Determining the flow regime in a biofilm carrier by means of magnetic resonance imaging

Time-lagged interspecies interactions prevail during biofilm development in moving bed biofilm reactor

Clarifying the essential succession dynamics of interspecies interactions during biofilm development is crucial for the regulation and application of biofilm-based processes. In this study, regular and time-series phylogenetic molecular ecological networks (pMENs) were constructed to investigate ordinary and time-lagged interspecies interactions during biofilm development in a moving bed biofilm reactor (MBBR). Positive interactions dominated both regular (89.78%) and time-series (77.04%) ecological networks, suggesting that extensive cooperative behaviors facilitated biofilm development. The pronounced directional interactions (72.52%) in the time-series network further indicated that time-lagged interspecies interactions prevailed in the biofilm development process. Specifically, the proportion of directional negative interactions was higher than that of positive interactions, implying that interspecific competition preferred to be time-lagged. The time-series network revealed that module hubs exhibited extensive time-lagged positive interactions with their neighbors, and most of them exhibited altruistic behaviors. Keystone species possessing more positive interactions were positively correlated with biofilm biomass, NO₃ ^- -N concentrations, and the removal efficiencies of NH₄ ⁺ -N and COD. However, keystone species and peripherals that were negatively targeted by their neighbors showed positive correlations with the concentrations of NO₂ ^- -N, polysaccharides, and proteins in the soluble microbial products. The data highlight that the time-series network can provide directional microbial interactions along with the biofilm development process, which would help to predict the tendency of community shifts and propose efficient strategies for the regulation of biofilm-based processes. This article is protected by copyright. All rights reserved.

3D printing polycaprolactone micro-nano copper scaffolds with a high antibacterial performance for potential sewage treatment

Effective application of micro-nano copper particles in elimination of the pathogenic microorganisms in the water remains a challenge. In this study, an optimum structural design was adopted in mathematical models to improve the efficiency of sewage filtration, and polycaprolactone/copper scaffold (PCs) was fabricated through a 3D printing method. The result shows that the micro-nano copper particles were physically embedded into the polycaprolactone scaffolds. In addition, the antibacterial efficiency of PCs against E. coli and S. aureus was up to 100% and the antibacterial performance could be remained in sewage filtration (copper: polycaprolactone = 1:2). The results suggest that PCs is a good candidate for application in the sewage treatment.

Role of carrier characteristics affecting microbial density and population in enhanced nitrogen and phosphorus removal from wastewater

This research aims to improve simultaneous nitrification-denitrification and phosphorus removal (SNDPR) using novel carriers and to demonstrate the effect of carrier characteristics on nutrient removal in a biofilm reactor. For this purpose, biofilms enriched with both polyphosphate-accumulating organisms (PAOs) and nitrifiers were cultivated in two parallel sequencing batch reactors containing conventional moving bed bioreactor carriers (MBBR) and a novel type of carriers (carbon-based moving carriers (CBMC)). The new carriers were produced based on recycled waste materials via a chemical-thermal process and their specific surface area were 10.4 times higher than typical MBBR carriers of similar dimensions. The results showed that the use of CBMC carriers increased bacterial adhesion by about 18.5% and also affected the microbial population inside the biofilms, leading to an increase in PAOs abundancy and thus an increase in biological phosphorus removal up to 12.5%. Additionally, it was corroborated that the volume of the anoxic zones with dynamic behavior is strictly influenced by the carrier structure and biofilm thickness due to a limitation in oxygen penetration. Accordingly, the formation of broader anoxic zones and shrinkage of these zones to a lesser extent resulted in the continuation of anoxic reactions for longer periods using the novel carriers. Thereby, an increase in nitrogen removal by about 15% was obtained mainly by denitrifying PAOs. The results also exhibited that a higher simultaneous nitrification-denitrification (SND) efficiency can be achieved by selecting an appropriate aeration program influencing the dynamic changes of anoxic zones. Overall, a biofilm system using the new carriers, with phosphorus and nitrogen removal efficiencies of 97.5% and 92.3%, was presented as an efficient, compact, and simple operation SNDPR process.

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.

3D biofilm visualization and quantification on granular bioanodes with magnetic resonance imaging

The use of microbial fuel cells (MFCs) for wastewater treatment fits in a circular economy context, as they can produce electricity by the removal of organic matter in the wastewater. Activated carbon (AC) granules are an attractive electrode material for bioanodes in MFCs, as they are cheap and provide electroactive bacteria with a large surface area for attachment. The characterization of biofilm growth on AC granules, however, is challenging due to their high roughness and three-dimensional structure. In this research, we show that 3D magnetic resonance imaging (MRI) can be used to visualize biofilm distribution and determine its volume on irregular-shaped single AC granules in a non-destructive way, while being combined with electrochemical and biomass analyses. Ten AC granules with electroactive biofilm (i.e. granular bioanodes) were collected at different growth stages (3 to 21 days after microbial inoculation) from a multi-anode MFC and T₁-weighted 3D-MRI experiments were performed for three-dimensional biofilm visualization. With time, a more homogeneous biofilm distribution and an increased biofilm thickness could be observed in the 3D-MRI images. Biofilm volumes varied from 0.4 μL (day 4) to 2 μL (day 21) and were linearly correlated (R² = 0.9) to the total produced electric charge and total nitrogen content of the granular bioanodes, with values of 66.4 C μL^-1 and 17 μg N μL^-1, respectively. In future, in situ MRI measurements could be used to monitor biofilm growth and distribution on AC granules.

NMR investigation of water diffusion in different biofilm structures

Mass transfer in biofilms is determined by diffusion. Different mostly invasive approaches have been used to measure diffusion coefficients in biofilms, however, data on heterogeneous biomass under realistic conditions is still missing. To non-invasively elucidate fluid-structure interactions in complex multispecies biofilms pulsed field gradient-nuclear magnetic resonance (PFG-NMR) was applied to measure the water diffusion in five different types of biomass aggregates: one type of sludge flocs, two types of biofilm, and two types of granules. Data analysis is an important issue when measuring heterogeneous systems and is shown to significantly influence the interpretation and understanding of water diffusion. With respect to numerical reproducibility and physico-chemical interpretation, different data processing methods were explored: (bi)-exponential data analysis and the Γ distribution model. Furthermore, the diffusion coefficient distribution in relation to relaxation was studied by D-T₂ maps obtained by 2D inverse Laplace transform (2D ILT). The results show that the effective diffusion coefficients for all biofilm samples ranged from 0.36 to 0.96 relative to that of water. NMR diffusion was linked to biofilm structure (e.g., biomass density, organic and inorganic matter) as observed by magnetic resonance imaging and to traditional biofilm parameters: diffusion was most restricted in granules with compact structures, and fast diffusion was found in heterotrophic biofilms with fluffy structures. The effective diffusion coefficients in the biomass were found to be broadly distributed because of internal biomass heterogeneities, such as gas bubbles, precipitates, and locally changing biofilm densities. Thus, estimations based on biofilm bulk properties in multispecies systems can be overestimated and mean diffusion coefficients might not be sufficiently informative to describe mass transport in biofilms and the near bulk.

Post carbon removal nitrifying MBBR operation at high loading and exposure to starvation conditions

This study investigates the performance of MBBR nitrifying biofilm post carbon removal at high loading and starvation conditions. The nitrifying MBBR, treating carbon removal lagoon effluent, achieved a maximum SARR of 2.13gN/m²d with complete conversion of ammonia to nitrate. The results also show the MBBR technology is capable of maintaining a stable biofilm under starvation conditions in systems that nitrify intermittently. The biomass exhibited a higher live fraction of total cells in the high loaded reactors (73-100%) as compared to the reactors operated in starvation condition (26-82%). For both the high loaded and starvation condition, the microbial communities significantly changed with time of operation. The nitrifying community, however, remained steady with the family Nitrosomonadacea as the primary AOBs and Nitrospira as the primary NOB. During starvation conditions, the relative abundance of AOBs decreased and Nitrospira increased corresponding to an NOB/AOB ratio of 5.2-12.1.

From biofilm ecology to reactors: a focused review

Biofilms are complex biostructures that appear on all surfaces that are regularly in contact with water. They are structurally complex, dynamic systems with attributes of primordial multicellular organisms and multifaceted ecosystems. The presence of biofilms may have a negative impact on the performance of various systems, but they can also be used beneficially for the treatment of water (defined herein as potable water, municipal and industrial wastewater, fresh/brackish/salt water bodies, groundwater) as well as in water stream-based biological resource recovery systems. This review addresses the following three topics: (1) biofilm ecology, (2) biofilm reactor technology and design, and (3) biofilm modeling. In so doing, it addresses the processes occurring in the biofilm, and how these affect and are affected by the broader biofilm system. The symphonic application of a suite of biological methods has led to significant advances in the understanding of biofilm ecology. New metabolic pathways, such as anaerobic ammonium oxidation (anammox) or complete ammonium oxidation (comammox) were first observed in biofilm reactors. The functions, properties, and constituents of the biofilm extracellular polymeric substance matrix are somewhat known, but their exact composition and role in the microbial conversion kinetics and biochemical transformations are still to be resolved. Biofilm grown microorganisms may contribute to increased metabolism of micro-pollutants. Several types of biofilm reactors have been used for water treatment, with current focus on moving bed biofilm reactors, integrated fixed-film activated sludge, membrane-supported biofilm reactors, and granular sludge processes. The control and/or beneficial use of biofilms in membrane processes is advancing. Biofilm models have become essential tools for fundamental biofilm research and biofilm reactor engineering and design. At the same time, the divergence between biofilm modeling and biofilm reactor modeling approaches is recognized.

Meso and micro-scale response of post carbon removal nitrifying MBBR biofilm across carrier type and loading

This study investigates the effects of three specific moving bed biofilm reactor (MBBR) carrier types and two surface area loading rates on biofilm thickness, morphology and bacterial community structure of post carbon removal nitrifying MBBR systems along with the effects of carrier type and loading on ammonia removal rates and effluent solids settleability. The meso and micro analyses show that the AOB kinetics vary based on loading condition, but irrespective of carrier type. The meso-scale response to increases in loading was shown to be an increase in biofilm thickness with higher surface area carriers being more inclined to develop and maintain thicker biofilms. The pore spaces of these higher surface area to volume carriers also demonstrated the potential to become clogged at higher loading conditions. Although the biofilm thickness increased during higher loading conditions, the relative percentages of both the embedded viable and non-viable cells at high and conventional loading conditions remained stable; indicating that the reduced ammonia removal kinetics observed during carrier clogging events is likely due to the observed reduction in the surface area of the attached biofilm. Microbial community analyses demonstrated that the dominant ammonia oxidizing bacteria for all carriers is Nitrosomonas while the dominant nitrite oxidizing bacteria is Nitrospira. The research showed that filamentous species were abundant under high loading conditions, which likely resulted in the observed reduction in effluent solids settleability at high loading conditions as opposed to conventional loading conditions. Although the settleability of the effluent solids was correlated to increases in abundances of filamentous organisms in the biofilm, analyzed using next generation sequencing, the ammonia removal rate was not shown to be directly correlated to specific meso or micro-scale characteristics. Instead post carbon removal MBBR ammonia removal kinetics were shown to be related to the viable AOB cell coverage of the carriers; which was calculated by normalizing the surface area removal rate by the biofilm thickness, the bacterial percent abundance of ammonia oxidizing bacteria and the percentage of viable cells.

Short and long term biosorption of silica-coated iron oxide nanoparticles in heterotrophic biofilms

The increased application of engineered nanoparticles (ENP) in industrial processes and consumer products has raised concerns about their impact on health and environmental safety. When ENP enter the global water cycle by e.g. wastewater streams, wastewater treatment plants (WWTP) represent potential sinks for ENP. During biological WWT, the attachment of ENP to biofilms is responsible for the desired removal of ENP from the water phase avoiding their release into the aquatic environment. However, the fundamental mechanisms guiding the interactions between ENP and biofilms are not yet fully understood. Therefore, this study investigates the behavior and biosorption of inorganic ENP, here magnetic iron oxide nanoparticles coated with silica (scFe3O4-NP), with heterotrophic biofilms at different time scales. Their magnetic properties enable to follow scFe3O4-NP in the biofilm system by a magnetic susceptibility balance and magnetic resonance imaging. Biofilms were exposed to scFe3O4-NP at short contact times (5 min) in flow cells and complementary, scFe3O4-NP were introduced into a moving bed biofilm reactor (MBBR) to be observed for 27 d. Mass balances revealed that scFe3O4-NP sorbed to the biofilm within a few minutes, but that the total biosorption was rather low (3.2 μg Fe/mg TSS). scFe3O4-NP mainly sorbed to the biofilm surface inducing the detachment of outer biofilm parts starting after an exposure time of 3h in the MBBR. The biosorption depended on the exposure concentration of scFe3O4-NP, but less on the contact time. Most scFe3O4-NP exited the flow cell (up to 65%) and the MBBR (57%) via the effluent. This effect was favored by the stabilization of scFe3O4-NP in the bulk liquid by organic matter leading to a low retention capacity of the MBBR system. The results contribute to improve our understanding about the fate of ENP in environmental and in technical biofilm systems and give indications for future investigations needed.

Direct surface visualization of biofilms with high spin coordination clusters using Magnetic Resonance Imaging

Magnetic Resonance Imaging is a powerful tool for the investigation of a biofilms' physical structure determining mass transport behavior which is of major importance in biofilm research. The entire biofilm is imaged in situ non-invasively and non-destructively on a meso-scale. In this study, different contrast agents were applied to study the biofilm's properties with the focus on mass transport, which is achieved by varying the contrast agents with respect to their NMR and interaction properties. The spatio-temporal tracking of these cluster, molecular and particulate contrast agents in biofilms was achieved by T1-, T2-weighted and proton density images during short (20h) and long (14 d) term exposures. The best biofilm surface visualization was observed when applying a new high spin coordination cluster (Fe10Gd10) showing a high affinity to the biofilm's surface and a fast immobilization within minutes. Contrarily, the small molecular contrast agents show no immobilization and fully penetrated into the biofilm. A concentration equilibrium was observed which was confirmed in back diffusion experiments. Interactions between larger nanoparticulate contrast agents and the biofilm required hours to achieve immobilization. Thus, the penetration depth into the biofilm is predominantly size-dependent. Here, it is shown that biofilm surface interactions can be observed in situ and spatio-temporarily resolved. The reported methodology demonstrates a new means to explore mass transfer of various substances in biofilms.

STATEMENT OF SIGNIFICANCE

In biofilm research, the investigation of the biofilms' physical structure is of high relevance for the understanding of mass transport processes. However, commonly used imaging techniques for biofilm imaging such as CLSM or electron microscopy rarely visualize the real biofilm due to their invasiveness and destructiveness. Magnetic Resonance Imaging (MRI) represents the ideal tool to image the biofilm in situ, non-invasively and non-destructively with a spatial resolution of several 10μm. To gain specific structural and functional information, a variety of MRI contrast agents (molecular and particulate) was applied with different properties for the first time. Results elucidate the interactions between the biofilms' surface and the contrast agents and open a new field for biotechnological applications by functional contrast enhancement.

Assessing the influence of biofilm surface roughness on mass transfer by combining optical coherence tomography and two-dimensional modeling

Imaging and modeling are two major approaches in biofilm research to understand the physical and biochemical processes involved in biofilm development. However, they are often used separately. In this study we combined these two approaches to investigate substrate mass transfer and mass flux. Cross-sectional biofilm images were acquired by means of optical coherence tomography (OCT) for biofilms grown on carriers. A 2D biofilm model was developed incorporating OCT images as well as a simplified biofilm geometry serving as structural templates. The model incorporated fluid flow, substrate transfer and biochemical conversion of substrates and simulated the hydrodynamics surrounding the biofilm structure as well as the substrate distribution. The method allowed detailed analysis of the hydrodynamics and mass transfer characteristics at the micro-scale. Biofilm activity with respect to substrate fluxes was compared among different combinations of flow, substrate availability and biomass density. The combined approach revealed that higher substrate fluxes at heterogeneous biofilm surface under two conditions: pure diffusion and when high flow velocity along the biofilms surface renders the whole liquid-biofilm interface to be highly active. In-between the two conditions the substrate fluxes across the surface of smooth biofilm geometry were higher than that of the heterogeneous biofilms.