Variable aeration in sequencing batch reactor with aerobic granular sludge

This study investigated the effects of reduced aeration in famine period on the performance of sequencing batch reactor (SBR) with aerobic granular sludge. Aerobic granules were first cultivated in two SBRs (R1 and R2) with acetate as sole carbon source. From operation day 27, aeration rate in R1 was reduced from 1.66 to 0.55 cm s(-1) from 110 min to the end of each cycle and further reduced from 30 min to the end of each cycle from day 63. R2 as a control was operated with a constant aeration rate of 1.66 cm s(-1) in the whole cycle during the entire experimental period. Results showed that changing trends of SVI, concentration, average size and VSS/SS of biomass with time in R1 and R2 were similar although different aeration modes were adopted. At steady state, SVI of aerobic granules and biomass concentration maintained at about 40 ml g(-1) and 6 g l(-1), respectively. Average size of granules was about 750 microm in R1 while 550 microm in R2. This is the first study to demonstrate that aerobic granular sludge could be stable at reduced aeration rate in famine period during more than 3-month operation. Such an operation strategy with reduced aeration rate will lead to a significant reduction of energy consumption, which makes the aerobic granular sludge technology more competitive over conventional activated sludge process. Furthermore, the stability of aerobic granular system with variable aeration further indicates that the difference of physiology and kinetics of aerobic granule in feast and famine periods results in the different requirements of oxygen and shear stress for the stability of granules, which will deepen the understanding of mechanism of aerobic granulation in sequencing batch reactor.     

Starvation is not a prerequisite for the formation of aerobic granules

Activated sludge with sludge volume index (SVI)₃₀ of 77 ml g⁻¹ and SVI₃₀ of 433 ml g⁻¹ was inoculated to start up reactors R1 and R2, respectively. In both R1 and R2, cycle time of 1 h and the influent chemical oxygen demand (COD) concentrations of 1,000 mg l⁻¹ were employed. Initial settling time of 2 min resulted in the loss of a substantial amount of biomass as wash-out and high effluent COD concentrations within the first week of operation. This implied that there was no starvation phase in each cycle of R1 and R2 during the first week of operation. However, aerobic granules with a size above 400 μm formed by day 7. Thus, it was concluded that starvation was not a prerequisite for the formation of aerobic granules. When cycle time was 1 h, the instability of aerobic granules was observed. When cycle time was prolonged to 1.5 h and granular sludge of 200 ml was used to start up reactor R3, the reactor R3 reached steady state within 1 week. SVI, size, and the morphology of granular sludge in R3 remained stable during the 47-day operation, which indicated that prolonged starvation time had positive effects on the stability of aerobic granules.     

Stoichiometry and kinetics of poly-beta-hydroxybutyrate metabolism in aerobic, slow growing, activated sludge cultures

This paper discusses the poly-beta-hydroxybutyrate (PHB) metabolism in aerobic, slow growing, activated sludge cultures, based on experimental data and on a metabolic model. The dynamic conditions which occur in activated sludge processes were simulated in a 2-L sequencing batch reactor (SBR) by subjecting a mixed microbial population to successive periods of external substrate availability (feast period) and no external substrate availability (famine period). Under these conditions intracellular storage and consumption of PHB was observed. It appeared that in the feast period, 66% to almost 100% of the substrate consumed is used for storage of PHB, the remainder is used for growth and maintenance processes. Furthermore, it appeared that at high sludge retention time (SRT) the growth rate in the feast and famine periods was the same. With decreasing SRT the growth rate in the feast period increased relative to the growth rate in the famine period. Acetate consumption and PHB production in the feast period both proceeded with a zero-order rate in acetate and PHB concentration respectively. PHB consumption in the famine period could best be described kinetically with a nth-order degradation equation in PHB concentration. The obtained results are discussed in the context of the general activated sludge models.     

Treatment of dairy effluents in an aerobic granular sludge sequencing batch reactor

Aerobic granular sludge can successfully be cultivated in a sequencing batch reactor (SBR) treating dairy wastewater. Attention has to be paid to the fact that suspended solids are always present in the effluent of aerobic granular sludge reactors, making a post-treatment step necessary. Sufficient post-treatment can be achieved through a sedimentation process with a hydraulic retention time of 15–30 min. After complete granulation and the separation of biomass from the effluent, removal efficiencies of 90% COD_total, 80% N_total and 67% P_total can be achieved at a volumetric exchange ratio of 50% and a cycle duration of 8 h. Effluent values stabilize at around 125 mg l^–1 COD_dissolved. The maximum applicable loading rate is nevertheless limited, as the stability of aerobic granules very much depends on the presence of distinct feast and famine conditions and the degradation of real wastewaters shows slower kinetics compared with synthetic wastewaters. As loading rate and volumetric exchange ratio are coupled in an SBR system, the potential of granular sludge for improving process efficiency is also limited.     

Properties of phenol-removal aerobic granules during normal operation and shock loading

The physical structure and activity of aerobic granules, and the succession of bacterial community within aerobic granules under constant operational conditions and shock loading were investigated in one sequencing batch reactor over ten months. While the maturation phase of the granulation process began on day 30, the structure of microbial community changed markedly until after three months of reactor operation under constant conditions with a loading rate of 1.5 g phenol L⁻¹ day⁻¹. A shock loading of 6.0 g phenol L⁻¹ day⁻¹ from days 182–192 led to divergence of bacterial community, an inhibition of the biomass activity, and a decrease in phenol removal rate in the reactor. However, phenol was still completely removed under this disturbance. After the shock loading, the mean sizes of aerobic granules increased, and the activity of the microbial population within the granules decreased, although there appeared highly resilient for the dominant bacterial community of aerobic granules which mainly included β-Proteobacteria. Correlation analysis suggested that biomass concentration and biomass loading were significantly related to the community composition of aerobic granules during the whole operational period. The development of a relatively stable bacterial community in aerobic granules implied that those distinct dominant microbes in aerobic granules were favorably selected and proliferated under the operational conditions.     

Biosorption and biodegradation of pentachlorophenol (PCP) in an upflow anaerobic sludge blanket (UASB) reactor

In order to understand the fate of PCP in upflow anaerobic sludge blanket reactor (UASB) more completely, the sorption and biodegradation of pentachlorophenol (PCP) by anaerobic sludge granules were investigated. The anaerobic granular sludge degrading PCP was formed in UASB reactor, which was seeded with anaerobic sludge acclimated by chlorophenols. At the hydraulic retention time (HRT) of 20–22 h, and PCP loading rate of 200–220 mg l⁻¹ d⁻¹, UASB reactor exhibited good performance in treating wastewater which containing 170–180 mg l⁻¹ PCP and the PCP removal rate of 99.5% was achieved. Sequential appearance of tetra-, tri-, di-, and mono-chlorophenol was observed in the reactor effluent after 20 mg l⁻¹ PCP introduction. Sorption and desorption of PCP on the anaerobic sludge granules were all fitted to the Freundlich isotherm equation. Sorption of PCP was partly irreversible. The Freundlich equation could describe the behavior of PCP amount sorbed by granular sludge in anaerobic reactor reasonably well. The results demonstrated that the main mechanism leading to removal of PCP on anaerobic granular sludge was biodegradation, not sorption or volatization.     

The effects of shear force on the formation, structure and metabolism of aerobic granules

The effect of shear force on aerobic granulation was studied in four column-type, sequential aerobic sludge blanket reactors. Hydrodynamic turbulence caused by upflow aeration served as the main shear force in the systems. Results showed that aerobic granulation was closely associated with the strength of shear force. Compact and regular aerobic granules were formed in the reactors with a superficial upflow air velocity higher than 1.2 cm s(-1). However, only typical bioflocs were observed in the reactor with a superficial upflow air velocity of 0.3 cm s(-1) during the whole experimental period. The characteristics of the aerobic granules in terms of settling ability, specific gravity, hydrophobicity, polysaccharide and protein content and specific oxygen utilization rate (SOUR) were examined. It was found that the shear force has a positive effect on the production of polysaccharide, SOUR, hydrophobicity of cell surface and specific gravity of granules. The hydrophobicity of granular sludge is much higher than that of bioflocs. Therefore, it appears that hydrophobicity could induce and further strengthen cell-cell interaction and might be the main force for the initiation of granulation. The shear-stimulated production of polysaccharides favors the formation of a stable granular structure. This research provides experimental evidence to show that shear force plays a crucial role in aerobic granulation and further influences the structure and metabolism of granules.     

Mathematical modeling of aerobic granular sludge: A review

Aerobic granulation may play an important role in the field of wastewater treatment due to the advantages of aerobic granules compared to the conventional sludge flocs, such as denser structure, better settleability and ensured solid-effluent separation, higher biomass concentration, and greater ability to withstand shock loadings, which is promising for a full-scale implementation. As an aid for this implementation, mathematical modeling would be an invaluable tool. In this paper, the existing mathematical models available in literature concerning aerobic granule systems are reviewed, including the modeling of the dynamic facets of the aerobic granulation process, the mass transfer and detachment in aerobic granules, the granule-based sequencing batch reactor, the fate of microbial products in granules, and the multi-scale modeling of aerobic granular sludge. An overview of the parameters used in the aerobic granular modeling approaches is also presented. Our growing knowledge on mathematical modeling of aerobic granule might facilitate the engineering and optimization of aerobic granular sludge technology as one of the most promising techniques in the biological wastewater treatment.     

Long-term impact of dissolved O(2) on the activity of anaerobic granules

The impact of influent dissolved O(2) on the characteristics of anaerobic granular sludge was investigated at various dissolved O(2) concentrations (0.5-8.1 ppm) in 1- and 5-L laboratory-scale upflow anaerobic sludge bed (UASB)-like anaerobic/aerobic coupled reactors with a synthetic wastewater (carbon sources containing 75% sucrose and 25% acetate). The rate of dissolved O(2) supplied to the coupled reactor was as high as 0.40 g O(2)/L(rx).d, and the anaerobic/aerobic coupled reactors maintained excellent methanogenic performances at a COD loading rate of 3 g COD/L(rx).d even after the reactors had been operated with dissolved O(2) for 3 months. The activities of granular sludge on various substrates (glucose, propionate, and hydrogen) were not impaired, and acetate activity was even improved over a short term. However, after 3 months of operation, slight declines on the acetoclastic activities of granules were observed in the coupled reactor receiving the recirculated fluid containing 8.1 ppm dissolved O(2).Methane yield in the anaerobic control reactor and anaerobic/aerobic coupled reactors revealed that a significant aerobic elimination (up to 30%) of substrate occurred in the coupled reactors, as expected. The presence of dissolved O(2) in the recirculated fluid resulted in the development of fluffy biolayers on the granule surface, which imposed a negative impact on the settleability of granular sludge and caused a slightly higher sludge washout. This research shows that the anaerobic/aerobic coupled reactor can be successfully operated under O(2)-limited conditions and is an ideal engineered ecosystem integrating oxic and anaerobic niches. (c) 1996 John Wiley & Sons, Inc.     

Denitrification with endogenous carbon source at low C/N and its effect on P(3HB) accumulation

The objective of the work reported here was to determine whether the ratio of COD/Nox has an impact on poly-beta-hydroxybutyrate (PHB) metabolism in activated sludge. Furthermore, it was tested if the ratio influenced the percentage use of organic compounds present in wastewater, for endogenous respiration, oxidation, accumulation and denitrification. Gas flow rate in SBR reactor was controlled by thermal mass flow controller (TMFC). Constant amount of air entering sequencing batch reactor was automatically adjusted to stable set-point 2mg O2 L(-1). It means that DO concentration in the reactor could change with oxygen uptake. During the filling period and part of the reaction time DO was nearly zero. Feast period of the external substrate availability and famine period of little amount or no external carbon availability were determined. At 23 h of the reaction time, and COD/Nox ratio 8, denitrification took place only during feast period. What was interesting, poly-beta-hydroxybutyrate degradation was observed in the feast period as well. However, at 11h of the reaction time and COD/Nox ratio 37, denitrification occurred in feast and famine period. In the feast period PHB was accumulated and in the famine period was used as the endogenous carbon source. COD consumption to reduce 1mg N-nitrate was ranging from 1.15 to 6.26 depending on carbon source and increased when exogenous and endogenous carbon were used by activated sludge. The increase in PHB content from 0.25 to 0.43 Cmol/Cmol resulted in a double increase in the amount of nitrogen removed due to denitrification was observed.     

Structure and stability of aerobic granules cultivated under different shear force in sequencing batch reactors

The cultivation of stable aerobic granules as well as granular structure and stability in sequencing batch reactors under different shear force were investigated in this study. Four column sequencing batch reactors (R1–R4) were operated under various shear force, in terms of superficial upflow air velocity of 0.8, 1.6, 2.4, and 3.2 cm s⁻¹, respectively. Aerobic granules were formed in all reactors in the experiment. It was found that the magnitude of shear force has an important impact on the granule stability. At shear force of 2.4 and 3.2 cm s⁻¹, granules can maintain a robust structure and have the potential of long-term operation. Granules developed in low shear force (R1, 0.8 cm s⁻¹ and R2, 1.6 cm s⁻¹) deteriorated to large-sized filamentous granules with irregular shape, loose structure and resulted in poor performance and operation instability. Granules cultivated under high shear force (R3, 2.4 cm s⁻¹ and R4, 3.2 cm s⁻¹) stabilized to clear outer morphology, dense and compact structure, and with good performance in 120 days operation. Fractal dimension (D_f) represents the internal structure of granules and can be used as an important indicator to describe the structure and stability of granules. Due to the combined effects of shear force and growth force, the mature granules developed in R3 and R4 also displayed certain differences in granular structure and characteristics.     

Characterization of aerobic granular sludge at various organic loading rates

Aerobic granular sludge was cultivated from activated sludge with two types of supports, namely bivalve shell carrier (BSC) and anaerobic granules (ANG). Granules were characterized at different organic loading rates (OLRs) ranging from 2.5 to 15 kg COD/m³ d and these granules were observed to withstand high OLRs. The physico-chemical characteristics of the aerobic granules were better than those of seed sludge. The granule formation with ANG support was found to be similar to that of non-support cultivation, i.e. formation from activated sludge only. By contrast, BSC support showed better performance in terms of faster settleability, compactness and especially resistance against organic shock loading. It also enabled self-cleaning effect by removing biofilm attached on the reactor wall during the start-up phase resulting rapid granulation process.     

Biodegradation and kinetics of aerobic granules under high organic loading rates in sequencing batch reactor

Biodegradation, kinetics, and microbial diversity of aerobic granules were investigated under a high range of organic loading rate 6.0 to 12.0 kg chemical oxygen demand (COD) m⁻³ day⁻¹ in a sequencing batch reactor. The selection and enriching of different bacterial species under different organic loading rates had an important effect on the characteristics and performance of the mature aerobic granules and caused the difference on granular biodegradation and kinetic behaviors. Good granular characteristics and performance were presented at steady state under various organic loading rates. Larger and denser aerobic granules were developed and stabilized at relatively higher organic loading rates with decreased bioactivity in terms of specific oxygen utilization rate and specific growth rate (μ _overall) or solid retention time. The decrease of bioactivity was helpful to maintain granule stability under high organic loading rates and improve reactor operation. The corresponding biokinetic coefficients of endogenous decay rate (k _d), observed yield (Y _obs), and theoretical yield (Y) were measured and calculated in this study. As the increase of organic loading rate, a decreased net sludge production (Y _obs) is associated with an increased solid retention time, while k _d and Y changed insignificantly and can be regarded as constants under different organic loading rates.     

Influence of substrate surface loading on the kinetic behaviour of aerobic granules

In the aerobic granular sludge reactor, the substrate loading is related to the size of the aerobic granules cultivated. This study investigated the influence of substrate surface loading on the growth and substrate-utilization kinetics of aerobic granules. Results showed that microbial surface growth rate and surface biodegradation rate are fairly related to the substrate surface loading by the Monod-type equation. In this study, both the theoretical maximum growth yield and the Pirt maintenance coefficient were determined. It was found that the estimated theoretical maximum growth yield of aerobic granules was as low as 0.2 g biomass g^–1 chemical oxygen demand (COD) and 10–40% of input substrate-COD was consumed through the maintenance metabolism, while experimental results further showed that the unit oxygen uptake by aerobic granules was 0.68 g oxygen g^–1 COD, which was much higher than that reported in activated sludge processes. Based on the growth yield and unit oxygen uptake determined, an oxidative assimilation equation of acetate-fed aerobic granules was derived; and this was confirmed by respirometric tests. In aerobic granular culture, about 74% of the input substrate-carbon was converted to carbon dioxide. The growth yield of aerobic granules was three times lower than that of activated sludge. It is likely that high carbon dioxide production is the main cause of the low growth yield of aerobic granules, indicating a possible energy uncoupling in aerobic granular culture.     

Biological nitrification–denitrification with alternating oxic and anoxic operations using aerobic granules

Efficient nitrification and denitrification of wastewater containing 1,700 mgl⁻¹ of ammonium-nitrogen was achieved using aerobic granular sludge cultivated at medium-to-high organic loading rates. The cultivated granules were tested in a sequencing batch reactor (SBR) fed with 6.4 or 10.2 kg NH₄⁺-N m⁻³ day⁻¹, a loading significantly higher than that reported in literature. With alternating 2 h oxic and 2 h anoxic operation (OA) modes, removal rate was 45.5 mg NH₄⁺-N g⁻¹ volatile suspended solids⁻¹ h⁻¹ at 6.4 kg NH₄⁺-N m⁻³ day⁻¹ loading and 41.3 ± 2.0 at 10.2 kg NH₄⁺-N m⁻³ day⁻¹ loading. Following the 60 days SBR test, granules were intact. The fluorescence in situ hybridization and confocal laser scanning microscopy results indicate that the SBR-OA granules have a distribution with nitrifers outside and heterotrophs outside that can effectively expose functional strains to surrounding substrates at high concentrations with minimal mass transfer limit. This microbial alignment combined with the smooth granule surface achieved nitrification–denitrification of wastewaters containing high-strength ammonium using aerobic granules. Conversely, the SBR continuous aeration mode yielded a distribution with nitrifers outside and heterotrophs inside with an unsatisfactory denitrification rate and floating granules as gas likely accumulated deep in the granules.     

Nitrifying granules cultivation in a sequencing batch reactor at a low organics-to-total nitrogen ratio in wastewater

It is possible to cultivate aerobic granular sludge at a low organic loading rate and organics-to-total nitrogen (COD/N) ratio in wastewater in the reactor with typical geometry (height/diameter = 2.1, superficial air velocity = 6 mm/s). The noted nitrification efficiency was very high (99%). At the highest applied ammonia load (0.3 ± 0.002 mg NH₄⁺–N g total suspended solids (TSS)⁻¹ day⁻¹, COD/N = 1), the dominating oxidized form of nitrogen was nitrite. Despite a constant aeration in the reactor, denitrification occurred in the structure of granules. Applied molecular techniques allowed the changes in the ammonia-oxidizing bacteria (AOB) community in granular sludge to be tracked. The major factor influencing AOB number and species composition was ammonia load. At the ammonia load of 0.3 ± 0.002 mg NH₄⁺–N g TSS⁻¹ day⁻¹, a highly diverse AOB community covering bacteria belonging to both the Nitrosospira and Nitrosomonas genera accounted for ca. 40% of the total bacteria in the biomass.     

Microbial community of aerobic granules for ammonium and sulphide removal in a sequencing batch reactor

Aerobic granules for sulphide and ammonium removal were cultivated in a sequencing batch reactor, and the microbial community of the aerobic granules was investigated by denaturing gradient gel electrophoresis. The loading rate increased from 0.15 to 0.9 kg S2? m?3 d?1, and the removal efficiencies of sulphide, chemical oxygen demand, and NH4 +-N were higher than 99, 80, and 98%, respectively. However, sludge settleability became poorer when the loading rate exceeded 0.3 kg S2? m?3 d?1. The denitrifying bacteria in the aerobic granules were Thauera sp., Pseudomonas alcaligenes, and uncultured planctomycetes, indicating that multiple N-removing processes occurred simultaneously in the aerobic granules. These processes could include nitrification and denitrification, aerobic denitrification, and anaerobic ammonia oxidation. Sludge settleability became poorer because of the overgrowth of uncultured Thiothrix sp.     

Effect of magnetic field on the accumulation of polyhydroxyalkanoates (PHAs) by microorganism in activated sludge

The effect of static magnetic field on polyhydroxyalkanoates (PHAs) syntheses by activated sludge under aerobic dynamic feeding (ADF) was evaluated in sequence batch reactors (SBR), with magnetic field intensities of 42, 21, 11 and 7 millitesla (mT) exposure in the feast, feast-famine and famine periods, respectively, and one control group without magnetic field exposure. Under each level of magnetic field intensity, the effect of magnetic field exposed in the famine period to PHAs syntheses was most significant in comparison with that in the feast or feast-famine period. Maximal hydroxybutyrate (HB) and (HV) yield occurred at 21 and 11 mT, respectively, and the minimal yield occurred at 42 mT during exposure in the famine period. The maximum biodegradable rate constant of PHA was noted at 11 mT during exposure in the famine period.     

Feast/famine growth environments and activated sludge population selection

The effect of feast/famine growth conditions on activated sludge cultures indicates that nonfilamentous cultures can be selected by providing proper substrate gradients and extended periods of endogenous metablism. Reactor operating strategies providing intermittently high substrate concentrations result in cultures characterized by high peak substrate and oxygen uptake activities, rapid settling rates, and high resistance to starvation. Sludge settleability can be manipulated using controlled variations in growth environment with corresponding changes noted in sludge activity. In combination with the low net growth rates associated with activated sludge systems, feast/famine environments would logically convey a selection advantage to microbes capable of readily assimilating substrate materials and maintaining viability during extended starvation periods.     

Aerobic granular sludge formation for high strength agro-based wastewater treatment

The present study investigates the formation of aerobic granular sludge in sequencing batch reactor (SBR) fed with palm oil mill effluent (POME). Stable granules were observed in the reactor with diameters between 2.0 and 4.0 mm at a chemical oxygen demand (COD) loading rate of 2.5 kg COD m⁻³ d⁻¹. The biomass concentration was 7600 mg L⁻¹ while the sludge volume index (SVI) was 31.3 mL g SS⁻¹ indicating good biomass accumulation in the reactor and good settling properties of granular sludge, respectively. COD and ammonia removals were achieved at a maximum of 91.1% and 97.6%, respectively while color removal averaged at only 38%. This study provides insights on the development and the capabilities of aerobic granular sludge in POME treatment.