Treatment of liquid effluents is a serious challenge owing to the high stability and colloidal nature of the particles. In many applications, microbubbles (< 150 µm) are employed for separation purposes due to their buoyancy and increased surface area to volume ratio. This property has been exploited in the water treatment industry for separation in a process known as dissolved air flotation (DAF). Though practically efficient, the process is energy intensive operating at >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.
