Cultivating Of Oleaginous Microalgae With Municipal Wastewater

Microalgae is one of the most promising lipid material for biodiesel preparation owing to its high photosynthesis efficiency, strong carbon fixation ability, high lipid content, growing independed on arable land. How to supply the requirement of huge water resources and reduce the cost of nutrients, expecially nitrogen and phosphrus as nutrients for cell cultivation are very important among those limitation factors for the industrial application of microalgal biofuels. On the other hand, treatment stress of municipal wastewater become more servious with the urbanization as the removal of nitrogen and phosphrus in municipal wastewater is difficult and costly. This work aims the combination of microalgae cultivation with municipal wastewater treatment. It is expected to achieve such win-win result that wastewater solves the shortage of water for algal cultivation and supplies free nitrogen and phosphorus as nutrients, and algal cells remove all nitrogen and phosphorus after its growth.To effectively couple advanced wastewater treatment and biodiesel production, selecting the proper microalgae species is important in this field. Two species of marine microalgae and four species of freshwater microalgae are cultivated in bubbled column photobioreactor with different water resources first. The dry weight and lipid content are evaluated for the growth. The results show that all Scenedesmus dimorphus (S. dimorphus), Pseudochlorococcum sp., Scenedesmus sp., can utilize municipal wastewater well, the lipid content could reach 0.25 g/(L·d) and the removal of TN is around of 97%, TP is around of 100%. Of all the algal species, S. dimorphus grows fastest and has strongest ability to removal nitrogen and phosphorus from wastewater.In order to determine an optimum culture medium of wastewater for oleaginous microalgae S. dimorphus, the inlfuences of different additives into wastewater on algal cells growth and lipid accumulation were investigated. The results showed that S. dimorphus could adapt municipal nutrients-rich wastewater by adding some nutrients as nitrogen, phosphorus, ferric ammonium citrate and trace elements, and the amounts of such nutrients have significant effects on cell growth, biomass yield and lipid accumulation. At optimum compositions of wastewater medium, the algal cell concentration could reach 8.0 g/L (dw), higher than that of 5.0 g/L (dw) in standard BG11, indicating that municipal wastewater has strong promotion effects on algal cells growth. Furthermore, S. dimorphus had strong capacity to absorb inorganic nitrogen and phosphorus from its culture water. There was almost no total nitrogen and phosphorus residues in culture medium after three days culturing when the adding amounts of nitrate and phosphate in waste water medium were no more than 185.2 mg/L and 16.1 mg/L respectively under the experimental conditions. As a conclusion, it was feasible to cultivate oleaginous microalgae with municipal nutrient-rich wastewater, not only producing feedstock for algal biodiesel, but also removing inorganic nitrogen and phosphorus from wastewater.Municipal wastewater has significant promotion for the growth of freshwater microalgae than standard BG11 medium, the dry weight increases up to nearly 50% during the cultivation of six species. Experimental results and analysis have testified that it is not the effects of nitrogen and phosphorus in wastewater. In order to detect what's the promoter of wastewater, The cultivation under different wastewater concentration (including dilution and inspissation) shows that the initial concentration offer the best effect for the growth of microalgae, and the cell concentration increase gradiently with the increasing of wastewater ratio. The dry matter after wastewater concentrated and freeze-dried is added to the medium, which has the same effect on the growth of microalgae with the initial wastewater, therefore, the volatile can not be the promoter. Obviously, the promoter is not the nitrogen or phosphorus. And more,the analysis of basic components in wastewater shows that there is no protein and reducing sugar in wastewater so that it could get rid of the possibility of promotion by heterotrophy. The presence of polysaccharide in waste water, which has the molecular weight of 6919 determined by infrared spectroscopy and by gel chromatography may cause the above promotion as the polysaccharide content in wastewater dry matter is only 20% but it can improve the dry weight of microalgae to 50%. In order to determine the type of polysaccharide, cultivation experiments of culture medium added dextran of molecular weight around 6919 instead extracts from waste water were carried out. It is found that dextran could also promote the growth of algal cells, though it is weaker to those by polysaccharide directly extracted from wastewater obviously, indicating the polysaccharide in wastewater whose promotion for algal cells growth is non-specific.