Strengthening Mass Transfer And Flash-light Effects In Photobioreactors To Improve Microalgal Biomass Growth With CO₂ Fixation

The global warming with emitted CO₂ and other greenhouse gases mainly from burning of fossil fuels are universal hazards.The CO₂ fixation can be done by different methods,but CO₂utilization by microalgae can simultaneously produce valuable biomass.Microalgae with fast growth rate can produce valuable biomass containing bioactive carbohydrates,proteins and lipids,which can be used as pharmaceuticals,health food,animal feed,chemicals and biofuels.Microalgae cultivated in efficient photobioreactors（PBR）are auto-phototrophic organisms and effective CO₂ consumers.However,the low cell density with high energy consumption is a major bottleneck in commercial viability of microalgal biomass.Self-rotary propellers（SRPs）with clockwise/counterclockwise blades were investigated to create spiral flow fields without external power to strengthen gas-liquid mixing and promote microalgal growth in an open raceway pond.The rotational flow around the propellers and spiral flow between the propellers generated extensive wall shear stress in three dimensions.Four-clockwise blades on the propellers exerted better mixing than three-counterclockwise blades.The bubble generation diameter was reduced by 69%and the mass transfer coefficient increased by49%when the propeller diameter was increased from 32 to 60 mm.The photochemical efficiency（φPSII）of Arthrospira platensis cells was enhanced by 25%,while the helix pitch and trichome lengths were enlarged by 7–16%.Self-rotary propellers（60mm diameter）with four-clockwise blades enhanced the growth rate of A.platensis biomass by 35%compared to that in an unmodified raceway pond without propellers.High growth rate of microalgae mainly depended on solution mixing and mass transfer properties can be achieved by installing baffle structures in photobioreactors（PBRs）.In this study,butterfly shaped baffles were installed to decrease mixing time and increase mass transfer coefficient to promote growth rate of Arthrospira platensis.The mixing time was reduced with increased baffle size and angle between wings,while mass transfer coefficient was simultaneously enhanced.The vertical flow vortices generated by butterfly baffles strengthened light/dark cycle between inner column and outer column in PBRs,which improved chlorophyll-a content,actual photochemical efficiency and electron transfer rate during photosynthesis.The biomass growth rate of A.platensis was increased by 33%with butterfly baffles compared to that without baffles,while helix pitch and trichome length were enlarged by 15-16%.In order to optimize light distribution for promoting biomass growth rate of Chlorella pyrenoidosa,concave walls were installed in plate photobioreactors（PBR）to generate rotational flow field of microalgal solution circulated from top inlets to bottom outlets.Flow vortices in four corners of concave-wall PBR resulted in decreased mixing time and increased mass transfer coefficient.The CO₂ bio-fixation by C.pyrenoidosa increased by 27%and chlorophyll-a concentration enhanced by 18.5%in concave-wall PBR compared to those in control（flat-wall）PBR.The concave walls diverge light rays to enhance frontal light exposure and supply more light photons into interior regions of PBRs.The promotion in light distribution and vortex flow field with concave walls enhanced light and nutrients utilization by microalgal cells,leading to an increased biomass growth rate by 21%.Microalgae are significantly affected by spectra compositions with various wavelengths.The development of light harvesting pigments can be controlled with specific wavelength of filtered light received by microalgae.Coverage of open raceway pond using transparent colored polyvinyl chloride sheets（PVCS）to filter light spectra was assessed for the capacity to enhance biomass growth rate.Results showed that orange PVCS filtering light spectra at wavelengths from480-665 nm increased biomass dry weight（3.3 g/L）by 61%compared with control condition（white PVCS=350-750 nm）.Light spectra filtering through orange PVCS were more easily absorbed by the light harvesting pigment protein complex（phycobilisome）of A.platensis cells and subsequently transferred to intracellular photosynthesis reaction centers.Therefore,A.platensis cells cultivated with light spectra filtering through orange PVCS contained 62.7 mg/L chlorophyll-a and 23.5 mg/L carotenoid,which were 40%and 29%higher than control conditions.