Highly heterogeneous interior structure of biofilm wastewater for enhanced pollutant removals

Response of biofilm-based systems for antibiotics removal from wastewater: Resource efficiency and process resiliency

Biofilm-based systems have efficient stability to cope-up influent shock loading with protective and abundant microbial assemblage, which are extensively exploited for biodegradation of recalcitrant antibiotics from wastewater. The system performance is subject to biofilm types, chemical composition, growth and thickness maintenance. The present study elaborates discussion on different type of biofilms and their formation mechanism involving extracellular polymeric substances secreted by microbes when exposed to antibiotics-laden wastewater. The biofilm models applied for estimation/prediction of biofilm-based systems performance are explored to classify the application feasibility. Further, the critical review of antibiotics removal efficiency, design and operation of different biofilm-based systems (e.g. rotating biological contactor, membrane biofilm bioreactor etc.) is performed. Extending the information on effect of various process parameters (e.g. hydraulic retention time, pH, biocarrier filling ratio etc.), the microbial community dynamics responsible of antibiotics biodegradation in biofilms, the technological problems, related prospective and key future research directions are demonstrated. The biofilm-based system with biocarriers filling ratio of ∼50-70% and predominantly enriched with bacterial species of phylum Proteobacteria protected under biofilm thickness of ∼1600 μm is effectively utilized for antibiotic biodegradation (>90%) when operated at DO concentration ≥3 mg/L. The C/N ratio ≥1 is best suitable condition to eliminate antibiotic pollution from biofilm-based systems. Considering the significance of biofilm-based systems, this review study could be beneficial for the researchers targeting to develop sustainable biofilm-based technologies with feasible regulatory strategies for treatment of mixed antibiotics-laden real wastewater.

Separate effect of turbulent pulsation on internal mass transfer in porous biofilms

Turbulence is considered to be the result of coupled time-averaged and pulsating velocities, making it difficult to distinguish the respective effects, and the quantitative effect of turbulent pulsation alone on mass transfer within biofilms has not been discussed in the literature. In this study, we constructed a special oscillating grid biofilm reactor combining Particle Image Velocimetry (PIV) measurements and Computational Fluid Dynamics (CFD) simulations to achieve nearly isotropic turbulence in a designed ambient without time-averaged velocity and shear stress. Subsequently, velocity and contaminant concentration distributions were obtained by solving a mass transfer model with a k-ε turbulence model, combined with measurements of biofilm structure parameters. The results showed that the increase in turbulent pulsation intensity led to a significant stratification of the percolation velocity gradient in biofilms, which enhanced convective mass transfer. The changes of biofilm density and porosity under turbulent pulsation were more strongly correlated with convective mass transfer. When the turbulent intensity (q) increased to 2.50 cm s^-1, the removal rate reached the highest value of 96.93%, accelerating the migration of contaminant concentration and the diffusive mass transfer effect was obvious. In addition, the trend of biofilm thickness under turbulent pulsation was consistent with the change of contaminant concentration distribution, and the correlation between them was greater. In summary, at q of 2.50 cm s^-1, there was a positive effect on both convection and diffusion mechanisms in biofilms, and the contaminant removal rate and biofilm thickness reached the maximum, which was the recommended turbulent pulsation conditions.

Blue Light Signaling Regulates Escherichia coli W1688 Biofilm Formation and l-Threonine Production

Escherichia coli biofilm may form naturally on biotic and abiotic surfaces; this represents a promising approach for efficient biochemical production in industrial fermentation. Recently, industrial exploitation of the advantages of optogenetics, such as simple operation, high spatiotemporal control, and programmability, for regulation of biofilm formation has garnered considerable attention. In this study, we used the blue light signaling-induced optogenetic system Magnet in an E. coli biofilm-based immobilized fermentation system to produce l-threonine in sufficient quantity. Blue light signaling significantly affected the phenotype of E. coli W1688. A series of biofilm-related experiments confirmed the inhibitory effect of blue light signaling on E. coli W1688 biofilm. Subsequently, a strain lacking a blue light-sensing protein (YcgF) was constructed via genetic engineering, which substantially reduced the inhibitory effect of blue light signaling on biofilm. A high-efficiency biofilm-forming system, Magnet, was constructed, which enhanced bacterial aggregation and biofilm formation. Furthermore, l-threonine production was increased from 10.12 to 16.57 g/L during immobilized fermentation, and the fermentation period was shortened by 6 h. IMPORTANCE We confirmed the mechanism underlying the inhibitory effects of blue light signaling on E. coli biofilm formation and constructed a strain lacking a blue light-sensing protein; this mitigated the aforementioned effects of blue light signaling and ensured normal fermentation performance. Furthermore, this study elucidated that the blue light signaling-induced optogenetic system Magnet effectively regulates E. coli biofilm formation and contributes to l-threonine production. This study not only enriches the mechanism of blue light signaling to regulate E. coli biofilm formation but also provides a theoretical basis and feasibility reference for the application of optogenetics technology in biofilm-based immobilized fermentation systems.

Homogeneously and heterogeneously structured biofilm models for wastewater treatment

Biofilm, a layer comprising extracellular polymeric substances, is the platform where the embedded living cells degrade the substances in the wastewaters. Biofilm models have been developed as part of the comprehensive models for the wastewater treatment process. This review summarizes the biofilm models applied in contemporary literature based on the spatial dimensions adopted for model build-up. The most commonly applied biofilm models are null-dimensional, considering the biofilm active biomass for the substrate sink's biological reaction. The one-dimensional, multi-species models are the second standard models for contemporary studies, providing transport and reaction resistances of substrates in the biofilm matrix and the interactions of competing or collaborating strains in the biofilm. The structural homogeneity of the biofilm challenges the validity of the uniformly structured models, highlighting the need to re-examine the validity of the uniformly structured models. The challenges and prospects of biofilm model developments and applications are outlined.

Biofilm with highly heterogeneous interior structure for pollutant removal: Effects of individual extracellular polymeric substance

For the first time, this study reveals the effects of an individual component of extracellular polymeric substances on the substrate consumption rates by the embedded cells based on the highly heterogeneous interior structures of a working biofilm. The flow-across mode in operation established a boundary-layer flow field with high transport resistance, making the uniformly structured model valid. Conversely, the flow field of the flow-through mode is determined by 46% jointly by proteins and β-d-glucopyranose polysaccharides. The substrate consumption rates hindered by β-d-glucopyranose polysaccharide is up to 60% over the 20%-40% biofilm height from the bottom, much lower than expected by the uniformly structured models. The strategies to maximize the biofilm performance have been suggested.

Nanocarriers for combating biofilms: advantages and challenges

Bacterial biofilms are highly resistant to antibiotics and pose a great threat to human and animal health. The control and removal of bacterial biofilms have become an important topic in the field of bacterial infectious diseases. Nanocarriers show great anti-biofilm potential because of their small particle size and strong permeability. In this review, the advantages of nanocarriers for combating biofilms are analyzed. Nanocarriers can act on all stages of bacterial biofilm formation and diffusion. They can improve the scavenging effect of biofilm by targeting biofilm, destroying extracellular polymeric substances, and enhancing the biofilm permeability of antimicrobial substances. Nanocarriers can also improve the antibacterial ability of antimicrobial drugs against bacteria in biofilm by protecting the loaded drugs and controlling the release of antimicrobial substances. Additionally, we emphasize the challenges faced in using nanocarrier formulations and translating them from a preclinical level to the clinical setting.

Integrating anaerobic digestion with bioelectrochemical system for performance enhancement: A mini review

Strategies for enhancing performance of anaerobic digestion (AD) process has been widely studied. The bioelectrochemical system (BES), including microbial fuel cell, microbial electrolysis cell (MEC), microbial desalination cell, and microbial electrosynthesis, had been proposed to integrate with AD for performance enhancement. This mini-review summarizes the current researches that integrated AD with BES to enhance the performance of the former. The working principles of BES were introduced. The integrated configurations of AD-BES as well as the associated applications were summarized. The statistics analysis for AD-MEC performances reported in literature were then performed to confirm the effects of reactor size and applied voltage on the methane productivity and enhancement. The challenges and prospects of the integrated AD-BES were delineated, and the potential scenarios of applying integrated AD-BES in field were discussed.

Biofilm with highly heterogeneous interior structure for pollutant removal: Cell distribution and manipulated mass transport

Biofilm wastewater treatment had been applied in practice. Conventionally the biofilm was modeled as a uniform structure to simplify the analysis. This study for the first time established a three-dimensional biofilm model with distributions separating living cells, Extracellular polymeric substances (EPS) and pores, based on which the local fluid flow velocity and pollutant diffusion and reaction fluxes inside the biofilm were numerically evaluated. Both the uniform structured and previously proposed heterogeneous models had been confirmed to overestimate the performances of a biofilm for wastewater treatment. The survival strategies of living cells in biofilm were discussed. Besides exposing to fresh pollutants for maximizing pollutant uptake, the tendency to form small aggregates of cells for shortening diffusion length so furnishing the pollutant with reduced diffusional resistance to living cells was also for the first time noted. This communication advanced the knowledge to comprehend the detailed processes in biofilm.

Impact of pH conditions and the characteristics of two electrodialysis membranes on biofilm development under semi-realistic conditions

The reuse of treated wastewater for irrigation is of increasing importance. The Ecosave farming project developed a new photocatalytic electrodialysis process for desalination and hygienization. However, membrane scaling significantly reduces filtration efficiency. This study investigated biofilm development on anion and cation exchange membranes at a wide pH range in pre-treated wastewater. Epifluorescence microscopic quantification of the biofilm by cell counts and surface coverage together with 16S rDNA gene copy numbers showed stronger biofilm development on the anion exchange membrane (AEM) compared with the cation exchange membrane (CEM) with up to 10⁵ cells mm^-2 and 20% surface coverage after three weeks. As the AEM biofilm developed best in neutral and a slightly alkaline pH, the CEM was colonized preferably at alkaline conditions. Extreme pH conditions strongly inhibited biofilm growth, which might help to minimize the maintenance effort by creating those conditions during the operation of the dialysis cell itself.

An Innovative in Situ Monitoring of Sulfate Reduction within a Wastewater Biofilm by H2S and SO42- Microsensors

Microelectrodes can be used to obtain chemical profiles within biofilm microenvironments. For example, sulfate (SO₄^2-) and hydrogen sulfide (H₂S) microelectrodes can be used to study sulfate reduction activity in this context. However, there is no SO₄^2- microelectrode available for studying sulfate reduction in biofilms. In this study, SO₄^2- and H₂S microelectrodes were fabricated and applied in the measurement of a wastewater membrane-aerated biofilm (MAB) to investigate the in situ sulfate reduction activity. Both the SO₄^2- and H₂S microelectrodes with a tip diameter of around 20 micrometers were successfully developed and displayed satisfying selectivity to SO₄^2- and H₂S, respectively. The Nernstian slopes of calibration curves of the fabricated SO₄^2- electrodes were close to -28.1 mV/decade, and the R² values were greater than 98%. Within the selected concentration range from 10^-5 M (0.96 mg/L) to 10^-2 M (960 mg/L), the response of the SO₄^2- microelectrode was log-linearly related to its concentration. The successfully fabricated SO₄^2- microelectrode was combined with the existing H₂S microelectrode and applied on an environmental wastewater biofilm sample to investigate the sulfate reduction activity within it. The H₂S and SO₄^2- microelectrodes showed stable responses and good performance, and the decrease of SO₄^2- with an accompanying increased of H₂S within the biofilm indicated the in situ sulfate reduction activity. The application of combined SO₄^2- and H₂S microelectrodes in wastewater biofilms could amend the current understanding of sulfate reduction and sulfur oxidation within environmental biofilms based on only H₂S microelectrodes.