Hydrogenase-based electrode for hydrogen sensing in a fermentation bioreactor.

The hydrogen-based economy will require not only sustainable hydrogen production but also sensitive and cheap hydrogen sensors. Commercially available H2 sensors are limited by either use of noble metals or elevated temperatures. In nature, hydrogenase enzymes present high affinity and selectivity for hydrogen, while being able to operate in mild conditions. This study aims at evaluating the performance of an electrochemical sensor based on carbon nanomaterials with immobilised hydrogenase from the hyperthermophilic bacterium Aquifex aeolicus for H2 detection. The effect of various parameters, including the surface chemistry, dispersion degree and amount of deposited carbon nanotubes, enzyme concentration, temperature and pH on the H2 oxidation are investigated. Although the highest catalytic response is obtained at a temperature around 60 °C, a noticeable current can be obtained at room temperature with a low amount of protein less than 1 µM. An original pulse-strategy to ensure H2 diffusion to the bioelectrode allows to reach H2 sensitivity of 4 µA cm-2 per % H2 and a linear range between 1 and 20%. Sustainable hydrogen was then produced through dark fermentation performed by a synthetic bacterial consortium in an up-flow anaerobic packed-bed bioreactor. Thanks to the outstanding properties of the A. aeolicus hydrogenase, the biosensor was demonstrated to be quite insensitive to CO2 and H2S produced as the main co-products of the bioreactor. Finally, the bioelectrode was used for the in situ measurement of H2 produced in the bioreactor in steady-state.

Metabolic Exchange and Energetic Coupling between Nutritionally Stressed Bacterial Species: Role of Quorum-Sensing Molecules.

Formation of multispecies communities allows nearly every niche on earth to be colonized, and the exchange of molecular information among neighboring bacteria in such communities is key for bacterial success. To clarify the principles controlling interspecies interactions, we previously developed a coculture model with two anaerobic bacteria, Clostridium acetobutylicum (Gram positive) and Desulfovibrio vulgaris Hildenborough (Gram negative, sulfate reducing). Under conditions of nutritional stress for D. vulgaris, the existence of tight cell-cell interactions between the two bacteria induced emergent properties. Here, we show that the direct exchange of carbon metabolites produced by C. acetobutylicum allows D vulgaris to duplicate its DNA and to be energetically viable even without its substrates. We identify the molecular basis of the physical interactions and how autoinducer-2 (AI-2) molecules control the interactions and metabolite exchanges between C. acetobutylicum and D. vulgaris (or Escherichia coli and D. vulgaris). With nutrients, D. vulgaris produces a small molecule that inhibits in vitro the AI-2 activity and could act as an antagonist in vivo Sensing of AI-2 by D. vulgaris could induce formation of an intercellular structure that allows directly or indirectly metabolic exchange and energetic coupling between the two bacteria.IMPORTANCE Bacteria have usually been studied in single culture in rich media or under specific starvation conditions. However, in nature they coexist with other microorganisms and build an advanced society. The molecular bases of the interactions controlling this society are poorly understood. Use of a synthetic consortium and reducing complexity allow us to shed light on the bacterial communication at the molecular level. This study presents evidence that quorum-sensing molecule AI-2 allows physical and metabolic interactions in the synthetic consortium and provides new insights into the link between metabolism and bacterial communication.

Treatment technologies and degradation pathways of glyphosate: A critical review.

Glyphosate is one of the most widely used post-emergence broad-spectrum herbicides in the world. This molecule has been frequently detected in aqueous environment and can cause adverse effects to plants, animals, microorganisms, and humans. This review offers a comparative assessment of current treatment methods (physical, biological, and advanced oxidation process) for glyphosate wastewaters, considering their advantages and drawbacks. As for other molecules, adsorption does not destroy glyphosate. It can be used before other processes, if glyphosate concentrations are very high, or after, to decrease the final concentration of glyphosate and its by-products. Most of biological and oxidation processes can destroy glyphosate molecules, leading to by-products (the main ones being AMAP and sarcosine) that can be or not affected by these processes. This point is of major importance to control process efficiency. That is the reason why a specific focus on glyphosate degradation pathways by biological treatment or different advanced oxidation processes is proposed. However, one process is usually not efficient enough to reach the required standards. Therefore, the combination of processes (for instance biological and oxidation ones) seems to be high-performance technologies for the treatment of glyphosate-containing wastewater, due to their potential to overcome some drawbacks of each individual process. Finally, this review provides indications for future work for different treatment processes to increase their performances and gives some insights into the treatment of glyphosate or other organic contaminants in wastewater.

Kinetic study of glyphosate degradation in wet air oxidation conditions.

Glyphosate is one of the most widely used herbicides in the world against perennial and annual weeds. It has been reported to be a micro pollutant, and its degradation in different wastewater treatment processes must be studied. For that purpose, the kinetics of wet air oxidation of glyphosate was studied in an autoclave reactor at a temperature range of 423-523 K and under a total pressure of 15 MPa. Oxidation reactions obeyed the first-order kinetics with respect to glyphosate concentration. The activation energy for glyphosate oxidation was found to be equal to 68.4 kJ mol-1. Furthermore, the possible reaction intermediates and main end products of glyphosate degradation in the wet air oxidation process were identified and quantified using UV-spectrophotometry and liquid chromatography coupled to high resolution mass spectrometry. A degradation pathway for glyphosate oxidation was proposed.

Experimental coupling and modelling of wet air oxidation and packed-bed biofilm reactor as an enhanced phenol removal technology.

Experimental coupling of wet air oxidation process and aerobic packed-bed biofilm reactor is presented. It has been tested on phenol as a model refractory compound. At 30 MPa and 250 °C, wet air oxidation batch experiments led to a phenol degradation of 97% and a total organic carbon removal of 84%. This total organic carbon was mainly due to acetic acid. To study the interest of coupling processes, wet air oxidation effluent was treated in a biological treatment process. This step was made up of two packed-bed biofilm reactors in series: the first one acclimated to phenol and the second one to acetic acid. After biological treatment, phenol and total organic carbon removal was 99 and 97% respectively. Thanks to parameters from literature, previous studies (kinetic and thermodynamic) and experimental data from this work (hydrodynamic parameters and biomass characteristics), both treatment steps were modelled. This modelling allows the simulation of the coupling process. Experimental results were finally well reproduced by the continuous coupled process model: relative error on phenol removal efficiency was 1 and 5.5% for wet air oxidation process and packed-bed biofilm reactor respectively.

Fermentative hydrogen production in an up-flow anaerobic biofilm reactor inoculated with a co-culture of Clostridium acetobutylicum and Desulfovibrio vulgaris.

Dark fermentation systems often show low H2 yields and unstable H2 production, as the result of the variability of microbial dynamics and metabolic pathways. Recent batch investigations have demonstrated that an artificial consortium of two anaerobic bacteria, Clostridium acetobutylicum and Desulfovibrio vulgaris Hildenborough, may redirect metabolic fluxes and improve H2 yields. This study aimed at evaluating the scale-up from batch to continuous H2 production in an up-flow anaerobic packed-bed reactor (APBR) continuously fed with a glucose-medium. The effects of various parameters, including void hydraulic retention time (HRTv), pH, and alkalinity, on H2 production performances and metabolic pathways were investigated. The results demonstrated that a stable H2 production was reached after 3-4days of operation. H2 production rates increased significantly with decreasing HRTv from 4 to 2h. Instead, H2 yields remained almost stable despite the change in HRTv, indicating that the decrease in HRTv did not affect the global metabolism.

Anaerobic biofilm reactors for dark fermentative hydrogen production from wastewater: A review.

Dark fermentation is a bioprocess driven by anaerobic bacteria that can produce hydrogen (H2) from organic waste and wastewater. This review analyses a relevant number of recent studies that have investigated dark fermentative H2 production from wastewater using two different types of anaerobic biofilm reactors: anaerobic packed bed reactor (APBR) and anaerobic fluidized bed reactor (AFBR). The effect of various parameters, including temperature, pH, carrier material, inoculum pretreatment, hydraulic retention time, substrate type and concentration, on reactor performances was investigated by a critical discussion of the results published in the literature. Also, this review presents an in-depth study on the influence of the main operating parameters on the metabolic pathways. The aim of this review is to provide to researchers and practitioners in the field of H2 production key elements for the best operation of the reactors. Finally, some perspectives and technical challenges to improve H2 production were proposed.

Modeling partial nitrification and denitrification in a hybrid biofilm reactor: calibration by retention time distribution and respirometric tests.

In this study, partial nitrification coupled with denitrification is modeled in a hybrid biofilm reactor with different hydraulic saturation conditions. The activated sludge model with two-step nitrification is implemented in GPS-X software. Hydrodynamic modeling by retention time distribution analysis and biokinetic measurement by respirometric tests are two significant parts of model calibration. By combining these two parts, partial nitrification in the aerobic part of the column is well simulated with a good agreement between experimental and modeled effluent concentrations of NH4 (+) and NO2 (-). Particularly, fully hydraulic saturation condition contributes to the large hydraulic volume of 1.9 L and high produced NO2 (-) concentration around 40 mg L(-1). However, modeling denitrification still needs to be improved with more calibrated parameters. Furthermore, three alternatives are proposed for the optimization of reactor design and operation.

Calibration of hydrodynamic behavior and biokinetics for TOC removal modeling in biofilm reactors under different hydraulic conditions.

In this study, total organic carbon (TOC) biodegradation was simulated by GPS-X software in biofilm reactors with carriers of plastic rings and glass beads under different hydraulic conditions. Hydrodynamic model by retention time distribution and biokinetic measurement by in-situ batch test served as two significant parts of model calibration. Experimental results showed that TOC removal efficiency was stable in both media due to the enough height of column, although the actual hydraulic volume changed during the variation of hydraulic condition. Simulated TOC removal efficiencies were close to experimental ones with low theil inequality coefficient values (below 0.15). Compared with glass beads, more TOC was removed in the filter with plastic rings due to the larger actual hydraulic volume and lower half saturation coefficient in spite of its lower maximum specific growth rate of biofilm, which highlighted the importance of calibrating hydrodynamic behavior and biokinetics.

Interpreting hydrodynamic behaviour by the model of stirred tanks in series with exchanged zones: preliminary study in lab-scale trickling filters.

In trickling filters for wastewater treatment, hydrodynamic behaviour is affected by the growth of biofilm on the porous medium. Therefore, modelling hydrodynamic behaviour is necessary and efficient to predict the biodegradation of pollutants. In this study, laboratory-scale trickling filters were filled with two different porous media (glass beads and plastic rings) and were fed by a synthetic substrate in batch mode. Total organic carbon (TOC) of the effluent was measured and retention time distribution (RTD) was determined by injecting NaCl. Results showed that medium had no significant effect on TOC removal rate (around 80% and 60% respectively for batch time of seven and two days). However, regarding the hydrodynamic behaviour, the effective volume ratio and hydraulic efficiency in the glass beads bed increased remarkably from 28% and 18% to 80% and 70%, respectively, with the reduction of dispersion coefficient (from 4.55 to 1.53). Moreover, the short batch time accelerated this change. Conversely, no variation of hydrodynamic behaviour in plastic rings bed was evident. Along with the feeding of synthetic substrate, biofilm concentration ranged from 1.5 to 10.1 g/L in the glass beads reactor and it achieved around 2.8 g/L in the plastic rings reactor. Hydrodynamic modelling indicated that the model of stirred tanks in series with exchanged zones fitted the experimental results well. These gave values of mobile and immobile volumes of 51 mL and 17 mL, respectively, in the glass beads filter and 25 mL and 15 mL, respectively, in the plastic rings filter.