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Optimization of microalgae growth and separation processes for the production of high added value products

Makaroglou Georgios

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Year 2023
Type of Item Doctoral Dissertation
Bibliographic Citation Georgios Makaroglou, "Optimization of microalgae growth and separation processes for the production of high added value products", Doctoral Dissertation, School of Chemical and Environmental Engineering, Technical University of Crete, Chania, Greece, 2023
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Microalgae are unicellular photosynthetic microorganisms that can grow in diverse environments, by developing adaptation mechanisms. They are found in freshwater, saline water, ice, soil, and even in wastewater. According to their pigmentation, arrangements of photosynthetic membranes or other morphological features, microalgae are classified as diatoms, green algae, golden algae, and cyanobacteria. Microalgae share similar growth mechanisms as terrestrial plants, with the difference that they are cellular microorganisms. They use sunlight to sequester CO2 dissolved in the growth medium for biomass and O2 production. Cultivation of microalgae takes place in open ponds or photobioreactors, comprising of various types. Sunlight or artificial light (e.g., fluorescent, LED) can be used for boosting photosynthesis. However, light can be a limiting factor for microalgae growth as high chlorophyll causes a “shadow effect” to microalgae growing near the center of photobioreactors. Microalgae have various applications such as biofuels, food supplements, cosmetics, and animal feed, being rich in proteins, lipids, carbohydrates, fatty acids, pigments, and vitamins. Production of microalgae biomass could be boosted by channeling CO2 supply during cultivation. In this way, food productivity can be increased and CO2 from flue gas be decreased. Furthermore, industrial flue gas typically contains 4-14% or more (v/v) CO2 concentration and toxic compounds such as SOx, NOx, and trace elements, which are emitted at high flow rate, having high temperatures ranging from 80 to 120 ℃ or above. For this reason, microalgae should be able to withstand this extreme environment, to mitigate CO2. Flue gas from natural gas could be used to lessen the negative effects on microalgae growth. Τhe present PhD thesis consists of four main research parts. The initial scope was the mutation of wild-type Stichococcus sp. microalgae strain which led to a new strain with reduced chlorophyll content, and increased biomass and lipids productivity (namely EMS1). Mutation was achieved by the chemical reagent ethyl methanesulfonate (EMS) which causes random mutations in the DNA. The purpose of the mutation is that high chlorophyll can inhibit light penetration in the inner part of the culture, cultivated in photobioreactors. Thus, leading to reduced biomass productivity. The mutant strain EMS1 showed 51% less chlorophyll, 12% higher biomass, and 45% higher lipids productivity, compared to the wild-type.The second scope was to further investigate the strains and select one which will be optimized on small scale with the ultimate goal to be scaled up in pilot photobioreactors and fixate CO2 from industrial flue gas. Experiments on wild-type and EMS1 Stichococcus sp. strains were conducted at laboratory scale (beaker vessels, 150 mL culture volume) with Bold’s Basal growth medium diluted in artificial seawater. Microalgae were grown attached on horizontal sandblasted glass tiles placed at the bottom of the vessels, as a measure to minimize harvesting costs. Agitation, CO2 capture efficiency, cultivation duration, nitrogen starvation duration, biomass, and bio-products production were examined. It was found that the absence of agitation had a minor effect on the reduction of biomass production. The addition of CO2 to reach 5% concentration in the gaseous phase resulted in up to 300% increase in biomass, while microalgae required 25 days to grow sufficiently. Also, the application of nitrogen starvation three days before harvesting enhanced intracellular lipids production. Screening of the two strains resulted in the selection of mutant EMS1 strain for optimization of its biomass and bio-products production (i.e., lipids, pigments, proteins, and carbohydrates). Taguchi’s Design of Experiments (DOE) was implemented to find the least required experimental runs. Also, microalgae were fed with synthetic flue gas (for CO2 fixation), to simulate real conditions from combustion of natural gas from industrial power plants. Five optimal combinations of the growth parameters were found from the statistical analysis of the data, maximizing each of the measured characteristics. Changes in aeration rate did not have a significant effect in biomass and total bioproducts production. Higher illuminance intensity (6,600 lux), continuous lighting and higher NaNO3 concentration (0.75 g L-1) resulted in higher yield in biomass and bio-products. Maximum value for biomass was equal to 45.7 ± 1.3 g m-2, while bio-products and their corresponding values were: lipids 11.6 ± 0.4 g m-2, pigments 0.22 ± 0.02 g m-2, proteins 9.5 ± 0.5 g m-2, carbohydrates 19.0 ± 1.5 g m-2, and total bio-products 34.8 ± 2.2 g m-2. Stichococcus sp. was found to be an excellent carbohydrates producer, having content up to 52%. Lipids production constituted up to 40% and the total bio-products yield was up to 91% of the biomass. Flashing light (1,000 Hz) effect triggered chlorophyll production by an average 8%, while three-day nitrogen starvation increased lipids production by an average 22%. Experimental maximum values were compared with regression analysis models for each of the five optimal conditions. Theoretical and experimental results were similar, with R2 of the models ranging from 84 to 99%. Following the lab-scale experiments, EMS1 Stichococcus sp. strain was scaled up in flat-panel photobioreactor (15 L culture volume) with the microalgae immobilized on sandblasted glass tiles. The experimental conditions were selected for biomass maximization and results showed biomass production equal to 49.2 g m-2, while total bio-products accounted for 41.7 g m-2. Microalgae were also fed with synthetic flue gas. The third scope of the PhD thesis was to test Stichococcus sp. in real conditions with an industrial flue gas supply for CO2 mitigation. Experiments were conducted in Lavrio power station, located in the region of Attica in Greece. EMS1 Stichococcus sp. strain adapted to this extreme environment and produced biomass values up to 50.5 g m-2 and the total bio-products measured were equal to 39.8 g m-2.As a final scope of the present thesis, dewatering and drying processes of microalgae grown i) suspended and ii) immobilized were evaluated. Cultures of suspended microalgae were harvested by vacuum filtration, centrifugation, and chitosan flocculation coupled with the aforementioned processes. Immobilized microalgae were harvested by scraping of the sandblasted glass tiles, as a means of cultivating microalgae in specific places, thus lowering harvesting costs. Based on the experimental results, even though biomass scraping showed lower recovery rates by 18%, compared to harvesting of suspended microalgae, it showed the lowest energy consumption (0.7  0.1 kWh kg-1 of dry biomass), which verified the scope of cultivating immobilized biomass. Growth medium removed from the bulk liquid of immobilized microalgae cultures could possibly be retained for future use, as it contains a portion of the previous culture and nutrients. Harvested biomass was further processed by drying of the excess liquid. Convective, solar, and freeze drying processes were tested for their efficacy and their influence on microalgae biomass and bio-products. Convective drying required the greater electrical energy consumption (58.5 kWh kg-1 of wet biomass), but it dried the biomass in the shorted period (5.5 hours). Extracted bio-products (i.e., lipids, pigments, proteins, and carbohydrates) were found to be linked with the drying processes. Lipids and total chlorophyll recovery were greater after the application of freeze-drying with their values equal to 0.26  0.01 g g-1 and 0.70  0.0410-2 g g-1 of biomass. Carbohydrates recovery was maximized by applying solar and convective drying, with a maximum value of 0.45  0.01 g g-1 biomass. In the case of proteins, each drying process showed similar recovery rates with an average value equal to 0.19  0.02 g g-1 biomass. The above indicate that drying processes should be considered for maximization of the end-products. The main conclusions that derived from the present PhD thesis is

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