Implications of cell composition and size on the performance of microalgae ultrasonic harvesting
Date
2018
Authors
Aligata, Alyssa Jean, author
Marchese, Anthony, advisor
Quinn, Jason, advisor
Peebles, Christie, committee member
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Abstract
Substantial economic challenges exist across the value chain for microalgae-based biofuels and bioproducts. Acoustic harvesting could dramatically reduce harvesting costs and directly address current energy barriers to separating algae from growth media. This technology utilizes ultrasonic standing waves to create an acoustic radiation force that, due to differences in the acoustic properties of the cells and media, causes the microalgae cells to agglomerate and settle out of the solution. The magnitude of the acoustic radiation force is directly related to the cell radius and acoustic contrast factor (ACF), the latter of which is a function of the density and compressibility of the cell. These properties can vary widely depending on the algae species, cultivation conditions, and growth stage—all of which affect the composition of the microalgae cells (e.g., lipid, carbohydrate, protein content). In this work, two methods were used to determine the ACF of microalgal cells: 1) a property measurement approach and 2) a particle tracking approach. The first method involved experimentally measuring the size distribution, density and compressibility of the cells and calculating the ACF. The second method utilized particle tracking velocimetry and a COMSOL Multiphysics model to estimate the ACF. The ACF was characterized, using both techniques, for three species—Chlamydomonas reinhardtii, Nannochloropsis salina, and Tetraselmis chuii—as a function of dynamic cellular composition over a 2-week growth period. For C. reinhardtii the lipid content increased from 26% ± 1% to 40% ± 1% from day 3 to 9, which resulted in a 43% decrease in ACF (0.056 ± 0.003 to 0.032 ± 0.001). For N. salina the lipid content increased from 25% ± 1% to 33% ± 1% from day 3 to 10, which also resulted in a 43% decrease in ACF (0.040 ± 0.002 to 0.023 ± 0.001). For T. chuii the lipid content remained relatively stable (~10%) throughout the growth period so the ACF (~0.3) did not change significantly. ACF decreases as lipid content increases because lipids have a negative ACF in growth media, whereas carbohydrates and proteins have a positive ACF. However, cell size can have a greater impact on an algal strains' responsiveness to acoustic harvesting because the net force is proportional to Φa2. Furthermore, acoustic harvesting works best for large diameter cells, provided that those cells have a nonzero ACF. T. chuii had the largest cell diameter of approximately 12 µm, while C. reinhardtii and N. salina had cell diameters of 8.5 µm and 4.3 µm, respectively. The Φa2 values for T. chuii were approximately 50× higher than the values for N. salina, which is largely due to T. chuii cells having a diameter that is 3× the diameter of N. salina cells. Composition also contributed to the higher Φa2 values for T. chuii since these cells were composed of mostly carbohydrates and had an ACF that was an order of magnitude higher than the ACF of N. salina. This research shows that acoustic harvesting has the potential to positively impact the algal biofuels value chain through the reduction of energy required for harvesting.
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Subject
acoustic harvesting
algae biofuels
acoustic properties
acoustic contrast factor