Study On Development Of Microalgae Photobioreactor And Forward Osmosis Separation Of Microalgae From Water

Since the 1990s,a large amount of greenhouse gases had been discharged into the environment,causing continuous growth of global temperature.As the most important greenhouse gas,the contribution of CO2 to greenhouse gases accounted for about 60%,so it is urgent to conduct CO2 emission reduction.As an autotrophic organism capable of efficient carbon fixation by photosynthesis,about 183 tons of CO2 could be fixed by per 100 tons of algal biomass and extremely high economic value microalgal biomass could be produced.However,in the process of actual large-scale microalgae cultivation and CO2 fixation,the industrial application of microalgae was severely restricted by the high investment cost of p hotobioreactor and low harvesting efficiency of microalgal biomass.In this paper,aiming at CO2 fixation and forward osmosis(FO)separation of microalgae from water and its pollution mechanism,the mass transfer efficiency and photosynthetic efficiency of microalgae in photobioreactor were studied,and a new type of enclosed photobioreactor was developed to improve the biomass production of microalgae.At the same time,the research on FO separation of microalgae from water was carried out to explore membrane fouling factors and mechanism based on FO membrane separation technology,and thus providing technical support for large-scale microalgal cultivation system.The main research contents and conclusions are listed as follows:(1)For low yield of microalgae in photobioreactor,a new type of column photobioreactor with internal light(ILC-CPBR)was developed with high CO2 tolerance species of mutated Chlorella mutant PY-ZU1 as the research object,which was used to analyze hydrodynamic characteristics,light intensity,microalgal growth and CO2 fixation rate.When aeration rate was 0.1 vvm,the mass transfer coefficient of ILC-CPBR was 24.3%and 21.5%higher than that of column photobioreactor with the same volume(CPBRv)and column photobioreactor with the same height(CPBRh).The adding of internal light column enhanced light intensity of microalgal growth by increasing light/dark ratio.Based on mutual enhancement of mass transfer coefficient and light intensity,the biomass increased by 82.4%and 64.4%in ILC-CPBR compared with those in CPBRv and CPBRh,and the peak of CO2 biofixation rate increased by 47.7%and 26.2%in ILC-CPBR compared with those in CPBRv and CPBRh.(2)For the fouling characteristics of FO separation of microalgae from water,the differences of water flux decline and contaminated components in FO dewatering of microalgal cells,soluble algal products(SAP)and microalgal broth were investigated with Chlorella vulgaris and Scenedesmus obliquus as the experimental algal species.Microalgal cells and SAP were the main causes of FO membrane fouling,and more serious water flux attenuation was induced by the interaction between algal cells and SAP.In the active layer facing feed solution(AL-FS)mode,the water flux caused by microalgal cells of Chlorella vulgaris was greater than that of SAP,and in the active layer facing draw solution(AL-DS)mode,the water flux caused by SAP was greater than that of algal cells.However,the effect of SAP on water flux was always greater than that of algal cells for Scenedesmus obliquus.In AL-FS mode,microalgal cells of Chlorella vulgaris were more likely to adsorp into the active layer of TFC membrane,and thus reducing the adsorption site of SAP on TFC membrane.Compared with AL-FS mode,the large amount adsorption of SAP on support layer of TFC membrane in AL-DS mode was the main reason for continuous attenuation of water flux.(3)For the fouling mechanism of FO separation of microalgae from water,the change of water flux,foulants composition on membrane and possible fouling mechanism were compared between thin film composite(TFC)and cellulose triacetate(CTA)membranes,which used SAP of Chlorella vulgaris as feed solution and used NaCl,MgCl2 and CaCl2 as draw solution.The results showed that TFC membrane exhibited higher water permeability but more loss of water flux in comparison with CTA.The SAP was inclined to be adsorbed by TFC membrane while adsorbate was easier to be removed by physical cleaning.By contrast,the pollutants accumulated on the membrane surface of CTA were much more irreversible.The interaction among calcium cations,SAP and R-COOH groups in the active layer of TFC membrane induced the formation of cake layer on the surface of TFC membrane while irreversible granular deposits of SAP were found accumulated on CTA membrane surface.In this study,a new type of column photobioreactor was developed,and possible membrane fouling mechanism in dewatering of SAP was proposed,providing theoretical basis for large-scale production of microalgae and separation of microalgae from water.