Research On The Lipid Accumulation Characteristics Of CHLORELLA VULGARIS And The Lipid Production With Wastewater Used As Substrate

Due to the challenge of energy crisis and greenhouse effect, microalgae energywhich renewable and do not increase the greenhouse gas emissions received lots ofattentions recently years. Microalgae has some advantages to be used as bioenergymaterial, such as higher growth rate and energy conversion ratio, larger potentialproduction scale. However, the expensive cost of nutrient in medium and energyconsumption in collection process restrict the commercial production of microalgaeenergy. The production cost of microalgae would be decreased significantly ifwastewater was used as medium, and wastewater was treated simultaneously.Chlorella vulgaris was used as model strain in this paper. The growthcharacteristics of C. vulgaris under various trophic mode were investigated.Thereafter, lipid content of algal cell with various nutrient concentration wasdetermined. Both mixotrophic mode and lipid accumulation of C. vulgaris wereanalyzed. Through the optimization and comparison of chemical coagulation andelectronical coagulation, the collection method for C. vulgaris was confirmed. Theeffect of drying temperature on biomass constitute was also investigated. Based onthe research of lipid accumulation in C. vulgaris with wastewater used as medium,C. vulgaris lipid production system was constituted. The results of system operationwas analyzed.BG11was used as basal medium, C. vulgaris was cultivated underphototrophic, heterotrophic and mixotrophic mode with various organic carbon andlight condition. The specific growth rates of autotrophic and heterotrophic with noaeration were only0.0018h^-1and0.0083h^-1, respectively. As for mixotrophic, itincreased to0.0204h^-1. Compare to ethanol and sodium acetate, glucose was moresuitable to be used as organic carbon source for C. vulgaris cultivation. The specificgrowth rates could promote significantly by aeration. It increased to0.1008h^-1undermixotrophic with air aeration, while slightly decreased to0.0930h^-1when20%CO₂used as air source. Analysis of various trophic mode of C. vulgaris indicated thatphotosynthetic metabolism and heterotrophic metabolism were occurredsimultaneously in algal cell under mixotrophic mode. The pathways were interacteach other, and codetermine the constitute and specific growth rate of algal cell.The characteristic of C. vulgaris lipid accumulation was investigated withvarious nutrient concentration in medium. Nitrogen deficiency would enhance lipidcontent significantly. The lipid content of algal cell increased from7.59%in3.75g/LNaNO₃to22.58%in0.1g/L NaNO₃. Phosphate limitation play a lessimportant role in lipid accumulation than nitrogen. Glucose play a very important role in lipid accumulation. When C. vulgaris was cultivated with5g/L glucose, thelipid content achieved17.40%at96h. However, only10.58%of lipid content wasobtained at240h due to glucose had been exhausted. The analysis of lipidaccumulate process under various nutrient condition indicated that nutrientdeficiency would restrict the growth rate of microalgae. If organic carbon and lightenergy were abundant in nutrient deficiency culture, then the major part ofmetabolic flux was converted into lipid accumulation, therefore lipid contentincreased.Based on the optimization of string rate, aeration rate, current density, initialcell density and pH, electronical coagulation （EC） for C. vulgaris collection wasestablished. The97%of collection efficiency was achieved at15min by EC, withinitial cell density0.241g/L, pH5.0, current density0.42mA/cm²and string rate50rpm. The energy consumption was only0.610kWh/kg. The C. vulgaris cultureunder different nutrient condition were collected by optimized aluminum sulfatecoagulation and EC, respectively. EC achieved higher collection efficiency and lipidrecovery efficiency than aluminum sulfate coagulation.99%¹00%of collectionefficiency and99%¹00%of lipid recovery efficiency were obtained by EC in20min. However, the Al3+concentration in effluent of EC was1.97².33times higherthan chemical coagulation. Constitute of algal cell would be changed under variousdrying temperature. Dry weight, lipid content and ester content of algal cell dryingin105°C decreased by3.30%,7.73%and26.30%compare to-70°C freeze drying.As for40°C drying, the constitute of algal cell was similar with freeze drying.C. vulgaris was cultivated in artificial wastewater and domestic wastewater,respectively. Due to the pollutants in domestic wastewater were more complex, thecell density decreased from1.496g/L in artificial wastewater to0.575g/L indomestic wastewater. The specific growth rates of C. vulgaris in sterilized domesticwastewater was higher than in non-sterilized wastewater, while pollutant removalefficiency was contrary. The lipid accumulation of C. vulgaris would be effect byaeration, light and nutrient concentration in wastewater. Cell density and lipidcontent of C. vulgaris would enhance with suitable CO₂concentration and aerationrate. Moreover, the increase of light intensity and time were also improved C.vulgaris cultivation. Cell density and lipid content of C. vulgaris also increasedalong with the increase of COD:TN. When COD:TN was50:1,1.181g/L of celldensity and14.38%of lipid content was obtained.Based on the results of previous research, the C. vulgaris lipid productionsystem was consisted. The domestic wastewater was pretreatment by filtration anddiluted, and then COD:TN:TP was adjusted to300:6:1by KH₂PO₄and glucoseaddition. After sterilization, the culture was used for C. vulgaris cultivation with5%CO₂aeration. When2000lx and7000lx light intensity was supplied, cell density reached0.954g/L and1.203g/L respectively, lipid content achieved14.49%and15.75%. As for COD, TN and TP, the removal efficiency were over70%,90%and96%, respectively. Cost analysis of the system indicated that the production cost ofC. vulgaris lipid with domestic wastewater used as medium was significantly lowerthan N-deficient medium. It was71.57yuan/kg with2000lx light intensity, only58.29%of N-deficient medium. If glucose was replaced by high organicconcentration wastewater, and sunlight used as light source, the profit of wastewatertreatment also calculated, the cost of C. vulgaris lipid could reduce to22.86yuan/kg.