Improving microalgal growth with reduced diameters of aeration bubbles and enhanced mass transfer of solution in an oscillating flow field

Recent progress on converting CO2 into microalgal biomass using suspended photobioreactors

Inhomogeneous light distribution and poor CO₂ transfer capacity are two critical concerns impeding microalgal photosynthesis in practical suspended photobioreactors (PBRs). To provide valuable guidance on designing high-performance PBRs, recent progress on enhancing light and CO₂ availabilities is systematically summarized in this review. Particularly, for the first time, the strategies on elevating light availability are classified and discussed from the perspectives of increasing incident light intensity, introducing internal illumination, optimizing flow field, regulating biomass concentrations, and enlarging illumination surface areas. Meanwhile, the strategies on enhancing CO₂ light availability are outlined from the aspects of generating smaller bubbles, extending bubbles residence time, and facilitating CO₂ dissolution using extra additives. Given the microalgal biomass production using current PBRs are still suffering from low productivity and economic feasibility, the possible future directions for PBRs implementation and development are presented. Altogether, this review is beneficial to furthering development of PBRs as a practical technology.

Sustainability and carbon neutrality trends for microalgae-based wastewater treatment: A review

As the global economy develops and the population increases, greenhouse gas emissions and wastewater discharge have become inevitable global problems. Conventional wastewater treatment processes produce direct or indirect greenhouse gas, which can intensify global warming. Microalgae-based wastewater treatment technology can not only purify wastewater and use the nutrients in wastewater to produce microalgae biomass, but it can also absorb CO₂ in the atmosphere or flue gas through photosynthesis, which demonstrates great potential as a sustainable and economical wastewater treatment technology. This review highlights the multifaceted roles of microalgae in different types of wastewater treatment processes in terms of the extent of their bioremediation function and microalgae biomass production. In addition, various newly developed microalgae cultivation systems, especially biofilm cultivation systems, were further characterized systematically. The performance of different microalgae cultivation systems was studied and summarized. Current research on the technical approaches for the modification of the CO₂ capture by microalgae and the maximization of CO₂ transfer and conversion efficiency were also reviewed. This review serves as a useful and informative reference for the application of wastewater treatment and CO₂ capture by microalgae, aiming to provide a reference for the realization of carbon neutrality in wastewater treatment systems.

Developing staggered woven mesh aerator with three variable-micropore layers in recycling water pipeline to enhance CO2 conversion for improving Arthrospira growth

A staggered woven mesh (SWM) aerator equipped with three variable-micropore layers was developed to enhance the CO₂ conversion into HCO₃^- in a recycling water pipeline for promoting CO₂ utilization efficiency and Arthrospira growth in large-scale raceway ponds. The input CO₂ gas was broken into smaller bubbles (0.78- 2.43 mm) through the first-stage shear with axial rectangles, second-stage shear with radial rectangles (equivalent pore diameter = 150 μm), and third-stage shear with uniform micropores. A high-speed camera (MotionXtra HG-100K CMOS) and an Image J image processing software were employed to capture the bubble pictures. Compared to the traditional steel pipe (TSP) aerator, the bubble generation diameter and time in the SWM aerator reduced by 72.3% and 48.6%, respectively. The optimized structure (ε = 14, pore = 23 μm) of the SWM aerator promoted the carbonization efficiency and HCO₃^- conversion efficiency into biomass by 78.6% and 64.6% than the TSP aerator. Further, the chlorophyll fluorescence and biomass measurements showed an increase in the actual photochemical efficiency (analyzed by Hansatech FMS1 chlorophyll fluorescence instrument) and biomass yield by 1.8 times and 80.1%.

Enhancing microalgal biomass productivity with an optimized flow field generated by double paddlewheels in a flat plate photoreactor with CO2 aeration based on numerical simulation

Computational fluid dynamics were used to analyze the flash light effect and CO₂ bubble behavior in an optimized flow field generated by double paddlewheels in a flat plate photoreactor to enhance microalgal biomass productivity. The increased D/w ratio significantly enhanced the average turbulent kinetic energy and flash cycle frequency. However, the effects became weak when the D/w ratio was over 0.67. Appliance of double paddlewheels increased flash cycle frequency from 0.035 to 0.121 Hz and increased light time ratio from 8.3% to 31.5%. Meanwhile, the bubble dynamic behavior was characterized using population balance model. The average bubble size reduced by 24.4% and the bubble rising velocity reduced by 10.6%, which facilitated CO₂ mixing and mass transfer in microalgal solution. Therefore, biomass accumulation of microalgae Chlorella in the photoreactor with double paddlewheels increased by 62.3% under 15% CO₂.

Development of a single helical baffle to increase CO2 gas and microalgal solution mixing and Chlorella PY-ZU1 biomass yield

A single helical baffle (SHB), consisting of twisted turns, was developed to convert straight flow into spiral flow in a Chlorella PY-ZU1 open raceway pond (ORWP) bubbled with 15% CO₂. Microalgal solution flowing through the SHB alternative helical interspaces generated whirling flow both vertically and horizontally, which decreased mixing and increased mass transfer rates. The optimized SHB had a pitch length to total SHB length ratio of 0.13 and SHB diameter to ORWP single channel width ratio of 0.30, which decreased mixing times and increased mass transfer coefficients by 41.1% and 38.4% respectively. SHB moved Chlorella PY-ZU1 from the ORWP bottom to the top, increasing light exposure for photosynthesis. Cellular electron transfer rates and photochemical efficiency (φPSII) increased by 18%, chlorophyll a content increased by 16% and variable to maximum fluorescence ratio increased by 13%. The microalgal biomass of SHB ORWP was 23% higher than that of conventional ORWP.

Photo-bioreactor design for microalgae: A review from the aspect of CO2 transfer and conversion

Photobioreactor (PBR) is the most critical equipment for microalgal photosynthetic fixation of CO₂. It provides suitable environmental conditions, such as CO₂, light and nutrients, for microalgal growth. As the major carbon source for microalgae, CO₂ gas is pumped into PBR with the formation of bubbles and formed gas-liquid flow. The gas-liquid flow affects CO₂ and nutrients transmission as well as microalgae cells distribution in PBR, thereby affecting the biochemical reaction of microalgae. While the migration and transport of biochemical reaction products affect the two-phase flow, phase distribution and flow resistance in the PBR in return, thus affecting the transport of light and nutrients. Therefore, microalgal photosynthetic rate is determined synthetically by two-phase flow and the transport of CO₂, light and nutrients in PBR. Deep understanding of gas-liquid two-phase flow, energy and mass transfer coupling with microalgal growth in PBR is the cornerstone for the design of an efficient microalgae PBR.

Developing microporous fibrous-diaphragm aerator to decrease bubble generation diameter for improving microalgal growth with CO2 fixation in a raceway pond

A novel microporous fibrous-diaphragm aerator (FDA) was proposed to generate pressurized gas with check valve to penetrate through millions of micropores (6-126 μm) of expanded fibrous diaphragm on internal support, thus decreasing bubble generation diameter and increasing bubble residence time for improving microalgal growth with CO₂ fixation in a raceway pond. When installation angle of FDA internal support increased from 0° to 45°, bubble generation time and diameter first decreased (to valley bottoms of 4 ms and 0.45 mm at 22°) and then increased. Compared to traditional strip aerator, bubble generation time and diameter decreased by 50% and 60% through FDA with support installation angle of 22° and average pore diameter of 28 µm, while gas-solution mixing time decreased by 22% and mass transfer coefficient increased by 40%, leading to increased actual photochemical efficiency (by 80%) and increased biomass yield (by 38.5%) of Arthrospira cells with pure CO₂ aeration through FDA.

Developing a CO2 bicarbonation absorber for promoting microalgal growth rates with an improved photosynthesis pathway

In order to solve the problems of the short residence time and low utilization efficiency of carbon dioxide (CO₂) gas added directly to a raceway pond, a CO₂ bicarbonation absorber (CBA) was proposed to efficiently convert CO₂ gas and sodium carbonate (Na₂CO₃) solution to sodium bicarbonate (NaHCO₃), which was dissolved easily in the culture medium and left to promote the microalgal growth rate. The CO₂ gas reacted with the Na₂CO₃ solution (initial concentration = 200 mM L⁻¹ and volume ratio in CBA = 60%) for 90 min at 0.3 MPa to give the optimized molar proportion (92%) of NaHCO₃ product in total inorganic carbon and increase the microalgal growth rate by 5.0 times. Quantitative label-free protein analysis showed that the expression levels of the photosystem II (PSII) reaction centre protein (PsbH) and PSII cytochrome (PsbV2) in the photosynthesis pathway increased by 4.8 and 3.4 times, respectively, while that of the RuBisCO enzyme (rbcL) in the carbon fixation pathway increased by 3.5 times in Arthrospira platensis cells cultivated with the NaHCO₃ product in the CBA at 0.3 MPa.

To increase the residence time of CO₂ gas added directly to the raceway pond, a CO₂ bicarbonation absorber was proposed to convert CO₂ gas and Na₂CO₃ to NaHCO₃, which was dissolved easily in the solution and left to promote the biomass growth rate.

Reduced generation time and size of carbon dioxide bubbles in a volute aerator for improving Spirulina sp. growth

A novel volute aerator was proposed to generate shear force and break gas flow through water centrifugation to promote mass transfer for CO₂ fixation with microalgae. The bubble evolution and gas-liquid mixing processes in volute aerator were numerically simulated. The bubble generation time and diameter were measured on a high-speed camera and assessed with level-set method. By optimizing gas inletpipe angle and water/gas inlet velocities of volute aerator, the bubble generation time decreased by 60.1% to 3.3 ms and outlet bubble diameter decreased by 50.7% to 1.8 mm, compared with traditional strip aerator. The corresponding gas-liquid mixing time reduced by 15.6% and mass transfer coefficient increased by 42.2%. The volute aerator was used as an alternative to traditional strip aerator to culture microalgae under high-purity CO₂ condition, which promoted average growth rate and biomass production by 26.6% and 50.7%, respectively.

Enhancing vorticity magnitude of turbulent flow to promote photochemical efficiency and trichome helix pitch of Arthrospira platensis in a raceway pond with conic baffles

In order to clarify vortex mechanisms under a turbulent flow field to explain enhanced biomass productivity, computational fluid dynamics and a miniature Doppler velocimeter were employed to investigate the promoted vorticity magnitude and turbulent kinetic energy to support the increased actual photochemical efficiency of Arthrospira platensis in a raceway pond with alternatively permutated conic baffles. Results showed that whereas the first two parameters increased by 5.9 and 13.9 times, respectively, the third rose on an average by 28% to the value of 0.59 measured on pulse-modulated fluorometer. Furthermore, it was detected on a Nikon inverted-fluorescence microscope that the average helix pitch and trichome length increased by 14% and 10% respectively, resulting in higher biomass productivity (34.8%).

Optimizing the gas distributor based on CO2 bubble dynamic behaviors to improve microalgal biomass production in an air-lift photo-bioreactor

Dynamic behavior of bubbles would significantly affect CO₂ mass transfer and may cause microalgae cells uneven distribution due to the bubble carrying effect. To improve microalgae growth, the gas distributor and aeration conditions was optimized according to the bubble rising behavior. The CO₂ bubble rising trajectory is similar to a Zigzag. The amplitude and wavelength of the Zigzag, which reflected the influenced zone of microalgae suspension in horizontal direction and disturbance intensity on culture, respectively, was controlled by the structure of gas distributor and aeration conditions. An optimized round gas distributor that full of holes with an inner diameter of 0.5mm and spacing of 1.5mm was designed. When cultivated with the optimized gas distributor aerating 5% CO₂ gas at 0.250vvm, the maximum biomass concentration of Chlorella pyrenoidosa achieved 2.88gL^-1, increased by 83.44% compared to that of 1.57gL^-1cultivated with the commercial micro-bubbles aerator.

Developing a water-circulating column photobioreactor for microalgal growth with low energy consumption

A water-circulating column photobioreactor (WCC-PBR) was developed to decrease bubble generation time and mixing time for growing microalgal biomass at low energy consumption. Bubble generation time was decreased by 60.4% and mixing time was decreased by 41.5% owing to an enhanced solution velocity with a water pump. Bubble residence time was decreased by 31.1% and mass transfer coefficient was decreased by 0.4% owing to a reduced distance between air aerator and solution surface. Microalgal growth rate was decreased by 12.7% from 128.9mg/Lday in an air-lifting column photobioreactor (ALC-PBR) to 112.6mg/Lday in a WCC-PBR because of the decrease in residence time of bubbles and an additional shear of cells in a water pump. However, total energy consumption of a WCC-PBR with an air compressor and a water pump was lower by 21.1% than that of an ALC-PBR with only an air compressor.

Improving microalgal growth with small bubbles in a raceway pond with swing gas aerators

A novel swing gas aerator was developed to generate small bubbles for improving the mass transfer coefficient and microalgal growth rate in a raceway pond. A high-speed photography system (HSP) was used to measure the bubble diameter and generation time, and online precise dissolved oxygen probes and pH probes were used to measure the mass transfer coefficient and mixing time. Bubble generation time and diameter decreased by 21% and 9%, respectively, when rubber gas aerators were swung in the microalgae solution. When water pump power and gas aeration rate increased in a raceway pond with swing gas aerators and oscillating baffles (SGAOB), bubble generation time and diameter decreased but solution velocity and mass transfer coefficient increased. The mass transfer coefficient increased by 25% and the solution velocity increased by 11% when SGAOB was used, and the microalgal biomass yield increased by 18%.