Culturing Of Oil-containing Scenedesmus Obliquus In Municipal Wastewater

Energy shortages and increasing eutrophication phenomenon are two problems in china. Culturing oil-containing microalgae in municipal wastewater for biodiesel production, not only can reduce the cost of biodiesel to alleviate the energy shortage; and can also transfer N and P from the sewage and purify the wastewater, helping to avoid eutrophication, and protect the water environment.By consulting relevant literatures, I chose four oil-containing microalgae. By examining the adaptability of four microalgae on wastewater and their fat yield, a kind of microalgae was chosen to be continuing studied. In further study, light, pH, nutrients were investigated for their effect on algae lipid production; and couture microalgae in the sewage from different stages of municipal wastewater treatment; in addition, I studied the effect of changing Water quality on the algae lipid production. Finally, fat yield were optimized with several major factors to identify the optimal condition for fat production, and was experimentally verified in the synthetic wastewater and actual sewage. The concrete results are as follows:1) Scenedesmus obliquus showed the highest specific growth rate (0.47d^-1) and the highest fat yield (0.25g/L) when cultured in the wastewater; the content of fat is about 20.8%, it is relatively high; It is easy to become a dominant species in Sewage; and Scenedesmus obliques'fat is dominated with saturated fatty acids and monounsaturated fatty acids, which met the standards for biodiesel production. Therefore, I chose Scenedesmus obliquus for further study.2) Environmental factors on the algae lipid production:The best light intensity for fat producing of Scenedesmus obliquus is 10 000lx; this microalgae has a capacity of resistance to high light intensity. Illumination time had little effect on the content of fat, but fat yield increased with increasing illumination time. Algal is suitable for neutral and alkaline conditions, the municipal wastewater will be suit for the growth of algae species on pH. The changing pH also cannot change the fat content, fat yield and algal production was proportional.When fixed nitrogen concentration (75mg/L), Initial phosphorus concentration which changes between 1mg/L 8mg/L has little effect on the algal fat content and fat yield. When the phosphors concentration is fixed at 3mg/L, The fat yield increased with the increasing nitrogen concentrations (between 20mg/L⁶0mg/L), I got the highest fat yield when the nitrogen concentration is 45mg/L. Ammonia is the best nitrogen source for algae growth and fat production. CO₂ enrichment can increase the growth rate of algae and fat production, but high concentration will inhibit the fat production. Carbonate is not suitable for culturing algae for fat production.Culturing algae in primary sedimentation tank effluent for biodiesel production can save the cost of sewage disposal, and gain high fat yield algae species, but need to improve the efficiency of CODcr removal. Besides, the changing quality of raw sewage (dilution ratio changed from 0% to 50%) can cause fluctuations in the final fat yield (ranged from 0.14⁰.20g/L).3) The culture condition of Scenedesmus obliquus for fat production was optimized. The optimum culture condition is: the light intensity is 14 500lx, the light cycle is 22:2, TN is 97mg/L, TP is 11mg/L, the content of CO₂ is 9.8%, the highest fat yield was 0.336g/L. The result that verified in sewage is 0.367g/L, which was higher than the fitted value in other literatures. The relative deviation is 8.34%. The verified result in municipal sewage is 0.38g/L, which was higher than the fitted values (0.22g/L) and the result of synthetic wastewater under the same conditions (0.275g/L), the relative deviation is 41.17%. Because that the bacteria can depredate CODcr and provide more CO₂ for microalgae in nutrient rich sewage. Therefore, the quadratic regression model need to be further optimized.