Cultivation of seven different species of marine microalgae using simulated flue gas mimicking effluents from paper mills as carbon source

Abstract

The use of fossil fuels is unsustainable, both due to limited supply and also due to large emissions of CO2 with effects on global warming. Biofuels is a viable alternative but can, as produced today, only provide a limited amount of fuels needed. Biofuels are currently mainly derived from terrestrial plants, requiring large amounts of arable land. Biofuels from microalgae on the other hand do not require arable land and can, theoretically, replace fossil fuels completely. In addition, biofuels from microalgae could use flue gases from industry as carbon source and waste waters as growth medium for nutrient supply. In Sweden, the large paper industry is a particularly interesting potential provider, both due to its extensive carbon dioxide emissions and available waste waters containing nitrogen and phosphorous. In this thesis, a simulated flue gas mimicking that of paper mills with 15 % CO2, 100 ppm NO and 10 ppm SO2 was used to cultivate seven species of marine microalgae and to study its effect on growth and cellular composition. Of the species tested, Nannochloropsis salina was identified to be the most promising candidate due to its tolerance to the simulated flue gas; the specific growth rate obtained from optical density measurements was barely affected by the increasing CO2 concentrations. The doubling time calculated from optical density was sustained at 79 hours throughout the experiment. However, the doubling time obtained from cell counts almost doubled from 42 to 87 hours when NO and SO2 was switched on. For the other species a general decline in specific growth rate could be observed with increasing CO2 concentrations. Dunaliella salina and Dunaliella tertiolecta were the species with the lowest doubling times, around 25 hours, but these were only sustained in the beginning of the experiments with CO2 levels between 0 - 8 %. The cell composition analyses performed showed to be problematic, possibly due to the marine medium or too low cell content in the samples. The constituents of the simulated flue gas all contribute to a lower pH and the marine species used in this study was very sensitive to pH below 7 as seen by decreased biomass levels in the cultures at these pH levels. Hence, the pH must be controlled to avoid cultivation collapse. In this study, NaOH additions and Hepes buffer was used to maintain pH around 7. The results indicate that the simulated flue gas is stressful for the microalgae tested but by using an adaptation period the gas tolerance might be improved. The waste waters of paper mills in Sweden contain on average levels of nitrogen and phosphorous that roughly corresponds to the levels in f/2 medium which indicates that these could be used as growth medium. Two types of waste waters from Nordic Paper Bäckhammar were tested in a separate experiment but further studies is needed to establish whether marine algae can grow in it.