Diffusivity of oxygen in aerobic granules

Implementing a completely autotrophic nitrogen removal over nitrite process using a novel umbrella basalt fiber carrier

The completely autotrophic nitrogen removal over nitrite (CANON) process is significantly hindered by prolonged start-up periods and unstable nitrogen removal efficiency. In this study, a novel umbrella basalt fiber (BF) carrier with good biological affinity and adsorption performance was used to initiate the CANON process. The CANON process was initiated on day 64 in a sequencing batch reactor equipped with umbrella BF carriers. During this period, the influent NH4+-N concentration gradually increased from 100 to 200 mg·L-1, and the dissolved oxygen was controlled below 0.8 mg L-1. Consequently, an average ammonia nitrogen removal efficiency (ARE) and total nitrogen removal efficiency (TNRE) of ∼90 and 80% were achieved, respectively. After 130 days, ARE and TNRE remained stable at 92 and 81.1%, respectively. This indicates a reliable method for achieving rapid start-up and stable operation of the CANON process. Moreover, Candidatus Kuenenia and Candidatus Brocadia were identified as dominant anammox genera on the carrier. Nitrosomonas was the predominant genus among ammonia-oxidizing bacteria. Spatial differences were observed in the microbial population of umbrella BF carriers. This arrangement facilitated autotrophic nitrogen removal in a single reactor. This study indicates that the novel umbrella BF carrier is a highly suitable biocarrier for the CANON process.

Diffusion of soluble organic substrates in aerobic granular sludge: Effect of molecular weight

Aerobic granular sludge (AGS) is an advanced biofilm-based technology for wastewater treatment. Diffusion of substrates into the granules is a key aspect of this technology. Domestic wastewater contains soluble organic substrates of different sizes that could potentially diffuse into the granules. In this study, the relation between the molecular weight of a substrate and its diffusion coefficient within the granule was studied with model substrates (polyethylene glycols (PEGs) with a molecular weight between 62 and 10 000 Da). The diffusion coefficients of the model substrates within granules from a full-scale installation were measured with the 'transient uptake of a non-reactive solute' method. The diffusion coefficients in the granules were not significantly different from the diffusion coefficients in water, at least up to 4000 Da molecular weight. This indicates that these PEGs were not obstructed by the granule matrix. The 10 kDa PEG behaved differently from the lighter PEGs, as it could not penetrate the entire granule. Furthermore, the granule structure was characterized with Environmental Scanning Electron Microscopy (ESEM). The granules displayed an open structure with large macropores and semi-solid regions, which contained microbial cells. The diffusion results suggest that most diffusing molecules were unobstructed in the macropores and barely obstructed in the semi-solid regions. Only the diffusion of the 10 kDa PEG seemed to be hindered by the semi-solid regions, but not by the macropores. Lastly, the apparent molecular weight distribution of domestic wastewater soluble COD was determined with ultrafiltration membranes of 100, 10, and 1 kDa molecular weight cut-off. The influent fractionation revealed that a large part (61-69%) of the influent soluble COD was lighter than 1 kDa. As molecules lighter than 1 kDa diffuse easily, the majority of the influent soluble COD can be considered as diffusible COD. These findings provide new insight into the availability of influent COD for granular sludge.

Effective nitrogen removal from ammonium-depleted wastewater by partial nitritation and anammox immobilized in granular and thin layer gel carriers

This study investigates the effect of physicochemical conditions on the partial nitritation and anammox treatment by immobilized ammonia oxidizers under ammonium-deplete conditions. The impact of oxygen and temperature was studied by measuring the activity of immobilized aerobic and anaerobic ammonia-oxidizing organisms (Ammonia-oxidizing bacteria (AOB) and archaea (AOA), and Anammox bacteria) embedded in polyvinyl alcohol - sodium alginate (PVA-SA) beads and in thin layer poly-ethylene glycol hydrogels. Beads and flat hydrogels were incubated in a fluidized bed reactor (FBR) and in two flow cells, respectively. Both systems were fed with synthetic wastewater (15 mg N-NH₄⁺/L) at different temperatures (20 °C and/or 30 °C) and different dissolved oxygen (DO) concentrations (0.1, 0.3, 0.5 and/or 1 mg/L) over 152 and 207 days, respectively. The FBR system had a maximum removal rate of 1.7 g-N/m³/d at 0.1 mg O₂/L, corresponding to 80% removal efficiency, while a high aerobic ammonia-oxidizing activity but a partial oxygen inhibition of Anammox bacteria were observed at higher DO concentrations. In both flow cells, nitrogen removal efficiency was highest (80%) at 30 °C and 1 mg O₂/L while removal was less favorable at lower DO and lower temperature. Our results indicate a potential use of hydrogel beads for an energy efficient technology with reduced aeration demand for treating low ammonia wastewater, while layered hydrogels are a possible first step for biological treatments of wastewater using tangential flow. In addition, we provide blueprint drawings of the flow cells, which may be used to 3D-print the apparatus for other applications.

Heterogeneous diffusion in aerobic granular sludge

Aerobic granular sludge (AGS) technology allows simultaneous nitrogen, phosphorus, and carbon removal in compact wastewater treatment processes. To operate, design, and model AGS reactors, it is essential to properly understand the diffusive transport within the granules. In this study, diffusive mass transfer within full‐scale and lab‐scale AGS was characterized with nuclear magnetic resonance (NMR) methods. Self‐diffusion coefficients of water inside the granules were determined with pulsed‐field gradient NMR, while the granule structure was visualized with NMR imaging. A reaction‐diffusion granule‐scale model was set up to evaluate the impact of heterogeneous diffusion on granule performance. The self‐diffusion coefficient of water in AGS was ∼70% of the self‐diffusion coefficient of free water. There was no significant difference between self‐diffusion in AGS from full‐scale treatment plants and from lab‐scale reactors. The results of the model showed that diffusional heterogeneity did not lead to a major change of flux into the granule (<1%). This study shows that differences between granular sludges and heterogeneity within granules have little impact on the kinetic properties of AGS. Thus, a relatively simple approach is sufficient to describe mass transport by diffusion into the granules.

Piggery wastewater treatment by aerobic granular sludge: Granulation process and antibiotics and antibiotic-resistant bacteria removal and transport

The aim of this work was to study the responses of aerobic granulation process to antibiotics and investigate the antibiotics and antibiotic-resistant bacteria (ARB) removal and transport. Results showed that aerobic granular sludge (AGS) was dominant in the bioreactor at day 45, and the relatively high protein content from tightly bound extracellular polymeric substances (TB-EPS) facilitated aerobic granulation and maintained biomass stabilization. The protein contents in EPS and TB-EPS were positively correlated with relative hydrophobicity, thereby improving the adsorption capacity among hydrophobic particles. The chemical oxygen demand (COD), NH₃-N, and total N removal efficiencies were 98.0%, 97.0%, and 92.4%, respectively. Five antibiotics, including kanamycin, tetracycline, ciprofloxacin, ampicillin, and erythromycin, were examined in piggery wastewater, with concentrations up to the concentration range of 29.4-44.1 µg/l, and the total antibiotics removal rate reached up to 88.4% ± 4.5%. A total of 5.2% of the total antibiotics were discharged from bioreactor, and 62.5% of the total antibiotics were degraded, and 32.3% of total antibiotics were adsorbed by aerobic granules. The presence of antibiotics rarely exhibited an influence on AGS formation, and the relatively high microbial activity of aerobic granules was beneficial to antibiotics removal. The ARB removal rate increased up to 89.4% ± 3.3%, but a large amount of ARB was enriched in aerobic granules.

Optimizing granules size distribution for aerobic granular sludge stability: Effect of a novel funnel-shaped internals on hydraulic shear stress

A novel funnel-shaped internals was proposed to enhance the stability and pollutant removal performance of an aerobic granular process by optimizing granule size distribution. Results showed up to 68.3±1.4% of granules in novel reactor (R1) were situated in optimal size range (700-1900μm) compared to less than 29.7±1.1% in conventional reactor (R2), and overgrowth of large granules was effectively suppressed without requiring additional energy. Consequently, higher total nitrogen (TN) removal (81.6±2.1%) achieved in R1 than in R2 (48.1±2.7%). Hydraulic analysis revealed the existence of selectively assigning hydraulic pressure in R1. The total shear rate (τtotal) on large granules was 3.07±0.14 times higher than that of R2, while τtotal of small granules in R1 was 70.7±4.6% in R2. Furthermore, large granules in R1 with intact extracellular polymeric substances (EPS) outer layer structure entrapped hydroxyapatite at center, which formed a core structure and further enhanced the stability of aerobic granules.

Microbial communities of aerobic granules: granulation mechanisms

Aerobic granulation is an advanced biological wastewater treatment technology. This study for the first time identified the microbial communities of sliced samples of mature granules by polymerase chain reaction (PCR) amplification and denaturing gradient gel electrophoresis (DGGE) technique and those of whole growing granules by high-throughput sequencing technique. The sliced sample study revealed that mature granules have a spherical core with anaerobic Rhodocyclaceae covered by an outer spherical shell with both aerobic and anaerobic strains. The growing granule study showed that the flocculated flocs were first transited to young granules with increased abundances of Flavobacteriaceae, Xanthomonadaceae, Rhodobacteraceae and Microbacteriaceae, then the abundances of anaerobic strains were increased owing to the formation of anaerobic core. Since the present granules were cultivated from flocculated flocs, the microbial community data suggested that granules were formed via a deterministic rather than via a random aggregation-disintegration mechanism.

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.

Aerobic sludge granulation: a tale of two polysaccharides?

Aerobic sludge granules are suspended biofilms with the potential to reduce the cost and footprint of secondary wastewater treatment. Attempts to answer how and why they form leads to a consideration of the role of their extracellular polymeric substances (EPS) in determining their physical and microbiological properties. The exopolysaccharide components of this matrix, in particular, have received attention as putative structural, gel-forming agents. Two quite different exopolysaccharides have been proposed as the gel-forming constituents, with their gel properties clearly different from those of activated sludge EPS. This review aims to address the question of whether more than one gel-forming exopolysaccharide exist in granules. Based on the available structural data, it seems likely that they are different gel-forming polymers and their differences are not artifacts of the analytical methods used. Nonetheless, both proposed structural gel polymers are extracted and purified based on procedures selecting for anionic polar polysaccharides soluble at high pH, and both contain hexuronic acids. Granulation does not result from EPS synthesis by any single microbial population, nor from production of a single exopolysaccharide. Future studies using solvents suitable for recalcitrant polysaccharides are likely to reveal important structural roles for other polysaccharides. It is hoped that this article will serve as a guide for subsequent studies into understanding the roles of exopolysaccharides in aerobic granular sludge.

Aerobic granular SBR systems applied to the treatment of industrial effluents

Four lab scale sequencing batch reactors (SBRs) were operated to remove organic matter and nitrogen from four different industrial wastewaters. The biomass grew in the reactors in the form of aerobic granules characterized by good settling properties. The high biomass concentrations achieved inside the reactors allowed reducing the solids concentration in the effluent down to 0.2 g VSS L(-1). The organic loading rates (OLR) applied to reactors ranged between 0.7 and 5.0 g CODL(-1)d(-1) with removal efficiencies of 60-95%. The nitrogen loading rates (NLR) applied varied between 0.15 and 0.65 g NH(4)(+)-NL(-1)d(-1) with variable removal efficiencies in the four systems (between 15% and 76%).

Biodegradation of p-cresol by aerobic granules in sequencing batch reactor

The cultivation of aerobic granules in sequencing batch reactor for the biodegradation of p-cresol was studied. The reactor was started with 100 mg/L of p-cresol. Aerobic granules first appeared within one month of start up. The granules were large and strong and had a compact structure. The diameter of stable granules was in the range of 1-5 mm. The integrity coefficient and granules density was found to be 96% and 1046 kg/m3, respectively. The settling velocity of granules was found to be in the range of 2x10(-2)-6x10(-2) m/sec. The aerobic granules were able to degrade p-cresol upto 800 mg/L at a removal efficiency of 88%. Specific p-cresol degradation rate in aerobic granules followed Haldane model for substrate inhibition. High specific p-cresol degradation rate up to 0.96 g p-cresol/(g VSS x day) were sustained upto p-cresol concentration of 400 mg/L. Higher removal efficiency, good settling characteristics of aerobic granules, makes sequencing batch reactor suitable for enhancing the microorganism potential for biodegradation of inhibitory compounds.

Microscale hydrodynamic analysis of aerobic granules in the mass transfer process

The internal structure of aerobic granules has a significant impact on the hydrodynamic performance and mass transfer process, and severely affects the efficiency and stability of granules-based reactors for wastewater treatment. In this study, for the first time the granule complex structure was correlated with the hydrodynamic performance and substrates reactions process. First, a series of multiple fluorescence stained confocal laser scanning microscopy images of aerobic granules were obtained. Then, the form and structure of the entire granule was reconstructed. A three-dimensional computational fluid dynamics study was carried out for the hydrodynamic analysis. Two different models were developed on the basis of different fluorescence stained confocal laser scanning microscopy images to elucidate the roles of the granule structure in the hydrodynamic and mass transfer processes of aerobic granules. The fluid flow behavior, such as the velocity profiles, the pathlines and hence the hydrodynamic drag force, exerted on the granule in a Newtonian fluid, was studied by varying the Reynolds number. Furthermore, the spatial distribution of dissolved nutrients (e.g., oxygen) was acquired by solving the convection-diffusion equations on the basis of the reconstructed granule structure. This study demonstrates that the reconstructed granule model could offer a better understanding to the mass transfer process of aerobic granules than simply considering the granule structure to be homogeneous.

Aerobic granules with inhibitory strains and role of extracellular polymeric substances

Microorganisms compete with other species by secreting antimicrobial compounds. The compact structure of aerobic granules was generally assumed to provide spatial isolation, resulting in the co-occurrence of diverse strains that have similar or dissimilar functions. No studies have investigated whether stable, mature aerobic granules can be formed with two mutually inhibitory strains. The strain Acinetobacter sp. I8 competes with Bacillus sphaericus I5 in a well-mixed environment, but can form stable and mature granules at 400 mg L(-1) phenol by repeatedly replenishing fresh medium in a sequencing batch reactor. The supernatants collected from the I8 medium in its exponential-growth phase or from the I5+I8 medium cultivated for 12 or 24h significantly inhibited I5 growth. Addition of tightly bound extracellular polymeric substances (TBEPS) or loosely bound extracellular polymeric substances (LBEPS) extracted from I5+I8 granules effectively suppressed the inhibitory effects of I8 on I5. The TBEPS or LBEPS physically separate strain I5 from I8 in the granule, and effectively adsorb the inhibitory substance(s) in the suspension.

Characterization, modeling and application of aerobic granular sludge for wastewater treatment

Recently extensive studies have been carried out to cultivate aerobic granular sludge worldwide, including in China. Aerobic granules, compared with conventional activated sludge flocs, are well known for their regular, dense, and strong microbial structure, good settling ability, high biomass retention, and great ability to withstand shock loadings. Studies have shown that the aerobic granules could be applied for the treatment of low- or high-strength wastewaters, simultaneous removal of organic carbon, nitrogen and phosphorus, and decomposition of toxic wastewaters. Thus, this new form of activate sludge, like anaerobic granular sludge, could be employed for the treatment of municipal and industrial wastewaters in near future. This chapter attempts to provide an up-to-date review on the definition, cultivation, characterization, modeling and application of aerobic granular sludge for biological wastewater treatment. This review outlines some important discoveries with regard to the factors affecting the formation of aerobic granular sludge, their physicochemical characteristics, as well as their microbial structure and diversity. It also summarizes the modeling of aerobic granule formation. Finally, this chapter highlights the applications of aerobic granulation technology in the biological wastewater treatment. It is concluded that the knowledge regarding aerobic granular sludge is far from complete. Although previous studies in this field have undoubtedly improved our understanding on aerobic granular sludge, it is clear that much remains to be learned about the process and that many unanswered questions still remain. One of the challenges appears to be the integration of the existing and growing scientific knowledge base with the observations and applications in practice, which this paper hopes to partially achieve.

Extracellular polymeric substances and structural stability of aerobic granule

The contributions of individual components in extracellular polymeric substances (EPSs) on structural stability of phenol-fed, aerobic granules were examined. The roles of proteins, alpha- and beta-polysaccharides, and lipids were studied via their selective hydrolysis using enzymes, and the structural changes of granule were probed using in situ fluorescent staining and confocal laser scanning microscopy. Selective enzymatic hydrolysis of proteins, lipids, and alpha-polysaccharides had a minimal effect upon the three-dimensional structural integrity of the granules. Conversely, selective hydrolysis of beta-polysaccharides fragmented the granules. The beta-polysaccharides were expected to form the backbone of a network-like outer layer with embedded proteins, lipids, alpha-polysaccharides, and cells to support the mechanical stability of granules.

Characterization of aerobic granules by microbial density at different COD loading rates

Aerobic granules were cultivated under temporal alternating aerobic and anoxic conditions without the presence of a carrier material in a sequencing batch reactor (SBR) with a high column height/column diameter ratio. The reactor was operated for 6h per cycle (aerobic: 4.75 h, anoxic: 1.25 h). To determine a new parameter for the definition of aerobic granules, a protocol of 4,6-diamidino-2-phenylindole hydrochloride staining and fluorescence image processing was developed. The d(tm) analysis showed that the increase in the chemical oxygen demand (COD) loading rate promoted no more growth of the aerobic granules. It was inconsistent with the results of the analysis of the sludge volume index (SVI) value but matched well with the results of the COD and nitrogen removal of the SBR and the particle size distribution by LS-PSA. The optimum COD loading rate for aerobic granulation in the SBR was 2.52 kg/m(3)d. When d(tm) was correlated with the biomass concentration and the SVI value during the period of granule formation, d(tm) could be used as a more sensitive and accurate parameter for classifying aerobic granules and optimizing the operational conditions for aerobic granulation processes.

Degradation of phenol by Acinetobacter strain isolated from aerobic granules

Aerobic granules effectively degrade phenol at high concentrations from which no Acinetobacter species, that can effectively degrade high concentrations of phenol, have ever been isolated from aerobic granules. The phenol-fed aerobic granule studied was made by merging several smaller granules, each with a core of proteins and nucleic acids surrounded by an outer layer enriched with polysaccharides. In the present study, a strain of Acinetobacter sp. was isolated from the phenol-fed aerobic granules and was identified using DNA sequencing. The fluorescent in situ hybridisation combined with the confocal laser scanning microscope test revealed that the isolated Acinetobacter strain was mainly distributed in the core regime of granule. Batch tests revealed that the suspended Acinetobacter strain could effectively degrade phenol at an initial phenol concentration of up to 1000 mg l(-1) with no cell growth taking place at a phenol concentration of 1500 mg l(-1). The Haldane model describes the inhibitory kinetics of the phenol degradation data. The suspended Acinetobacter strain had a propensity to attach to the surface of sterilized polyurethane foam at a concentration of 12.3mg dry cells mg(-1) dry foam. The immobilized cells could not only degrade phenol at a rate similar to the suspended cells at phenol concentration of 500 mg l(-1), but also effectively degraded phenol at 1500 mg l(-1). The polysaccharides outer layer protected the Acinetobacter strain from phenol's toxicity; while the strain may also contribute to bioaggregation of the granule for its high propensity to attach to solid surface.

Oxygen diffusion and consumption in active aerobic granules of heterogeneous structure

The interior structure of aerobic granules is highly heterogeneous, hence, affecting the transport and reaction processes in the granules. The granule structure and the dissolved oxygen profiles were probed at the same granule in the current work for possible estimation of transport and kinetic parameters in the granule. With the tested granules fed by phenol or acetate as carbon source, most inflow oxygen was consumed by an active layer thickness of less than 125 microm on the granule surface. The confocal laser scanning microscopy scans also revealed a surface layer thickness of approximately 100 microm consisting of cells. The diffusivities of oxygen transport and the kinetic constant of oxygen consumption in the active layers only were evaluated. The theoretical models adopted in literature that ignored the contributions of the layered structure of aerobic granule could have overlooked the possible limitations on oxygen transport.

Oxygen diffusion in active layer of aerobic granule with step change in surrounding oxygen levels

High biomass density and large size limit the transfer of dissolved oxygen (DO) in aerobic granules. In the literature, the oxygen diffusivity is often employed as an input parameter for modeling transport processes in aerobic granules. The interior of an aerobic granule was observed to be highly heterogeneous. In this work, the distributions of extracellular polymeric substances (EPS) and cells in the interior of phenol-fed and acetate-fed granules were built up using a five-fold staining scheme, combined with the use of a confocal laser scanning microscope (CLSM). The steady-state and transient DO with step changes in surrounding DO levels at various depths were measured in the granules using microelectrodes. Cells were probed in a surface layer of thickness 125-375 microm. A marked fall in DO was also noted over this surface layer. No aerobic oxidation could occur beneath the active layer, indicating the oxygen transfer limit. Fitting the steady-state and transient DO data over the active surface layer yielded apparent diffusivities of oxygen were (9.5+/-3.5)x10(-10)m(2)s(-1) for the phenol-fed granule and (3.5+/-1.0)x10(-10)m(2)s(-1) for the phenol-fed granule. These values were lower than those adopted in models in the literature.

Distribution of extracellular polymeric substances in aerobic granules

Extracellular matrix provides an architectural structure and mechanical stability for aerobic granules. Distributions of cells and extracellular polymeric substances (EPS), including proteins, alpha- and beta-D-glucopyranose polysaccharides, in acetate-fed granules and phenol-fed granules were probed using a novel quadruple staining scheme. In acetate-fed granules, protein and beta-D-glucopyranose polysaccharides formed the core, whereas, the cells and alpha-D-glucopyranose polysaccharides accumulated in the granule outer layers. Based on these experimental findings, this study indicated that different conclusions can be obtained regarding EPS distributions when granules were stained differently. The core of phenol-fed granules, conversely, was formed principally by proteins; whereas, the cells and alpha- and beta-D-glucopyranose polysaccharides were accumulated at an outer filamentous layer. Using a series of confocal laser scanning microscope (CLSM) images whose threshold values were determined via Otsu's scheme, the three-dimensional distributions of cells and EPS were produced using a polygonal surface model. Structural information extracted can be applied in further development of comprehensive granule models.

Degradation of phenol by aerobic granules and isolated yeastCandida tropicalis

Aerobic granules effectively degrade phenol at high concentrations. This work cultivated aerobic granules that can degrade phenol at a constant rate of 49 mg-phenol/g x VSS/h up to 1,000 mg/L of phenol. Fluorescent staining and confocal laser scanning microscopy (CLSM) tests demonstrated that an active biomass was accumulated at the granule outer layer. A strain with maximum ability to degrade phenol and a high tolerance to phenol toxicity isolated from the granules was identified as Candida tropicalis via 18S rRNA sequencing. This strain degrades phenol at a maximum rate of 390 mg-phenol/g x VSS/h at pH 6 and 30 degrees C, whereas inhibitory effects existed at concentrations >1,000 mg/L. The Haldane kinetic model elucidates the growth and phenol biodegradation kinetics of the C. tropicalis. The fluorescence in situ hybridization (FISH) and CLSM test suggested that the Candida strain was primarily distributed throughout the surface layer of granule; hence, achieving a near constant reaction rate over a wide range of phenol concentration. The mass transfer barrier provided by granule matrix did not determine the reaction rates for the present phenol-degrading granule.