DEVELOPMENT OF A FLUIDIC OSCILLATOR-DRIVEN FLOTATION SYSTEM

Treatment of liquid effluents is a serious challenge owing to the high stability and colloidal nature of the particles. In many applications, microbubbles (5 bars and consequently consuming ~90% of the total energy required in water purification plants. Other approaches in generating microbubbles for separation are not without challenges. One example is dispersed air flotation, which generates bubbles several orders of magnitude larger than the bubble exit pore and consequently unsuitable for flotation of these colloidal particles. These two concerns have been addressed in this research with the designing and development of a microbubble diffuser driven by a fluidic oscillator to facilitate microbubble generation suitable for flotation as well as investigating its performance for flotation applications. This fluidic oscillator converts continuous air supply into oscillatory flow with a regular frequency to generate bubbles of the scale of the exit pore. Bubble characterisation results showed that average bubble size generated under oscillatory air flow state from a 50 µm pore membrane was 86 µm, ~ twice the size of the diffuser pore size of 38 µm. In contrast, continuous airflow at the same rate through the same diffusers yielded an average bubble size of 1059 µm, 28 times larger than the pore size. In the first application, fluidic oscillator generated microbubbles were investigated for the separation of emulsified oil using Aluminium sulphate as the coagulant. The effect of surfactant concentration on oil droplet size was investigated. It was found that oil droplet size varied inversely proportional to surfactant concentration. In addition, it was found that the oil removal efficiency also depends on the surfactant concentration. The maximum oil removal efficiency by Microflotation was found to be 91% under lowest surfactant concentration tested (0.3 wt%) whilst at highest surfactant concentration used (10 wt%); lowest recovery efficiency (19.4%) was recorded. In the second application, the separation of algal cells under fluidic oscillator generated microbubbles was investigated by varying metallic coagulant types, concentration and pH. Best performances were recorded at the highest coagulant dose (150 mg/L) applied under acidic conditions (pH 5). Amongst the three metallic coagulants studied, ferric chloride yielded the overall best result of 99.2% under the optimum conditions followed closely by ferric sulphate (98.1%) and aluminium sulphate with 95.2%. The third application investigated the performance of Microflotation for the recovery of yeast cells from their growth medium at different pH levels, flocculant dose and varying bubble sizes. In this study, the food-grade-constituent- Chitosan was used as the flocculant. Results reaching 99% cell recovery were obtained under various conditions examined. Bubble size profiling showed an increase in average bubble size with diffuser pore size. Also, cell recovery efficiency was a function of both bubble size and particle size (cell size). For smaller particles (<50 μm), relatively smaller bubbles (<80 μm) were found to be more effective for recovery, otherwise, relatively larger bubbles (80-150 μm) proved to be efficient in recovering larger particles (particle size: ~250 μm). Acidic and neutral pHs were effective in separation as hydrophobic particles were formed. As pH tends towards alkalinity, flocs become more hydrophilic, leading to low recovery from the aqueous solution. In addition, separation efficiency was dependent on flocculant dose as increase in concentration improved flocculation and consequently, yeast recovery. However, above a critical concentration, overdosing occurred and inadvertently, recovery efficiency decreased. The results compare well with conventional dissolved air flotation (DAF) benchmarks, but has a highly turbulent flow, whereas Microflotation is laminar with several orders of magnitude lower energy density

Development of an optical microscopy system for automated bubble cloud analysis

Recently, the number of uses of bubbles has begun to increase dramatically, with medicine, biofuel production, and wastewater treatment just some of the industries taking advantage of bubble properties, such as high mass transfer. As a result, more and more focus is being placed on the understanding and control of bubble formation processes and there are currently numerous techniques utilized to facilitate this understanding. Acoustic bubble sizing (ABS) and laser scattering techniques are able to provide information regarding bubble size and size distribution with minimal data processing, a major advantage over current optical-based direct imaging approaches. This paper demonstrates how direct bubble-imaging methods can be improved upon to yield high levels of automation and thus data comparable to ABS and laser scattering. We also discuss the added benefits of the direct imaging approaches and how it is possible to obtain considerable additional information above and beyond that which ABS and laser scattering can supply. This work could easily be exploited by both industrial-scale operations and small-scale laboratory studies, as this straightforward and cost-effective approach is highly transferrable and intuitive to use

Intensification of yeast production with microbubbles

Yeast requires and consumes a high amount of oxygen rapidly during growth. Maintaining yeast cultures under sufficient aeration, however, is a significant challenge in yeast propagation. Due to their high surface area, microbubbles are more efficient in mass transfer than coarse bubbles. The performance of an airlift loop bioreactor equipped with a fluidic oscillator generated microbubbles in yeast propagation is presented here. The approach is compared with a conventional bubble generation method that produces coarse bubbles. Dosing with microbubbles transferred more oxygen to the cultures, achieving non-zero dissolved O2 levels and consequently, eliminating the starvation state of yeast in contrast to coarse bubble sparging. The average cell growth yield obtained under microbubble sparging reached 0.31 mg/h (±0.02) while 0.22 mg/h (±0.01) was recorded for cells grown with coarse bubbles during the log phase. The percent difference in average growth yield after 6 hours was 18%. Additionally, the use of microbubbles in yeast harvest from growth medium proved effective, yielding >99% cell recovery. The result of this study is crucial for the biofuel industry but also, the food, nutraceutical and pharmaceutical industry for which end product purity is premium

Exploiting ozonolysis-microbe synergy for biomass processing: Application in lignocellulosic biomass pretreatment

Pretreating lignocellulosic biomass is an energy and time consuming process. This study presents an alternative pretreatment technique, which explores a synergistic approach between ozonolysis and cellulolytic microorganism-Pseudomonas putida at room temperature. Ozone is a strong oxidative agent that reacts with lignin by attacking the carbon-carbon double bonds, while P. putida preferentially hydrolyses the exposed cellulolytic parts of the biomass to simple sugars. The results from SEM and FTIR show a significant reduction in lignin and cellulose contents, leading to relatively high sugar recovery. The glucose concentration increases coincidentally with the ozonation duration and After 24 h however, the concentration reached 1.1 mg/ml, a 323% increase compared with results after 2 h. Increasing the ozonation time to 24 h reduced the biological pretreatment time by 50% but crucially, increases microbial biomass. This approach has potentially high ramifications particularly for industries exploiting lignocellulosic biomass as a feedstock for bioethanol production

In-situ disinfection and a new downstream processing scheme from algal harvesting to lipid extraction using ozone-rich microbubbles for biofuel production

The scaling up and downstream processing costs of biofuels from microalgae are major concerns. This study focuses on reducing the cost by using energy efficient methods in the production of microalgae biomass and the downstream processes (biomass harvesting and lipid extraction). Ozonation of Dunaliella salina (green alga) and Halomonas (Gram-negative bacterium) mixed cultures for 10 min at 8 mg/L resulted in a reduction in the bacterial contaminant without harming the microalga. Harvesting of D. salina cells through microflotation resulted in a 93.4% recovery efficiency. Ozonation of the harvested microalgal cells for 60 min produced three main saturated hydrocarbon compounds (2-pentadecanone, 6, 10, 14-trimethyl; hexadecanoic acid; octadecanoic acid) consisting of 16 to 18 carbons. By systematically switching the carrier gas from CO2 to O3, the microbubble-driven airlift loop bioreactor (ALB) delivers nutrient to the culture and in-situ disinfection respectively. Further, modulating the bubble size to match particle size ensures recovery of the cells after culture. All three key operations (disinfection, harvesting and lipid extraction) are assembled in a scalable, relatively energy efficient process

Microflotation performance for algal separation

The performance of microflotation, dispersed air flotation with microbubble clouds with bubble size about 50 µm, for algae separation using fluidic oscillation for microbubble generation is investigated. This fluidic oscillator converts continuous air supply into oscillatory flow with a regular frequency to generate bubbles of the scale of the exit pore. Bubble characterization results showed that average bubble size generated under oscillatory air flow state was 86 µm, approximately twice the size of the diffuser pore size of 38 µm. In contrast, continuous air flow at the same rate through the same diffusers yielded an average bubble size of 1,059 µm, 28 times larger than the pore size. Following microbubble generation, the separation of algal cells under fluidic oscillator generated microbubbles was investigated by varying metallic coagulant types, concentration and pH. Best performances were recorded at the highest coagulant dose (150 mg/L) applied under acidic conditions (pH 5). Amongst the three metallic coagulants studied, ferric chloride yielded the overall best result of 99.2% under the optimum conditions followed closely by ferric sulfate (98.1%) and aluminum sulfate with 95.2%. This compares well with conventional dissolved air flotation (DAF) benchmarks, but has a highly turbulent flow, whereas microflotation is laminar with several orders of magnitude lower energy density. Biotechnol. Bioeng. 2012; 109:1663–1673. © 2012 Wiley Periodicals, Inc

Enhanced Mass Transfer in Microbubble Driven Airlift Bioreactor for Microalgal Culture

In this study, the effect of microfluidic microbubbles on overall gas-liquid mass transfer (CO2 dissolution and O2 removal) was investigated under five different flow rates. The effect of different liquid substrate on CO2 mass transfer properties was also tested. The results showed that the KLa can be enhanced by either increasing the dosing flowrate or reducing the bubble size; however, increasing the flow rate to achieve a higher KLa would ultimately lower the CO2 capture efficiency. In order to achieve both higher CO2 mass transfer rate and capture efficiency, reducing bubble size (e.g. using microbubbles) has been proved more promising than increasing flow rate. Microbubble dosing with 5% CO2 gas showed improved KLa by 30% - 100% across different flow rates, compared to fine-bubble dosing. In the real algal culture medium, there appears to be two distinct stages in terms of KLa, divided by the pH of 8.4

Aerator design for microbubble generation

Fine bubbles are a key component in improving the performance of gas-liquid reactors, particularly in situations where reactions are mass transfer limited. Many aerator types exist for different reactor applications; however conventional aerators are mostly suited to coarse bubble generation. A new aerator suitable for microbubble generation by fluidic oscillation has been designed and tested with the view of getting a uniform bubble distribution across the aerator. Microbubbles generated from various membrane pore sizes and oscillation frequencies were characterized for this aerator to determine the optimum operating parameters. It was evident that the introduction of a flow distributor plate to the plenum chamber improved gas distribution from the inlet to the porous membrane leading to uniform bubble generation across the entire aerator The resultant average bubble size from this new design under oscillatory flow was found to be approximately 2-3 times the membrane pore size. This outcome has a great potential to promote the efficiency of multiphase reactors where mass transfer plays a key role

Exploiting microbubble-microbe synergy for biomass processing: Application in lignocellulosic biomass pretreatment

The potential of lignocellulosic biomass as a sustainable biofuel source is substantial. The development of an efficient and cost effective pretreatment approach remains challenging. In this study, we have explored a new, relatively cheap pretreatment option that works at ambient temperatures. By using microbubbles generated by fluidic oscillation, free radicals around the gas-liquid interface of the microbubble readily attack and degrade lignocellulosic biomass, rendering it more amenable to digestion. The combination of microbubbles and Pseudomonas putidada robust delignification and cellulolytic microbe, further improved biomass degradation and consequently, increased glucose production from wheat straw in comparison to solo pretreatment of the biomass with microbubbles and Pseudomonas putida respectively. The microbubble-microbe approach to make biomass more amenable to sugar production is potentially a valuable alternative or complementary pretreatment techniqu

Harvesting Environmental Microalgal Blooms for Remediation and Resource Recovery: A Laboratory Scale Investigation with Economic and Microbial Community Impact Assessment

A laboratory based microflotation rig termed efficient FLOtation of Algae Technology (eFLOAT) was used to optimise parameters for harvesting microalgal biomass from eutrophic water systems. This was performed for the dual objectives of remediation (nutrient removal) and resource recovery. Preliminary experiments demonstrated that chitosan was more efficient than alum for flocculation of biomass and the presence of bacteria could play a positive role and reduce flocculant application rates under the natural conditions tested. Maximum biomass removal from a hyper-eutrophic water retention pond sample was achieved with 5 mg·L-1 chitosan (90% Chlorophyll a removal). Harvesting at maximum rates showed that after 10 days, the bacterial diversity is significantly increased with reduced cyanobacteria, indicating improved ecosystem functioning. The resource potential within the biomass was characterized by 9.02 μg phosphate, 0.36 mg protein, and 103.7 μg lipid per mg of biomass. Fatty acid methyl ester composition was comparable to pure cultures of microalgae, dominated by C16 and C18 chain lengths with saturated, monounsaturated, and polyunsaturated fatty acids. Finally, the laboratory data was translated into a full-size and modular eFLOAT system, with estimated costs as a novel eco-technology for efficient algal bloom harvesting