SURFACTANT-AIDED DISPERSED AIR FLOTATION AS A HARVESTING AND PRE-EXTRACTION TREATMENT FOR CHLORELLA SACCHAROPHILA

Abstract

Microalgae are a promising feedstock for the production of biodiesel and other value added products, however, this biotechnology is hampered by high processing costs. An approach for reducing these expenses is to combine two or more processes in one technique. Thus, this thesis aimed to investigate an effective means for microalgae harvest, pre-treatment and product extraction, suitable for large scale. From comparative analyses, surfactant-aided dispersed air flotation was identified as a promising microalgae harvesting technique, with lower energy requirements compared to other techniques, and with potential to also contribute to cell wall disruption. Supercritical CO2 (SC-CO2) lipid extraction was determined most suitable as it is considered an environmentally friendly approach which requires only limited post processing of the extract. Experimental investigation of these techniques on Chlorella saccharophila showed that surfactant-aided dispersed air flotation with surfactant CTAB resulted in a recovery and enrichment ratio of 94 % and 13, respectively, with an improvement in total suspended solids (TSS) from 0.02 to 0.30% (w/w). Although, a secondary centrifugation step was necessary to further concentrate the biomass to a TSS of 3.6% (w/w), the use of dispersed air flotation prior to secondary treatment resulted in a 14 fold reduction in working volume, which is significant as it utilizes up to 500 times less energy compared to centrifugation. SC-CO2 lipid extraction from harvested biomass resulted in maximum fatty acid methyl ester (FAME) yields of 20.4% (dcw) using a reaction time of 90 min, 3500 psi, 73°C and 89% moisture (w/w). However, good FAME yields (16.5% dcw) could also be obtained when the run time was reduced to 30 min and the biomass was used without drying. These conditions are more industrially favorable, as they would eliminate drying expenses and potentially reduce the operational costs by processing three times the biomass within 90 min. The presence of surfactant CTAB improved the recovery of FAME in C. saccharophila using SC-CO2 extraction, and a hold time of 24 h after harvest further improved yields. This demonstrates how CTAB can simultaneously work to improve microalgae harvest and product recovery, potentially improving the process economics by process reduction.