Effects of nutrients on microalgae growth in anaerobic digestate

Today it is known that the world population has passed seven billion, and by 2050 it is predicted that the population will be 9.72 billion. In the world, 80% of energy demand are still met by fossil fuels such as oil, coal and natural gas. In addition, 85% of crude oil is used for transportation, and 10% is used for chemical industries. This has led to worldwide interest in biomass production related to biofuel concept. In present, various products are being produced in different industrial areas from the biomass obtained from various organisms. Bioethanol, biomethane, biohydrogen and biodiesel are obtained from the biomass of the organisms. For biofuels, it is known that plants such as corn, soybean and sugar cane are used. The most important advantage of biofuel production from biomass is the reduction of CO2 emissions in the air. By using solar energy, CO2 is converted into organic mass, and biofuel is obtained from this organic biomass. However, the need for a large area for the cultivation of crops, the reduction of agricultural land, and the consumption of too much water are among the reasons why biomass is not kept warm. Algae, who needs less space and water, is now being worked on biofuel studies and many different studies. Especially the ability of microalgae to adapt to different environments has made it possible to obtain various products, including biofuels. The most important problem in obtaining products from biomass of microalgae is the low microalgal biomass productivity. The microalgal biomass is not sufficient, especially for biofuel such as biodiesel. There are many factors that limit biomass formation. For photosynthetic microalgae, light, temperature, amount of CO2 and nutrients are main limiting factors. In present, microalgae can be optimized by changing light intensity, temperature, and CO2 in order to increase its biomass. However, the issue of how much nutrient required or nutrient optimization are still debated. Either the amount of some nutrients may be useful for some microalgae or may be inhibitory or toxic for another microalgae. In addition, nutrient needs may be variable depending on the microalgae species. It is known that microalgae is cultivated in the anaerobic digestate. The obtained biomass is also used as co-digestion or directly for methane production. The biofuel obtained from microalgae biomass are known to be suitable for biodiesel production, but the obtained biomass is not sufficient in the industrial view. There are many factors affecting microalgae biomass concentration like the dark color and particulate matter of the digestate that limit the penetration of light. Although this problem can be overcome by diluting the digestate or decreasing the reactor volume. The another approach is thought that this problem can be solved by adding nutrients. Some studies made for this purpose and showed that the increase of biomass in wastewater by adding nutrients such as Mg and Fe. The use of wastewater is not only important for the treatment, it is also important factor that reduces water footprint and nutrient requirement to produce microalgae. Because one of the major problems in the world is the need for clean water. It's now known that using drinking water resources such as lake water for obtaining microbial biomass for industrial purposes is not sustainable environmental way. In addition, drinking water resources is not nutrient enough to produce microalgae. Therefore, microalgae produce in seawater seems to be more attractive because sea water contains nutrients such as Mg and K. However, possible negative impacts on marine ecosystems are still a big question when large facilities are constructed to produce microalgae in the seabed. Considering the flexible adaptation capabilities of microalgae, the use of wastewater in terms of nutrient requirements is now considered as sustainable environmental approach. In this study, the aim is to increase biomass of the freshwater mixed microalgae cultured in digestate by adding various nutrients referred to BG11 media. For this aim, 600 ml batch cultures were prepared contained digestate with 1:10 dilution ratio. HEPES buffer was used and pH maintained between 7 and 7.5. Different nutrients were added to the each culture and microalgae growth was observed by VSS, chlorophyll measurement and cell counting techniques. Specific growth rate and doubling time were determined based on cell counting data. Additionally, their NH3-N, organic C, TKN and PO43--P removal efficiencies were also determined by using standart methods, whether there was an effect of removal efficiency or not. During the experiments, the growth rate and doubling time of the microalgae cultivated in the digestate were lower than BG11 media. Fe source, Mg source and EDTA were added to solutions separately in terms of observing their effect on growth. Fe source was "Ferric Ammonium Citrate" and Mg source was MgSO4 .When Fe source was increased to 10 times as 3.0mM referred to BG11 media, inhibition was observed in growth. Mg was expected to support growth well, but Mg solutions' results showed lower values compared to EDTA. Among the Mg solutions, only 3.0 mM Mg increased the growth rate and doubling time like 0.1585 μ -1 and 4.3 day respectively. When Mg and EDTA added to solutions in different concentrations, Mg 0.6 +EDTA 0.024 mM had the highest growth rate and doubling time like 0.1668 μ -1 and 4.1.day respectively. Chlorella sp. and Scenedesmus sp. were observed as dominant species. Rather than microalgae, protozoa with silia or flagella were observed in the digestate media. COD removals were found very low compared to ammonical nitrogen and phosphorus removals. Based on low COD removals, it was considered that there was probably no mixotrophic growth. The ammonical nitrogen removal rate and the theoretical biomass production were determined for each solution in terms of TKN values via the mass balance calculations. The theoretical biomass production were found higher then experimental biomass production in all solutions. EDTA was known as a chelator in the media for microalgae and was intended to ensure metal ions remain dissolved in the media to be taken into by microalgae. Among the nutrients, digestate solution with 0,024mM EDTA had the highest biomass concentration with the value 1410 mg /l VSSalgae with 0.1700 μ(d-1) and 4 days doubling time. It had also the highest net VSS value as 1000 mg/L. And the ammonical nitrogen removal rate and nitrogen removal efficiency showed in EDTA 0.024 mM as 11.4 mg/l/d and 77% respectively.