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Optimization of growth separation and drying processes of microalgae cultures of Stichococcus sp. strain

Tsantopoulou Eleftheria

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Summary

This thesis compares the different growth, separation and drying conditions of the microalgae strain Stichococcus sp. in order to find the best methods to maximize the production of dry biomass and bio-products (proteins, lipids, carbohydrates, total chlorophyll, β-carotene). In addition, it examines whether the methods are energetically beneficial.Two types of the Stichococcus sp. strain, the wild type and the mutant strain, were tested. Microalgae were grown in beakers and in a bioreactor (wild strain only) in both cases using a sandblasted glass bottom. During the growth of the microalgae in the beakers, tests were performed regarding the type of lighting (steady, flashing), NaNO3 concentration (0.75 g L-1, 0.25 g L-1) and nitrogen deficiency (3 days). It was shown that the production of dry biomass and total bio-products is optimized when the growth conditions are: constant lighting and 0.75 g L-1 NaNO3. It is worth noting that the type of lighting and lack of nitrogen do not have a decisive role in the final results. Growth in the bio-reactor was done under flash lighting conditions and 0.75 g L-1 NaNO3. The dry biomass concentration from the bio-reactor is 49.17 g L-1 and the total bio-products is 41.70 g L-1.Then, to optimize biomass separation, a comparison was made between centrifugation, filtration, flocculation with biomass removal from an immobilized cell culture system. Biomass scraping, although with a low recovery (0.22 g/150 ml), appears to be the most energy efficient, requiring the lowest electricity consumption (0.7 kWh kg-1), and the culture supernatant could be reused as a pre-culture in subsequent cycles cultivation.To find the optimal biomass drying method, three ways, convection, solar drying and freeze-drying, were tested. Convection proved to be the fastest way (5 h) but it was also the way with the highest energy requirement (58.5 kWh kg-1). On the other hand, solar drying requires 6.5 h and zero energy. Freeze-drying is a method that is completed in 8 h and has an energy requirement of 49.7 kWh kg-1. Neither method can be called optimal with these results alone, as convection is energy inefficient, solar drying is a method you cannot rely on year-round, and freeze-drying is time-consuming.Thus, the final concentrations of each bio-product separately resulting from each of the drying methods are taken into account. It is observed that the maximum production of lipids (0.26 g g-1 dry biomass), total chlorophyll (6.99 mg g-1 dry biomass) and β-carotene (1.44 mg g-1 dry biomass) comes from the freeze-drying method. Then, the optimal concentration of carbohydrates comes from convection (0.45 g g-1 dry biomass) while it seems that the production of proteins is not affected by the drying method.

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