Hemaka C.H. Bandulasena

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.

Fluidic oscillator-mediated microbubble generation to provide cost effective mass transfer and mixing efficiency to the wastewater treatment plants.

Aeration is one of the most energy intensive processes in the waste water treatment plants and any improvement in it is likely to enhance the overall efficiency of the overall process. In the current study, a fluidic oscillator has been used to produce microbubbles in the order of 100 μm in diameter by oscillating the inlet gas stream to a pair of membrane diffusers. Volumetric mass transfer coefficient was measured for steady state flow and oscillatory flow in the range of 40-100l/min. The highest improvement of 55% was observed at the flow rates of 60, 90 and 100l/min respectively. Standard oxygen transfer rate and efficiency were also calculated. Both standard oxygen transfer rate and efficiency were found to be considerably higher under oscillatory air flow conditions compared to steady state airflow. The bubble size distributions and bubble densities were measured using an acoustic bubble spectrometer and confirmed production of monodisperse bubbles with approximately 100 μm diameters with fluidic oscillation. The higher number density of microbubbles under oscillatory flow indicated the effect of the fluidic oscillation in microbubble production. Visual observations and dissolved oxygen measurements suggested that the bubble cloud generated by the fluidic oscillator was sufficient enough to provide good mixing and to maintain uniform aerobic conditions. Overall, improved mass transfer coefficients, mixing efficiency and energy efficiency of the novel microbubble generation method could offer significant savings to the water treatment plants as well as reduction in the carbon footprint.

Azimuthally Oscillating Membrane Emulsification for Controlled Droplet Production

A novel membrane emulsification (ME) system is reported consisting of a tubular metal membrane, periodically azimuthally (tangentially) oscillated with frequencies up to 50 Hz and 7 mm displacement in a gently cross flowing continuous phase. A computational fluid dynamics (CFD) analysis showed consistent axial shear at the membrane surface, which became negligible at distances from the membrane surface greater than 0.5 mm. For comparison, CFD analysis of a fully rotating ME system showed local vortices in the continuous phase leading to a variable shear along the axis of the membrane. Using an azimuthally oscillating membrane, oil-in-water emulsions were experimentally produced with a median diameter of 20–120 μm, and a coefficient of variation of droplet size of 8%. The drop size was correlated with shear stress at the membrane surface using a force balance. In a single pass of continuous phase, it was possible to achieve high dispersed phase concentrations of 40% v/v.

Microalgae recovery by microflotation for biofuel production using metallic coagulants

Background: Harvesting algal biomass is an important unit operation in the production of biofuel from algae. However, many of the known techniques available for harvesting and dewatering microalgal biomass are energy intensive and in some cases intrusive and inefficient. Here we show that microflotation mediated by fluidic oscillation is an approach that differs from dissolved air flotation by the nonintrusive laminar flow approach, as well as low energy consumptions, and is also different from dispersed air flotation by the method of bubble generation. Results and discussion: Using three metallic coagulant types, recovery efficiencies of 99.2, 98.1 and 95.2% were obtained for ferric chloride, ferric sulfate and aluminum sulfate, respectively. The benchmarks in the literature for dissolved air flotation are 40–98%. Conclusion: Biofuel production from microalgae can be facilitated by the improvement of a key unit operation such as harvesting and dewatering. The study outcome strongly recommends the technique as a technology for microalgal harvesting.

An inverse method for rheometry of power-law fluids

This paper is concerned with the determination of the constitutive viscous parameters of dilute solutions of xanthan gum by means of an inverse method used in conjunction with finite element modeling of the governing system of partial differential equations. At low concentrations xanthan gum behaves as a shear-thinning, power-law non-Newtonian fluid. Finite element modeling is used to simulate the pressure-driven flow of xanthan gum solutions in a microchannel T-junction. As the flow is forced to turn the corner of the T-junction a range of shear rates, and hence viscosities, is produced. It is shown that the statistical properties of the velocity field are sensitive to the constitutive parameters of the power-law model. The inverse method is shown to be stable and accurate, with measurement error in the velocity field translating to small errors in the rheological parameter estimation. Due to the particular structure of the inverse map, the error propagation is substantially less than the estimate from the Hadamard criterion.

An inverse methodology for the rheology of a power-law non-Newtonian fluid

The current paper presents a novel methodology for calculating the rheological para-meters for dilute aqueous solutions of a power-law non-Newtonian fluid, xanthan gum (XG). Previous studies have verified the fidelity of finite-element modelling of the Navier—Stokes equations for reproducing the velocity fields of XG solutions in a microfluidic T-junction with experimental observations obtained using micron resolution particle image velocimetry (μ-PIV). As the pressure-driven fluid is forced to turn the corner of the T-junction, a range of shear rates, and therefore viscosities, are produced within the flow system. Thus, a setup that potentially establishes the rheological profile of XG from a single experiment is selected. An inverse method based on finding the mapping between the statistical moments of the velocity field and the constitutive parameters of the viscosity profile demonstrated that such a system could potentially be used for the design of an efficient microfluidic rheometer. However, μ-PIV technology is expensive and the equipment is bulky. The current paper investigates whether different flow features could be used to establish the rheological profile.

Dielectric barrier discharge plasma microbubble reactor for pretreatment of lignocellulosic biomass

A novel lignocellulosic biomass pretreatment reactor has been designed and tested to investigate pretreatment efficacy of miscanthus grass. The reactor was designed to optimize the transfer of highly oxidative species produced by dielectric barrier discharge plasma to the liquid phase immediately after generation, by arranging close proximity of the plasma to the gas‐liquid interface of microbubbles. The reactor produced a range of reactive oxygen species and reactive nitrogen species, and the rate of production depended on the power source duty cycle and the temperature of the plasma. Ozone and other oxidative species were dispersed efficiently using energy efficient microbubbles produced by fluidic oscillations. A 5% (w/w) miscanthus suspension pretreated for 3 h at 10% duty cycle yielded 0.5% acid soluble lignin release and 26% sugar release post hydrolysis with accelerated pretreatment toward the latter stages of the treatment demonstrating the potential of this approach as an alternative pretreatment method.

Quantification of the Ozone Dose Delivered into a Liquid by Indirect Plasma Treatments: Method and Calibration of the Pittsburgh Green Fluorescence Probe

Determination of the ozone dose delivered into liquids by plasma systems is of importance in many emerging plasma applications, such as plasma medicine. Quantification of this dose remains extremely challenging due to the complex physico-chemical processes encountered in the gas plasma, the plasma–liquid interface and the liquid itself. Chemical probes have the potential to address the limitation of more traditional plasma diagnostic techniques but most commercial chemical probes are not specific enough to be used in plasma applications. Here we report on the development of a method for the quantification of the ozone delivered into a liquid using Pittsburgh Green, a novel ozone-selective fluorescence probe. Entailed within this work is a method for the preparation of the probe solutions, the design of a calibration system and a normalized calibration curve correlating fluorescence intensity to actual ozone dose delivered to the liquid. This enables the quantitative comparison of ozone measurements performed with different spectrofluorometers and in different institutions.