explore the mechanism of photogranulation on account of high similarity between cryoconites and OPGs. We contend that there is no universal external cause for photogranulation. However, cryoconites and OPGs, as well as their intravariations, which are all under different stress fields, are the outcome of universal physiological processes of the Oscillatoriales interfacing with goldilocks interactions of stresses. Finding the rules of photogranulation may enhance engineering of glacier and wastewater systems to manipulate their ecosystem impacts.

Treatment of real domestic sewage in a pilot-scale aerobic granular sludge reactor: Assessing start-up and operational control

Aerobic granular sludge (AGS) has been considered a breakthrough in the wastewater treatment sector given its key characteristics, such as excellent settleability, simultaneous removal of organic and nutrient pollutants, and compactness. However, the formation of granules often delays the start-up of granular-based systems, especially in large-scale settings. This study addressed the start-up of a pilot-scale AGS sequencing batch reactor (SBR) treating domestic sewage, monitored for over 280 days. The challenges faced during aerobic granulation using a mixture of activated sludge and anaerobic granular sludge as inoculum and the performance of the reactor on organic matter, nitrogen, and phosphorus removal were discussed. Results showed that robust and stable granules were formed after an initial period of around six months, with the settling time playing a key role on granules development. At least 80% of granules had a diameter greater than 0.2 mm and 60% >1 mm. In general, the reactor achieved high nitrogen removal efficiency, as well as satisfactory removal of soluble COD. However, total COD abatement was impaired by the various episodes of suspended solids loss with the effluent. Overall, this study demonstrated that the reactor was efficient in the treatment of domestic sewage, but its performance was adversely affected from sudden changes in the influent quality. PRACTITIONER POINTS: Aerobic granular sludge (AGS) applied to small-scale domestic sewage treatment. The control of sludge age in AGS can be a problem due to short sedimentation times. High DO to maintain aerobic granulation can economically make the process economically unfeasible in tropical countries. A sludge with excellent sedimentation properties was obtained. However, maintaining the granule over time is a challenge.

Microbial predation accelerates granulation and modulates microbial community composition

BACKGROUND

Bacterial communities are responsible for biological nutrient removal and flocculation in engineered systems such as activated floccular sludge. Predators such as bacteriophage and protozoa exert significant predation pressure and cause bacterial mortality within these communities. However, the roles of bacteriophage and protozoan predation in impacting granulation process remain limited. Recent studies hypothesised that protozoa, particularly sessile ciliates, could have an important role in granulation as these ciliates were often observed in high abundance on surfaces of granules. Bacteriophages were hypothesized to contribute to granular stability through bacteriophage-mediated extracellular DNA release by lysing bacterial cells. This current study investigated the bacteriophage and protozoan communities throughout the granulation process. In addition, the importance of protozoan predation during granulation was also determined through chemical killing of protozoa in the floccular sludge.

RESULTS

Four independent bioreactors seeded with activated floccular sludge were operated for aerobic granulation for 11 weeks. Changes in the phage, protozoa and bacterial communities were characterized throughout the granulation process. The filamentous phage, Inoviridae, increased in abundance at the initiation phase of granulation. However, the abundance shifted towards lytic phages during the maturation phase. In contrast, the abundance and diversity of protozoa decreased initially, possibly due to the reduction in settling time and subsequent washout. Upon the formation of granules, ciliated protozoa from the class Oligohymenophorea were the dominant group of protozoa based on metacommunity analysis. These protozoa had a strong, positive-correlation with the initial formation of compact aggregates prior to granule development. Furthermore, chemical inhibition of these ciliates in the floccular sludge delayed the initiation of granule formation. Analysis of the bacterial communities in the thiram treated sludge demonstrated that the recovery of 'Candidatus Accumulibacter' was positively correlated with the formation of compact aggregates and granules.

CONCLUSION

Predation by bacteriophage and protozoa were positively correlated with the formation of aerobic granules. Increases in Inoviridae abundance suggested that filamentous phages may promote the structural formation of granules. Initiation of granules formation was delayed due to an absence of protozoa after chemical treatment. The presence of 'Candidatus Accumulibacter' was necessary for the formation of granules in the absence of protozoa.

Long-term enrichment of anaerobic propionate-oxidizing consortia: Syntrophic culture development and growth optimization

Propionate is a quantitatively important methanogenic intermediate in anaerobic digesters and only limited number of microbes can utilize it under syntrophic association with methanogens. The syntrophic propionate oxidizing bacterias are known to be slow growers due to the low energy yield. Thus, propionate get accumulated frequently in anaerobic digesters and it negatively affect digester performance. In this study, propionate degrading consortia from four different seeding sources were enriched in sequential bath mode in two phases; first adaption phase with 1 g/L of propionate concentration and later, high-strength phase with 3 g/L. From 16s rRNA gene based metagenomics analysis of the former phase, four syntrophic microbial groups, Syntrophaceae, Syntrophomonadaceae, Methanobacterium and Methanosaeta were found to be dominant with complete degradation of propionate. The substrate accelerated microbial shifts were observed at high-strength phase with significant decrease of Syntrophaceae up to 26.9 %. Using Response Surface Methodology, pH 6.8-6.9 and temperature 34.5-34.9 °C were found to be optimum growth conditions for the propionate degradation culture. Observed results could be useful to improve degradation efficiencies and obtained enriched culture can be used to recover propionate-accumulated digesters by bio-augmentation.

Feast/famine ratio determined continuous flow aerobic granulation

Plug flow reactors (PFRs) made of multiple completely stirred tank reactors (CSTRs) in series were used to cultivate aerobic granules in real domestic wastewater. Theoretically, changing the number of CSTR chambers in series will change the nature of plug flow, and thus alter the pattern of the feast/famine condition and impact the aerobic granulation progress. Therefore, PFRs were operated in 4-, 6-, and 8-chamber mode under the same gravity selection pressure (a critical settling velocity of 9.75 m h^-1) and hydraulic retention time (6.5 h) until steady states were reached to evaluate the effect of the feast/famine condition on continuous flow aerobic granulation. The sludge particle size, circularity, settleability, specific gravity, zone settling velocity, and extracellular polymeric substance contents were analyzed to evaluate the role that a feast/famine regime plays in aerobic granulation. It was found that aerobic granulation failed whenever the feast/famine ratio was greater than 0.5. The results support a conclusion that the feast/famine condition is likely a prerequisite for continuous flow aerobic granulation.

Decolourization of azo, anthraquinone and triphenylmethane dyes using aerobic granules: Acclimatization and long-term stability

The long-term stability of aerobic granules is critical for decolourization of different dyes in textile wastewater. Here, we investigated dye decolourization and the stability of acetate-cultivated granules after exposure to dyes. Results show that granules can maintain excellent structure stability with the presence of azo and triphenylmethane dyes during a 200-day operation period, achieving biomass concentrations as high as 8-12 g L^-1 and 90% and 100% decolourization efficiency, respectively. Aerobic granules, however, partially disintegrated after exposure to anthraquinone, resulting in dye decolourization efficiency ranging from 50 to 80% and a biomass concentration as low as around 0.5 g L^-1 due to biomass wash-out. The study indicates that long-term granule stability is much dependent on the dye classes. The enrichment of specific species in granules for dye decolourization has not been affected by the granule structure. The specific dye decolourization rate and dye to microorganism ratio for anthraquinone were 5-6.5 and 13.5-16.4 times, respectively, higher than those for azo and triphenylmethane dyes, but the total reactor performance for anthraquinone decolourization is much poorer than azo and triphenylmethane dyes due to low biomass retention in the reactor. The results suggest the importance of stability of aerobic granules for biomass retention to achieve better treatment performance of dye-containing wastewater. For the first time, the long-term stability and decolourization performance of aerobic granules for treating anthraquinone and triphenylmethane dyes are reported here and compared with azo dye, which can be used to guide the treatment of real textile wastewater containing azo, anthraquinone and triphenylmethane dyes by aerobic granules.

Factors affecting aerobic granule sludge formation in leachate treatment - a systematic review

The biological treatment of landfill leachate due to high concentration of Chemical Oxygen Demand (COD), ammonia, and other toxic compounds is so difficult. One of the leachate treatment technology is the sludge biogranulation, that containing the two aerobic and anaerobic process. The aim of this study was conducted for determining the main factors affecting aerobic granule sludge formation in leachate treatment. In this study, all related papers in international databases were evaluated including Google Scholar, Science Direct, and PubMed, Also Open Access Journal Directory from 1990 until 2020 were investigated. The keywords used included Aerobic Granule Sludge (AGS), leachate treatment, Wastewater treatment, Granular Sequential Batch Reactors (GSBR), Formation Extracellular polymeric substance (EPS). Overall, 2,658 articles were retrieved of which 71 were selected after revising the titles and abstracts. Aerobic granulation has been only lately studied and a limited number of studies have been devoted to identification aspects of the process such as the organic source, and other factor affecting on formation granules. Some factors as shear stress, settling time, and the effluent discharge site have direct effect on the efficiency of aerobic granules reactor and other factors such as divalent metal ions, dissolved oxygen concentration, the ratio of height to diameter of the reactor, temperature affecting on the granulation process. If suitable conditions provide, the aerobic granule sludge process can be useful for leachate treatment.

Novel aerobic granular sludge culture strategy: Using granular sludge Anammox process effluent as a biocatalyst

The effluent from granular sludge Anammox process was used as a biocatalyst to improve the culture rate of aerobic granular sludge, and the internal causes of this effect were studied. In this study, in sequencing batch reactor, the formation and changes of AGS was monitored with granular sludge Anammox process effluent added before and after. The community structure of AGS was analyzed by molecular biology methods. The results showed ammonia utilizing rate increased from 3.41 to 5.96 mgNH₄⁺-N/(g VSS·h), NO₂^--N maintained a high accumulation rate, and the denitrification performance remained stable. On 40th day, the diameter of the AGS reached 3.5 mm, and the concentrations of PN and PS reached 330.5 and 62.9 mg/gVSS, respectively. The community structure has changed. Nitrosomonas (31.7%) became the new dominant bacteria. Signal molecules contained in the effluent as inter-species signal molecules could enhance the formation of AGS.

Continuous-flow aerobic granulation in plug-flow bioreactors fed with real domestic wastewater

This pilot study was designed to explore the feasibility of achieving successful aerobic granulation in continuous flow infrastructure like that existing in modern wastewater treatment plants (WWTPs). Results demonstrated that aerobic granulation of activated sludge can be achieved in plug-flow reactors (PFRs) fed with primary effluent from a domestic WWTP with seasonal temperature variation between 10 and 22.5 °C. It took about 90 days during the reactor startup to reach a state of sustained aerobic granulation. The characteristics of aerobic granules formed were comparable to those measured in sequential batch reactors (SBRs). The feast-to-famine concentration profiles measured in the plug-flow pilot reactors were found to be in line with those present in the full-scale treatment trains, lending support to the feasibility of converting existing infrastructure to continuous flow aerobic granulation systems. A selection pressure based on settling velocity (V_s) was applied in a V_s selector to retain bioparticles with V_s greater than ~9-9.75 m h^-1. It was theorized that an external V_s selection pressure would be necessary but would not be the sole condition sufficient to drive aerobic granulation. The alternating feast-to-famine internal selection provided by the PFRs is also believed to be a required condition to transform biomass from flocs toward dense and compact aerobic granules. While the pilot-scale Plug-flow Aerobic Granulation (PAG) reactor achieved similar COD and NH₃ removal efficiencies as the full-scale WWTP treatment train, its effluent from V_s selector contained an average of 138 mg L^-1 total suspended solids (TSS) as a result of the biomass 'wash-out" by the V_s selection pressure. Pilot results suggest a second clarifier for polishing, in addition to the V_s selector, may be needed in a full-scale application of the technique unless other downstream processes (flocculation, sedimentation, filtration) are provided to reach final water quality goals.

Overcoming the instability of aerobic granular sludge under nitrogen deficiency through shortening settling time

This study investigated the short settling time strategy to overcome the instability of aerobic granular sludge (AGS) under nitrogen deficiency. AGS variations in its physical and chemical characteristics and microbial community were investigated. Results showed that nitrogen deficiency led to the instability of AGS, while short settling time strategy could overcome the instability of AGS under nitrogen deficiency. Extracellular polymeric substances (EPS), especially, the increased secretion of polysaccharide and proteins with amide III groups at the short settling time enhanced the stability of the granules under nitrogen deficiency. Unclassified_f_Microbacteriaceae shifted to be the major bacteria group at short settling time, along with the decrease of Meganema and Rhodobacter and the increase of Lysobacter, which may play an important role in enhancing AGS stability. Therefore, shortening settling time supports an effective strategy for applications of AGS under nitrogen deficiency.

Microbial community dynamics during aerobic granulation in a sequencing batch reactor (SBR)

Microorganisms in aerobic granules formed in sequencing batch reactors (SBR) remove contaminants, such as xenobiotics or dyes, from wastewater. The granules, however, are not stable over time, decreasing the removal of the pollutant. A better understanding of the granule formation and the dynamics of the microorganisms involved will help to optimize the removal of contaminants from wastewater in a SBR. Sequencing the 16S rRNA gene and internal transcribed spacer PCR amplicons revealed that during the acclimation phase the relative abundance of Acinetobacter reached 70.8%. At the start of the granulation phase the relative abundance of Agrobacterium reached 35.9% and that of Dipodascus 89.7% during the mature granule phase. Fluffy granules were detected on day 43. The granules with filamentous overgrowth were not stable and they lysed on day 46 resulting in biomass wash-out. It was found that the reactor operation strategy resulted in stable aerobic granules for 46 days. As the reactor operations remained the same from the mature granule phase to the end of the experiment, the disintegration of the granules after day 46 was due to changes in the microbial community structure and not by the reactor operation.

Using high frequency and low-intensity ultrasound to enhance activated sludge characteristics

In this study, high-frequency ultrasound wave (1.8 MHz) at low intensity was applied to improve activated sludge settleability at high MLSS concentration. The effect of irradiation intensity, sonication mode, MLSS concentration and sample volume on the physical characteristics of sludge in a pilot scale settling column were investigated for optimizing the conditions. The obtained results showed that high-frequency ultrasound decreased the height of sludge (44%) and effluent turbidity (82.2%) and increased sludge settling velocity about 3 times at high biomass concentration. Irradiation intensity of 0.4 w/cm² and sonication mode with interval times of 10 s showed the best results on the performance of the system at MLSS concentration of 8000 mg/L with a sample volume of 3 L.

Effect of nitrogen deficiency on the stability of aerobic granular sludge

To assess the stability of aerobic granular sludge (AGS) under nitrogen deficiency conditions, three sequence batch reactors were operated with chemical oxygen demand (COD) to nitrogen (N) ratios of 100/5, 100/2.5, and 100/2, while COD concentration was kept consistent. AGS variations in its physicochemical characteristics, microbial community, and treatment performance were investigated. The results indicated that good treatment performance and stable AGS were achieved under nitrogen deficiency. Extracellular polymeric substances (EPS) regulating mechanism preserved AGS stability under nitrogen deficiency, especially through increased secretion of polysaccharide. In addition, members of the Anaerolineaceae were the major filamentous bacteria, which are strictly anaerobic organism, providing a possible explanation to the lack of filamentous bacteria outgrowth under N deficient condition. Insights from this study could help lower chemical costs in AGS applications for specific industrial wastewater treatments.

Organic loading rate (OLR) regulation for enhancement of aerobic sludge granulation: Role of key microorganism and their function

According to unique growth characteristics of various environmental microorganism specially with different substrates and their levels, aerobic sludge granulation are studied under different operation mode of influent organic loading rate (OLR), and the EPS component, sludge surface characters and functional microbes are analyzed to achieve a novel process for stable sludge granulation. Results showed that activated sludge cultivated under gradient influent OLR decreasing from 5.5 to 3.5 kgCOD m^-3 d^-1 achieved complete granulation with average size of 438 μm and exopolysaccharide (PS) to protein (PN) ratio over 2.0. Meanwhile, these granules had excellent flocculability and hydrophobicity with Zeta potential and contact angle of -15 mV and 110°, respectively. Principal component analysis (PCA) illustrated that microbes with function of EPS secretion enriched with decreased OLR regulation for their suitable specific growth characteristics, then promoted other microbes aggregation and sludge granulation along with the improvement of cellular surface characters and microbial niche.

Comparison of aerobic granulation in SBR and continuous-flow plants

Up to now, aerobic granulation of activated sludge is only realised in SBRs, where the discontinuous feed and sedimentation allow the formation of dense granules with excellent settling properties. However, aerobic granulation in continuous-flow plants (CFP) is gaining more and more interest in order to exploit the advantages of these excellent sludge properties to construct compact and efficient WWTP. Within the scope of this project, a SBR and CFP were operated in parallel to investigate the aerobic granulation of activated sludge and to compare the biomass in terms of their structure and settling behavior. CFP operation included two experimental phases with different reactor designs. The use of synthetic wastewater during phase I led to a biomass with a SVI of 42 ml g^-1, whereby the SVI declined only to 85 ml g^-1 in the second phase and the use of municipal sewage. After the start-up period, microscopic images of the biomass from CFP comprised small compact granules with a high flocculent fraction. Particle size distribution for phase II confirm, that 72% of the particles had a size over 200 μm. A strong correlation was observed between the appearance of NO_x-N in the first reactor and the SVI. The results illustrate, that the anaerobic conditions during feeding are essential to keep stable granules.

The difference of morphological characteristics and population structure in PAO and DPAOgranular sludges

We examined how long-term operation of anaerobic-oxic and anaerobic-anoxic sequencing batch reactors (SBRs) affects the enhanced biological phosphorus removal (EBPR) performance and sludge characteristics. The microbial characteristics of phosphorus accumulating organism (PAO) and denitrifying PAO (DPAO) sludge were also analyzed through a quantitative analysis of microbial community structure. Compared with the initial stage of operation characterized by unstable EBPR, both PAO and DPAO SBR produced a stable EBPR performance after about 100-day operation. From day 200 days (DPAO SBR) and 250 days (PAO SBR) onward, sludge granulation was observed, and the average granule size of DPAO SBR was approximately 5 times larger than that of PAO SBR. The DPAO granular sludge contained mainly rod-type microbes, whereas the PAO granular sludge contained coccus-type microbes. Fluorescence in situ hybridization analysis revealed that a high ratio of Accumulibacter clade I was found only in DPAO SBR, revealing the important role of this organism in the denitrifying EBPR system. A pyrosequencing analysis showed that Accumulibacter phosphatis was present in PAO sludge at a high proportion of 6%, whereas it rarely observed in DPAO sludge. Dechloromonas was observed in both PAO sludge (3.3%) and DPAO sludge (3.2%), confirming that this organism can use both O₂ and NO₃^- as electron acceptors. Further, Thauera spp. was identified to have a new possibility as denitrifier capable of phosphorous uptake under anoxic condition.

Investigation of rapid granulation in SBRs treating aniline-rich wastewater with different aniline loading rates

In this work, aerobic granules were cultivated in two reactors which were denoted as R_L and R_H under 0.6 and 1.8 kg m^-3 d^-1 of aniline loading rates, respectively. The aerobic granular sludge (AGS) in the two sequential batch reactors for treating aniline-rich wastewater was compared. The results showed that the AGS could be rapidly formed with sludge volume index below 30 mL g^-1. The AGS in R_L had more filamentous bacteria than that in R_H by microstructural observations while the secretion of protein in extracellular polymeric substances was improved in R_H and in turn increased relative hydrophobicity of AGS. Within 4-h cycle, the excellent removal of aniline and chemical oxygen demand (COD) were achieved in the two reactors. The removal efficiencies of aniline and COD were consistently over 99.7%, 89.6%, respectively in R_L and 98.6%, 86.6%, respectively in R_H. As for nitrogen removal, NH₄⁺-N released from aniline biodegradation could also be reduced efficiently via nitrification and no nitrite accumulation occurred in both the reactors. Total nitrogen removal performance in R_H was better, due to a more compact structure of AGS. The investigation of microbial community succession by pyrosequencing showed that the diversity of microorganisms decreased when AGS was developed. Proteobacteria especially Gammaproteobacteria significantly increased during aerobic granulation in both reactors. It was also found that the relative abundance of Actinobacteria was higher in R_H than that in R_L. Furthermore, the strains responsible for aniline biodegradation, nitrification, denitrification, and phosphorous accumulation were detected in the systems.

Application of grey system theory on the influencing parameters of aerobic granulation in SBR

Aerobic granulation is a promising technology for wastewater treatment. Four operational parameters were selected as influencing factors for this study. Aerobic granulation was experimented with three different values of organic loading rate (3, 6 and 9 kg COD m^-3 d^-1), superficial upflow air velocity (SUAV) (2, 3 and 4 cm s^-1), settling time (3, 5 and 10 min) and volume exchange ratio (25%, 50% and 75%) in sequencing batch reactor in nine trials for the optimal performance of aerobic granulation. The influence of compared parameters on five reference parameters (sludge volume index (SVI), time taken for the appearance of granules, size and specific gravity of granules and chemical oxygen demand (COD) removal) was analyzed using grey system theory. The grey relational coefficients and grey entropy relational grade of each parameter were calculated. Hydrodynamic shear force in terms of SUAV was found to have the greatest influence on granule appearance, specific gravity of granules and COD removal efficiency. SVI is greatly affected by settling time. The optimal scopes of all the compared parameters were found.

Granulation of Non-filamentous Bulking Sludge Directed by pH, ORP and DO in an Anaerobic/Aerobic/Anoxic SBR

In an anaerobic/aerobic/anoxic (A/O/A) sequencing batch reactor (SBR), non-filamentous bulking sludge granulated after the adjustment of cycle duration and influent composition directed by pH, oxidation-reduction potential (ORP) and dissolved oxygen (DO). The turning points and plateaux of pH, ORP and DO profiles indicated the end of biochemical reactions, such as chemical oxygen demand (COD) consumption, P release, ammonium oxidation, P uptake and denitrification. The difference of nutrient concentration between the beginning and turning points represented the actual treatment capability of the sludge. Non-filamentous bulking with SVI30 of 255 mL g(-1) resulted in a huge biomass loss. After regulation, the cycle duration was shortened from 310 to 195 min without unnecessary energy input. In addition, the settling ability was obviously improved as SVI30 reduced to 28 mL g(-1). Moreover, matured granules with an average diameter of 600 μm were obtained after 45 days, and simultaneous COD, ammonium and phosphate (P) removal was also realized after granulation.

Full scale performance of the aerobic granular sludge process for sewage treatment

Recently, aerobic granular sludge technology has been scaled-up and implemented for industrial and municipal wastewater treatment under the trade name Nereda(®). With full-scale references for industrial treatment application since 2006 and domestic sewage since 2009 only limited operating data have been presented in scientific literature so far. In this study performance, granulation and design considerations of an aerobic granular sludge plant on domestic wastewater at the WWTP Garmerwolde, the Netherlands were analysed. After a start-up period of approximately 5 months, a robust and stable granule bed (>8 g L(-1)) was formed and could be maintained thereafter, with a sludge volume index after 5 min settling of 45 mL g(-1). The granular sludge consisted for more than 80% of granules larger than 0.2 mm and more than 60% larger than 1 mm. Effluent requirements (7 mg N L(-1) and 1 mg P L(-1)) were easily met during summer and winter. Maximum volumetric conversion rates for nitrogen and phosphorus were respectively 0.17 and 0.24 kg (m(3) d)(-1). The energy usage was 13.9 kWh (PE150·year)(-1) which is 58-63 % lower than the average conventional activated sludge treatment plant in the Netherlands. Finally, this study demonstrated that aerobic granular sludge technology can effectively be implemented for the treatment of domestic wastewater.

Fast formation of aerobic granules by combining strong hydraulic selection pressure with overstressed organic loading rate

The combined strong hydraulic selection pressure (HSP) with overstressed organic loading rate (OLR) as a fast granulation strategy was used to enhance aerobic granulation. To investigate the wide applicability of this strategy to different scenarios and its relevant mechanism, different settling times, different inoculums, different exchange ratios, different reactor configurations, and different shear force were used in this study. It was found that clear granules were formed within 24 h and steady state reached within three days when the fast granulation strategy was used in a lab-scale reactor seeded with well settled activated sludge (Reactor 2). However, granules appeared after 2-week operation and reached steady state after one month at the traditional step-wise decreased settling time from 20 to 2 min with OLR of 6 g COD/L·d (Reactor 1). With the fast granulation strategy, granules appeared within 24 h even with bulking sludge as seed to start up Reactor 3, but 6-day lag phase was observed compared with Reactor 2. Both Reactor 2 and Reactor 3 experienced sigmoidal growth curve in terms of biomass accumulation and granule size increase after granulation. In addition, the reproducible results in pilot-scale reactors (Reactor 5 and Reactor 6) with diameter of 20 cm and height/diameter ratio (H/D) of 4 further proved that reactor configuration and fluid flow pattern had no effect on the aerobic granulation when the fast granulation strategy was employed, but biomass accumulation experienced a short lag phase too in Reactor 5 and Reactor 6. Although overstressed OLR was favorable for fast granulation, it also led to the fluffy granules after around two-week operation. However, the stable 6-month operation of Reactor 3 demonstrated that the rapidly formed granules were able to maintain long-term stability by reducing OLR from 12 g COD/L·d to 6 g COD/L·d. A mechanism of fast granulation with the strategy of combined strong HSP and OLR was proposed to explain results and guide the operation with this fast strategy.

Decreasing effect and mechanism of moisture content of sludge biomass by granulation process

Disposal of a high volume of sludge significantly raises water treatment costs. A method for cultivating aerobic granules in a sequencing batch airlift bioreactor to significantly produce lower moisture content is described. Results indicate that optimization of settling time and control of the shear stresses acted on the granules. The diameter of the granule was within the range of 1.0-4.0 mm, and its sludge volume index was stabilized at 40-50 mL g(-1). Its specific gravity was increased by a factor of 0.0392, and specific oxygen uptake rate reached 60.126 mg h(-1) g(-1). Moreover, the percentage of its moisture content in the reactor ranged from 96.73% to 97.67%, and sludge volume was reduced to approximately 60%, greatly due to the presence of extracellular polymeric substances in the granules, as well as changes in their hydrophobic protein content. The removal rate of chemical oxygen demand and [Formula: see text] reaches up to 92.6% and 98%, respectively. The removal rates of total phosphorus is over 85%. Therefore, aerobic granular sludge process illustrates a good biological activity.

Rapid start-up and improvement of granulation in SBR

BACKGROUND

The aim of this study is to accelerate and improve aerobic granulation within a Sequencing Batch Reactor (SBR) by cationic polymer addition.

METHODS

To identify whether the polymer additive is capable of enhancing granule formation, two SBRs (R1 and R2, each 0.15 m in diameter and 2 m in height) are used by feeding synthetic wastewater. The cationic polymer with concentration of 30 to 2 ppm is added to R2, while no cationic polymer is added to R1.

RESULTS

Results show that the cationic polymer addition causes faster granule formation and consequently shorter reactor start-up period. The polymer-amended reactor contains higher concentration of biomass with better settling ability (23% reduction in SVI15) and larger and denser granules (112% increase of granular diameter). In addition, the results demonstrate that the cationic polymer improve the sludge granulation process by 31% increase in Extracellular Polymer Substance(EPS) concentration, 7% increase in Specific Oxygen Uptake Rate(SOUR), 18% increase in hydrophobicity, and 17% reduction in effluent Mixed Liquor Suspended Solid(MLSS) concentration.

CONCLUSIONS

Concludingly, it is found that using the cationic polymer to an aerobic granular system has the potential to enhance the sludge granulation process.

Polyhydroxyalkanoate recovery and effect of in situ extracellular polymeric substances removal from aerobic granules

Polyhydroxyalkanoate (PHA) recovery from aerobic granules was investigated using four cell digestion agents, namely, sodium hypochlorite, sodium hydroxide, acetone and sodium chloride. Simultaneously, the removal of extracellular polymeric substances (EPS) and its effect on PHA yield were investigated. The highest PHA recovery yield was obtained using sodium hypochlorite, accounting for 89% cell dry weight (CDW). The highest PHA was recovered after the sodium hypochlorite completely removed the EPS from the aerobic granules. The average molecular weight (Mw) of the PHA recovered using sodium hypochlorite was 5.31 × 10(5)g/mol with only 1.8% molecular weight degradation. The energy and duration analysis for PHA recovery revealed that the sodium hypochlorite method required the least amount of energy and time at 0.0561 MJ/g PHA and 26 h, respectively. The PHA that was recovered was a P3(HB-co-HV) co-polymer.

A proposed aerobic granules size development scheme for aerobic granulation process

Aerobic granulation is increasingly used in wastewater treatment due to its unique physical properties and microbial functionalities. Granule size defines the physical properties of granules based on biomass accumulation. This study aims to determine the profile of size development under two physicochemical conditions. Two identical bioreactors namely Rnp and Rp were operated under non-phototrophic and phototrophic conditions, respectively. An illustrative scheme was developed to comprehend the mechanism of size development that delineates the granular size throughout the granulation. Observations on granules' size variation have shown that activated sludge revolutionised into the form of aerobic granules through the increase of biomass concentration in bioreactors which also determined the changes of granule size. Both reactors demonstrated that size transformed in a similar trend when tested with and without illumination. Thus, different types of aerobic granules may increase in size in the same way as recommended in the aerobic granule size development scheme.

Physicochemical and biological characterization of long-term operated sulfate reducing granular sludge in the SANI® process

The SANI(®) process (Sulfate reduction, Autotrophic denitrification and Nitrification Integrated) is a treatment system with low energy demands. The major bioreactor of this new technology is a sulfate-reducing up-flow sludge bed (SRUSB) that converts organics and provides electron donors for subsequent autotrophic denitrification. This research characterizes the granules inside the SRUSB, with the aim of improving its efficiency, maximizing its operational flexibility, and minimizing its footprint. The unique sulfate-reducing bacteria (SRB) granules serving in the SRUSB were found to increase the resilience and compactness of the SRUSB. The granules, with a compact and porous structure, showed high cohesion resisting breakage with a shear force G > 3400 s(-1). The hydrophobicity of the external surface of the mature granules remained stable at around 70% and acid volatile sulfide (AVS) accumulated at the bottom of the SRUSB. 16s rRNA gene analysis of the microbial communities revealed that Desulfobulbus (42.1%), Prosthecochloris (19%) and Trichococcus (12%) dominated the mature granular sludge. Fluorescence in situ hybridization (FISH) further showed that SRB organisms were located internally and then surrounded by non-SRB. According to the FISH results, the spatial distribution of extracellular polymeric substances (EPS) displayed protein and α-polysaccharides in the exterior and β-polysaccharide in the core of the granules. Such biological structure suggests that each SRB granule acts as an efficient and independent unit, capable of achieving both fermentation and organic conversion. The present investigation sheds light on the physicochemical and biological characteristics of the SRB granulate. This information provides valuable information for scaling-up the SANI(®) process to treat real saline sewage in Hong Kong.

Rapid formation of nitrifying granules treating high-strength ammonium wastewater in a sequencing batch reactor

Short initial settling time and rapidly increased ammonium nitrogen loading were employed to cultivate nitrifying granular sludge treating inorganic wastewater with 1000 mg/L ammonium nitrogen. It was found that the nitrifying granule-dominant sludge was formed in a sequencing batch reactor (SBR) with influent ammonium concentration increased from 200 to 1000 mg N/L within 55 days. During the following 155-day operation period, nitrifying granules exhibited good performance with an ammonium removal efficiency of 99%. In the meantime, sludge volume index (SVI) decreased from 92 to 15 mL/g and the mean size of the nitrifying granules increased from 106 to 369 μm. Mixed liquor suspended solids (MLSS) decreased from the initial 6.4 to around 3 g/L during the granulation period and increased to over 10 g/L at the end of the operation. The long-term stability of nitrifying granules and the reactor performance were not negatively affected by inhibition from free ammonia (FA) and free nitrous acid (FNA) in this study. This makes the granule sludge technology promising in treating high-strength ammonium wastewater in practice.

Impact of influent COD/N ratio on disintegration of aerobic granular sludge

Disintegration of aerobic granular sludge (AGS) is a challenging issue in the long-term operation of an AGS system. Chemical oxygen demand (COD)-to-nitrogen (N) ratio (COD/N), often variable in industrial wastewaters, could be a destabilizing factor causing granule disintegration. This study investigates the impact of this ratio on AGS disintegration and identifies the key causes, through close monitoring of AGS changes in its physical and chemical characteristics, microbial community and treatment performance. For specific comparison, two lab-scale air-lift type sequencing batch reactors, one for aerobic granular and the other for flocculent sludge, were operated in parallel with three COD/N ratios (4, 2, 1) applied in the influent of each reactor. The decreased COD/N ratios of 2 and 1 strongly influenced the stability of AGS with regard to physical properties and nitrification efficiency, leading to AGS disintegration when the ratio was decreased to 1. Comparatively the flocculent sludge maintained relatively stable structure and nitrification efficiency under all tested COD/N ratios. The lowest COD/N ratio resulted in a large microbial community shift and extracellular polymeric substances (EPS) reduction in both flocculent and granular sludges. The disintegration of AGS was associated with two possible causes: 1) reduction in net tyrosine production in the EPS and 2) a major microbial community shift including reduction in filamentous bacteria leading to the collapse of granule structure.

Enhancement of aerobic granulation by zero-valent iron in sequencing batch airlift reactor

This study elucidates the enhancement of aerobic granulation by zero-valent iron (ZVI). A reactor augmented with ZVI had a start-up time of aerobic granulation (43 days) that was notably less than that for a reactor without augmentation (64 days). The former reactor also had better removal efficiencies for chemical oxygen demand and ammonium. Moreover, the mature granules augmented with ZVI had better physical characteristics and produced more extracellular polymeric substances (especially of protein). Three-dimensional-excitation emission matrix fluorescence showed that ZVI enhanced organic material diversity. Additionally, ZVI enhanced the diversity of the microbial community. Fe(2+) dissolution from ZVI helped reduce the start-up time of aerobic granulation and increased the extracellular polymeric substance content. Conclusively, the use of ZVI effectively enhanced aerobic granulation.

Optimization of operation conditions for the startup of aerobic granular sludge reactors biologically removing carbon, nitrogen, and phosphorous

The transformation of conventional flocculent sludge to aerobic granular sludge (AGS) biologically removing carbon, nitrogen and phosphorus (COD, N, P) is still a main challenge in startup of AGS sequencing batch reactors (AGS-SBRs). On the one hand a rapid granulation is desired, on the other hand good biological nutrient removal capacities have to be maintained. So far, several operation parameters have been studied separately, which makes it difficult to compare their impacts. We investigated seven operation parameters in parallel by applying a Plackett-Burman experimental design approach with the aim to propose an optimized startup strategy. Five out of the seven tested parameters had a significant impact on the startup duration. The conditions identified to allow a rapid startup of AGS-SBRs with good nutrient removal performances were (i) alternation of high and low dissolved oxygen phases during aeration, (ii) a settling strategy avoiding too high biomass washout during the first weeks of reactor operation, (iii) adaptation of the contaminant load in the early stage of the startup in order to ensure that all soluble COD was consumed before the beginning of the aeration phase, (iv) a temperature of 20 °C, and (v) a neutral pH. Under such conditions, it took less than 30 days to produce granular sludge with high removal performances for COD, N, and P. A control run using this optimized startup strategy produced again AGS with good nutrient removal performances within four weeks and the system was stable during the additional operation period of more than 50 days.

Aerobic sludge granulation in a full-scale sequencing batch reactor

Aerobic granulation of activated sludge was successfully achieved in a full-scale sequencing batch reactor (SBR) with 50,000 m(3) d(-1) for treating a town's wastewater. After operation for 337 days, in this full-scale SBR, aerobic granules with an average SVI30 of 47.1 mL g(-1), diameter of 0.5 mm, and settling velocity of 42 m h(-1) were obtained. Compared to an anaerobic/oxic plug flow (A/O) reactor and an oxidation ditch (OD) being operated in this wastewater treatment plant, the sludge from full-scale SBR has more compact structure and excellent settling ability. Denaturing gradient gel electrophoresis (DGGE) analysis indicated that Flavobacterium sp., uncultured beta proteobacterium, uncultured Aquabacterium sp., and uncultured Leptothrix sp. were just dominant in SBR, whereas uncultured bacteroidetes were only found in A/O and OD. Three kinds of sludge had a high content of protein in extracellular polymeric substances (EPS). X-ray fluorescence (XRF) analysis revealed that metal ions and some inorganics from raw wastewater precipitated in sludge acted as core to enhance granulation. Raw wastewater characteristics had a positive effect on the granule formation, but the SBR mode operating with periodic feast-famine, shorter settling time, and no return sludge pump played a crucial role in aerobic sludge granulation.

Analysis of aerobic granular sludge formation based on grey system theory

Based on grey entropy analysis, the relational grade of operational parameters with aerobic granular sludge's granulation indicators was studied. The former consisted of settling time (ST), aeration time (AT), superficial gas velocity (SGV), height/diameter (H/D) ratio and organic loading rates (OLR), the latter included sludge volume index (SVI) and set-up time. The calculated result showed that for SVI and set-up time, the influence orders and the corresponding grey entropy relational grades (GERG) were: SGV (0.9935) > AT (0.9921) > OLR (0.9894) > ST (0.9876) > H/D (0.9857) and SGV (0.9928) > H/D (0.9914) > AT (0.9909) > OLR (0.9897) > ST (0.9878). The chosen parameters were all key impact factors as each GERG was larger than 0.98. SGV played an important role in improving SVI transformation and facilitating the set-up process. The influence of ST on SVI and set-up time was relatively low due to its dual functions. SVI transformation and rapid set-up demanded different optimal H/D ratio scopes (10-20 and 16-20). Meanwhile, different functions could be obtained through adjusting certain factors' scope.

The competition between flocculent sludge and aerobic granules during the long-term operation period of granular sludge sequencing batch reactor

The long-term operational stability of aerobic granular sludge reactor was investigated in this study. It was found that the fraction of flocculent sludge fluctuated from 5 to 35%, even with a settling time of less than 5 minutes and manual discharge of flocculent sludge during a steady state of more than 400 days. Although the microbial community structure of flocculent sludge was similar to that of granular sludge co-existing in the reactor, the specific growth rate, the observed biomass yield and the specific oxygen consumption rate of flocculent sludge were much higher than those of granular sludge with identical microbial community structures. Therefore, the presence offlocculent sludge in the granular sludge reactor is mainly because of the kinetic superiority of flocculent sludge over granular sludge, rather than microbial competition. Increasing mass transfer in the feast period or discharging excess flocculent sludge could enhance the growth of granular sludge and improve the stability of the long-term operation of the granular sludge reactor.

Cultivation of aerobic granules in a novel configuration of sequencing batch airlift reactor

Aerobic granules can be formed in sequencing batch airlift reactors (SBAR) and sequencing batch reactors (SBR). Comparing these two systems, the SBAR has excellent mixing condition, but due to a high height-to-diameter ratio (H/D), there is no performance capability at full scale at the present time. This research examined a novel configuration of SBAR at laboratory scale (with a box structure) for industrial wastewater treatment. To evaluate chemical oxygen demand (COD) removal efficiency and granule formation of the novel reactor (R1), in comparison a conventional SBAR (R2) was operated under similar conditions during the experimental period. R1 and R2 with working volumes of 3.6 L and 4.5 L, respectively, were used to cultivate aerobic granules. Both reactors were operated for 4 h per cycle. Experiments were done at different organic loading rates (OLRs) ranging from 0.6-4.5 kg COD/m3.d for R1 and from 0.72-5.4 kg COD/m3.d for R2. After 150 days of operation, large-sized black filamentous granules with diameters of 0.5-2 mm and 2-11 mm were formed in R1 and R2, respectively. In the second part of the experiment, the efficiency of removal of a toxic substance by aerobic granules was investigated using aniline as a carbon source with a concentration in the range 1.2-6.6 kg COD/m3.d and 1.44-7.92 kg COD/m3.d in R1 and R2, respectively. It was found that COD removal efficiency of the novel airlift reactor was over 97% and 94.5% using glucose and aniline as carbon sources, respectively. Sludge volume index (SVI) was also decreased to 30 mL/g by granulation in the novel airlift reactor.