| Nowadays, the continuous, adequate and inexpensive supply of biomass is the main challenge for the large-scale commercial application of microalgae biomass. The microalgae cultivation and reactor development technology are crucial to achieve efficient and low cost large-scale cultivation of microalgae. CO2 in the flue gases and nitrogen, phosphorus and other nutrients in the sewage can effectively promote the growth of microalgae. Raceway bioreactor is a kind of open type reactor for microalgae scale cultivation, but at present the optimization design and scaling-up lack of theoretical basis, and few studies on the fluid flow and mass transfer inside the reactor. In this study, based on the microalgae cultivation, combined with biological carbon sequestration and nitrogen and phosphorus removal for wastewater, the microalgae culture optimization and bioreactor optimization design were researched to provide technical supports for the microalgae scale application.11 strains of energy microalgae currently available in the laboratory were used to screen the algae with strong ability of CO2 tolerance. Two strains, which have the potential of biological carbon sequestration and biodiesel production, had been selected under the 5% and 10% CO2 concentrations via single factor experiment method. The results showed that Chlorella sp.2 and Scenedesmus dimorphus (FACHB-496) had better CO2 tolerance. Under the 5% and 10% CO2 concentrations, the biomass yield of Scenedesmus dimorphus was 280 and 214 mg·L-1·d-1, respectively; and the Chlorella sp. was 244 and 246 mg·L-1·d-1, respectively.The influences of light intensity, initial total nitrogen concentration (ITNC), aeration rate, aeration period and culture duration on CO2 fixing rate (CFR) of Chlorella sp. were investigated via single factor experiment method. The results showed that the maximum CO2 fixing rate was 564.67 mg·L-1·day-1 with biomass concentration of 3.50 g·L-1 and nitrogen/phosphors removing rate of 66.72%/55.95% under the optimal conditions:light intensity of 240 μmol·m-2·s-1, ITNC of 128 mg·L-1, aeration rate of 0.3 vvm, aeration period of 15 s/60 s (on/off) and culture duration of 10 days.The seven factors affect CO2 fixation, i.e. light intensity, ITNC, aeration rate, aeration period, inoculum density, pH and magnesium concentration, were screened by using Plackett-Burman design. Second-order polynomial model was then developed to examine the effects and interactions between significant variables (light intensity, ITNC and inoculum density) on CO2 fixation using central composite design of response surface methodology. The results showed that quadratic model is a suitable model to describe the relationship between the CFR of Scenedesmus dimorphus and the major influencing factors. The determination coefficient and experimental precision were respectively 96.09% and 14.734, which confirmed the excellent accuracy of the model. Optimum culture conditions were predicted at light intensity of 238μmol·m-2·s-1, ITNC of 152 mg·L-1, inoculum density of 0.3 g·L-1, under which the maximum CFR of 638.13 mg·L-1·d-1. This was in good agreement with the theoretical predicted value of 619.64 mg·L-1·d-1. The results offered a significantly favorable guidance for culture conditions optimization of biological carbon sequestration. Subsequently, the influence of nitrogen starvation strategy on lipid content were investigated, and the results showed that 5-days nitrogen starvation strategy when replacing 80% of the culture solution with tap water, the lipid content was increased to 24.2% from 11.9%.The advantage of the CFD was applied to optimize the structure and size of the airlift raceway bioreactor via simulation analysis the velocity distribution of the internal fluid of the raceway bioreactor, in order to improve the microalgae biomass accumulation. The calculated results showed that the airlift-driven raceway bioreactor had maximum average velocity when setting two U-tubes (of 350 mm in length) in the middle, furthermore adding the inducing baffle structure. Under this condition, minimum dead zone ratio was obtained, which was advantageous to the microalgae biomass accumulation. Subsequently, using the optimized bioreactor to do a medium-scale outdoor microalgae cultivation experiment in the optimum culture conditions, which have got in the early study. The results showed that the optimization solution of bioreactor was useful to improve microalgae biomass production, after 10 days culture duration, the biomass production and lipid content of Chlorella sp.2 was 185.71 mg·L-1·d-1 and 23.49%, respectively. |
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