| At present,the excessive consumption of fossil fuels has been continuously aggravating the trends of energy crisis and global warming.The third generation biofuel derived from microalgae biomass has been regarded as a promising alternative for the current fossil fuels.In addition,microalgae can converte CO2 into organic matters through photosynthesis at efficiencies 10-50 times of terrestrial plants.As a consequence,the microalgae-based carbon fixation and energy utilization technology has the double effects of alleviating the fossil energy shortage and reducing greenhouse gas emissions,which has been attracting great attention worldwide.Notably,the high density large-scale microalgae photoautotrophic cultivation is the key to realize this technology.Light is the fundamental driving force for microalgae photoautotrophic cultivation.However,due to the absorption,scattering and mutual shading effects between microalgae cells,the light intensity decreases exponentially along light path when light penetrates a microalgae suspension,resulting in a heterogeneous light distribution within microalgae culture,which is unfavorable to microalgae growth.In this regard,the improvement on light transfer and optimization of light distribution within microalgae suspension is crucial to improve microalgae photoautotrophic growth.Based on the technology of microalgae suspension photoautotrophic cultivation,the objective of this work is to enhance light transfer and hence optimize the light distribution within microalgae photobioreactor.To fufill this purpose,firstly,a cultivation mode with enhanced light transfer and microalgae growth by regulating biomass concentrations within the photobioreactor was proposed.The light penetarbility within microalgae culture was elevated by periodically regulating the biomass concentrations within the photobioreactor,therefore we analyzed the light distribution characteristics within the photobioreactor and investigated the effect of pre-harvesting ratio,biomass concentration regulation frequency and incident light intensity on the performance of the proposed cultivation mode.From the perspective of improving the light environment of microalgae cells far from the light incident surface inside the photobioreactor,we constructed a flat-plate photobioreactor embedded with hollow light guides.The improvement on light distribution characteristics resulted from the introduction of hollow light guides were analyzed.Besides,the effect of incident light intensity and aeration rate on the performance of the proposed photobioreactor were also studied.Next,in order to further optimize the light distribution characteristics throughout the entire photobioreactor,we constructed the planar waveguide airlift flat-plate photobioreactor and the lab-scale open raceway pond with built-in planar waveguide modules by introducing the planar waveguides doped with light scattering nanoparticles into the field of microalgae cultivation.The light distribution characteristics within the proposed flat-plate photobioreactor and open raceway pond were analyzed and the effect of planar waveguide length,interval between adjacent planar waveguides,initial nitrate concentration and light intensity emitted from waveguide surface on microalgae growth and lipid accumulation were studied.Thereafter,on account of the superior light distribution characteristics within the planar waveguide airlift flat-plate photobioreactor,we systematically studied the enhancement of microalgae growth and lipid accumulation employing growth-phase based light-feeding strategies and light spectrum composition manipulation in the planar waveguide airlift flat-plate photobioreactor.The main conclusions obtained in this work are as follows:(1)A cultivation mode with enhanced light transfer and microalgae growth by regulating biomass concentrations within the photobioreactor was proposed.During the microalgae batch cultivation,partial microalgae cells were periodically withdrawn from the photobioreactor followed by pre-harvested by the microfiltration membrane,afterwards,the transparent culture medium returned back to the photobioreactor and diluted the biomass concentration.Consequently,the microalgae growth was enhanced under a higher light penetrability resulted from the diluted biomass concentrations.Experimental results indicate that the average light intensities in the photobioreactor employing the proposed cultivation mode were 27.05–122.06% higher relative to those in the control photobioreactor during the whole cultivation period,resulting in a 46.48% increase in Chlorella vulgaris FACHB-31 biomass production.(2)The flat-plate photobioreactor embedded with hollow light guides was constructed.On account of the fact that the light attenuation characteristics within air are much lower than those within microalgae suspension,the ends of the hollow light guides functioned as a secondary light source for microalgae cells in light-deficient regions and hence improve the light environment of microalgae cells far away from the light incident surface,which offset the adverse effect of light attenuation on microalgae growth to a certain extent.Results indicate that the introduction of hollow light guides improved the light conditions of microalgae cells in regions 3-6 cm from light incident surface significantly.After 3 days cultivation,the average light intensities of interior regions(i.e.,3–6 cm from light incident surface)in the proposed hollow light guide flat-plate photobioreactor were enhanced by 2–6.5 times compared with those of in the conventional flat-plate photobioreactor.Consequently,the C.vulgaris FACHB-31 biomass concentration in the proposed hollow light guide flat-plate photobioreactor was 23.42% higher than that of obtained in the conventional flat-plate photobioreactor.(3)To further optimize the light distratibiton characteristics throughout the entire photobioreactor,we constructed the planar waveguide airlift flat-plate photobioreactor and the lab-scale open raceway pond with built-in planar waveguide modules by introducing the planar waveguides doped with light scattering nanoparticles into the field of microalgae cultivation.The planar waveguides functioned as diluting and redistributing the intense incident light within microalgae culture more homogeneously.Experimental results indicate that the illumination surface area per unit volume and light distribution coefficient in the proposed planar waveguide airlift flat-plate photobioreactor were 11.3 times and 7.22 times of those in the flat-plate photobioreactor without waveguides,respectively.In addition,during the whole cultivation period,the illuminated volume fractions in the proposed planar waveguide airlift flat-plate photobioreactor were 21.4–410% higher than those in the flat-plate photobioreactor without waveguides.Consequently,with the width of the photobioreactor being 25 cm,a 220% improvement in C.vulgaris FACHB-31 biomass production was obtained relative to that in the flat-plate photobioreactor without waveguides.Afterwards,through the optimization of reactor structure,initial nitrate concentration and output light intensity emitted from planar waveguide,the maximum microalgae biomass concentration,lipid content per dry cell weight,total lipid yield and average lipid productivity in the airlift flat-plate photobioreactor integrated with planar waveguides reached 5.57 g L-1,41.66%,2200.25 mg L-1,273.02 mg L-1 d-1.With respect to the lab-scale open raceway pond with built-in planar waveguides,the introduction of planar waveguides into open raceway pond improved the light distributions within Nannochloropsis oculata suspension significantly.As compared to the conventional open raceway pond,the illumination surface area to volume ratio and effective illuminated volume percentage in the proposed open raceway pond were respectively improved by 5.53 times and 19.68-172.72%.Consequently,the superior light distribution characteristics in the proposed open raceway pond contributed to 193.33% increase in biomass concentration.And,the obtained average lipid productivity of N.oculata reached 113.06 mg L-1 d-1.(4)On account of the superior light distribution characteristics within the planar waveguide airlift flat-plate photobioreactor under a 10 mm interval between adjacent planar waveguides and considering the variations of optimal light intensity required by microalgae cells along with growth phases,growth-phase based light-feeding strategies on the synchronous enhancement of microalgae growth and lipid accumulation were proposed and verified.By adjusting the output light intensity emitted from the planar waveguide surface along with microalgae growth phases,microalgae growth and lipid accumulation were enhanced.Results indicate that the growth-phase based light-feeding strategies showed significant influence on C.vulgaris FACHB-31 growth and lipid accumualtion.The light-feeding strategy characterized by stepwise incremental light intensities improved C.vulgaris FACHB-31 growth while light-feeding strategy characterized by stepwise decremental light intensities could not improve C.vulgaris FACHB-31 growth.On the basis of this,stage based light-feeding strategies were therefore put forward which further enhanced C.vulgaris FACHB-31 growth and lipid accumulation simultaneously.Consequently,the maximum biomass concentration and lipid yield of C.vulgaris FACHB-31 respectively reached 5.32 g L-1 and 1892.54 mg L-1.Correspondingly,the average lipid productivity was 312.92 mg L-1 d-1.(5)Based on the planar waveguide airlift flat-plate photobioreactor characterized by superior light distribution characteristics within microalgae culture,the enhancement of microalgae growth and lipid accumulation employing light spectrum composition manipulation was studied.Results indicate that the light spectrum composition showed sigificant influence on C.vulgaris FACHB-31 growth and lipid accumulation.For the monochromatic light,C.vulgaris FACHB-31 cells cultivated under red light showed the highest growth rate,biomass concentration and lipid content.C.vulgaris FACHB-31 cells cultivated under the mixture of red and blue reached the stable phase much earlier,which was unfavorable to microalgae biomass accumulation.The maximum and minimum lipid yields of C.vulgaris FACHB-31 were obtained under the mixture of red and green under the ratio of 9:1(879.33 mg L-1)and white light(591.51 mg L-1),respectively.As a consequence,the optimum light spectrum for C.vulgaris FACHB-31 growth and lipid accumulation was the mixture of red and green light under a ratio of 9:1. |
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