Scaling of the cultivation and flocculation of marine microalgae Nannochloropsis oculata
Microalgae are attracting researchers' interest in the production of food, feed, chemicals and biofuels. Nannochloropsis oculata is a marine species with a high growth rate, tolerates wide environmental conditions and can produce more than 50% of its dry weight in the form of lipids. Currently, commercial cultivation of microalgae is carried out in open systems because closed systems have a high production cost. However, in open systems it is not possible to control environmental parameters, which reduces their productivity. In Chapter 1 of this Thesis, a semi-closed system was compared with open systems, on a pilot scale (1,200 L). The semi-closed system consisted of circular tanks installed in an agricultural greenhouse, which provided better conditions for the cultivation ofN. oculata , mainly in low temperature and high rainfall seasons. However, although it is relatively easy to grow microalgae, the collection of biomass is one of the main bottlenecks for its large-scale production, responsible for up to 30% of the total cost. Flocculation is a low-cost technology proposed for the concentration of microalgae. Thus, the collection of N. oculata by flocculation was also studied. In Chapter 2, twenty-five natural and synthetic polymers of low and high molecular weight and with different charge density were analyzed. Comparing the results with Chlorella vulgaris, a freshwater species, it was observed that only natural polymers were efficient for both species, while synthetic polymers showed low efficiency for N. oculata. In general, increasing the charge density of the polymers resulted in an increase in efficiency. Comparing cost and performance, natural polymers showed the best results. In Chapter 3, the best synthetic and natural polymers were selected and the effects of different factors were assessed. In general, the presence of organic matter affected everyone's efficiency, while salinity and pH affected synthetic and natural polymers, respectively. The dose effect was observed only in synthetic polymers, where the increase resulted in a decrease in efficiency.None of the tested polymers showed toxicity toN. oculata . However, as they are not affected by salinity, only natural polymers were recommended for the species. In Chapter 4, N. oculata flocculation was scaled using a natural polymer. There was no difference between the results of the bench experiments (300 mL) and the pilot scale (250 L). However, despite the excellent results obtained previously with synthetic water, the natural polymer showed a decrease in efficiency when natural water was used. By reducing salinity from 30 to 10, the efficiency of the polymer increased from 50% to 98%. The results obtained indicate that the staggering of the cultivation and flocculation of N. oculata was reached. However, future studies must be carried out to optimize the flocculation efficiency ofN. oculata using natural seawater.