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

Today a lot of carbon dioxide is released into the atmosphere due to human activities. A way of recycling the carbon and reuse waste for something beneficial is needed. Microalgae have the capacity to fixate inorganic CO₂ into organic matters and purify flue gases from this component. A desirable characteristic of many species of microalgae is the capability of producing and accumulating lipids suitable for biodiesel. Some algal species can also accumulate polysaccharides as <I>e.g</I>. starch for bioethanol purposes. A large and cheap carbon source, as CO₂ enriched air, is needed for a large scale production of algal biomass and exhaust gases from many industries could be possible alternatives. Potentially for future use, wastewater from an industry could be growth medium and provide nutrients, mainly nitrogen and phosphorous, needed for growth of microalgae. The risk of eutrophication would then be minimized.<p></p> This project involved screening and investigation of growth of eleven different freshwater microalgae species with simulated flue gas, mimicking flue gas from Swedish pulp- and paper mills, as carbon source. Batch cultures in Erlenmeyer flasks (250 ml) were used for cultivation and a gas distribution system was set up for providing artificially produced flue gas of 15 % CO₂, 100 ppm NO and 10 ppm SO₂ which was bubbled through the cell suspensions. Growth was followed by OD₇₅₀and cell count and higher specific growth rate means higher carbon fixation capacity. All freshwater species showed a quality of being able to grow in the high concentrations of the three constituents in the flue gas and can thereby work as a carbon source for microalgae. At the stated highest levels of the flue gas components the specific growth rates were generally decreased. <I>Scenedesmus</I> simris002, <I>Chlamydomonas reinhardtii</I> and <I>Nannochloropsis salina</I> showed high specific growth rates; they were pH tolerant and seemed to resist all components in the flue gas. The pH was a function of added flue gas constituents and concentrations (lowering the pH) and growth of algae (increasing the pH). The pH must be controlled when cultivating algae as both literature and experiments during this project indicate that too low pH kills the algae. Lipids, carbohydrates and protein content (% g/g) were investigated at late exponential phase, as potential oil or starch producer is of interest for biofuel purposes. The most promising species for carbohydrates were <I>Chlorella emersonii</I> (45 %) and <I>Chlorella sorokiniana</I> (34 %). The lipid content was estimated by a correlation where the values for proteins (g/g %), carbohydrates (g/g %) and assumed ash weight of 10 (g/g %) were subtracted from 100 % and the rest was assumed to be the proportion of lipids. The species with highest estimated lipid content were <I>Botryococcus braunii</I> (57 %) and <I>Chlorella vulgaris</I> (52 %). As for the protein content <I>N. salina</I> (66 %) and Nedsjön isolate (64 %) obtained the highest values.<p></p> Two types of wastewater from two different purification steps, obtained from <I>Nordic Paper Bäckhammar AB</I>, were tested as growth medium in small flasks with loose caps. The waters were the only source of nutrients for growth of the cells. The freshwater species cultivations were followed visually and <I>S</I>. simris002, <I>Scenedesmus obliquus</I> and <I>C. sorokiniana</I> showed the most promising results of being able to grow in the wastewater, even though growth seemed to be limiting due to low exchange of gas and CO₂ or too low nutrient concentration. The strain of <I>N. salina</I> showed qualities of growing in freshwater, brackish water and marine water which is a desirable characteristic if diluting the growth medium with a nearby lake or sea