Master of Science in Biological and Agricultural Engineering (MSBAE)
Biological and Agricultural Engineering
Separation of microalgal cells from culture medium is a challenging phase of algal production. Application of microfiltration as Stage 1 harvesting technology to pre-concentrate algal suspension from 0.2 g-dry/L to 10 g-dry/L (1% solids) prior to the final dewatering stage by centrifugation (Stage 2) was studied in this research. A bench-scale filtration setup with three tubular crossflow membrane modules (pore sizes: 0.05 µm, 0.1 µm, 0.5µm) equipped with data acquisition systems was constructed. In phase-1 experiments, the influence of algal species and the size of particles in the culture media on the filterability of microalgae were studied. It was found that Nannochloris sp. with the biggest particles yielded highest average fluxes with all the three tested membranes. The selected algal species all yielded highest fluxes when filtered by the membrane with the smallest pore size (0.05 µm). Phase-2 experiments were aimed at maximizing the permeation flux by optimizing the transmembrane pressure (TMP) and crossflow velocity (CFV). The highest steady state flux was obtained at TMP of 172.4 kPa (25 psi) and CFV of 5.53 m/s, at a feeding flow rate of 42 L/min. In the final step, algal culture was concentrated to 10.89 g-dry/L (~1.1% solid), reaching a 54.45X volumetric reduction. Air-assisted backwash was employed as a physical flux enhancement technique. The most effective backwashing regime involved three periodic backwashes per hour. This frequency of backwashing enhanced the flux by 32%. For a maximum flux scenario, filtration of Nannochloris sp. yielded a maximum permeation flux of 326.04 ± 4.14 L/m2.h. At the maximum flux setting, the pumping energy required for a large scale microfiltration process was estimated to be 5.11 kWh/m3 of pond water. However the same system required much lower energy at lower CFVs. For the TMP of 137.9 kPa and at the lowest CFV (3.55 m/s), the pumping energy reached the minimum 1.18 kWh/m3, which was comparable to the energy requirement of centrifugation. Despite the high operational costs of the tested microfiltration system in both maximum flux and minimum energy scenarios, it can be a valuable Stage 1 harvesting technology for specific applications that cannot tolerate chemical coagulant or those that require a high degree of purity.
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Khademi, Mostafa, "Application of Tubular Crossflow Microfiltration in Harvesting Microalgae" (2014). LSU Master's Theses. 2458.