Rate limiting factors in trichloroethylene co-metabolic degradation by phenol-grown aerobic granules

The potential of aerobic granular sludge in co-metabolic removal of recalcitrant substances was evaluated using trichloroethylene (TCE) as the model compound. Aerobic granules cultivated in a sequencing batch reactor with phenol as the growth substrate exhibited TCE and phenol degradation activities lower than previously reported values. Depletion of reducing energy and diffusion limitation within the granules were investigated as the possible rate limiting factors. Sodium formate and citrate were supplied to the granules in batch studies as external electron sources. No significant enhancing effect was observed on the instant TCE transformation rates, but 10 mM formate could improve the ultimate transformation capacity by 26 %. Possible diffusion barrier was studied by sieving the biomass into five size fractions, and determining their specific TCE and phenol degradation rates and capacities. Biomass in the larger size fractions generally showed lower activities. Large granules of >700 μm diameter exhibited only 22 % of the flocs’ TCE transformation capacity and 35 % of its phenol dependent SOUR, indicating the possible occurrence of diffusion limitation in larger biomass. However, the highest specific TCE transformation rate was observed with the fraction that mostly consisted of small granules (150–300 μm), suggesting an optimal size range while applying aerobic granules in TCE co-metabolic removal.     

Biological hydrogen production in a UASB reactor with granules. I: Physicochemical characteristics of hydrogen-producing granules

Hydrogen-producing granules with an excellent settling ability were cultivated in an upflow anaerobic sludge blanket reactor treating a sucrose-rich synthetic wastewater. The physicochemical characteristics of granules were evaluated in this study. The mature granules had a diameter ranging from 1.0 to 3.5 mm and an average density of 1.036 +/- 0.005 g/mL, whereas they had good settling ability and a high settling velocity of 32-75 m/h. The low ratio of proteins/carbohydrates for the extracellular polymeric substances (EPS) in the granules suggests that carbohydrates rather than proteins, might play a more important role in the formation of the H(2)-producing granules. The contact angle of the mature granules, 54 +/- 2 degrees , was larger than that of the seed sludge (38 +/- 2 degrees ), indicating that the microbial cells in the H(2)-producing granules had higher hydrophobicity. The granules had fractal nature with a fractal dimension of 1.78. Their porosities were in the range of 0-0.70, and increased with increasing granule size. The ratios between the observed and predicted settling velocities by Stokes' law were in a range of 1.00-1.50, and the fluid collection efficiency of the granules ranged from 0 to 0.19, indicating that their permeabilities were lower and that there was little advective flow through their interior. Experimental results also suggest that molecular diffusion appeared to play an important role in the mass transfer through the H(2)-producing granules.     

保藏温度对厌氧氨氧化颗粒污泥特性的影响

为考察保藏温度对厌氧氨氧化污泥颗粒特性的影响,同时优化保藏厌氧氨氧化颗粒污泥温度参数,本试验首先通过HRT调控进水基质负荷培养厌氧氨氧化颗粒污泥,并采用KHCO3和Na HCO3交替提供无机碳源。然后分别在–40℃、4℃、(27±4)℃室温和35℃条件下避光保藏。结果表明,Na HCO3可代替KHCO3作为厌氧氨氧化菌生长的无机碳源。相比于其他保藏温度,4℃保藏能够较好地维持生物量和生物活性,同时能较好地维持颗粒污泥的沉降性能、颗粒污泥和细胞结构完整性。在保藏过程中,一阶衰减指数模型可拟合厌氧氨氧化颗粒污泥生物量及活性的衰减过程,衰减指数与胞溶程度正相关,而且生物量的衰减比活性的衰减更快。同时,颗粒污泥胞外聚合物中蛋白质与多糖的比值(PN/PS)和血红素不能有效指示保藏过程中颗粒污泥沉降性能和活性的变化,而生物活性与胞溶程度呈负相关。     

Specific aerobic granules can be developed in a completely mixed tank reactor by bioaugmentation using micro-mycelial pellets of Phanerochaete chrysosporium

Aerobic granules were firstly developed in a completely mixed tank reactor (CMTR) by seeding micro-mycelial pellets (MMPs) of Phanerochaete chrysosporium. During phenol wastewater treatment, sludge granulation rate reached 67 % after 15-day operation. The granules in CMTR are different from aerobic granules described in literature in morphology, and a majority of them are rod-shaped or rodlike sludge besides spherical granules. The polymorphic granules, having no essential difference with aerobic granules previously reported, achieve advantages over conventional activated sludge in settling ability, biomass concentration, density, integrity coefficient and removal ability to phenol wastewater. The optimized parameters for sludge granulation in CMTR including temperature, inoculum quantity, rotary speed and superficial air upflow velocity are 30 °C, 5–7 g/l, 150 rpm, and 0.5 cm/s, respectively. Analysis on sludge granulation mechanism indicates that MMPs not only result in the formation of aerobic granules containing MMPs as nuclei, but also induce the formation of biogranules which do not have MMP at their cores. The work challenges the general belief that the homogenous circular flow pattern of microbial aggregates is necessary for aerobic sludge granulation.     

The effect of hydraulic retention time on the stability of aerobically grown microbial granules

AIMS: The aim of this study is to evaluate the effect of hydraulic retention time (HRT) on the development of aerobically grown microbial granules. METHODS AND RESULTS: Five column-shaped sequential aerobic sludge blanket reactors (SASBRs) were seeded with aerobically grown microbial granules and operated in a cyclic mode at different HRTs. At the shortest HRT of 1 h, the strong hydraulic pressure triggered biomass washout and led to reactor failure. At the longest HRT of 24 h, which represented the weakest hydraulic selection in this study, aerobic granules were gradually substituted by bioflocs because of the lower frequency of volumetric exchange. Within the optimum range of HRTs from 2 to 12 h, however, aerobic granules became stabilized in the presence of adequate hydraulic selection in the reactors, with good mixed liquor volatile suspended solids (MLVSS) retention, high volumetric chemical oxygen demand (COD) removal, low sludge volume index (SVI) values, good effluent quality, low sludge production rate, stronger and more compact structures, high cell hydrophobicity and high ratios of extracellular polysaccharides (PS) to extracellular proteins (PN). CONCLUSIONS: HRTs between 2 and 12 h provided the hydraulic selection pressures favourable for the formation and maintenance of stable aerobic granules with good settleability and activity. SIGNIFICANCE AND IMPACT OF THE STUDY: This is the first systematic study on the effect of HRT on heterotrophic aerobic granules. The results of the investigation are useful in understanding how aerobic granules can be applied for wastewater treatment.     

PVA-gel beads enhance granule formation in a UASB reactor

PVA-gel beads were used as a biocarrier in a lab-scale UASB reactor treating synthetic wastewater composed of corn steep liquor (CSL) with the aim of evaluating its use as a growth nucleus to enhance granule formation. Over 117 days of operation, the organic loading rate was increased to 22.5kgCOD/m(3)/day with an influent COD of about 10.8g/L at an HRT of 12h with COD removal efficiencies greater than 87%. By the end of the study period, the PVA-gel turned black and granule formation was achieved as compared with the formation of much fewer natural granules without the PVA-gel nucleus. No filamentous bacteria were found on the surface or interior of the PVA-gel beads. The PVA-gel granules had an average settling velocity 200m/h (5cm/s), and a biomass attachment of 0.93g VSS/g PVA-gel. The required time for formation of PVA-gel granules was thus demonstrated to be shorter than that of ordinary sludge granules under the experimental conditions used in this study.     

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.     

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.     

Required minimum granule size in UASB reactor and characteristics variation with size

The objective of this study was to define minimum size of bioparticles that could be classified as granules, to offer all advantages of granular sludge. Based on the theory of sedimentation, the minimum diameter bioparticles, which should be considered as granules was found out for specific gravity of sludge ranging between 1.01 and 1.05. For example, for specific gravity of 1.035 the minimum diameter of granules required for better sludge retention was 0.34 mm. The diameter based on this theory was evaluated by carrying out settling column analysis of a granular sludge obtained from lab-scale UASB reactor and verified with microscopic observation. To find out the effect of granules size on the nature of biodegradability, specific methanogenic activity (SMA) was carried out. It was observed that SMA increased with size of bioparticles tested in the range of 0.27-3.03 mm. The change in VSS/SS ratio and specific gravity was observed with size of granules. Consideration of variation in specific gravity with size of granules increased the degree of validation of sedimentation theory for the calculation of granules diameter.     

Microscopic observation of aerobic granulation in sequential aerobic sludge blanket reactor

AIMS: This paper attempts to provide visual evidence of how aerobic granulation evolves in sequential aerobic sludge blanket reactors. METHODS AND RESULTS: A series of experiments were conducted in two column-type sequential aerobic sludge reactors fed with glucose and acetate as sole carbon source, respectively. The evolution of aerobic granulation was monitored using image analysis and optical and scanning electron microscopy. The results indicated that the formation of aerobic granules was a gradual process from seed sludge to compact aggregates, further to granular sludge and finally to mature granules with the sequential operation proceeding. Glucose- and acetate-fed granules have comparable characteristics in terms of settling velocity, size, shape, biomass density and microbial activity. However, the microbial diversity of the granules was associated with the carbon source supplied. In this work, an important aerobic starvation phase was identified during sequential operation cycles. It was found that periodical aerobic starvation was an effective trigger for microbial aggregation in the reactor and further strengthened cell-cell interaction to form dense aggregates, which was an essential step of granulation. The periodical starvation-induced aggregates would finally be shaped to granules by hydrodynamic shear and flow. CONCLUSION: Aerobic granules can be formed within 3 weeks in the systems. The periodical starvation and hydrodynamic conditions would play a crucial role in the granulation process. SIGNIFICANCE AND IMPACT OF THE STUDY: Aerobic granules have excellent physical characteristics as compared with conventional activated sludge flocs. This research could be helpful for the development of an aerobic granule-based novel type of reactor for handling high strength organic wastewater.     

Nitrogen removal performance of a hybrid anammox reactor

The anaerobic ammonium oxidation (anammox) process has attracted considerable attention in recent years as an alternative to conventional nitrogen removal technologies. In this study, an innovative hybrid reactor combining fluidized and fixed beds for anammox treatment was developed. The fluidized bed was mechanically stirred and the gaseous product could be rapidly released from the anammox sludge to prevent washout of the sludge caused by floatation. The fixed bed comprising a non-woven biomass carrier could efficiently catch sludge to reduce washout. During the operation, nitrogen loading rates to the reactor were increased to 27.3 kg N/m³/d, with total nitrogen removal efficiencies of 75%. The biomass concentration in the fluidized bed reached 26-g VSS/L. Anammox granules were observed in the reactors, with settling velocities and sludge volumetric index of 27.3 ± 6.5 m/h and 23 mL/g, respectively. Quantification of extracellular polymeric substances revealed the anammox granules contained a significant amount of extracellular proteins.     

Effects of physico-chemical factors on the viscosity evolution of anaerobic granular sludge

The rheological properties of anaerobic granular sludge samples from four full-scale and one lab-scale anaerobic bioreactors were characterized by determining their “limit viscosity” values. These values were deducted from the evolution of the apparent viscosity of granular sludge samples (20 mL) at steady shear rate (200 s⁻¹) recorded using rotation tests with a wings type measurement cell stirrer Anton-Parr reference: ST24-1D/2V-Q0. The limit viscosity values depended on the applied shear rate, indicating a non-Newtonian behavior of the anaerobic granular sludge types investigated. The effect of variations of physico-chemical parameters such as pH (involving surface charge change), size, surface roughness and TSS content on the evolution of the limit viscosity of an anaerobic granular sludge suspension was investigated. This showed the importance of both quantitative (number of particles in a given volume) as well as qualitative (surface charge or shape) granule-granule interactions on this rheological parameter. Moreover, the origin of the granular sludge strongly influenced the limit viscosity value according with different granules characteristics. This work confirms the ability of the rheological parameter “limit viscosity” as an overall parameter to describe the physico-chemical characteristics (TSS, granulometry, origin, and charge) of anaerobic granular sludge and showed this holds for both sieved (500 μm) and unsieved sludges.     

Influence of aeration intensity on mature aerobic granules in sequencing batch reactor

Aeration intensity is well known as an important factor in the formation of aerobic granules. In this research, two identical lab-scale sequencing batch reactors with aeration intensity of 0.8 (R1) and 0.2 m³/h (R2) were operated to investigate the characteristics and kinetics of matured aerobic granules. Results showed that both aeration intensity conditions induced granulation, but they showed different effects on the characteristics of aerobic granules. Compared with the low aeration intensity (R2), the aerobic granules under the higher aeration intensity (R1) had better physical characteristics and settling ability. However, the observed biomass yield (Y _obs) in R1 [0.673 kg mixed liquor volatile suspended solids (MLVSS)/kg chemical oxygen demand (COD)] was lower than R2 (0.749 kg MLVSS/kg COD). In addition, the maximum specific COD removal rates (q _max) and apparent half rate constant (K) of mature aerobic granular sludge under the two aeration intensities were at a similar level. Therefore, the matured aerobic granule system does not require to be operated in a higher aeration intensity, which will reduce the energy consumption.     

Sedimentological evolution in an UASB treating SYNTHES, a new representative synthetic sewage, at low loading rates

The changes in the sedimentological attributes of the sludge bed in an upflow anaerobic sludge blanket (UASB) reactor fed with a low-strength wastewater mimicking raw domestic sewage were assessed in this study. The reactor was inoculated with 250 ml of granular sludge from a full-scale UASB reactor. The organic loading rate (OLR) varied from 1 to 2 g COD/ld. During the half-year long study, the reactor was operated at hydraulic retention times (HRTs) of 4.8 and 10 h, at 33 degrees C. Sludge sedimentology showed that the original granular sludge experienced serious instability and disintegration, leading to a much finer final grain assemblage, mainly due to substrate transfer limitation and cell starvation at the interior of larger granules. With time, the size uniformity tended to decrease, sphericity tended to increase, the skewness of the granule size distribution became negative, and the kurtosis became peaked and leptokurtic. In spite of the observed size reduction, reactor efficiency increased to a CODtotal removal of 96%. Biomass (sludge) yield was 0.012 g VS/g COD removed. The CH4 content of the biogas was high (up to 96%). This study thus highlights the treatment of a new type of wastewater with the deployment of the UASB reactor. It also reports the evolutionary trend of the biomass particle size distribution, making reference to a classic sedimentological appraisal.     

Bioremediation of wastewaters with recalcitrant organic compounds and metals by aerobic granules

Compared to activated sludge flocs, aerobic granules have a regular shape, and a compact and dense structure which enhances settleability, higher biomass retention, multi-microbial functions, higher tolerance to toxicity, greater tolerance to shock loading, and relatively low excess sludge production. The potential for improved process efficiency and cost-effectiveness can be attractive when it is applied to both municipal and industrial wastewaters. This review discusses potential applications of aerobic granulation technology in wastewater treatment while drawing attention to relevant findings such as diffusion gradients existing in aerobic granules which help the biomass cope with inhibitory compounds and the ability of granules to continue degradation of inhibitory compounds at extreme acid and alkaline pHs.     

Effect of Lime Pretreatment on Granulation of Switchgrass

Densification of switchgrass into consistent and high-density solid feedstock will reduce the cost of transport, handling, and storage to produce fuels and chemicals. Development a novel, low-cost densification technology is critical for reducing the delivered cost of feedstock while improving the bulk flow properties of densified products. In this paper, a novel wet granulation technology was proposed to investigate the effect of lime pretreatment on the production of switchgrass granules. Granulation is a process of agglomerating fine powders by wetting powder surfaces with liquid binders and mild application of shear/vibrating forces. Switchgrass was size reduced into fine powders using a knife mill and pretreated with three lime loading rates (0.05, 0.1, 0.2 g/g of biomass) at 121 °C for 30 min and at room temperature (25 °C) for 72 h. The structural modification of pretreated samples was analyzed by scanning electron microscopy and autofluorescence microscopy. Pretreated samples were granulated using a pan granulator with pre-formulated starch binder. Granules made from 20 % (0.2 g/g of biomass) lime loading rate had significantly higher single granule density and angle of repose with lower binder requirement than that of untreated granules. Lime treatment did not significantly increase the bulk density and hardness of granules. Lime-treated granules had significantly higher ash content and lower gross calorific value than that of untreated granules. In overall, lime treatment was not attractive to produce granules for thermochemical conversion platform, but lime-treated granules could be used to produce liquid biofuels and platform chemicals in biochemical conversion platform.     

Precipitation potential as a major factor in the formation of granular sludge in an upflow sludge-blanket reactor for denitrification of drinking water

The effects of the chemical composition of water on granular sludge formation and characteristics in a denitrifying upflow sludge-blanket (USB) reactor were studied. Denitrification of drinking water showed different biomass sludge characteristics when the reactor was fed with groundwater as opposed to surface water. USB reactors fed with groundwater produced granules with good settling characteristics, SVI (sludge volume index) values lower than 30 ml/g, and high reactor biomass concentrations (20–25 g/l), while surface-water-fed reactors exhibited lower biomass concentrations (10–15 g/l) due to poor settling characteristics (SVI values of 50–90 ml/g). Sludge granules from the reactor fed with surface water had a low mineral content of between 10% and 20% as compared to a mineral content of 25%–50% in the groundwater reactor. The larger mineral content in the groundwater-fed reactor was due to a greater precipitation potential, i.e. higher concentrations of calcium and alkalinity present in groundwater combined with the release of alkalinity and subsequent increase in pH caused by biological denitrification. Verification for this phenomenon was established by enriching surface water with calcium and alkalinity, which increased the reactor's precipitation potential from 15 mg/1 to 40 mg/1 (as CaCO₃). The granules obtained from the reactor fed with enriched surface water had a high mineral content of between 40% and 50% and very low SVI values, contributing to improved granule-settling characteristics and reactor stability.     

Size-effect on the physical characteristics of the aerobic granule in a SBR

Owing to a fast growth rate, aerobic granules display a wide range of sizes, approximately 0.3-5.0 mm in diameter. As the diameter increases, the aerobic granule undergoes serial morphological and physical changes that could cause problems to the reactor operation, a phenomenon which, however, has not been fully studied hitherto. In this study, aerobic granules from a sequencing batch reactor were mechanically separated into various size-categories in order to investigate their physical properties, including sludge volumetric index (SVI), settling velocity (sv), specific surface hydrophobicity, granule strength, total solids, percentage volatile solids and other structural properties. Also, the live and dead biomass distribution was examined under a confocal laser scanning microscope after treatment with nucleic acid viability stains. Regardless of size, the biomass (both live and dead) was densest in the outer layer of the granule, which was about 600+/-50 microm thick. The live cells appeared only in the peripheral zone, while dead biomass spread into the inner zone. The biomass distribution pattern justified the changing physical properties of the granules as they grew bigger. As size increased, the sv, granule total density and biomass density increased but not in parallel with the size increment, while the granule strength, specific surface hydrophobicity and SVI decreased. Nonetheless, beyond a threshold size (4.0 mm diameter), the granules presented peculiar values in those properties, deviating from the initial trends. This was due to both inner and outer structural changes. The physical properties associated significantly with the size factor, for which the correlation coefficients were above 0.67. In view of biological viability and physical properties, the operational size-range suggested for optimal performance and economically effective aerobic SBR granular sludge is a diameter of 1.0-3.0 mm.     

Reactivation of effluent granular sludge from a high-rate Anammox reactor after storage

In this study, effluent sludge from a high-rate Anammox reactor was used to re-start new Anammox reactors for the reactivation of Anammox granular sludge. Different start-up strategies were evaluated in six upflow anaerobic sludge blanket (UASB) reactors (R₁–R₆) for their effect on nitrogen removal performance. Maximal nitrogen removal rates (NRRs) greater than 20 kg N/m³/day were obtained in reactors R₃–R₅, which were seeded with mixed Anammox sludge previously stored for approximately 6 months and 1 month. A modified Boltzmann model describing the evolution of the NRR fit the experimental data well. An amount of sludge added to the UASB reactor or decreasing the loading rate proved effective in relieving the substrate inhibition and increasing the NRR. The modified Stover–Kincannon model fit the nitrogen removal data in the Anammox reactors well, and the simulation results showed that the Anammox process has great nitrogen removal potential. The observed inhibition in the Anammox reactors may have been caused by high levels of free ammonia. The sludge used to seed the reactors did not settle well; sludge flotation was observed even after the reactors were operated for a long time at a floating upward velocity (F_s) of greater than 100 m/h. The settling sludge, however, exhibited good settling properties. Scanning electron microscopy showed that the Anammox granules consisted mainly of spherical and elliptical bacteria with abundant filaments on their surface. Hollows in the granules were also present, which may have contributed to sludge floatation.     

High rate biological treatment of sulfate-rich wastewater in an acetate-fed EGSB reactor

An expanded granular sludge bed reactor, inoculated with acclimated sulfidogenic granular sludge, was operated at 33 °C and fed with acetic acid as COD source and sulfate as electron acceptor. The bioreactor had a sulfate conversion efficiency of 80–90% at a high sulfate loading rate of 10.4 g SO₄ ^2--S/l.d after only 60 days of start-up. This was achieved by implementing a dual operational strategy. Firstly acetic acid was dosed near stoichiometry (COD over sulfur ratio = 2.0 to 2.2) which allowed almost complete sulfate removal. Secondly the pH in the bioreactor was kept slightly alkaline (7.9 ± 0.1) which limited the concentration of the inhibitory undissociated hydrogen sulfide H₂S (pK_a = 7). This allowed the acetotrophic sulfate reducing bacteria to predominate throughout the long term experiment. The limitations of the EGSB technology with respect to the sulfate conversion rate appeared to be related to the biomass wash-out and granule deterioration occurring at superficial upflow velocities above 10 m/h. Increasing the recirculation flow caused a drop in the sulfate reduction rate and efficiency, an increase of the suspended sludge fraction and a considerable loss of biomass into the effluent, yielding bare mainly inorganic granules. Elemental analysis revealed that a considerable amount of the granular sludge dry matter at the end of the experiment, at an upflow velocity of 20 m/h, consisted of calcium (32%), mainly in the form of carbonate deposits, while organic matter only represented 7%.