The influence of short-term starvation on aerobic granules

The stable operation of nitritation process with the continuous granular sludge-type reactor and microbial community analysis

The nitritation step is the fundament for the biological nitrogen removal regardless of the traditional nitrification and denitrification process, the nitrite shunt process or the anammox process. Thus, exploring the effective nitritation performance is an important aspect of biological nitrogen removal. This study explored the upper limit of nitritation rate by increasing hydraulic residence time with the well-mixed and continuous granular sludge-type reactor characterized with low complexity and easy operation. The results showed that with the nitrogen loading rate of 1.0 kg/m3/d, the nitrite production rate could reach up to 0.65 kg/m3/d with the nitrite production efficiency of 63.49%, which is remarkable compared to that in the previously similar research. The microbial analysis indicated that ammonia-oxidizing bacteria was successfully enriched (13.27%) and genus Nitrosomonas was the dominant bacteria type. Besides, the activity of ammonium oxidizing bacteria in the continuous flow reactor was higher than that of other reactor types. The growth of vorticella on the sludge was also found in the reactor. The test of specific sludge activity and the microbe analysis both indicated that the nitrite-oxidizing bacteria was well inhibited during the whole experiment, which indicated the strategy of simply adjusting the dissolved oxygen is effective for running of nitritation process. The phosphorus removal performance was also achieved with a removal efficiency of 23.53%. The functional composition of the microbial community in the samples was predicted and finally transformation mechanism of nitrogen in sludge was drawn. In sum, this study indicated the superior performance of the granule sludge-type nitritation process and give a reference for the application of biological nitrogen removal technology.

Potential causes of partial-denitrification (PD) granular sludge breakdown under high nitrate loading rates

The granule instability has been frequently reported during the operation of high loading rates. While, there no research was performed on the recently developed anoxic partial-denitrification (PD) granules, a novel pathway in producing nitrite from nitrate for anammox process. Herein, this work, for the first time, investigated the influence of nitrate loading rates on the instability of PD granules and identified the key causes. Two lab-scale sequencing batch reactors (SBRs) were operated with nitrate loading rates (NLR) increased from 0.48 to 3.84 kg N/m³/d (R1, 8 cycles/d), and 0.96 to 7.68 kg N/m³/d (R2, 16 cycles/d) by gradually elevating the influent nitrate concentration. Results showed that nitrite production rates increased with the NLRs, with a maximal value of 5.26 kg N/m³/d obtained. However, the compact regular PD granules were not stable and broke down when NLR was above 3.84 kg N/m³/d, which resulted in serious sludge washing out from SBR. The high NLRs led to the extracellular polymeric substances (EPS) transformation in terms of its composition and structure, which the protein content in the EPS and the tightly bound EPS (T-EPS) fraction was significantly decreased, this was supposed to be the major reason causing the breakdown of PD granules. Besides, it was found the PD granule in R2 was more deteriorated than that in R1 under the same high NLR, suggesting the short starvation (idle) times in SBR cycle was likely another reason impairing the stability of PD granules. Overall, this research provides useful information in development of granule-based PD systems and sheds light on achieving high-rate nitrite production in SBR with great stability.

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.

Temporal dynamics of antibiotic resistant bacteria and antibiotic resistance genes in activated sludge upon exposure to starvation

The activated sludge represents a huge reservoir for antibiotic resistant bacteria (ARB) and antibiotic resistance genes (ARGs). Owing to the wastewater fluctuation, annual maintenance and storage requirement, the activated sludge in wastewater treatment plants (WWTPs) may suffer from substrate deficiency (i.e., starvation). Whereas the starvation has been confirmed to regulate the antibiotic resistance in numerous pure bacteria, its impacts on the antibiotic resistance in activated sludge remain unclear. Here, the dynamics of sulfonamide and tetracycline ARB and corresponding ARGs in three forms including intracellular ARGs (iARGs), adsorbed extracellular ARGs (aeARGs) and free extracellular ARGs (feARGs) in activated sludge upon exposure to starvation were investigated. The results showed that, among the different electron donors (i.e., carbon, nitrogen and phosphate), carbon starvation could effectively reduce the absolute abundance of ARB and aeARGs by up to 1.68 lgs and 2.62 lgs, respectively, and released a small amount of feARGs in wastewater with the maximum value of 1.1 × 10⁵ copies/mL due to the high degree of sludge cell lysis and DNA adsorption/degradation. For the different acceptor conditions (that is, alternating anaerobic-aerobic, anaerobic, anoxic and aerobic), the anaerobic-aerobic starvation obviously mitigated the absolute abundance of ARB, aeARGs and iARGs by 0.71 lgs, 3.41 lgs and 1.35 lgs, respectively, via the substantial sludge cell lysis and DNA degradation. These findings demonstrated the response patterns and mechanisms of bacterial resistance in activated sludge to starvation stress, and thus provide clues to control the risk of antibiotic resistance in WWTPs by the starvation strategy.

Differences in exopolysaccharides of three microbial aggregates

Different microbial aggregates show substantial differences in morphology, and extracellular polymer substances have been confirmed to play a key role in the formation of aggregates. In this study, three different microbial aggregates and their exopolysaccharides were compared. The results show that the granular sludge was largest in size and the most compact in shape. Biofilms with a certain thickness had the next greatest density, and flocculent sludge, with the smallest particle size, was the loosest. The extended Derjaguin-Landau-Verwey-Overbeek analysis shows that hydrogen bonding, hydrophobic and electrostatic interactions affect the aggregation of microorganisms. A comparison of exopolysaccharides shows that granular sludge exopolysaccharides show the highest hydrophobicity (38.08%) and lowest surface charge (-20.5 mV), followed by biofilm exopolysaccharides (27.9% and -24.8 mV respectively). The results of Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy show that the contents of hydrophilic and hydrophobic functional groups and charged functional groups of exopolysaccharides affect the above properties of exopolysaccharides, thereby affecting microbial aggregation. In addition, the hydrogen bond content of exopolysaccharides in granular sludge (19.3%), biofilm (19.2%) and activated sludge (18.9%) decreased sequentially. This also affects the cross-linking of microbial exopolysaccharides to form hydrogels. Finally, the results of confocal laser scanning microscopy showed that, different from the other two aggregates, the extracellular α-polysaccharides of granular sludge are mainly distributed in the nucleus, which is more conducive to aggregation. The research results of this thesis provide a new understanding of the differences in the aggregation morphology of different aggregates from the perspective of exopolysaccharides.

Ionic response of algal-bacterial granular sludge system during biological phosphorus removal from wastewater

Biological phosphorus removal (BPR) from wastewater can be generally realized through alternative non-aeration and aeration operation to create anaerobic and aerobic conditions respectively for P release and uptake/accumulation by polyphosphate accumulating organisms (PAOs), with P removal finally achieved by controlled discharge of P-rich sludge. In this study, the response of algal-bacterial aerobic granular sludge (AB-AGS) during BPR to main ions including Ac^- (acetate), Cl^-, SO₄^2-, NH₄⁺, K⁺, Mg²⁺, Ca²⁺ and Na⁺ in wastewater was investigated with conventional bacterial AGS (B-AGS) as control and acetate as the sole carbon source. Results show that BPR process mainly involved the changes of Ac^-, K⁺, Mg²⁺, and Ca²⁺ rather than Cl^-, SO₄^2-, NH₄⁺ and Na⁺. The mole ratio of ΔP/ΔAc kept almost unchanged during the non-aeration (P release) phase in both B-AGS and AB-AGS systems (ΔP_B-AGS/ΔAc_B-AGS > ΔP_AB-AGS/ΔAc_AB-AGS), and it was negatively influenced by the light in AB-AGS systems, in which 62% of acetate was not utilized for P release at the high illuminance of 81 k lux. During the entire non-aeration/aeration period, both ΔK/ΔP and ΔMg/ΔP remained constant, while ΔK_AB-AGS/ΔP_AB-AGS > ΔK_B-AGS/ΔP_B-AGS and ΔMg_AB-AGS/ΔP_AB-AGS ≈ ΔMg_B-AGS/ΔP_B-AGS. The presence of algae seemed not beneficial for PAOs to remove P, while more K⁺ and P uptake by algae in AB-AGS suggest its great potential for manufacturing biofertilizer.

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.

Effect of starvation on the nitrification performance of constructed rapid infiltration systems

Three constructed rapid infiltration (CRI) systems (C¹, C² and C³) were operated under 7, 14 and 21 days of continuous starvation, respectively. The effect of starvation on the ammonia removal efficiency (ARE), nitrite accumulation rate (NAR), bioactivity of nitrifiers and content of extracellular polymeric substances (EPS) was investigated. The results showed that the activity of nitrite-oxidizing bacteria (NOB) was higher than that of ammonia-oxidizing bacteria (AOB) in stabilization periods, leading to a complete nitrification in CRI systems. During starvation periods, the activity decay rates of AOB (k_AOB) for C¹, C² and C³ were 0.172 ± 0.008, 0.132 ± 0.009 and 0.128 ± 0.009 d^-1, respectively, and those of NOB (k_NOB) were 0.159 ± 0.005, 0.152 ± 0.009 and 0.150 ± 0.005 d^-1, respectively, implying that k_AOB was higher than k_NOB in a 7-day starvation period, while showing a contrasting result in a 14- or 21-day starvation period. When resuming wastewater supply, AOB activity as well as the ARE in C¹, C² and C³ gradually restored to their initial levels within 6, 10 and 23 days, respectively. However, NOB activity was unable to fully restore after a 14- or 21-day starvation period, causing the final NAR of C² and C³ to remain at 25% and 60%, respectively. Furthermore, EPS could be used as the source of carbon and energy for hungry microorganisms to guarantee the metabolic activity of living cells in a starvation environment. These findings could provide a theoretical foundation for operational optimization of CRI systems under starvation conditions.

State of the art of aerobic granulation in continuous flow bioreactors

In the wake of the success of aerobic granulation in sequential batch reactors (SBRs) for treating wastewater, attention is beginning to turn to continuous flow applications. This is a necessary step given the advantages of continuous flow treatment processes and the fact that the majority of full-scale wastewater treatment plants across the world are operated with aeration tanks and clarifiers in a continuous flow mode. As in SBRs, applying a selection pressure, based on differences in either settling velocity or the size of the biomass, is essential for successful granulation in continuous flow reactors (CFRs). CFRs employed for aerobic granulation come in multiple configurations, each with their own means of achieving such a selection pressure. Other factors, such as bioaugmentation and hydraulic shear force, also contribute to aerobic granulation to some extent. Besides the formation of aerobic granules, long-term stability of aerobic granules is also a critical issue to be addressed. Inorganic precipitation, special inocula, and various operational optimization strategies have been used to improve granule long-term structural integrity. Accumulated studies reviewed in this work demonstrate that aerobic granulation in CFRs is capable of removing a wide spectrum of contaminants and achieving properties generally comparable to those in SBRs. Despite the notable research progress made toward successful aerobic granulation in lab-scale CFRs, to the best of our knowledge, there are only three full-scale tests of the technique, two being seeded with anammox-supported aerobic granules and the other with conventional aerobic granules; two other process alternatives are currently in development. Application of settling- or size-based selection pressures and feast/famine conditions are especially difficult to implement to these and similar mainstream systems. Future research efforts needs to be focused on the optimization of the granule-to-floc ratio, enhancement of granule activity, improvement of long-term granule stability, and a better understanding of aerobic granulation mechanisms in CFRs, especially in full-scale applications.

Effects of thiocyanate on granule-based anammox process and implications for regulation

The feasibility of using anaerobic ammonium oxidation (anammox) process to treat industrial wastewater containing thiocyanate (SCN^-) was examined in this study. Anammox activity decreased with increasing thiocyanate concentration and pre-exposure time in batch tests. A typical noncompetitive model was used to fit the data for thiocyanate inhibition, and the 50% inhibition concentration (IC₅₀) of thiocyanate on anammox was 620.4mgL^-1 at 200mgL^-1 total nitrogen level. The influent thiocyanate concentration of test reactor (R₁) in phase II gradually increased from 10 to 120mgL^-1, and the average nitrogen removal efficiency (NRE) of R₁ was maintained at 83.0±7.82%. This robustness was attributed to the self-adaptation ability of anammox biomass through long-term acclimatization. The NRE was decreased to 57.1% under 130mgL^-1 thiocyanate within two days. However, the NRE of control reactor (R₀) in absence of thiocyanate was 91.23±4.11% in this phase. Under thiocyanate stress, the specific anammox activity, settling velocity and heme c content of the granules significantly decreased, and the extracellular polymeric substances content slightly increased. The short- and long-term performance inhibition could be reversed in the presence of 10mgL^-1 Fe(III).

Insight into the short- and long-term effects of Cu(II) on denitrifying biogranules

This study aimed to investigate the short- and long-term effects of Cu(2+) on the activity and performance of denitrifying bacteria. The short-term effects of various concentrations of Cu(2+) on the denitrifying bacteria were evaluated using batch assays. The specific denitrifying activity (SDA) decreased from 14.3 ± 2.2 (without Cu(2+)) to 6.1 ± 0.1 mg N h(-1)g(-1) VSS (100 mg Cu(2+)L(-1)) when Cu(2+) increased from 0 to 100 mg L(-1) with an increment of 10 mg Cu(2+)L(-1). A non-competitive inhibition model was used to calculate the 50% inhibition concentration (IC50) of Cu(2+) on denitrifying sludge (30.6 ± 2.5 mg L(-1)). Monod and Luong models were applied to investigate the influence of the initial substrate concentration, and the results suggested that the maximum substrate removal rate would be reduced with Cu(2+) supplementation. Pre-exposure to Cu(2+) could lead to an 18.2-46.2% decrease in the SDA and decreasing percentage of the SDA increased with both exposure time and concentration. In the continuous-flow test, Cu(2+) concentration varied from 1 to 75 mg L(-1); however, no clear deterioration was observed in the reactor, and the reactor was kept stable, with the total nitrogen removal efficiency and total organic carbon efficiency greater than 89.0 and 85.0%, respectively. The results demonstrated the short-term inhibition of Cu(2+) upon denitrification, and no notable adversity was observed during the continuous-flow test after long-term acclimation.

The role of autoinducer-2 in aerobic granulation using alternating feed loadings strategy

Quorum sensing (QS) plays an important role in aerobic granulation while how QS system regulates the formation of aerobic granules needs further discussion. This study cultivated activated sludge in two identical sequencing batch reactors (R1 and R2) at different influent organic loading rate (OLR) strategies: R1 was operated using constant OLR (around 8.0kg/m(3)d), while R2 was operated at alternating OLR (4.0-17.0kg/m(3)d). Microbial aggregates appeared in R2 on day 19, while the morphology of sludge in R1 changed little compared with the initial sludge. The concentration of autoinducer-2 (AI-2) in R2 showed an ascending trend, along with the increase of cell adhesiveness. The total extracellular polymeric substances (EPS) amount and large molecular weight EPS of R2 rose steadily, which was different from R1. Some bacteria able to self-aggregate and promote EPS secretion were exclusive in R2. A mechanism about aerobic granulation at alternating OLR was proposed.

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.

Influence of carbon source on nutrient removal performance and physical-chemical characteristics of aerobic granular sludge

The impact of carbon source variation on the physical and chemical characteristics of aerobic granular sludge and its biological nutrient (nitrogen and phosphorus) removal performance was investigated. Two identical sequencing batch reactors, R1 and R2, were set up. Granular biomass was cultivated to maturity using acetate-based synthetic wastewater. After mature granules in both reactors with simultaneous chemical oxygen demand (COD), ammonium and phosphorus removal capability were achieved, the feed of R2 was changed to municipal wastewater and R1 was continued on synthetic feed as control. Biological phosphorus removal was completely inhibited in R2 due to lack of readily biodegradable COD; however, the biomass maintained high ammonium and COD removal efficiencies. The disintegration of the granules in R2 occurred during the first two weeks after the change of feed, but it did not have significant impacts on settling properties of the sludge. Re-granulation of the biomass in R2 was then observed within 30 d after granules' disintegration when the biomass acclimated to the new substrate. The granular biomass in R1 and R2 maintained a Sludge Volume Index close to 60 and 47 mL g(-1), respectively, during the experimental period. It was concluded that changing the carbon source from readily biodegradable acetate to the more complex ones present in municipal wastewater did not have significant impacts on aerobic granular sludge characteristics; it particularly did not affect its settling properties. However, sufficient readily biodegradable carbon would have to be provided to maintain simultaneous biological nitrate and phosphorus removal.

Sequentially alternating pollutant scenarios of phenolic compounds in a continuous aerobic granular sludge reactor performing simultaneous partial nitritation and o-cresol biodegradation

Industrial wastewater treatment plants must operate properly during the transient-state conditions often found in the industrial production. This study presents the performance of simultaneous partial nitritation and o-cresol biodegradation in a continuous aerobic granular reactor under sequentially alternating pollutant (SAP) scenarios. Three SAP scenarios were imposed during the operation of the granular reactor. In each one, a secondary recalcitrant compound (either p-nitrophenol (PNP), phenol or 2-chlorophenol (2CP)) were added for a short period of time to the regular influent containing only ammonium and o-cresol. Partial nitritation and o-cresol biodegradation were not inhibited by the presence of PNP or phenol and both compounds were fully biodegraded. On the contrary, the presence of 2CP strongly inhibited both processes within 2days. However, the reactor was recovered in a few days. These findings demonstrate that treatment of complex industrial wastewaters with variable influent composition is feasible in a continuous aerobic granular reactor.

Aerobic granules: microbial landscape and architecture, stages, and practical implications

For the successful application of aerobic granules in wastewater treatment, granules containing an appropriate microbial assembly able to remove contaminants should be retained and propagated within the reactor. To manipulate and/or optimize this process, a good understanding of the formation and dynamic architecture of the granules is desirable. Models of granules often assume a spherical shape with an outer layer and an inner core, but limited information is available regarding the extent of deviations from such assumptions. We report on new imaging approaches to gain detailed insights into the structural characteristics of aerobic granules. Our approach stained all components of the granule to obtain a high quality contrast in the images; hence limitations due to thresholding in the image analysis were overcome. A three-dimensional reconstruction of the granular structure was obtained that revealed the mesoscopic impression of the cavernlike interior of the structure, showing channels and dead-end paths in detail. In "old" granules, large cavities allowed for the irrigation and growth of dense microbial colonies along the path of the channels. Hence, in some areas, paradoxically higher biomass content was observed in the inner part of the granule compared to the outer part. Microbial clusters "rooting" from the interior of the mature granule structure indicate that granules mainly grow via biomass outgrowth and not by aggregation of small particles. We identify and discuss phenomena contributing to the life cycle of aerobic granules. With our approach, volumetric tetrahedral grids are generated that may be used to validate complex models of granule formation.

Application of high OLR-fed aerobic granules for the treatment of low-strength wastewater: performance, granule morphology and microbial community

Aerobic granules, pre-cultivated at the organic loading rate (OLR) of 3.0 kg COD/(m3 x day), were used to treat low-strength wastewater in two sequencing batch reactors at low OLRs of 1.2 and 0.6 kg COD/(m3 x day), respectively. Reactor performance, evolution of granule morphology, structure and microbial community at low OLRs under long-term operation (130 days) were investigated. Results showed that low OLRs did not cause significant damage to granule structure as a dominant granule morphology with size over 540 microm was maintained throughout the operation. Aerobic granules at sizes of about 750 microm were finally obtained at the low OLRs. The granule reactors operated at low OLRs demonstrated effective COD and ammonia removals (above 90%), smaller granule sizes and less biomass. The contents of extracellular polymeric substances in the granules were decreased while the ratios of exopolysaccharide/exoprotein were increased (above 1.0). The granules cultivated at the low OLRs showed a smoother surface and more compact structure than the seeded granules. A significant shift in microbial community was observed but the microbial diversity remained relatively stable. Confocal Laser Scanning Microscopy observation showed that the live cells were spread throughout the whole granule, while the dead cells were mainly concentrated in the outer layer of the granule, and the proteins, polysaccharides and lipids were mainly located in the central regime of the granule. In conclusion, granules cultivated at high OLRs show potential for treating low-strength organic wastewater steadily under long-term operation.

Ammonium adsorption in aerobic granular sludge, activated sludge and anammox granules

The ammonium adsorption properties of aerobic granular sludge, activated sludge and anammox granules have been investigated. During operation of a pilot-scale aerobic granular sludge reactor, a positive relation between the influent ammonium concentration and the ammonium adsorbed was observed. Aerobic granular sludge exhibited much higher adsorption capacity compared to activated sludge and anammox granules. At an equilibrium ammonium concentration of 30 mg N/L, adsorption obtained with activated sludge and anammox granules was around 0.2 mg NH4-N/g VSS, while aerobic granular sludge from lab- and pilot-scale exhibited an adsorption of 1.7 and 0.9 mg NH4-N/g VSS, respectively. No difference in the ammonium adsorption was observed in lab-scale reactors operated at different temperatures (20 and 30 °C). In a lab-scale reactor fed with saline wastewater, we observed that the amount of ammonium adsorbed considerably decreased when the salt concentration increased. The results indicate that adsorption or better ion exchange of ammonium should be incorporated into models for nitrification/denitrification, certainly when aerobic granular sludge is used.

Variations of both bacterial community and extracellular polymers: the inducements of increase of cell hydrophobicity from biofloc to aerobic granule sludge

To investigate the inducements of increase of cell hydrophobicity from aerobic biofloc (ABF) and granular sludge (AGS), in this study, as the first time the hydrophilic and hydrophobic bacterial communities were analyzed independently. Meanwhile, the effect of extracellular polymers (EPS) on the cell hydrophobicity is also studied. Few Bacteroidetes were detected (1.35% in ABF and 3.84% in AGS) in hydrophilic bacteria, whereas they are abundant in the hydrophobic cells (47.8% and 43% for ABF and AGS, respectively). The main species of Bacteroidetes changed from class Sphingobacteria to Flavobacteria in AGS. On the other hand, EPS is directly responsible to cell hydrophobicity. For AGS, cell hydrophobicity was sharply decreased after EPS extraction. Both quantity and property of the extracellular protein are related to hydrophobicity. Our results showed the variation of cell hydrophobicity was resulted from variations of both bacterial population and EPS.

Structural determination of a key exopolysaccharide in mixed culture aerobic sludge granules using NMR spectroscopy

Nuclear magnetic resonance (NMR) techniques were used to elucidate the structure of an exopolysaccharide material previously revealed to be important in formation of aerobic granules. The 1D NMR spectral data acquired showed that this gel-forming polysaccharide was a major component of granular EPS, while 1D and 2D NMR spectra showed it consisted of eight sugar residues. These were assigned as α-galactose, α-rhamnose, 2-acetoamido-2-deoxy-α-galactopyranuronic acid, β-mannose, β-galactose, β-glucuronate, β-glucosamine, and N-acetyl β-galactosamine. With the exception of 2-acetoamido-2-deoxy-α-galactopyranuronic acid, a highly unusual sugar, their presence was confirmed with high-performance anion-exchange chromatography with pulsed amperometric detection (HPAEC-PAD). Carbon and proton shifts were assigned for each sugar. Heteronuclear multiple bond correlation (HMBC) and nuclear Overhauser enhancement spectroscopy (NOESY) were used to identify linkage sites between individual sugar residues. This gel-forming exopolysaccharide appeared to be a highly complex single heteropolysaccharide with a repeat sequence of α-galactose, β-mannose, β-glucosamine, N-acetyl-β-galactosamine, and 2-acetoamido-2-deoxy-α-galactopyranuronic acid. It has a disaccharide branch of β-galactose and β-glucuronic acid attached to 2-acetoamido-2-deoxy-α-galactopyranuronic acid and an α-rhamnose branch attached to α-galactose.

Characterization of alginate-like exopolysaccharides isolated from aerobic granular sludge in pilot-plant

To understand functional gel-forming exopolysaccharides in aerobic granular sludge, alginate-like exopolysaccharides were specifically extracted from aerobic granular sludge cultivated in a pilot plant treating municipal sewage. The exopolysaccharides were identified by the FAO/WHO alginate identification tests, characterized by biochemical assays, gelation with Ca(2+), blocks fractionation, spectroscopic analysis as UV-visible, FT-IR and MALDI-TOF MS, and electrophoresis. The yield of extractable alginate-like exopolysaccharides was reached 160+/-4mg/g (VSS ratio). They resembled seaweed alginate in UV-visible and MALDI-TOF MS spectra, and distinguished from it in the reactions with acid ferric sulfate, phenol-sulfuric acid and Coomassie brilliant blue G250. Characterized by their high percentage of poly guluronic acid blocks (69.07+/-8.95%), the isolated exopolysaccharides were capable to form rigid, non-deformable gels in CaCl(2). They were one of the dominant exopolysaccharides in aerobic granular sludge. We suggest that polymers play a significant role in providing aerobic granular sludge a highly hydrophobic, compact, strong and elastic structure.

Gel-forming exopolysaccharides explain basic differences between structures of aerobic sludge granules and floccular sludges

The sol-gel transition of extracellular polymeric substances (EPS) derived from sludge flocs and granules is investigated in order to explain basic differences between the two aggregates. A reversible, pH dependent sol-gel transition was observed at pH 9.0-12.0 in EPS extracted from granules. At pH <9 granule EPS existed as a strong gel, indicating that their EPS exist in a gel state at normal operating pH of a wastewater treatment system (i.e. 6.0-8.5). This characteristic transition from solution to strong gel was not observed in any of the EPS samples derived from floccular sludges. A transition to a weak gel was however, observed at pH 4.0-5.0. Enriched exopolysaccharides from the granular EPS exhibited rheological behaviour analogous to the granules and the granule EPS. The critical overlap concentration (c*) of the exopolysaccharide concentrate was 0.33% w/w, similar to the c* of other known bacterial exopolysaccharides. Additionally, the protein content was found to be not contributing to the storage modulus of granule EPS gels. These factors suggest that exopolysaccharides or glycosides were the gelling agent in aerobic sludge granules. Given that EPS derived from aerobic sludge granules and flocs are distinguished by such a sol-strong gel transition, these exopolysaccharides therefore likely play an important role in granulation.

Proteolytic activity in stored aerobic granular sludge and structural integrity

Aerobic granules lose stability during storage. The goal of this work was to highlight the main cause of stability loss for stored granules as intracellular protein hydrolysis. The quantity of extracellular proteins was noted to be significantly lower during granule storage, and protease enzyme activities were correspondingly higher in the cores of stored granules. The proteolytic bacteria, which secrete highly active protease enzymes, were for the first time isolated and characterized by analyzing 16S rDNA sequences. The proteolytic bacteria belonged to the genera Pseudomonas, Raoultella, Acinetobacter, Pandoraea, Klebsiella, Bacillus and uncultured bacterium, and were grouped into Proteobacteria, Enterobacteria and Firmicutes. The PB1 (Pseudomonas aeruginosa) strain, which exhibited very high proteolytic activity during the skim milk agar test, was located at the core regime with active protease enzymes, and was close to the obligate anaerobic strain Bacteroides sp. Hence, the extracellular proteins in stored granules were proposed to be hydrolyzed by enzymes secreted by proteolytic bacteria with the hydrolyzed products ultimately being used by nearby anaerobic strains. This process gradually digests the protein core, and eventually consumes the entire granule.

Influence of starvation time on formation and stability of aerobic granules in sequencing batch reactors

Three sequencing batch reactors, R1, R2 and R3, with a 1.5-h, 4-h and 8-h cycle time, respectively, were used to cultivate aerobic granules with the same synthetic wastewater containing 1000 mg l(-1) COD. As the initial COD concentrations in the cycles were the same, three different cycle times led to three different starvation times in repeated cycles of the three reactors. It was found that 63 cycles were needed to form granules with the longest starvation time in R3 while it took 256 cycles in R1 with the shortest starvation time. However, as far as the formation time was concerned, granules were formed on day 16 with 1.5-h cycle time while on day 21 with 8-h cycle time, which indicated that a shorter cycle time with a shorter starvation time speeded up the granulation. This was mainly due to the stronger hydraulic selection pressure at shorter cycle time. However, it was found that granules formed with cycle time of 1.5h were unstable. Fluffy granules with poor settling ability were observed in R1 in the 4th month, which led to the collapse of R1 after 160-day of operation. Granules in R2 and R3 showed good stability during the long-term operation. Therefore, a reasonable starvation time was necessary to maintain the long-term stability of aerobic granules.

Optimization of main factors associated with nitrogen removal in hybrid sludge sequencing batch reactor with step-feeding of swine wastewater

To attain a high nitrogen removal efficiency and good sludge settleability in a step-fed sequencing batch reactor (SFSBR) treating swine wastewater, L(9)(3(4)) orthogonal experiments were carried out to optimize main factors associated with nitrogen removal, namely, the influent C/N ratio, feeding volume ratio, nitrogen loading rate and aeration intensity. Results showed that nitrogen loading rate contributed most for the build-up of NO(2)(-)-N, NO(3)(-)-N and NH(4)(+)-N in the effluent, while aeration intensity was the most important factor for net nitrogen removal efficiency based on the initial and final nitrogen concentrations in the SFSBR cycle. Additionally, the periodic starvation created by stepwise feeding was the major inducing force for granulation in the SFBSR process and the influent C/N ratio had a profound influence on sludge settleability and granular sludge stability in terms of sludge volume index (SVI) and the fraction of granular sludge with diameter over 0.5 mm (f(0.5 mm)), respectively. Considering the most and secondary important control factor for individual response index, the optimal operating condition for nitrogen removal of SFSBR treating swine wastewater was determined as A(3)B(3)C(1)D(2), i.e., influent C/N ratio 7.0 mg COD/mg NH(4)(+)-N, feeding volume ratio 3:1, nitrogen loading rate 0.026 g NH(4)(+)-N/gVSS . d and aeration intensity 4.2 L/m(3) . s, respectively. Under the optimal operating conditions, inorganic nitrogen concentration in the effluent, net nitrogen removal efficiency, SVI and f(0.5 mm) reached 21 mg/L, 72 %, 40.7 mL/g and 4.3 %, respectively.