Determination of the profile of DO and its mass transferring coefficient in a biofilm reactor packed with semi-suspended bio-carriers

Lipid production from biofilms of Marinobacter atlanticus in a fixed bed bioreactor

BACKGROUND

Biotechnologies that utilize microorganisms as production hosts for lipid synthesis will enable an efficient and sustainable solution to produce lipids, decreasing reliance on traditional routes for production (either petrochemical or plant-derived) and supporting a circular bioeconomy. To realize this goal, continuous biomanufacturing processes must be developed to maximize productivity and minimize costs compared to traditional batch fermentation processes.

RESULTS

Here, we utilized biofilms of the marine bacterium, Marinobacter atlanticus, to produce wax esters from succinate (i.e., a non-sugar feedstock) to determine its potential to serve as a production chassis in a continuous flow, biofilm-based biomanufacturing process. To accomplish this, we evaluated growth as a function of protein concentration and wax ester production from M. atlanticus biofilms in a continuously operated 3-D printed fixed bed bioreactor. We determined that exposing M. atlanticus biofilms to alternating nitrogen-rich (1.8 mM NH4+) and nitrogen-poor (0 mM NH4+) conditions in the bioreactor resulted in wax ester production (26 ± 5 mg/L, normalized to reactor volume) at a similar concentration to what is observed from planktonic M. atlanticus cells grown in shake flasks previously in our lab (ca. 25 mg/L cell culture). The wax ester profile was predominated by multiple compounds with 32 carbon chain length (C32; 50-60% of the total). Biomass production in the reactor was positively correlated with dilution rate, as indicated by protein concentration (maximum of 1380 ± 110 mg/L at 0.4 min-1 dilution rate) and oxygen uptake rate (maximum of 4 mmol O2/L/h at 0.4 min-1 dilution rate) measurements at different flow rates. Further, we determined the baseline succinate consumption rate for M. atlanticus biofilms to be 0.16 ± 0.03 mmol/L/h, which indicated that oxygen is the limiting reactant in the process.

CONCLUSION

The results presented here are the first step toward demonstrating that M. atlanticus biofilms can be used as the basis for development of a continuous flow wax ester biomanufacturing process from non-sugar feedstocks, which will further enable sustainable lipid production in a future circular bioeconomy.

Exposure of the static magnetic fields on the microbial growth rate and the sludge properties in the complete-mix activated sludge process (a Lab-scale study)

BACKGROUND

In this study, the effect of static magnetic fields (SMFs) on improving the performance of activated sludge process to enhance the higher rate of microbial growth biomass and improve sludge settling characteristics in real operation conditions of wastewater treatment plants has been investigated. The effect of SMFs (15 mT), hydraulic retention time, SRT, aeration time on mixed liquor suspended solids (MLSS) concentrations, mixed liquor volatile suspended solids (MLVSS) concentrations, α-factor, and pH in the complete-mix activated sludge (CMAS) process during 30 days of the operation, were evaluated.

RESULTS

There were not any differences between the concentration of MLSS in the case (2148.8 ± 235.6 mg/L) and control (2260.1 ± 296.0 mg/L) samples, however, the mean concentration of MLVSS in the case (1463.4 ± 419.2 mg/L) was more than the control samples (1244.1 ± 295.5 mg/L). Changes of the concentration of MLVSS over time, follow the first and second-order reaction with and without exposure of SMFs respectively. Moreover, the slope of the line and, the mean of α-factor in the case samples were 6.255 and, - 0.001 higher than the control samples, respectively. Changes in pH in both groups of the reactors were not observed. The size of the sluge flocs (1.28 µm) and, the spectra of amid I' (1440 cm-1) and II' (1650 cm-1) areas related to hydrogenase bond in the case samples were higher than the control samples.

CONCLUSIONS

SMFs have a potential to being considered as an alternative method to stimulate the microbial growth rate in the aeration reactors and produce bioflocs with the higher density in the second clarifiers.

Improvement of MBBR-MBR Performance by the Addition of Commercial and 3D-Printed Biocarriers

Moving bed biofilm reactor combined with membrane bioreactor (MBBR-MBR) constitute a highly effective wastewater treatment technology. The aim of this research work was to study the effect of commercial K1 biocarriers (MBBR-MBR K1 unit) and 3D-printed biocarriers fabricated from 13X and Halloysite (MBBR-MBR 13X-H unit), on the efficiency and the fouling rate of an MBBR-MBR unit during wastewater treatment. Various physicochemical parameters and trans-membrane pressure were measured. It was observed that in the MBBR-MBR K1 unit, membrane filtration improved reaching total membrane fouling at 43d, while in the MBBR-MBR 13X-H and in the control MBBR-MBR total fouling took place at about 32d. This is attributed to the large production of soluble microbial products (SMP) in the MBBR-MBR 13X-H, which resulted from a large amount of biofilm created in the 13X-H biocarriers. An optimal biodegradation of the organic load was concluded, and nitrification and denitrification processes were improved at the MBBR-MBR K1 and MBBR-MBR 13X-H units. The dry mass produced on the 13X-H biocarriers ranged at 4980-5711 mg, three orders of magnitude larger than that produced on the K1, which ranged at 2.9-4.6 mg. Finally, it was observed that mostly extracellular polymeric substances were produced in the biofilm of K1 biocarriers while in 13X-H mostly SMP.

Parametric optimization of additive manufactured biocarrier submerged in sequencing batch reactor for domestic wastewater treatment

The high nutrient concentration in domestic wastewater effluent can endanger the aquatic life via eutrophication. Thus, research have been carried out to prevent harm to aquatic life. In regard biofilm reactors have been successful by far with few limitations. Bio-carrier fabrication of desired shape is one of the limitations. Recently, the invention of additive manufacturing (AM) of object made it feasible to fabricate the desired shape. In this study additive manufactured bio‒carrier (AMB) was printed using AM technique, with high surface area to volume ratio as well as density higher than water. The submerged attach growth sequencing batch biofilm reactor (SAGSBBR) for organic and nutrient removal from domestic wastewater (DWW) was conducted to determine the optimum bio‒carrier filling ratio (FR) and cycle time (CT) by using response surface methodology (RSM) with CT ranging between 12 h and 24 h and FR ranging between 0 and 20%. The maximum chemical oxygen demand (COD), ammonia-nitrogen (NH₄ ⁺‒N), and total phosphorus (TP) removal was 96.8 mg/L, 93.32 mg/L, and 88.89 mg/L respectively, which was achieved in submerged attached growth sequential biofilm batch reactor with 10% FR (SAGSBBR‒10). The optimization study determined the optimal solution of CT and FR to be 17.07 h and 12.38% respectively, with desirability of 0.987. The predicted mean of responses for the optimal solution were 96.64%, 94.40% and 89.94% for COD removal, NH₄ ⁺‒N removal and TP removal, respectively. The rate of biomass attachment at the first stage in SAGSBBR‒10 and SAGSBBR‒20 was about 11.39 mg/carrier.d and 8.64 mg/carrier.d, whereas the highest accumulation achieved was 98.27 mg/carrier and 80.15 mg/carrier respectively. Thus, this study can assist us to achieve sustainable development goal (SDG) 6.

Effect of hydrodynamics on the transformation of nitrogen in river water by regulating the mass transfer performance of dissolved oxygen in biofilm

Biofilms drive crucial ecosystem processes in rivers. This study provided the basis for overall quantitative calculations about the contribution of biofilms to the nitrogen cycle. At the early stage of biofilm formation, dissolved oxygen (DO) could penetrate the biofilms. As the biofilm grew and the thickness increased, then the mass transfer of DO was restricted. The microaerobic layer firstly appeared in biofilm under the turbulent flow conditions, with the appearance of the microaerobic and anaerobic layer, the nitrification and denitrification reaction could proceed smoothly in biofilm. And the removal efficiency of total nitrogen (TN) increased as the biofilm matured. Under the turbulent flow conditions, mature biofilms had the smallest thickness, but the highest proportion the anaerobic layer to the biofilm thickness, the highest density, and the highest nitrogen removal efficiency. However, the nitrogen removal efficiency of biofilm was the lowest under laminar flow conditions. The difference of layered structure of biofilm and the DO flux in biofilm explained the difference of nitrogen migration and transformation in river water under different hydrodynamic conditions. This study would help control the growth of biofilm and improve the nitrogen removal capacity of biofilm by regulating hydrodynamic conditions.

Mixotrophic denitrification processes in basalt fiber bio-carriers drive effective treatment of low carbon/nitrogen lithium slurry wastewater

Lithium battery slurry wastewater was successfully treatedby using basalt fiber (BF) bio-carriers in a biological contact oxidation reactor. This resulted in a significant reduction of COD (93.3 ± 0.5 %) and total nitrogen (77.4 ± 1.0 %) at 12 h of HRT and dissolved oxygen (DO) of 0-1 mg/L. The modified Stover-Kincannon model indicated that the total nitrogen removal rate was 4.462 kg/m3/d in R-BF while the substrate maximum specific reaction rate (qmax) in the Monod model was 0.323 mg-N/mgVSS/d. A stable internal environment was established within the bio-nest. Metataxonomic analysis revealed the presence of denitrification and decarbonization bacteria, combined heterotrophic nitrification-aerobic denitrification bacteria, nitrite-oxidizing bacteria, and ammonia-oxidizing bacteria. Functional analysis displayed changes related to (aerobic)chemoheterotrophy, nitrogen respiration, nitrate reduction, respiration/denitrification of nitrite, and nitrate in R-BF. The study proposes a novel approach to achieve denitrification for the treatment of lithium slurry wastewater at low C/N conditions.

Can the carbon metabolic activity of biofilm be regulated by the hydrodynamic conditions in urban rivers?

Hydrodynamic regulation is widely used to improve the water quality of urban rivers. However, it is yet to explore substantially whether hydrodynamics could regulate the metabolic activity of biofilm in such aquatic systems. Herein, the pilot experiment of hydrodynamics in the rotation tanks was designed, including two experiment phases, namely constant flow and adjusting flow for 21 days and 14 days, respectively. In constant flow phase, biofilms grew in five shear stress gradients (R1-R5, 0.0044- 0.12 Pa). The carbon metabolic rate (k) of mature biofilms evaluated by BIOLOG ECO microplates showed a hump-shaped relationship with increasing shear stress, with R3 (0.049 Pa) the highest, while R5 (0.12 Pa) the lowest. To verify whether the metabolic activity of biofilm cultured at constant flow phase can be regulated by shear stress, we initiated the adjusting flow phase, and shear stress in reactors was reset uniformly at 0.049 Pa (with the highest k). Results showed the carbon metabolic activity of biofilm in reactor R4 and R5 increased rapidly by day 3, and there was no significant difference between the carbon metabolic rates among the five treatments by day 14. Meanwhile, the utilization levels of polymers and carbohydrates by biofilms were significantly different among the five treatments after hydrodynamic regulations. These results suggested that the total carbon metabolic activity of biofilm can be regulated by hydrodynamics, while the divergent changes of the specific carbon source category might affect the biofilm-mediated carbon biogeochemical processes, which should be considered for the application of hydrodynamic regulation in river ecological restoration projects.

Insights into the role of concentration polarization on the membrane fouling and cleaning during the aerobic granular sludge filtration process

The aerobic granular sludge (AGS) effectively mitigates the membrane fouling of a membrane bioreactor. However, the role and effects of the concentration polarization (CP), induced during the AGS filtration process on the membrane fouling and membrane cleaning efficiency, remain unclear. In the present study, the AGS resulted in a higher CP proportion (>50%) and a lower CP resistance (<3 × 10¹² m^-1), compared with the flocculent sludge, owing to the synergistic effect of the hydraulic shear and AGS scouring development, which improved the AGS in suspension and also minimized its deposition on the membrane. High-frequency interactions (contact and collision) between the AGS and membrane enhanced the CP resistance by returning more granular sludge from the cake layer to the CP, which proportionally increased the fouling resistance. Based on the correlation of CP and fouling resistance, the CP resistance was divided into 3 categories: high-intensity (2.76 × 10¹² m^-1), medium-intensity (1.74 × 10¹² m^-1), and low-intensity (0.62 × 10¹² m^-1). At the high-intensity CP, most membrane pores were "sealed" (complete pore blocking [R² > 0.9015]) and the pore blocking condition was the most serious (K-value = 0.0622 s^-1), while the membrane surface became denser and rougher. As a result, the permeability loss after the long-term filtration increased. In the chemical cleaning investigation, the alkaline detergents yielded an enhanced membrane cleaning efficiency to recover permeability. By reducing the CP, the membrane cleaning efficiency was marginally improved. The present study reveals the quantitative role of CP and offers insights into the mechanisms that govern membrane fouling in a membrane bioreactor.

Enhancement of wastewater treatment performance using 3D printed structures: A major focus on material composition, performance, challenges, and sustainable assessment

In order to enhance the performance and sustainability of wastewater treatment technologies, researchers are showing keen interest in the development of novel materials which can overcome the drawbacks associated with conventional materials. In this context, 3D printing gained significant attention due to its capability of fabricating complex geometrics using different material compositions. The present review focuses on recent advancements of 3D printing applications in various physicochemical and biological wastewater treatment techniques. In physicochemical treatment methods, substantial research has been aimed at fabricating feed spacers and other membrane parts, photocatalytic feed spacers, catalysts, scaffolds, monoliths, and capsules. Several advantages, such as membrane fouling mitigation, enhanced degradation efficiency, and recovery and reusability potential, have been associated with the aforementioned 3D printed materials. While in biofilm-based biological treatment methods, the use of 3D printed bio-carriers has led to enhanced mass transfer efficiency and microbial activities. Moreover, the application of these bio-carriers has shown better removal efficiency of chemical oxygen demand (∼90%), total nitrogen (∼73%), ammonia nitrogen (95%), and total phosphorous (∼100%). Although the removal efficiencies were comparable with conventional carriers, 3D printed carriers led to ∼40% reduction in hydraulic retention time, which could significantly save capital and operational expenditures. This review also emphasizes the challenges and sustainability aspects of 3D printing technology and outlines future recommendations which could be vital for further research in this field.

Sulfur transformation and bacterial community dynamics in both desulfurization-denitrification biofilm and suspended activated sludge

Types of microbial aggregates have essential effects on bacterial communities' characteristics, thus affecting the pollutants removal. An up-flow biofilm reactor was used to study the different performances of S^2-/NO₂^- removal and functional genes in suspended sludge and biofilms. The metabolic pathways of sulfurous and nitrogenous pollutants in the desulfurization-denitrification process were proposed. The results showed that S⁰ formation dominated the reactor with a high S^2- concentration. Autotrophic Sulfurovum responsible for S^2-/S⁰ oxidation was the only dominant bacteria in suspended sludge. Heterotrophic Desulfocapsa responsible for SO₄^2- reduction coexisted with Sulfurovum and dominated in biofilms. S^2- oxidation to S⁰ was catalyzed via fccA/B and sqr genes in suspended sludge. S₃^2-/S⁰ oxidation to SO₄^2- was catalyzed via dsrA/B gene in biofilms. SO₄^2- and NO₂^- were removed via the dissimilatory sulfate reduction and denitrification pathway, respectively. This work provides a fundamental and practical basis for optimizing suspended sludge/biofilm systems for S^2-/NO₂^- removal.

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.

Characteristics of oxygen mass transfer and its impact on pollutant removal performance and microbial community structure in an aerobic fluidized bed biofilm reactor for treatment of municipal wastewater

A laboratory-scale aerobic fluidized bed biofilm reactor (AFBBR) was established to evaluate the oxygen mass transfer (OMT) process and its impact on municipal wastewater treatment performance. Aeration rates had different effects on the OMT of the wastewater and biofilm. In the wastewater, oxygenation performance, oxygen uptake rate (OUR), and volumetric OMT coefficient (k_La) improved under high aeration rates. However, within the biofilm, the OMT process under the aeration rate of 0.096 L/(min·L) were higher than under other conditions [0.064 L/(min·L) and 0.128 L/(min·L)]. The denitrifying bacteria (DNB) abundance under the aeration rate of 0.096 L/(min·L) were improved so that total nitrogen (TN, 66.98 ± 4.23%) and ammonia nitrogen (NH₄⁺-N, 74.70 ± 2.30%) removal were higher than those under other aeration conditions. These results showed that suitable aeration could improve wastewater treatment efficiency through changing the OMT process and microbial community structure.

Treatment of high-load organic wastewater by novel basalt fiber carrier media

The carrier medium plays a key role in improving existing remediation potential of conventional biological contact oxidation reactors. In this study, a biological contact oxidation reactor was constructed using basalt fiber (R-BF) as a biological carrier. The bioreactor performance was investigated in terms of reduction in chemical oxygen demand (COD), ammonium nitrogen (NH₄⁺-N), and total nitrogen (TN) at organic loadings rate of 15.243 kg/m³·d and nitrogen loading rate of 1.068 kg/m³·d. We found that COD, NH₄⁺-N, and TN were reduced to 99.1%, 97.9%, and 97.8%, respectively. Within the R-BF, a bio-nest was developed which had abundant pores and channels and supported successful movement of nutrients, resulting in high biological activity (55.78%). The microbial communities within the bio-nest were diverse and rich and sludge production during operation was minimal. This makes BF a promising application for wastewater treatment. This research might be useful in the construction of integrated bioreactors that can operate under high organic and nitrogen loadings rates with reduced energy consumption, i.e. 75% in this study.

Contrasting distribution of antibiotic resistance genes and microbial communities in suspended activated sludge versus attached biofilms in an integrated fixed film activated sludge (IFAS) system

Suspended activated sludge (AS) and carrier-attached biofilms simultaneously exist in an integrated fixed film activated sludge (IFAS) system. However, the differentiation of antibiotic resistance genes (ARGs) and microbial communities in different types of biofilms is rarely reported. In this study, successions of ARGs and microbial communities of AS and two types of suspended carrier-attached biofilms over seasons were investigated in the IFAS system of one municipal wastewater treatment plant. Results showed that substantial differences were found in the distribution pattern of ARGs, bacterial communities, and predicted microbial function between AS and attached biofilms. The relative abundances of all detected ARGs in AS were significantly higher than those in attached biofilms. ARGs with higher relative abundances generally existed in K3 carrier (surface area ≥ 800 m²/m³) attached biofilms than those in K1 carrier (surface area ≥ 450 m²/m³) biofilms. The relative abundances of ARGs were negatively correlated with temperature and biochemical oxygen demand (BOD₅) and positively correlated with ammonium nitrogen contents for AS but not for attached biofilms. No significant relationship was found between the extracellular polymeric substance (EPS) content and ARG abundance for all samples. Temperature, BOD₅, and ammonium nitrogen contents were closely connected to microbial communities. The Bray-Curtis distance of bacterial communities between two adjacent sampling seasons for AS was larger than those of two attached biofilms. Network analysis indicated that the AS network had more positive links and intense connections than the attached biofilm networks, potentially facilitating the dissemination of ARGs. The differential distribution of ARGs among the three types of samples was significantly correlated with the microbial co-occurrence network topological properties. Bray-Curtis distance and network analysis suggest that microbial community is more robust in attached biofilms than in suspended AS. This work provides a more in-depth understanding of ARGs and microbial community distributions in wastewater biofilms.

Recent progress in integrated fixed-film activated sludge process for wastewater treatment: A review

Integrated fixed-film activated sludge (IFAS) process is considered as one of the leading-edge processes that provides a sustainable solution for wastewater treatment. IFAS was introduced as an advancement of the moving bed biofilm reactor by integrating the attached and the suspended growth systems. IFAS offers advantages over the conventional activated sludge process such as reduced footprint, enhanced nutrient removal, complete nitrification, longer solids retention time and better removal of anthropogenic composites. IFAS has been recognized as an attractive option as stated from the results of many pilot and full scales studies. Generally, IFAS achieves >90% removals for combined chemical oxygen demand and ammonia, improves sludge settling properties and enhances operational stability. Recently developed IFAS reactors incorporate frameworks for either methane production, energy generation through algae, or microbial fuel cells. This review details the recent development in IFAS with the focus on the pilot and full-scale applications. The microbial community analyses of IFAS biofilm and floc are underlined along with the special emphasis on organics and nitrogen removals, as well as the future research perspectives.

Insights into spatial effects of ceria nanoparticles on oxygen mass transfer in wastewater biofilms: Interfacial microstructure, in-situ microbial activity and metabolism regulation mechanism

Growing international exploitation of ceria nanoparticles (CeO₂ NPs) for varied applications has increased their release into wastewater treatment plants. Mass transfer of oxygen (MTO) in wastewater biofilm is of considerable importance to influence the activity and purification ability of biofilm. Herein, we investigated the spatial distribution of oxygen in gas-liquid-biofilm phases, the microstructure of interfaces and the in-situ microbial activity to reveal the impacts of CeO₂ NPs on MTO in wastewater biofilm and the related mechanisms. After exposure to 1 and 10 mg/L CeO₂ NPs, the oxygen transfer coefficient (K_La) from gas to wastewater increased by 28.1% and 75.3% with a reduction of thickness in gas-liquid boundary layer, indicating the enhanced MTO in gas-liquid interface. In contrast, the MTO in liquid-biofilm interface was negatively affected and the thickness of liquid-biofilms boundary layer significantly increased, which was mainly attributed to the smoother surface and the decreased surface area difference of biofilm. Within biofilm, the microbial activity was inhibited by 10 mg/L CeO₂ NPs, whereas the production of extracellular polymeric substance (EPS) was significantly improved, leading to a decline of 35.0% in the internal effective diffusivity (D_B) and a 300-μm reduction of oxygen penetration depth. Moreover, the relative activities of key enzymes involved in glycometabolism indicated the transition of Embden-Meyerhof pathway to pentose phosphate pathway, which probably contributed to the enhanced EPS production and consequently increased mass transfer resistance in liquid-biofilm interface and inner biofilm. These results could potentially expand the knowledge on mass transfer of nutrients or pollutants in wastewater biofilm in response to NPs exposure.

Enhancement of biological nitrogen removal performance using novel carriers based on the recycling of waste materials

This study aims to enhance biological nitrogen removal performance by the innovative carbon-based carriers. The new carriers were produced based on recycling waste materials. In these carriers, the advantages of the hybrid system and physicochemical properties of activated carbon were integrated to promote microbial attachment. To verify the performance of the new carriers compared to the conventional moving carriers, the experiments were conducted in two parallel laboratory-scale sequencing batch reactors under various operating conditions. The analysis revealed that the specific surface area of the new carrier with a total pore volume of 0.0015cm³/gr was 10.9 times the specific surface area of a conventional carrier. Further, the comparative results indicated that the new highly porous carriers made a major contribution to increasing the attached active biomass up to 20.2%. From the data analysis (DO, ORP, and pH), it was also confirmed that the new carriers had a positive effect on the creation of a greater anoxic zone within the biofilm. Consequently, the simultaneous nitrification-denitrification and total nitrogen removal efficiencies enhanced significantly up to 14.3% and 16.8%, respectively. From the environmental and economic viewpoints, the benefits of the novel carrier showed that it is a practical alternative for the conventional carrier providing a cost-effective wastewater treatment technology.

Promoting the granulation process of aerobic granular sludge in an integrated moving bed biofilm-membrane bioreactor under a continuous-flowing mode

This investigation demonstrated that aerobic granular sludge (AGS) could be cultivated rapidly in a single continuous-flowing membrane bioreactor (MBR) by introducing freely moved bio-carriers with a filling ratio of 10%. By operating the bioreactor for 28 days, AGS was successfully cultivated and kept stable for >2 months with a compact structure and clear shape, in which, extracellular polymeric substances played a key role in maintaining the stability of granular sludge structure. The microbial composition between AGS and attached biofilm was quite different, which indicated that the introduced bio-carriers improved the biodiversity within the bioreactor. Additionally, an explicit internal circulation was formed by the introduced bio-carriers, which was the main reason leading to the rapid formation of AGS. This is an interesting discovery and a novel approach to promote the rapid granulation of biomass in an MBR. Moreover, combining the biodegradation of AGS and filtration of membrane module, the bio-reactor achieved an excellent performance in removing COD_Cr (>90%) and TN (>85%) during the whole process.

Mass transfer of nanobubble aeration and its effect on biofilm growth: Microbial activity and structural properties

It is necessary to improve the performance and reduce the aeration cost is of wastewater treatment by aerobic biofilm systems. Nanobubble aeration is supposed to be a promising method to achieve these goals. Compared with coarse bubbles, dissolved oxygen profiling showed that the nanobubbles provided more oxygen to biofilms, offering superior oxygen supply capacity and 1.5 times higher oxygen transfer efficiency. Nanobubble aeration accelerated the growth of the biofilm and achieved better removal efficiencies of chemical oxygen demand and ammonia, with as maximum as six times higher dehydrogenase activity, and more extracellular polymeric substance content than when using the traditional aeration mode. This is attributed to the enhancement of metabolism and the proliferation of microorganisms. Confocal laser-scanning microscopy imaging confirmed that nanobubble aeration affected the components of biofilm by shifting the microbial community and changing its metabolic pathways of biofilms, such as carbohydrate synthesis. Nanobubble aeration resulted in an energy saving of approximately 80%. The assessment of nanobubble aerated biofilm growth suggests that this technique can offer a rapid-initiation, high efficiency, and low-cost strategy for aerobic biofilm systems in wastewater treatment.

Optimized aeration strategies for nitrogen removal efficiency: application of end gas recirculation aeration in the fixed bed biofilm reactor

Aeration strategy played an important role in reactor performance. In this study, when superficial upflow air velocity (SAV) decreased from 0.16 to 0.08 cm s^-1, low dissolved oxygen concentration (DO) of 2.0 mg L^-1 occurred in reactor. The required depth for anoxic microenvironment in biofilm decreased from 902.3 to 525.9 μm, which enhanced the growth of denitrifying bacteria and total nitrogen (TN) removal efficiency. However, decreasing aeration intensity resulted in insufficient hydraulic shear stress, which led to weak biofilm matrix structure. Mass biofilm detachment and reactor deterioration then occurred after 87 days of operation. An end gas recirculation aeration strategy was proposed to separately manipulate DO and aeration intensity. Low DO and high aeration intensity were simultaneously achieved, which enhanced the metabolism of denitrifying bacteria (such as Flavobacterium sp., Pseudorhodobacter sp., and Dok59 sp.) and EPS-producing bacteria (such as Zoogloea sp. and Rhodobacter sp.). Consequently, high TN removal performance (82.1 ± 2.7%) and stable biofilm structure were achieved.

Zeolite powder based polyurethane sponges as biocarriers in moving bed biofilm reactor for improving nitrogen removal of municipal wastewater

This study aims to enhance nitrogen removal efficiency of a moving bed biofilm reactor (MBBR) by developing a new MBBR with zeolite powder-based polyurethane sponges as biocarriers (Z-MBBR). Results indicated the total nitrogen (TN) removal efficiency and simultaneous nitrification and denitrification (SND) performance in Z-MBBR were nearly 10% higher than those in the conventional MBBR with sponges as biocarriers (S-MBBR). About 84.2 ± 4.8% of TN was removed in Z-MBBR compared to 75.1 ± 6.8% in S-MBBR. Correspondingly, the SND performance in Z-MBBR and S-MBBR was 90.7 ± 4.1% and 81.7 ± 6.5%, respectively. The amount of biofilm attached to new biocarriers (0.470 ± 0.131 g/g carrier) was 1.3 times more than that of sponge carriers (0.355 ± 0.099 g/g carrier). Based on the microelectrode measurements and microbial community analysis, more denitrifying bacteria existed in the Z-MBBR system, and this can improve the SND performance. Consequently, this new Z-MBBR can be a promising option for a hybrid treatment system to better nitrogen removal from wastewater.

Insight into the microbial community and its succession of a coupling anaerobic-aerobic biofilm on semi-suspended bio-carriers

This work aims at establishing a coupling anaerobic-aerobic biofilm within a single bioreactor and revealing its microbial community and succession. By using a semi-suspended bio-carrier fabricated with 3D printing technique, an obvious DO gradient was gradually created within the biofilm, which demonstrated that a coupling anaerobic-aerobic biofilm was successfully established on the surface of bio-carriers. The results of metagenomic analysis revealed that the microbial community on the bio-carriers experienced a continuous succession in its structure and dominant species along with the operational time. The formed coupling biofilm created suitable micro multi-habitats for the co-existence of these microorganisms, including nitrifying and denitrifying bacteria, which were beneficial to the removing of organic pollutants and converting nutrients. Along with the succession, the microbial community was gradually dominated by several functional microorganisms. Overall, the results presented an approach to improve the microbial biodiversity by constructing a new structure and floating status of bio-carriers.

Variation of the characteristics of biofilm on the semi-suspended bio-carrier produced by a 3D printing technique: Investigation of a whole growing cycle

The presented investigation focused on exploring the characteristics of the biofilm formed on a novel semi-suspended bio-carrier and revealing their variation during the whole growing cycle. This used semi-suspended bio-carrier was designed to be a spindle shape, and then fabricated by using a 3D printing technique. Results indicated the bio-carrier provided a suitable environment for the attachment of diverse microorganisms. During the experimental period lasted for 45days, the biofilm quickly attached on the surface of the bio-carrier and grew to maturity, but its characteristics, including the chemical compositions, adhesion force, surface roughness, structure of microbial communities, varied continuously along with the operational time, which greatly influenced the performance of the bioreactor. The shape and structure of bio-carrier, and the shearing force caused by the aeration are important factors that influence the microbial community and its structure, and also heavily affect the formation and growth of biofilm.