| Ever since the Industrial Revolution, the concentrations of greenhouse gases, especially CO2, in the atmosphere has been steadily increased because of the increasing consumption of fossil fuels, and beyond the extent that the ecosystem can accept, which will arise a range of environmental problems, such as changes in average temperature, sea level rise and inundation of coastal lands. Although researchers have tried a variety of approaches mitigating and restricting CO2 emission, however, problems such as high costs and complex operation restrict the application of these technologies. Except high moisture content, sewage sludge contains many other ingredients, including massive nutrients such as N, P, K and trace elements. Cultivation of marine microalgae using sewage sludge not only may save the massive culture medium by providing inexpensive raw material, but also can accelerate the reclamation of sewage sludge by preparing the industrial chemicals with microalgae.We isolated a species of marine microalgae from twelve common species of marine microalgae, which could be resistant to acid and high CO2 concentration. Meanwhile the sludge extraction fluid was used instead of the traditional culture media to reduce the cost of carbon fixation. On the basis of this, the environmental factors affecting Chaetoceros gracilis growth in the mixture media were optimized. The marine microalgae was cultivated with a photobioreator, and the CO2 utilization efficiency was determined. The main conclusions are as follows:(1)In this research, alkaline thermal hydrolysis, ultrasonication and ultrasonic-alkali extraction were used to pretreat the sludge. On the basis of detecting the influences of process conditions on the extractions of NO3--N,NO2--N,NH4+-N, PO43--P from the sludge, the best working conditions for each method were optimized. The results showed that, the optimum conditions were pH=13, T=100℃, t=9h for alkaline thermal hydrolysis; for ultrasonic treatment, the effect of extraction was the best at P=400W and t=40min; while for ultrasonic-alkali extraction method, the maximum concentrations of inorganic nitrogen(IN) and phosphorus(PO43--P) were obtained at pH=13, P=400W and t=20min. Under each optimum condition, the effect for extracting IN and PO43--P from WAS followed the order of ultrasonic-alkali extraction-ultrasonication> alkaline thermal hydrolysis. The extraction rate of nitrogen and phosphorus for ultrasonic treatment reached up to 33.48% and 57.5% respectively. Meanwhile, considering the influence of pH on microalgae cultivation, Ultrasonic treatment was proposed to be the optimal method for extracting nutrients from the waste activated sludge (WAS).(2)Changes of algal cell density of each species grown in F/2 culture medium at pH from 2 to 8 were investigated using micro-panel method, and their tolerance of H2SO4 and HNO3 was investigated. And then the more acid resistant species of microalgae were cultivated in F/2 culture medium with an aeration of 15%(v/v) CO2 at the rate of 75ml/min for 7 days. Normal room air was used as the control. The species with highest CO2 tolerance was selected based on the changes of algal cell density and biomass as compared with the control. The results showed that seven species of microalgae, Dunaliella tertiolecta, Spirulina platensis, Chlorella pacifica, Nitzschia Closterium, Platymonas subcordiformis, Chaetoceros gracilis, Pavlovaviridis Tseng had stronger acid resistance than the others and could grow well at pH 4. Moreover, Chaetoceros gracilis presented the highest tolerance of 15% CO2, the biomass of which was 102.47% of the control. It is suggested that Chaetoceros gracilis had the potential for fixing CO2 from flue gas and reducing greenhouse effect.(3)In the mixture medium, the best growth for Chaetoceros gracilis was observed at 30℃, illumination strength of 60001x,10% CO2 and the volume ratio of sludge extraction fluid to seawater was 1:29. (4)A photobioreactor with the volume of 10 L was designed and made up. Batch cultivation experiments were carried out in this photobioreactor, the optimum inoculum volume, the optimum circulation rate and the best CO2 flow rate were determined to be 1×106cells/ml,1200ml/h and 20ml/min, respectively. Under the best growth condition, the velocity of Chaetoceros gracilis fixing CO2 from flue gas achieved 0.67g CO2/L·d. During the experimental period, Chaetoceros gracilis could absorb inorganic nutrients from the mixture medium quickly. 96.92% of NO3--N,77.48% of NO2--N,93.32% of NH4+-N and 88.52% of PO43--P were removed from the mixture medium, respectively. After cultivation for 7 days, the nutrient concentrations in the mixture medium were below the Integrated Wastewater Discharge Standard of China, and could be discharged directly without causing pollution in the water.In conclusion, Chaetoceros gracilis can be resistant to acid and high CO2 concentration, cultivation of Chaetoceros gracilis using sewage sludge from municipal wastewater treatment plants, not only can largely reduce the cost for microalgae production, but also are beneficial to the reclamation of sewage sludge. It is suggested that Chaetoceros gracilis has the potential for fixing CO2 from flue gas and reducing greenhouse effect.
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