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

The acceleration of aerobic sludge granulation by alternating organic loading rate: Performance and mechanism

As a highly promising treatment technology for wastewater, long start-up time is one of the bottlenecks hindering the widespread application of aerobic granular sludge (AGS). This study focused on exploring the possibility of alternating organic loading rate (OLR) in promoting AGS granulation. Under alternating OLR (3.6-14.4 kgCOD/m3·d), AGS granulation was significantly accelerated. The mean granule size under alternating load reached 234.6 μm at 17 d, while under constant OLR (7.2 kgCOD/m3·d), the mean granule size was only 179.2 μm. Moreover, the granule size maintained continuous growth even when the alternating OLR was changed to constant OLR. Alternating load significantly increased the content of extracellular polymeric substances (EPS), especially proteins (PN) in tightly bound EPS (TB-EPS), which was likely the main reason for accelerating AGS granulation. Moreover, alternating load reduced the hydrophilicity of EPS and promoted the content of proteins secondary structures that favored aggregation in TB-EPS, which were also beneficial for granulation. Microbial community results showed that alternating load might promote the enrichment of EPS producing bacteria, such as Thauera, Brevundimonas and Shinella. Meanwhile, the content of enzymes that regulated amino acids metabolism also increased under alternating load, which might be related to the increase of PN in EPS. These results further demonstrated that alternating load promoted granulation through EPS.

Microbial population properties in the hierarchically structured aerobic granular sludge: Phenotype and genotype

Aerobic granular sludge (AGS) is a layered microbial aggregate formed by the ordered self-assembly of different microbial populations. In this study, the outer layer (OL), middle layer (ML), and the inner layer (IL) of matured AGS were obtained by circular cutting. The adhesion of microorganisms in IL was significantly higher than that in OL and ML during the famine period, while the adhesion of microorganisms in ML and OL was significantly higher than that in IL during the feast period, confirming that the formation of AGS started in the famine period, and the feast period promoted the increase of particle size. Microorganisms in the three-layer structure were highly diverse and rich in genes for cytochrome c oxidase synthesis with oxygen as the electron acceptor. G_Pseudoxanthomonas was the dominant bacterium in OL. Its spatial distribution increased gradually from the inside to the outside. G_Rhodanobacter was the dominant bacterium in IL. Its spatial distribution gradually decreased from the inside to the outside. The microorganisms in IL contained abundant pili genes. During the self-assembly process of particle formation, G_ Rhodanobaker adhered stronger than G_ Pseudoxanthomonas. The interface between aerobic and anoxic was about 0.6 mm away from the granule surface. Combined with the electron mediator properties of the extracellular polymeric substance (EPS) in granules, it was speculated that the degradation of organic substrates located in the anoxic layer relied on EPS as a mediator for long-range electron transfer, and finally transferred electrons to O₂. This study provides a new viewpoint on the formation mechanism of AGS from the perspective of the ordered self-assembly of microorganisms, offering a theoretical basis for the optimal selection of culture conditions and the application of AGS technology.

How to measure diffusion coefficients in biofilms: A critical analysis

Biofilm and granular sludge processes depend on diffusion of substrates. Despite their importance for the kinetic description of biofilm reactors, biofilm diffusion coefficients reported in literature vary greatly. The aim of this simulation study was to determine to what extent the methods that are used to measure diffusion coefficients contribute to the reported variability. Granular sludge was used as a case study. Six common methods were selected, based on mass balances and microelectrodes. A Monte Carlo simulation was carried out to determine the theoretical precision of each method, considering the uncertainty of various experimental parameters. A model-based simulation of a diffusion experiment was used to determine the theoretical accuracy as a result of six sources of error: solute sorption, biomass deactivation, mass transfer boundary layer, granule roughness, granule shape, and granule size distribution. Based on the Monte Carlo analysis, the relative standard deviation of the different methods ranged from 5% to 61%. In a theoretical experiment, the six error sources led to an 37% underestimation of the diffusion coefficient. This highlights that diffusion coefficients cannot be determined accurately with existing experimental methods. At the same time, the need for measuring precise diffusion coefficients as input value for biofilm modeling can be questioned, since the output of biofilm models has a limited sensitivity toward the diffusion coefficient.

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.

Effects of light intensity on oxygen distribution, lipid production and biological community of algal-bacterial granules in photo-sequencing batch reactors

The effects of light intensity (0-225 µmol m^-2 s^-1) on oxygen distribution, lipid production and biological community structure of algal-bacterial granules were investigated in six identical photo-sequencing batch reactors (with a dark/light cycle of 12 h/12 h). Typically green algal-bacterial granules could be developed at a light intensity of ≥135 µmol m^-2 s^-1. The lipid content was significantly increased under higher light intensity, while the percentage of saturated fatty acid methyl esters was remarkably decreased. Results showed that light intensity ≥90 µmol m^-2 s^-1 yielded enough O₂ production from algae, creating aerobic/anoxic zone (0.3-0.6 mg-O₂/L) in the core of granules and thus efficient algal-bacterial symbiosis system. Enhanced nitrogen and phosphorus removals were achieved in the reactors with stronger light illumination, probably attributable to the enrichment of ammonia oxidizing bacteria (Comamonadaceae and Nitrosomonadaceae) and algae (Navicula and Stigeoclonium). Illuminance ≥180 µmol m^-2 s^-1 was found to be unfavorable for Nitrospiraceae.

Stability of aerobic granular sludge under condition of low influent C/N ratio: Correlation of sludge property and functional microorganism

Aerobic granular sludge process treating domestic wastewater with low C/N ratio is necessary to be studied for rapid urbanization in China and other countries. In this study, two parallel reactors with different influent C/N ratio (15 in R1, 5 in R2) were established. Compared to the disintegrated granule in R1 with high influent C/N ratio, granules with large size (650 μm) and compact structure (integrity coefficient <0.1) were stable in R2 along with influent C/N ratio decreased to 5. High-through sequencing illustrated the functional microbes like Thauera and Paracoccus enriched under low influent C/N ratio, and principal component analysis further showed these microbes were positive correlation with tryptophan and protein-like substances in extracellular polymeric substances (EPS) and granular strength. It was indicated that under low influent C/N ratio, several resistant microbes like Thauera (19.5%) enriched and then secreted tryptophan and protein-like substances, and stable granules with multi-functional microbes could be formed finally.

Microbial Population Dynamics and Ecosystem Functions of Anoxic/Aerobic Granular Sludge in Sequencing Batch Reactors Operated at Different Organic Loading Rates

The granular sludge process is an effective, low-footprint alternative to conventional activated sludge wastewater treatment. The architecture of the microbial granules allows the co-existence of different functional groups, e.g., nitrifying and denitrifying communities, which permits compact reactor design. However, little is known about the factors influencing community assembly in granular sludge, such as the effects of reactor operation strategies and influent wastewater composition. Here, we analyze the development of the microbiomes in parallel laboratory-scale anoxic/aerobic granular sludge reactors operated at low (0.9 kg m^-3d^-1), moderate (1.9 kg m^-3d^-1) and high (3.7 kg m^-3d^-1) organic loading rates (OLRs) and the same ammonium loading rate (0.2 kg NH₄-N m^-3d^-1) for 84 days. Complete removal of organic carbon and ammonium was achieved in all three reactors after start-up, while the nitrogen removal (denitrification) efficiency increased with the OLR: 0% at low, 38% at moderate, and 66% at high loading rate. The bacterial communities at different loading rates diverged rapidly after start-up and showed less than 50% similarity after 6 days, and below 40% similarity after 84 days. The three reactor microbiomes were dominated by different genera (mainly Meganema, Thauera, Paracoccus, and Zoogloea), but these genera have similar ecosystem functions of EPS production, denitrification and polyhydroxyalkanoate (PHA) storage. Many less abundant but persistent taxa were also detected within these functional groups. The bacterial communities were functionally redundant irrespective of the loading rate applied. At steady-state reactor operation, the identity of the core community members was rather stable, but their relative abundances changed considerably over time. Furthermore, nitrifying bacteria were low in relative abundance and diversity in all reactors, despite their large contribution to nitrogen turnover. The results suggest that the OLR has considerable impact on the composition of the granular sludge communities, but also that the granule communities can be dynamic even at steady-state reactor operation due to high functional redundancy of several key guilds. Knowledge about microbial diversity with specific functional guilds under different operating conditions can be important for engineers to predict the stability of reactor functions during the start-up and continued reactor operation.

Aerobic Granular Sludge

Aerobic Granular Sludge (AGS) has been successfully applied for carbon, nitrogen and phosphorous removal from wastewaters, in a single tank, reducing the space and energy requirements. This is especially beneficial for, often space restricted, industrial facilities. Moreover, AGS holds a promise for the toxic pollutants removal, due to its layered and compact structure and the bacteria embedding in a protective extracellular polymeric matrix. These outstanding features contribute to AGS tolerance to toxicity and stability. Strategies available to deal with toxic compounds, namely granulation with effluents containing toxics and bioaugmentation, are addressed here. Different applications for the toxics/micropollutants removal through biosorption and/or biodegradation are presented, illustrating the technology versatility. The anthropogenic substances effects on system performance and bacterial populations established within AGS are also addressed. Combination of contaminants removal to allow water discharge, and simultaneous valuable products recovery are presented as final remark.

Nanoband array electrode as a platform for high sensitivity enzyme-based glucose biosensing

We describe a novel glucose biosensor based on a nanoband array electrode design, manufactured using standard semiconductor processing techniques, and bio-modified with glucose oxidase immobilized at the nanoband electrode surface. The nanoband array architecture allows for efficient diffusion of glucose and oxygen to the electrode, resulting in a thousand-fold improvement in sensitivity and wide linear range compared to a conventional electrode. The electrode constitutes a robust and manufacturable sensing platform.

Rheological and fractal hydrodynamics of aerobic granules

The structural and hydrodynamic features for granules were characterized using settling experiments, predefined mathematical simulations and ImageJ-particle analyses. This study describes the rheological characterization of these biologically immobilized aggregates under non-Newtonian flows. The second order dimensional analysis defined as D2=1.795 for native clusters and D2=1.099 for dewatered clusters and a characteristic three-dimensional fractal dimension of 2.46 depicts that these relatively porous and differentially permeable fractals had a structural configuration in close proximity with that described for a compact sphere formed via cluster-cluster aggregation. The three-dimensional fractal dimension calculated via settling-fractal correlation, U∝l(D) to characterize immobilized granules validates the quantitative measurements used for describing its structural integrity and aggregate complexity. These results suggest that scaling relationships based on fractal geometry are vital for quantifying the effects of different laminar conditions on the aggregates' morphology and characteristics such as density, porosity, and projected surface area.

Toxic and inhibitory effects of trichloroethylene aerobic co-metabolism on phenol-grown aerobic granules

Aerobic granule, a form of microbial aggregate, exhibits good potential in degrading toxic and recalcitrant substances. In this study, the inhibitory and toxic effects of trichloroethylene (TCE), a model compound for aerobic co-metabolism, on phenol-grown aerobic granules were systematically studied, using respiratory activities after exposure to TCE as indicators. High TCE concentration did not exert positive or negative effects on the subsequent endogenous respiration rate or phenol dependent specific oxygen utilization rate (SOUR), indicating the absence of solvent stress and induction effect on phenol-hydroxylase. Phenol-grown aerobic granules exhibited a unique response to TCE transformation product toxicity, that small amount of TCE transformation enhanced the subsequent phenol SOUR. Granules that had transformed between 1.3 and 3.7 mg TCE gSS(-1) showed at most 53% increase in the subsequent phenol SOUR, and only when the transformation exceeded 6.6 mg TCE gSS(-1) did the SOUR dropped below that of the control. This enhancing effect was found to sustain throughout several phenol dosages, and TCE transformation below the toxicity threshold also lessened the granules' sensitivity to higher phenol concentration. The unique toxic effect was possibly caused by the granule's compact structure as a protection barrier against the diffusive transformation product(s) of TCE co-metabolism.

Accelerated aerobic granulation using alternating feed loadings: alginate-like exopolysaccharides

Alginate-like exopolysaccharides (ALE) likely contribute markedly to strength of aerobic granules. This study cultivated aerobic granules from propionate wastewaters using strategies with different organic loading rates (OLRs) (4.4-17.4 kg/m(3)-d). When the OLR increased suddenly, the constituent cells (Pseudomonas, Clostridium, Thauera and Arthrobacter) were stimulated to secret extracellular cyclic diguanylate (c-di-GMP) and produced excess ALE, which formed a large quantity of sticky materials that served as the precursor of aerobic granules. Formation of excess ALE was the prerequisite for accelerated granulation. Conversely, this study observed no enrichment of poly guluronic acid blocks in ALE during granulation.

CH4 emission and conversion from A2O and SBR processes in full-scale wastewater treatment plants

Wastewater treatment systems are important anthropogenic sources of CH4 emission. A full-scale experiment was carried out to monitor the CH4 emission from anoxic/anaerobic/oxic process (A20) and sequencing batch reactor (SBR) wastewater treatment plants (WWTPs) for one year from May 2011 to April 2012. The main emission unit of the A2O process was an oxic tank, accounting for 76.2% of CH4 emissions; the main emission unit of the SBR process was the feeding and aeration phase, accounting for 99.5% of CH4 emissions. CH4 can be produced in the anaerobic condition, such as in the primary settling tank and anaerobic tank of the A2O process. While CH4 can be consumed in anoxic denitrification or the aeration condition, such as in the anoxic tank and oxic tank of the A2O process and the feeding and aeration phase of the SBR process. The CH4 emission flux and the dissolved CH4 concentration rapidly decreased in the oxic tank of the A2O process. These metrics increased during the first half of the phase and then decreased during the latter half of the phase in the feeding and aeration phase of the SBR process. The CH4 oxidation rate ranged from 32.47% to 89.52% (mean: 67.96%) in the A2O process and from 12.65% to 88.31% (mean: 47.62%) in the SBR process. The mean CH4 emission factors were 0.182 g/ton of wastewater and 24.75 g CH4/(person x year) for the A2O process, and 0.457 g/ton of wastewater and 36.55 g CH4/(person x year) for the SBR process.

Operational strategies for nitrogen removal in granular sequencing batch reactor

This study investigated the effects of different operational strategies for nitrogen removal by aerobic granules with mean granule sizes of 1.5mm and 0.7 mm in a sequencing batch reactor (SBR). With an alternating anoxic/oxic (AO) operation mode without control of dissolve oxygen (DO), the granular sludge with different size achieved the total inorganic nitrogen (TIN) removal efficiencies of 67.8-71.5%. While under the AO condition with DO controlled at 2mg/l at the oxic phase, the TIN removal efficiency was improved up to 75.0-80.4%. A novel operational strategy of alternating anoxic/oxic combined with the step-feeding mode was developed for nitrogen removal by aerobic granules. It was found that nitrogen removal efficiencies could be further improved to 93.0-95.9% with the novel strategy. Obviously, the alternating anoxic/oxic strategy combined with step-feeding is the optimal way for TIN removal by granular sludge, which is independent of granule size.

Biodegradation of phenol using Corynebacterium sp. DJ1 aerobic granules

The single-culture Corynebacterium sp. DJ1 aerobic granules were cultivated and were utilized to degrade high-strength phenolic wastewater. These granules can degrade phenol at sufficient high rate without severe inhibitory effects up to phenol concentration of 2000 mg l(-1). Furthermore, the kinetic characteristic noted for these granules yields a zero-order phenol degradation behavior with 500-1500 mg l(-1) phenol, which facilitates reactor design and scale up. With added acetate to promote cell growth, this single-culture aerobic granular system yields the highest phenol degradation rate reported in granular literature.

Applications of Anammox based processes to treat anaerobic digester supernatant at room temperature

The supernatant of an anaerobic digester was treated at 20 degrees C in two systems. The first one is a two units configuration, conformed by two sequencing batch reactors (SBR), carrying out partial nitrification and Anammox processes, respectively. Partial nitrification was achieved by granular biomass with a mean diameter of 3 mm, operating at a dissolved oxygen concentration of 2.7 mg/L. The combined system allowed the removal of nitrogen loading rates around 0.08 g N/(Ld). Afterwards, Anammox biomass was spontaneously developed in the inner core of the nitrifying granules of the SBR and therefore, partial nitrification and Anammox process were carried out in a single unit. Once the stable CANON process was established, a mean nitrogen removal rate of 0.8 g N/(Ld) was registered. The settling velocities of the granules ranged from 70 to 150 m/h with sludge volumetric index values lower than 50 mL/g VSS during the whole operation.

Possible role of denitrification on aerobic granular sludge formation in sequencing batch reactor

The aim of this work was to evaluate and quantify the influence of denitrification of nitrate on the aerobic granular sludge development. Two parallel sequencing batch reactors (SBRs) were fed with synthetic wastewater, the first reactor (R1) receiving nitrate at a fixed concentration whereas the second reactor (R2) did not receive any external source of nitrate. Both systems were working at the same operational conditions with a low specific air velocity (0.07cms(-1)), a relatively low and acceptable oxygen concentration (1.8+/-0.8mgL(-1)) and without significant biomass selection pressure (minimal velocity 0.4mh(-1)). The results showed that the sludge settling properties of R1 were comparable to granular sludge (settling velocity approximately 10.6mh(-1), SVI(5) approximately 13-30mLg(-1), SVI(30) approximately 11-28mLg(-1) and SVI(5)/SVI(30) ratio approximately 1.05-1.1). In the opposition, biomass of the reactor R2 reveals more traditional properties. Nitrogen measurements revealed that a significant denitrification (up to 80% of fed nitrate) occurred in R1 in the core of aggregates, as aerobic condition was maintained in the bulk. After the mean particle size was reduced from 250 to 125microm by an increase of mechanical stirring rate, it was shown that denitrification decreased from 80% to 20%. These parallel experiments definitively proved that the presence of nitrate in SBRs can assist the densification of the biological aggregates in aerobic condition. The presence of nitrate (commonly provided by nitrification) should be considered as a possible factor which helps to maintain the granulation process.

Measurement of dissolved oxygen and its diffusivity in aerobic granules using a lithographically-fabricated microelectrode array

A novel gold microelectrode array (MEA) was manufactured with microfabrication techniques and applied on the measurement of dissolved oxygen profile in an aerobic granule. The MEA contained five gold microelectrodes, which had a good linear response to dissolved oxygen and typically had a lifetime of more than 10 days. Dissolved oxygen microprofiles near the surface of an aerobic granule were monitored with this MEA. Based on the measurements, an oxygen effective diffusivity in the upper 100 microm layer of the aerobic granule was estimated to be 1.19 x 10(-9) m2/s. The experimental results demonstrate that the MEA was able to measure the DO levels in aerobic granules accurately and precisely and that the MEA could be used to determine constituents, profiles, and functions in situ in small spaces. Moreover, since the device shape and microelectrode arrangement were all defined by photolithography, the proposed fabrication procedure was flexible and appropriate for fabrication of various types of MEAs.

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.