Performance Of Photobioreactor Research Articles

CFD modeling of CO2 fixation by microalgae cultivated in a lab scale photobioreactor

The green microalga Chlorella vulgaris has been studied for efficient fixation of carbon dioxide, CO2, from elevated concentrations of in-feed gas flows. In this work, a Computational Fluid Dynamics (CFD) model for algae cultivation including the dependence on CO2 concentration in liquid has been derived based on cultivation experiments of Chlorella vulgaris. The experiments were performed in a laboratory scale cylindrical stirred tank reactor with a range of enriched CO2 concentrations in-feed (0.04–50 %). The model provided by Béchet et al. (2015) for algae cultivation considering dependence on local light, algae concentration and temperature has been implemented for CFD and modified for comprising CO2 dependence. The model has been verified with experimental results for the algae productivity showing the proper trends for dissolved CO2 concentrations and algae concentrations. Thus, the developed CFD model can serve as a tool for evaluating photobioreactor performance for CO2 capture.

A reduced-order hybrid model for photobioreactor performance and biomass prediction

This paper introduces a hybrid approach for photobioreactor modeling tailored to microalgae cultivation, combining data-driven and mechanistic concepts to improve modeling efficiency and practicality for industrial scale-up applications. Most growth models for microalgae are nonlinear and require experimental measurement of several parameters. The aim of this work is to develop linear practical models for monitoring purposes. A model based on linear coefficients and polynomial features is proposed, balancing interpretability with non-linear representation focusing on model transparency. To simplify the growth model, Taylor series expansion is applied to the Monod and logistic population models. Two scale-specific models are developed and evaluated, offering practical solutions for monitoring microalgae growth in photobioreactors. Therefore, this reduced order representation allows the biomass growth rate to be dependent directly on the biomass concentration. These models do not require exhaustive data collection of substrate concentration over time, making them cost-effective and efficient for industrial applications. This work provides a step forward in photobioreactor modeling, contributing to the sustainable production of microalgae.

Hydrodynamic and mass transfer characterization of an internally illuminated airlift photobioreactor for growing Spirulina platensis

Abstract The dimensions of the of a photobioreactor play a major role in its hydrodynamic and mass transfer characteristics that also affect its growth performance. In this study, a bench-scale airlift photobioreactor (PBR) was designed and fabricated to have a riser-to-diameter ratio (Ar/Ad) equal to 0.42 and an aspect ratio (H/D) equal to 1.53. These dimension ratios are different from typical values wherein most airlift photobioreactors are designed to have Ar/Ad of at least 1.0 and H/D of at least 2.0. It was hypothesized, based on the results of Hwang and Cheng [1], the fabricated PBR in this study will have better hydrodynamic and mass transfer characteristics such as lesser mixing time, better liquid circulation and lesser gas entrainment that may lead to better growth performance. Due to the different dimension ratios from other studies, characterization of the PBR and testing of its performance are required. The hydrodynamic and mass transfer characteristics of the fabricated airlift photobioreactor in this study were measured at varying superficial gas velocities (UGR) from 0.0017 ms−1 to 0.0124 ms−1. The growth parameters of the Spirulina platensis were then tested at different aeration rates to relate the hydrodynamic and mass transfer characteristics to the photobioreactor performance. This study has proven that a photobioreactor that is designed and operated to have the best hydrodynamic and mass transfer characteristics will have the best growth performance at the same light intensity and initial optical density.

Leveraging the dynamics of microalgal CO2 capture to estimate the maximum inherent photosynthetic potential

AbstractBACKGROUNDNatural photosynthesis, utilizing intelligent molecular machinery to harness sunlight, stands as the benchmark for efficient energy generation. Despite its high quantum efficiency compared to synthetic methods, challenges persist due to dynamic nutritional and light requirements in plant, microalgae, and cyanobacteria‐driven processes. Reported microalgae CO2 uptake rates rely on biomass dry cell weight measurement after certain incubation period and systematic study of required CO2 concentrations for optimal photobioreactor operation remaining unexplored. This research is thus aimed at evaluating the critical dissolved CO2 (Ccrit ∙ CO2) concentration and specific CO2 uptake rates (SCUR) of Chlorella minutissima (CM) culture in the presence of surplus light and other nutrients by online monitoring and measuring the dynamic CO2 uptake rates which will help in maintaining the optimal rate of CO2 delivery for continuous cultivation of microalgae while minimizing the escape of CO2.RESULTSDissolved Ccrit ∙ CO2 was determined to be in the range of 16–20 mg/L, and the maximum SCUR in BBM (Bold Basal Medium) was found to be 0.73 mg CO2 (mg dcw ∙ h)−1. Interestingly, this SCUR value is nearly three times greater than that of the most efficient in vitro artificial photosynthetic novel pathway reported so far.CONCLUSIONFrom the calculated SCUR values, it may be noted that 9.6 g of biomass can be obtained from 1 g of microalgal inoculum in a day, thereby setting a benchmark for the scientists working in the areas of bioprocess engineering, synthetic biology, and metabolic engineering to comply. © 2024 Society of Chemical Industry (SCI).

Fast development of purple nonsulfur bacteria (PNSB) in a bubble column photobioreactor: Influence of carbon source and dissolved O2 availability on wastewater treatment performance

Photobioreactors with purple nonsulfur bacteria (PNSB) constitute a promising technology for wastewater treatment, gas purification, and high value-added bioproducts. In this work, different operational strategies were tested to investigate their impacts on photobioreactor performance, biomass properties, and microbial community. It showed that the operational conditions significantly influence the performance and microbial composition of PNSB photobioreactors. Anaerobic conditions favored the dominance of Rhodopseudomonas spp. and moderate COD removal efficiency (36.7 %), while aerobic conditions significantly enhanced COD removal efficiency (91.7 %) but altered the microbial community composition from PNSB to aerobic heterotrophic microorganisms, such as Delftia sp. and Microbacterium sp.. The removal ratio of chemical oxygen demand (COD):N:P was 100:10:2 under anaerobic conditions with continuous CO2 supply, while it was 100:1:1 under aerobic conditions with continuous supply of a CO2/O2 mixture. Net CO2 removal efficiencies of 2.8 % and 5.4 % were recorded under anaerobic conditions with and without the addition of organic carbon, respectively. The outlet CO2 concentration showed that CO2 uptake occurred under dark and anaerobic conditions without organic carbon. The reduced production of extracellular polymeric substances (EPS) was observed after COD was removed from the influent, preventing granule formation.

Mathematical Model-Based Optimization of Continuous Flow Photobioreactor Operating at Steady State Using MATLAB Optimization Function

AbstractMicroalgae have received great attention recently due to its potential for sustainable CO2 emission reduction and production of biofuel. These applications are highly dependent on microalgae biomass productivity of photobioreactor (PBR). The objective of this work is to optimize continuous flow PBR at steady state using a mathematical model describes growth of the microalgae species Chlorella vulgaris. The operating conditions that affect the performance of PBR are determined as pH, CO2 percentage in the gas feed, light intensity, and dilution rate. The MATLAB optimization function (fmincon) is used to find the best design and operating variables in the tested region. The maximum biomass productivity is achieved at the upper-bound of the range for pH, CO2 percentage in the gas feed, and light intensity, while for dilution rate, there exists an optimum value that guarantees maximum productivity. The maximum biomass productivity and specific growth rate were estimated as 4.03 and 0.196 billion cells/L h, respectively. This is achieved at optimum dilution rates of 0.049 and 1.0 h−1, respectively. The fmincon optimization results agree well with the literature that used different optimization methods. The model-based optimization predicts the best performance of PBR without conducting experiments. Sensitivity analysis of model constants on biomass concentration showed that mass transfer coefficient KLa has the highest sensitivity followed by µmax, KCL, and KE which has the lowest sensitivity for biomass production. The obtained results are of technological significance for processes such as CO2-fixation and biofuel production. Research effort is needed to exploit optimization results in large-scale cultivation.

A robust multinutrient kinetic model for enhanced lutein and biomass yields in mixotrophic microalgae cultivation: A step towards successful large-scale productions.

Mixotrophic cultivation holds great promise to significantly enhance the productivities of biomass and valuable metabolites from microalgae. In this study, a new kinetic model is developed, explicitly describing the effect of the most influential environmental factors on both biomass growth and the production of the high-value product lutein. This extensive study of multinutrient kinetics for Tetradesmus obliquus in a mixotrophic regime covers various nutritional conditions. Crucial nutrients governing the model include nitrate, phosphate, and glucose. Using seven state variables and 13 unknown parameters, the model's accuracy was ensured through a well-designed two-factor, four-level experimental setup, providing ample data for reliable calibration and validation. Results accurately predict dynamic concentration profiles for all validation experiments, revealing broad applicability. Optimizing nitrogen availability led to significant increases in biomass (up to fourfold) and lutein production (up to 12-fold), with observed maximum biomass concentration of 6.80 g L-1 and lutein reaching 25.58 mg L-1. Noticeably, the model exhibits a maximum specific growth rate of 4.03 day-1, surpassing reported values for photoautotrophic and heterotrophic conditions, suggesting synergistic effects. Valuable guidance is provided for applying the method to various microalgal species and results are large-scale production-ready. Future work will exploit these results to develop real-time photobioreactor operation strategies.

Study of Chlorella sorokiniana Cultivation in an Airlift Tubular Photobioreactor Using Anaerobic Digestate Substrate

Microalgae offer a promising solution for efficiently treating high-nitrogen wastewater and recovering valuable nutrients. To optimize microalgae growth and nutrient assimilation, case-dependent studies are essential to demonstrate the process’s potential. This study aimed to evaluate the treatment capacity of high-nitrogen anaerobic digestion effluent as a nutrient source for a C. sorokiniana microalgal culture in a tubular photobioreactor. The study had two primary objectives: to assess how the concentration and composition of the digestate influence microalgae growth, and to identify the preferred nitrogen forms assimilated by the microalgae during long-term, continuous operation. A 20 L tubular airlift bioreactor was constructed and used in batch mode; various digestate concentrations were examined with ammonia nitrogen levels reaching to 160 mg/L. These experiments revealed a biomass growth rate of up to 130 mg/L/d and an ammonia nitrogen assimilation rate ranging from 8.3 to 12.5 mg/L/d. The presence of phosphorous proved essential for microalgae growth, and the growth entered a stationary phase when the initial phosphorous was fully assimilated. A nitrogen-to-phosphorous (N/P) ratio of 10 supported efficient species growth. While ammonia was the preferred nitrogen form for microalgae, they could also utilize alternative forms such as organic and nitrate nitrogen, depending on the specific digestate properties. The results from the continuous photobioreactor operation confirmed the findings from the batch mode, especially regarding the initial nitrogen and phosphorous content. An important condition for nearly complete ammonia removal was the influent dilution rate, to balance the nitrogen assimilation rate. Moreover, treated effluent was employed as dilution medium, contributing to a more environmentally sustainable water management approach for the entire process, at no cost to the culture growth rate.

Microalgae biofuels: illuminating the path to a sustainable future amidst challenges and opportunities.

The development of microalgal biofuels is of significant importance in advancing the energy transition, alleviating food pressure, preserving the natural environment, and addressing climate change. Numerous countries and regions across the globe have conducted extensive research and strategic planning on microalgal bioenergy, investing significant funds and manpower into this field. However, the microalgae biofuel industry has faced a downturn due to the constraints of high costs. In the past decade, with the development of new strains, technologies, and equipment, the feasibility of large-scale production of microalgae biofuel should be re-evaluated. Here, we have gathered research results from the past decade regarding microalgae biofuel production, providing insights into the opportunities and challenges faced by this industry from the perspectives of microalgae selection, modification, and cultivation. In this review, we suggest that highly adaptable microalgae are the preferred choice for large-scale biofuel production, especially strains that can utilize high concentrations of inorganic carbon sources and possess stress resistance. The use of omics technologies and genetic editing has greatly enhanced lipid accumulation in microalgae. However, the associated risks have constrained the feasibility of large-scale outdoor cultivation. Therefore, the relatively controllable cultivation method of photobioreactors (PBRs) has made it the mainstream approach for microalgae biofuel production. Moreover, adjusting the performance and parameters of PBRs can also enhance lipid accumulation in microalgae. In the future, given the relentless escalation in demand for sustainable energy sources, microalgae biofuels should be deemed a pivotal constituent of national energy planning, particularly in the case of China. The advancement of synthetic biology helps reduce the risks associated with genetically modified (GM) microalgae and enhances the economic viability of their biofuel production.

Communication mediated interaction between bacteria and microalgae advances photogranulation

Recently, photogranules composed of bacteria and microalgae for carbon-negative nitrogen removal receive extensive attention worldwide, yet which type of bacteria is helpful for rapid formation of photogranules and whether they depend on signaling communication remain elusive. Varied signaling communication was analyzed using metagenomic method among bacteria and microalgae in via of two types of experimentally verified signaling molecule from bacteria to microalgae, which include indole-3-acetic acid (IAA) and N-acyl homoserine lactones (AHLs) during the operation of photo-bioreactors. Signaling communication is helpful for the adaptability of bacteria to survive with algae. Compared with non-signaling bacteria, signaling bacteria more easily adapt to the varied conditions, evidenced by the increased abundance in the operated reactors. Signaling bacteria are easier to enter the phycosphere, and they dominate the interactions between bacteria and algae rather than non-signaling bacteria. The co-abundance groups (CAGs) with signaling bacteria possess higher abundance than that without signaling bacteria (22.27 % and 6.67 %). Importantly, signaling bacteria accessibly interact with microalgae, which possess higher degree centralities and 32.50 % of them are keystone nodes in the network, in contrast to only 18.66 % of non-signaling bacteria. Thauera carrying both IAA and AHLs synthase genes are highly enriched and positively correlated with nitrogen removal rate. Our work not only highlights the essential roles of signaling communication between microalgae and bacteria in the development of photogranules, but also enriches our understanding of microbial sociobiology.

Towards Optimisation of Microalgae Cultivation through Monitoring and Control in Membrane Photobioreactor Systems

This research lays a foundation for optimised membrane photobioreactor performance and introduces novel control parameters crucial for advancing microalgae cultivation techniques and promoting environmental sustainability. Particularly, this study presents an innovative solids retention time (SRT) controller designed for a pilot-scale membrane photobioreactor. Employing a fuzzy-logic knowledge-based approach, this controller uses the first derivative of pH data dynamics (pH′) as an input variable, directly correlated with nitrogen recovery rate and biomass productivity when normalised by average light irradiance (I2). Through a feedback mechanism, it regulates daily SRT variations, ensuring stable reactor operation, optimal volatile suspended solids concentration, efficient nitrogen removal, and enhanced biomass productivity. Normalised nitrogen recovery rate, considering solar light irradiance and volatile suspended solids concentration, increased by 51% compared to previous studies employing fixed SRT and hydraulic retention time (HRT). Combining this SRT controller with a previously studied HRT controller could potentially amplify biomass productivity efficiency. In addition, controlling or not controlling the HRT and SRT are assessed in terms of filtration performance and GHG emissions. Finally, a new dissolved-oxygen-based parameter shows promise for continuous microalgae culture control.

Interchangeable modular design and operation of photo-bioreactors for Chlorella vulgaris cultivation towards a zero-waste biorefinery

This study explores diverse cultivation modes for Chlorella vulgaris within a biorefinery at pilot scale that produces both biodiesel by transesterification of waste frying oils and syngas by gasification of organic wood waste. Given microalgae's comparatively modest biofuel yield relative to principal biorefinery products, the microalgae cultivation process is designed on the biofuels production rates. Liquid and gaseous waste streams are recycled inside the biorefinery: crude glycerol is mixed with wood to enhance the quality of syngas, wastewater is fed to microalgae so as flue gas. Also, the oil extracted from microalgae contributes to produce biodiesel and the waste cells are gasified.Considering that the optimal fit for each cultivation mode varies with the shape of the reactor, we propose a modular approach to assemble them in batteries of tubular, bubble flow, and airlift reactors, and present an operating design criterion that can fulfill the mass balance of the plant by adding/transforming the number of units inside the different batteries. Methods to adjust the operating conditions and control the operating parameters are also discussed.The designed configurations were operated recycling nominal waste streams of about 30 L d−1 of wastewater and 90 Nm3 h−1 of flue gas. Results confirm that the most advantageous one, in terms of volume per recycled waste streams, is a battery of 16 airlift reactors, operating in mixotrophic mode, with growing rate of 0.427 d−1, yield of 3.06, glycerol conversion 39 %, CO2 removal 64 % of inlet 6–10 %(mol) concentration. The same nominal waste streams can also be managed by 40 tubular reactors in almost heterotrophic conditions coupled with 12 bubble columns in autotrophic conditions; working respectively at growing rates of 0.395 d−1 and 0.362 d−1 and yields of 2.94 and 2.84. The battery of tubular reactors reached a glycerol conversion of 45 % and the array of bubble columns removed about 51 % of inlet 12–20 %(mol) CO2 concentration. A complete comparison is reported also in terms of dimensionless numbers and pumping/mixing requirements.

A modelling workflow for quantification of photobioreactor performance

In this work we have developed a comprehensive modelling workflow for the quantification of photobioreactor performance. Computational Fluid Dynamics (CFD) modelling combined with Lagrangian particle tracking was used to characterise the flow field inside the reactor; this information was combined with a Monte-Carlo model of light attenuation and a kinetic growth model to predict the performance of the system over the duration of the entire batch. The CFD model was validated against measurements of the overall hold-up, local hold-up and mixing time for superficial velocities between 0.6 and 6 cm s−1 in a pilot-scale bubble column photobioreactor, with the CFD predictions agreeing with the experimental data. Comparison was also made between the predicted biomass concentration and experimental measurements using the diatom Phaeodactylum tricornutum, with the model predictions being in good agreement with the experimental results. The model was used to investigate a range of operating conditions and reactor designs, with the most promising predicted to give a 40 % increase in the biomass productivity. Results from this work can be used for the in-silico design and optimisation of photobioreactor systems, thereby enabling their wider use as a sustainable production technology.

Achieving Discharge Limits in Single-Stage Domestic Wastewater Treatment by Combining Urban Waste Sources and Phototrophic Mixed Cultures.

This work shows the potential of a new way of co-treatment of domestic wastewater (DWW) and a liquid stream coming from the thermal hydrolysis of the organic fraction of municipal solid waste (OFMSW) mediated by a mixed culture of purple phototrophic bacteria (PPB) capable of assimilating carbon and nutrients from the medium. The biological system is an open single-step process operated under microaerophilic conditions at an oxidative reduction potential (ORP) < 0 mV with a photoperiod of 12/24 h and fed during the light stage only so the results can be extrapolated to outdoor open pond operations by monitoring the ORP. The effluent mostly complies with the discharge values of the Spanish legislation in COD and p-values (<125 mg/L; <2 mg/L), respectively, and punctually on values in N (<15 mg/L). Applying an HRT of 3 d and a ratio of 100:7 (COD:N), the presence of PPB in the mixed culture surpassed 50% of 16S rRNA gene copies, removing 78% of COD, 53% of N, and 66% of P. Furthermore, by increasing the HRT to 5 d, removal efficiencies of 83% of COD, 65% of N, and 91% of P were achieved. In addition, the reactors were further operated in a membrane bioreactor, thus separating the HRT from the SRT to increase the specific loading rate. Very satisfactory removal efficiencies were achieved by applying an HRT and SRT of 2.3 and 3 d, respectively: 84% of COD, 49% of N, and 93% of P despite the low presence of PPB due to more oxidative conditions, which step-by-step re-colonized the mixed culture until reaching >20% of 16S rRNA gene copies after 49 d of operation. These results open the door to scaling up the process in open photobioreactors capable of treating urban wastewater and municipal solid waste in a single stage and under microaerophilic conditions by controlling the ORP of the system.

Hybrid photobioreactor operation for the intensified production of Haslea ostrearia and marennine in function of strain variability

Marennine is a high-value compound produced by the benthic diatom Haslea ostrearia, which could be used as an alternative to synthetic antibiotics in aquaculture. Over the last few years, however, low production rates have been impeding its exploitation. Recent progress on H. ostrearia culture devices led to the design of a particular photobioreactor (PBR) for high yield marennine production, which remained to be further studied: the objective of this study is thus to evaluate the advantages of using a submerged membrane photobioreactor (MPBR) for the culture of H. ostrearia by testing the reliability of its performances when changing strains. In addition, the impact of the decrease in cell size through time, as experienced by H. ostrearia along its life cycle, was studied in regard to marennine productivity, thus considering inter- and intra-strain variability. Using the 1 L MPBR proposed by Gargouch et al. (2022) three different strains of H. ostrearia were cultivated in a fed-batch mode and marennine was collected in a semicontinuous way (D = 0,1 d−1) with an ultrafiltration membrane immersed in the culture. Biometrics of the inocula were measured, and biomass development and marennine production were followed over 20 days of culture. Results confirmed the advantage brought by the addition of a membrane inside the culture chamber enabling to maintain a minimal productivity of 2 mg d−1, using only 3 L of fresh culture medium over a 20-day culture. Results also showed that cell downsizing along H. ostrearia life cycle did impact marennine productivity per cell, with smaller cells (i.e. older strains), displaying a 4 to 15 times increase in biovolumetric marennine productivity depending on the strain. Hypotheses explaining this increase in marennine production along the life cycle of H. ostrearia are discussed, considering also changes in the microbiome or epigenetic variations along strain life cycle.

Valorization of potato starch wastewater using anaerobic acidification coupled with Chlorella sorokiniana cultivation

Wastewater from the potato processing industry called protamylasse is rich in proteins and carbohydrates that potentially can be valorized through cultivation of microalgae by mixotrophic metabolism. However, the complex organic compounds are a challenge, as algae grow best on simple compounds such as volatile fatty acids (VFA). This study demonstrates a new two-stage system. First, VFA production was achieved by testing mesophilic and thermophilic anaerobic acidification (AA) at a short hydraulic retention time (HRT; 3.3 and 5 days) resulting in the release of ammonium and phosphate. HRT of 5 days and thermophilic conditions was optimal considering the high acetate yield of 0.23 g and 22 ml CH4 per g volatile solids (VS). Then, Chlorella sorokiniana was chosen based on the obtained growth rate, and better adaption in ammonium-rich AA effluent after screening several tested microalgae (Chlorella sorokiniana, Chlorella vulgaris, Scenedesmus obliquus, and Haematococcus pluvialis). It was cultivated for valorization of nutrients and organics and successfully upscaled to 25 L photobioreactor (PBR) scale under both batch and continuous operation with high dosage of 25% (8.2 g L−1 of VS) of AA effluent at an HRT of 5 days in the PBR. Chlorella sorokiniana removed more than 99% of the chemical oxygen demand (COD) and the VFA during continuous flow PBR operation. This approach contributed to the final removal efficiency of 71%, 91%, and 78% for phosphorus, nitrate, and ammonia, respectively, and production of microalgae biomass with more than 73% protein. Thus, a promising process for simultaneous treatment of high strength wastewater for microalgal protein production.

Performance of membrane photobioreactor for integrated Spirulina strain cultivation and nutrient removal of membrane bioreactor effluent

Water reuse by wastewater treatment techniques is implemented to promote a circular economy strategy and alleviate water shortage. The microalgae-based wastewater treatment is an environmentally friendly tool to treat secondary or tertiary wastewater by removing the remaining nutrients, primarily dissolved nitrogen (N) and phosphorus (P). This research aimed to study the membrane photobioreactor (MPBR) performance to treat nitrate nitrogen (NO3-N) and phosphate (PO4) that remained in the membrane bioreactor (MBR) effluent by varying hydraulic retention times (HRTs) and solids retention times (SRTs). Bench-scale of MPBR (63 L) was continuously operated by cultivating Spirulina sp. TISTR 8875. The LED light intensity of 3000–5000 lux was provided 24 h daily. The experimental studies evaluated HRTs in the range of 6.4–9.7 days and SRTs at 20, 40, 60, and 80 days, and the microalgae cells were not removed (infinite SRT). The results showed that the HRT range of 7.7–7.8 days and SRT range of 60–80 days exhibited NO3-N and PO4 removal efficiencies of 39.3–40.9% and 43.8–46.6%, respectively, with the NO3-N and PO4 removal rates of 2.47–2.55 and 0.22–0.23 mg/L-d, respectively. In addition, iron (Fe) in the form of FeEDTA, has been added to the microalgae cultivation at a concentration of 0.07 g/L. It was found that Fe plays an essential role in the growth of microalgae and increases nutrient removal efficiency. The experimental results indicate that the cultivation of Spirulina sp. TISTR 8875 is expected to treat nutrients by the MPBR system as a promising water reuse and nutrient recovery potential.

Photobioreactors for utility-scale applications: effect of gas-liquid mass transfer coefficient and other critical parameters.

Cultivation of microalgae and controlling its growth and performance in closed photobioreactors (PBRs) are easier than open pond systems for wastewater treatment. The performance of PBRs is influenced by geometry, hydrodynamic behavior, and mass transfer. Horizontal and vertical configurations as common designs of PBR are reviewed based on their features, advantages, and disadvantages. However, vertically operated PBRs like bubble columns are preferably used for utility-scale applications of microalgae-based processes. Moreover, an appropriate reactor design reduces the inhibitory effect of dissolved oxygen concentration produced by microalgae and consequently increases the level of available CO2 in the medium. Medium properties, superficial gas velocity, gas holdup, bubble sizes, shear stress, mixing time, sparger design, and the ratio of inner diameter to effective height are shown to influence the overall volumetric mass transfer coefficient (KLa) and PBR's performance. The vertical PBRs like bubble columns provide a high mass transfer, a short liquid circulation time, and a long frequency of light/dark cycle for utility application of microalgae. Different flow regimes are obtained in PBRs based on the gas flow rate, inner diameter, and medium properties. Hydraulic retention time as the main operational parameter is determined in a batch mode for continuous wastewater treatment.

Mapping the performance of photobioreactors for microalgae cultivation. Part III: subtropical and mid‐latitudes climate zone

AbstractBACKGROUNDIdentifying geographic locations with favorable climates for large‐scale openwork microalgae cultivation is a trend. In light of that, what we propose in this study is to map the performance of photobioreactors for microalgae culture in the subtropical (25° to 35° N and S) and mid‐latitudes (35° to 55° N and S) climate zones. Twenty geographic locations were chosen to represent 14 types of climates in the four seasons of the year. The study is a latitudinal, climatological and seasonal assessment.RESULTSThe results showed that the subtropical climatic zone presents better conditions for microalgae growth with higher levels of biomass productivity and lipid productivity. The highest annual average biomass productivity (0.210 g L−1 d−1) and annual average lipid productivity (0.025 g L−1 d−1) were recorded in a simulation of hot desert climate (BWh). The winter season presents a minor potential for microalgae cultivation.CONCLUSIONThe results of this study shed light on the impact of subtropical and mid‐latitudes climate zone and seasons of the year on the performance of photoautotrophic microalgae‐based processes. © 2023 Society of Chemical Industry (SCI).

A cleaner and smarter way to achieve high microalgal biomass density coupled with facilitated self-flocculation by utilizing bicarbonate as a source of dissolved carbon dioxide

Microalgae feedstock-based bioprocesses are increasingly unfolding as promising futuristic technologies towards achieving some of the sustainable development goals, mostly pertaining to clean energy and environment. The current technologies that involve sparging flue-gas carbon dioxide (CO2) directly into closed or open photobioreactors suffer from very limited bioavailability of sparingly soluble CO2 in dissolved form leading to less algal biomass production. Thus, the present study was designed to assess the ability of Chlorella vulgaris as a model microalga to capture carbon from a dissolved inorganic carbon (DIC) source like sodium bicarbonate (NaHCO3) and grow in photobioreactors to high biomass density. With the aim to develop a proof of this concept, Chlorella vulgaris was grown in minimal medium supplemented with varying doses of initial bicarbonate (50–250 mM). The results demonstrated that with increasing initial bicarbonate (HCO3−) concentration, CO2 fixation rate and algal biomass productivity were enhanced. At 150 mM initial HCO3−, biomass concentration and CO2 fixation rate were increased by about 145% and 38%, respectively, as compared to minimal medium. Biomass concentration was further improved by implementing a smart intermittent bicarbonate feeding strategy, which resulted in an overall enhancement by about 255%, as opposed to the case of minimal medium. It was interesting to observe that Chlorella vulgaris facilitated its own harvesting by self-flocculation-cum-settling mechanism within a short period of time in presence of HCO3−. The efficiency of self-flocculation was found to increase by about 55% in presence of 150 mM initial HCO3−, when compared with minimal medium. High resolution microscopic analyses revealed that microalgal cells were embedded within a self-produced matrix of extracellular polymeric substances (EPS) and underwent alterations in topological complexity that may have facilitated nutrient uptake and gaseous exchange across the cell surface, thereby resulting in enhanced biomass production and self-settling. Therefore, strategic utilization of bicarbonate as a source of DIC convincingly establishes the novelty of our rational approach towards enhancing microalgal biomass coupled with efficient CO2 fixation and self-harvesting. This innovative and smart process may be scaled up and translated to efficiently sequester flue-gas CO2 released by thermal power plants and other fossil fuel-based industries through algal cultivation.