Development of an integrated process of membrane filtration for harvesting microalgae and recovery of high-value compounds

Abstract

The work presented in this PhD thesis aims at developing an integrated method for harvesting Dunaliella salina, both in the non-carotenogenic and in the carotenoid-rich phases, using membrane processes and centrifuges. Additionally, the ultimate goal is the recovery of high-value compounds for application in the food, cosmetic and pharmaceutical industries. Dunaliella salina is a halotolerant green microalga, which under low nitrogen and high salinity conditions, produces carotenoids. This microalga is very fragile due to lack of a rigid cell wall and, consequently, can be easily ruptured when an excessive shear stress is applied. Firstly, the pre-concentration of Dunaliella salina by membrane filtration prior to a final harvesting step by centrifugation was performed. The aim of the integrated harvesting process was to minimise the overall energy expenditure and reduce capital costs, while assuring a minimal loss of cell integrity. For the harvesting experiments the main parameters were optimised, namely: cross-flow velocity (vcross-flow), transmembrane pressure (TMP), membrane molecular weight cut-off (MWCO), backflush protocol and permeate volumetric flux (JV). The harvesting of non-carotenogenic Dunaliella salina by ultrafiltration, under controlled transmembrane pressure, allowed a final concentration factor of 5.9 and an average permeate flux of 31 L/(m2.h). The harvesting of carotenoid-rich Dunaliella salina, under controlled permeate flux, reached a concentration factor of 10, at a permeate volumetric flux of 21 L/(m2.h). The integrated process led to a reduction of Total Cost of Ownership (TCO) and energy consumption of 70% and 76%, respectively, when comparing with harvesting only with centrifuges (one-step approach). Additionally, several oxidation processes were studied for treatment of the permeate recovered, after membrane harvesting, in order to assess its recycle as cultivation medium for Dunaliella salina. The permeate was treated either by ozonation or by UV radiation, and the different types of permeate were tested as cultivation media. The results obtained indicate that the permeate treated by UV with addition of H2O2 was best suited for the growth of carotenoid-rich Dunaliella salina. Nevertheless, the use of untreated permeate seems a reasonable alternative, considering that it might be the most cost efficient solution. Finally, a Dunaliella salina biorefinery process was developed for the recovery and fractionation of added-value compounds, with a particular focus on the recovery of polar lipids. This work combined extraction with REACH solvents, a saponification reaction and organic solvent nanofiltration processes to obtain distinct fractions of compounds. Overall, high recoveries were obtained (85% of carotenoids, 86% of glycerol, 94% of polar lipids and 95% of proteins), as well as fractions with high product enrichment in polar lipids and in glycerol.