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Biocatalysis has widened its scope and relevance since new molecular tools, including improved expression systems for proteins, protein and metabolic engineering, and rational techniques for immobilization, have become available. However, applications are still sometimes hampered by low productivity and difficulties in scaling up. A practical and reasonable step to improve the performances of biocatalysts (including both enzymes and whole-cell systems) is to use them in flow reactors. This review describes the state of the art on the design and use of biocatalysis in flow reactors. The encouraging successes of this enabling technology are critically discussed, highlighting new opportunities, problems to be solved and technological advances. Biocatalyzed reactions with different classes of enzymes can be implemented with the integration of flow reactor technology, potentially leading to sustainable and highly productive continuous processes. The combination of biocatalysis and flow chemistry opens the door to extensive application in cascade reactions. Biocatalyzed flow reactions can occur either in monophasic flow or in segmented (slug) flow, where two or more immiscible phases are present. Limitation of substrate/product inhibition effects, in-line purification with easy recovery of the product, and no mechanical mixing are among the most distinctive advantages of flow-based biocatalysis. Automated machines and devices for in-line product recovery are now available at relatively low prices, making flow-based biocatalysis an easy-to-use technology.

Aerobic granular sludge (AGS) in continuous-flow reactors (CFRs) has attracted significant interest, with notable progress in research and application over the past two decades. Cumulative studies have shown that AGS-CFRs exhibit comparable morphology, settleability, and pollutant removal efficiency to AGS cultivated in sequencing batch reactors, despite their smaller particle sizes. Shear force and selection pressure are the primary drivers of granulation. While not mandatory for granulation, feast/famine conditions play a crucial role in ensuring long-term stability and nutrient removal. Additionally, bioaugmentation can facilitate the granulation process. Furthermore, this paper comprehensively assesses the application of AGS-CFRs in full-scale wastewater treatment plants (WWTPs). Currently, AGS-CFRs have been implemented in nine WWTPs, encompassing two distinct processes. Hydrocyclone-based densified activated sludge significantly enhances sludge density, settleability, and biological phosphorus removal efficiency, thus increasing treatment capacity. The microaerobic–aerobic configuration with internal separators can induce granulation, ensuring long-term stability, eliminating the need for external clarifiers, and reducing land and energy requirements. This review demonstrates the high potential of AGS-CFRs for intensifying existing WWTPs with minimal retrofitting needs. However, further research is required in granulation mechanisms, long-term stability, and nutrient removal to promote the widespread adoption of AGS.

This study investigated the effects of ammonium and nitrite on ammonia-oxidizing bacteria (AOB) from an activated sludge process in laboratory-scale continuous-flow reactors. AOB communities were analyzed using specific PCR followed by denaturing gel gradient electrophoresis, cloning and sequencing of the 16S rRNA gene, and AOB populations were quantified using real-time PCR. To study the effect of ammonium, activated sludge from a sewage treatment system was enriched in four reactors receiving inorganic medium containing four different ammonium concentrations (2, 5, 10 and 30 mM NH₄⁺-N). One of several sequence types of the Nitrosomonas oligotropha cluster predominated in the reactors with lower ammonium loads (2, 5 and 10 mM NH₄⁺-N), whereas Nitrosomonas europaea was the dominant AOB in the reactor with the highest ammonium load (30 mM NH₄⁺-N). The effect of nitrite was studied by enriching the enriched culture possessing both N. oligotropha and N. europaea in four reactors receiving 10-mM-ammonium inorganic medium containing four different nitrite concentrations (0, 2, 12 and 22 mM NO₂⁻-N). Nitrosomonas oligotropha comprised the majority of AOB populations in the reactors without nitrite accumulation (0 and 2 mM NO₂⁻-N), whereas N. europaea was in the majority in the 12- and 22-mM NO₂⁻-N reactors, in which nitrite concentrations were 2.1-5.7 mM (30-80 mg N L⁻¹).

This study evaluates the performance of continuous flow and drop-based microfluidic devices for the synthesis of silver nanoparticles (AgNPs) under identical hydrodynamic and chemical conditions. Flows at low values of Dean number (De < 1) were investigated, where the contribution of the vortices forming inside the drop to the additional mixing inside the reactor should be most noticeable. In the drop-based microfluidic device, discrete aqueous drops serving as reactors were generated by flow focusing using silicone oil as the continuous phase. Aqueous solutions of reagents were supplied through two different channels merging just before the drops were formed. In the continuous flow device, the reagents merged at a Tee junction, and the reaction was carried out in the outlet tube. Although continuous flow systems may face challenges such as particle concentration reduction due to deposition on the channel wall or fouling, they are often more practical for research due to their operational simplicity, primarily through the elimination of the need to separate the aqueous nanoparticle dispersion from the oil phase. The results demonstrate that both microfluidic approaches produced AgNPs of similar sizes when the hydrodynamic conditions defined by the values of De and the residence time within the reactor were similar.

Many research papers describe selective hydrogenation of functional groups, such as nitro groups, carbonyl groups, or unsaturated carbon bonds to obtain fine chemicals or precursors of pharmaceuticals. Quite often, the catalyst performance is investigated in batch or continuous flow reactors without finding advantages and disadvantages of this or that regime. At the same time, the transition from batch processes to continuous flow occurs on the industrial scale. However, the batch process can be preferable for some reactions, in spite of its drawbacks. This review article aims to identify all publications that consider selective hydrogenation of functional groups in organic compounds, both in batch and continuous flow reactors, at the same reaction conditions that allow making conclusions about the benefits of one of the regimes in a particular case.

Abstrac The new identity of alginate composites with bentonite as sustainable catalytic natural-based material for methylene blue (MB) decolorization in a continuous flow-packed-bed reactor was investigated. The small-scale materials were produced by drop-wise coupled freeze drying with natural organic raw ingredients, namely bentonite and alginate. The decolonization experiments were conducted by manipulating the bed depth, flow rate, and temperature systematically. The evaluation revealed that a substantial 66 mm diameter and a flow rate of 0.7 mL/min were required to attain a high removal effectiveness (99%) of continuous MB-colored dyes at a temperature of 45 °C. Alginate-based composites were very appropriate because of their facile manufacture, cost-effectiveness, biocompatibility, renewability, easy separability, absence of secondary pollutants, and ecological benignity. Estimation of the cost and environmental impact of raw material supply and processing were evaluated by using embodied energy and embodied CO 2 criteria. The implementation of scale adjustments and material supply was anticipated to result in a reduction of overall expenses. Regarding the environmental impact of the material, the embodied energy values for our alginate-clay production process confirmed that freeze-drying had the highest proportion, similar to cost analysis. In summary, products based on alginate showed potential for effective decolorization in the dye industry. Further research was required to thoroughly assess the technical and commercial viability of these materials, including their unique material properties and appropriate manufacturing methods.

Tuning the reaction parameters to maximize products yield is one of the major needs for any process. The goal of this research is to increase the reduction of CO2 with water by examining the operating parameters of a meso-scale continuous-flow type photochemical reactor over hydrothermally synthesized photocatalysts such as Pt/TiO2 and Pt/TiO2/RGO. Effects of catalyst type, weight of catalyst utilized, photochemical reactor temperature, retention time by variating the liquid water flow rate, and cocatalyst loading were investigated to increase the concentration of total organic carbon compounds including HCHO and CH3OH. The effect of titanium dioxide phase ratio (anatase: rutile) presence at the Pt/TiO2/RGO photocatalysts was also studied. The results revealed that the 0.3 wt.% Pt/TiO2/RGO5% photocatalyst which includes a phase ratio of 81:19 for anatase: rutile respectively has the superior photocatalytic activity to other studied photocatalysts. The physciochemical properties of different prepared photocatalytic samples were determined using various characterization techniques. Analyzing the liquid products on gas chromatography, it was found that CH3OH represents the major product whereas HCHO was the minor one. This reactor exhibits a great performance towards CO2 photocatalytic reduction under the optimized conditions.

Aerobic granular sludge (AGS) comprises an aggregation of microbial cells in a tridimensional matrix, which is able to remove carbon, nitrogen and phosphorous as well as other pollutants in a single bioreactor under the same operational conditions. During the past decades, the feasibility of implementing AGS in wastewater treatment plants (WWTPs) for treating sewage using fundamentally sequential batch reactors (SBRs) has been studied. However, granular sludge technology using SBRs has several disadvantages. For instance, it can present certain drawbacks for the treatment of high flow rates; furthermore, the quantity of retained biomass is limited by volume exchange. Therefore, the development of continuous flow reactors (CFRs) has come to be regarded as a more competitive option. This is why numerous investigations have been undertaken in recent years in search of different designs of CFR systems that would enable the effective treatment of urban and industrial wastewater, keeping the stability of granular biomass. However, despite these efforts, satisfactory results have yet to be achieved. Consequently, it remains necessary to carry out new technical approaches that would provide more effective and efficient AGS-CFR systems. In particular, it is imperative to develop continuous flow granular systems that can both retain granular biomass and efficiently treat wastewater, obviously with low construction, maintenance and exploitation cost. In this review, we collect the most recent information on different technological approaches aimed at establishing AGS-CFR systems, making possible their upscaling to real plant conditions. We discuss the advantages and disadvantages of these proposals and suggest future trends in the application of aerobic granular systems. Accordingly, we analyze the most significant technical and biological implications of this innovative technology.

Achieving nitrogen removal in a single device has attracted great attention of researchers in recent years due to its small footprint and low energy consumption. In this study, a novel continuous-flow air-lifting reactor is designed to establish multi-functional zones, including an aerobic zone, an anoxic zone, and a settling zone. With an alternation aeration of double-layer aerators (DLAs), multiple dissolved oxygen (DO) environments are maintained and denitrification and carbon removal in the same reactor are achieved. Results demonstrate that frequent alternating aeration with a short duration (aeration of the upper-layer aerators for 30 min + aeration of the lower-layer aerators for 0.6 min) can achieve better mixing of microorganisms and pollutants, and lead to a good nitrogen removal efficiency of over 90%. This new reactor presents a good TN removal performance in treating real domestic wastewater with an effluent TN concentration lower than 10 mg/L without external carbon source addition.