Comparison of flotation techniques to harvest microalgae for aquaculture

Aquaculture is one of the fastest-growing food production sectors in the world and microalgae play a very important role. Microalgae are used as feed in all stages of growth of bivalves, in some larval stages of crustaceans and fish, and also for feeding zooplankton. Moreover, microalgae can be employed to treat wastewater from aquaculture and even for biotechnological applications, such as production of bioactive compounds.

However, harvesting is still one of the major obstacles to their low-cost massive production. The methods currently employed have limitations due to the high culture volumes that must be handled. Centrifugation, for example, despite being highly efficient is also highly expensive. If the volume to be centrifuged could be reduced, this would reduce costs. Flotation is an attractive pre-concentration step as the biomass can be readily skimmed from the surface, contrarily to sedimentation. This project is intended to evaluate different flotation techniques to harvest microalgae commonly employed in aquaculture (Chaetoceros muelleri, Conticribra weissflogii, Nannochloropsis oculata and Thalassiosira pseudonana).

As microalgae differ in cell properties and dissolved organic matter released, all of which can influence the flotation process, it is important to characterize each species. For that, cells size, shape and zeta potential will be characterized. The dissolved organic matter will be characterized for organic carbon, protein and carbohydrate contents by LC-OCD, TOC and FEEMs. To investigate the effect of organic matter on flotation, cultures with and without will be compared. Flotation will be achieved by chemical flocculation, comparing synthetic polyacrylamide and natural tannin polymers. The polymers will be characterized by charge density, zeta potential, surface tension, dispersity and molecular weight. Dissolved and dispersed air flotation and foam fractionation will be employed and evaluated by harvesting efficiency, concentration factor and bubble size. Flotation will be compared to sedimentation and a cost analysis will compare the economic viability of these techniques. By analyzing the residual flocculant in the water, we will evaluate the environmental sustainability of flotation.