Industrial Photobioreactors and Scale-Up Concepts

Regulation of microclimate and shading effects of microalgal photobioreactors on rooftops: Microalgae as a promising emergent for green roof technology

Urban areas remarkably affect global public health due to their emissions of greenhouse gases and poor air quality. Although urban areas only cover 2% of the Earth's surface, they are responsible for 80% of greenhouse gas emissions. Dense buildings limit vegetation, leading to increased air pollution and disruption of the local and regional carbon cycle. The substitution of urban gray roofs with microalgal green roofs has the potential to improve the carbon cycle by sequestering CO2 from the atmosphere. Microalgae can fix 15-50 times more CO2 than other types of vegetation. Advanced microalgal-based green roof technology may significantly accelerate the reduction of atmospheric CO2 in a more effective way. Microalgal green roofs also enhance air quality, oxygen production, acoustic isolation, sunlight absorption, and biomass production. This endeavor yields the advantage of simultaneously generating protein, lipids, vitamins, and a spectrum of valuable bioactive compounds, including astaxanthin, carotenoids, polysaccharides, and phycocyanin, thus contributing to a green economy. The primary focus of the current work is on analyzing the ecological advantages and CO2 bio-fixation efficiency attained through microalgal cultivation on urban rooftops. This study also briefly examines the idea of green roofs, clarifies the ecological benefits associated with them, discusses the practice of growing microalgae on rooftops, identifies the difficulties involved, and the positive aspects of this novel strategy.

Transcriptomic and metabolomic analyses revealed regulation mechanism of mixotrophic Cylindrotheca sp. glycerol utilization and biomass promotion

BACKGROUND

Diatoms have been viewed as ideal cell factories for production of some high-value bioactive metabolites, such as fucoxanthin, but their applications are restrained by limited biomass yield. Mixotrophy, by using both CO₂ and organic carbon source, is believed effective to crack the bottleneck of biomass accumulation and achieve a sustainable bioproduct supply.

RESULTS

Glycerol, among tested carbon sources, was proved as the sole that could significantly promote growth of Cylindrotheca sp. with illumination, a so-called growth pattern, mixotrophy. Biomass and fucoxanthin yields of Cylindrotheca sp., grown in medium with glycerol (2 g L^-1), was increased by 52% and 29%, respectively, as compared to the autotrophic culture (control) without compromise in photosynthetic performance. As Cylindrotheca sp. was unable to use glycerol without light, a time-series transcriptomic analysis was carried out to elucidate the light regulation on glycerol utilization. Among the genes participating in glycerol utilization, GPDH1, TIM1 and GAPDH1, showed the highest dependence on light. Their expressions decreased dramatically when the alga was transferred from light into darkness. Despite the reduced glycerol uptake in the dark, expressions of genes associating with pyrimidine metabolism and DNA replication were upregulated when Cylindrotheca sp. was cultured mixotrophically. Comparative transcriptomic and metabolomic analyses revealed amino acids and aminoacyl-tRNA metabolisms were enhanced at different timepoints of diurnal cycles in mixotrophic Cylindrotheca sp., as compared to the control.

CONCLUSIONS

Conclusively, this study not only provides an alternative for large-scale cultivation of Cylindrotheca, but also pinpoints the limiting enzymes subject to further metabolic manipulation. Most importantly, the novel insights in this study should aid to understand the mechanism of biomass promotion in mixotrophic Cylindrotheca sp.

Hydrodynamics and mass transfer of concentric-tube internal loop airlift reactors: A review

The concentric-tube internal loop airlift reactor is a typical reactor configuration which has been adopted for a myriad of chemical and biological processes. The reactor hydrodynamics (including mixing) and the mass transfer between the gas and liquid phases remarkably affect the operational conditions and thus are crucial to the overall reactor performance. Hence, this study aims at providing a thorough description of the basic concepts and a comprehensive review of the relevant reported studies on the hydrodynamics and mass transfer of the concentric-tube internal loop airlift reactors, taking microalgae cultivation as an exemplary application. In particular, the reactor characteristics, geometry, CFD modeling, experimental characterization, and scale up considerations are elucidated. The research gaps for future research and development are also identified.

Mass cultivation of marine diatoms using local salts and its impact on growth and productivity

Diatoms are of great interest for many biotechnological applications. The present study highlights the comparative analysis for mass cultivation under the effect of seawater made from table salt (TS), rock salt (RS), and synthetic seawater in the presence of normal silica and induction coupled plasma (ICP) nanosilica (Nano Si) for inducing diatom growth. Out of all the test formulations, RS-f/2 Nano Si showed the best results with maximum cell density (3.16x107±0.04 and 3.24x107±0.05 cells mL^-1), carbohydrate (403.0±3.4 and 398.0±8.1 mg g^-1), and chrysolaminarin yield (66.2±5.5 and 49.3±5.1 mg g^-1) in both Chaetoceros gracilis and Thalassiosira weissflogii respectively. The presence of a rich pigment profile and lipids further highlights the importance of TS and RS for cost-effective mass culturing. Results reveal that mass cultivation of marine diatoms with TS and RS in the presence of nanosilica not only reduces costs but also enhances metabolite production.

Outflow from a Biogas Plant as a Medium for Microalgae Biomass Cultivation—Pilot Scale Study and Technical Concept of a Large-Scale Installation

Microalgae-based technologies have huge potential for application in the environment sector and the bio-energy industry. However, their cost-efficiency has to be improved by drawing on design and operation data for large-scale installations. This paper presents a technical concept of an installation for large-scale microalgae culture on digestate liquor, and the results of a pilot-scale study to test its performance. The quality of non-treated digestate has been shown to be insufficient for direct use as a growth medium due to excess suspended solids, turbidity, and organic matter content, which need to be reduced. To that end, this paper proposes a system based on mechanical separation, flotation, and pre-treatment on a biofilter. The culture medium fed into photobioreactors had the following parameters after the processing: COD—340 mgO2/dm3, BOD5—100 mgO2/dm3, TN—900 mg/dm3, and TP—70 mg/dm3. The installation can produce approx. 720 kgVS/day of microalgal biomass. A membrane unit and a thickening centrifuge (thickener) were incorporated into the design to separate and dehydrate the microalgal biomass, respectively. The total energy consumption approximated 1870 kWh/day.

Submerged membrane photobioreactor for the cultivation of Haslea ostrearia and the continuous extraction of extracellular marennine

Haslea ostrearia is a marine diatom known to produce and excrete the marenine blue pigment. Its controlled, continuous and intensified cultivation remains a challenge. Thus, a submerged membrane photobioreactor (MPBR) was implemented in order to simultaneously and continuously cultivate H. ostrearia and extract marennine. The MPBR was compared with a similar air-lift photobioreactor (without membrane), both working at a dilution rate equal to 0.1, 0.3 and 0.5 d^-1. Contrary to the air-lift photobioreactor, the MPBR successfully operated at high dilution rate without biomass washout. The MPBR allowed continuously recovering marennine and reaching high cell density (555 ± 25 × 10⁶ cells L^-1 at D = 0.1 d^-1), marennine concentration (36.00 ± 0.02 mg L^-1 at D = 0.1 d^-1) and marenine productivity (7.20 ± 0.01 mg L^-1 d^-1 at D = 0.5 d^-1).

Lab-scale photobioreactor systems: principles, applications, and scalability

Phototrophic microorganisms that convert carbon dioxide are being explored for their capacity to solve different environmental issues and produce bioactive compounds for human therapeutics and as food additives. Full-scale phototrophic cultivation of microalgae and cyanobacteria can be done in open ponds or closed photobioreactor systems, which have a broad range of volumes. This review focuses on laboratory-scale photobioreactors and their different designs. Illuminated microtiter plates and microfluidic devices offer an option for automated high-throughput studies with microalgae. Illuminated shake flasks are used for simple uncontrolled batch studies. The application of illuminated bubble column reactors strongly emphasizes homogenous gas distribution, while illuminated flat plate bioreactors offer high and uniform light input. Illuminated stirred-tank bioreactors facilitate the application of very well-defined reaction conditions. Closed tubular photobioreactors as well as open photobioreactors like small-scale raceway ponds and thin-layer cascades are applied as scale-down models of the respective large-scale bioreactors. A few other less common designs such as illuminated plastic bags or aquarium tanks are also used mainly because of their relatively low cost, but up-scaling of these designs is challenging with additional light-driven issues. Finally, this review covers recommendations on the criteria for photobioreactor selection and operation while up-scaling of phototrophic bioprocesses with microalgae or cyanobacteria.

Limnospira indica PCC8005 growth in photobioreactor: model and simulation of the ISS and ground experiments

The Arthrospira-B experiment is the first experiment in space ever allowing the online measurements of both oxygen production rate and growth rate of Limnospira indica PCC8005 in batch photobioreactors running on-board ISS. Four bioreactors were integrated in the ISS Biolab facility. Each reactor was composed of two chambers (gas and liquid) separated by a PTFE membrane and was run in batch conditions. Oxygen production was monitored by online measurement of the total pressure increase in the gas chamber. The experiments are composed of several successive batch cultures for each reactor, performed in parallel on ISS and on ground. In this work, a model for the growth of the cyanobacterium Limnospira indica PCC8005 (also known as Arthrospira or spirulina) in these space membrane photobioreactors was proposed and the simulation results obtained are compared to the experimental results gathered in space and on ground. The photobioreactor model was based on a light transfer limitation model, already used to describe and predict the growth and oxygen production in small to large scale ground photobioreactors. It was completed by a model for pH prediction in the liquid phase allowing assessment of the pH increase associated to the bicarbonate consumption for the biomass growth. A membrane gas-liquid transfer model is used to predict the gas pressure increase in the gas chamber. Substrate limitation is considered in the biological model. A quite satisfactory fit was achieved between experimental and simulation results when a suitable mixing of the liquid phase was maintained. The data showed that microgravity has no first order effect on the oxygen production rate of Limnospira indica PCC8005 in a photobioreactor operating in space in zero gravity conditions.

Effect of design dark fraction on the loss of biomass productivities in photobioreactors

Design dark fraction reflects the unlit part of a microalgal culture system, as for example a hydraulic loop used for temperature or pH regulation, or a circulating pump for mixing purposes. This study investigates the impact of design dark fraction on photosynthetic biomass productivity of the eukaryotic microalgae Chlorella vulgaris. The effect of the volume of the dark fraction and the residence time spent in this dark fraction was investigated with two different nitrogen sources (N-NH₄⁺, N-NO₃^-). Results showed a decrease of biomass productivity when the volume of the dark fraction and the dark residence time increased. Up to 47% loss of biomass productivity could be reached for a design dark fraction [Formula: see text] = 30% of the total culture system volume. This loss was explained as a result of metabolic reactions related to an increase of respiration activity or a decrease of photosynthetic activity in the cells.

Design and Bench-Scale Hydrodynamic Testing of Thin-Layer Wavy Photobioreactors

In a thin-volume photobioreactor where a concentrated suspension of microalgae is circulated throughout the established spatial irradiance gradient, microalgal cells experience a time-variable irradiance. Deploying this feature is the most convenient way of obtaining the so-called “flashing light” effect, improving biomass production in high irradiance. This work investigates the light flashing features of sloping wavy photobioreactors, a recently proposed type, by introducing and validating a computational fluid dynamics (CFD) model. Two characteristic flow zones (straight top-to-bottom stream and local recirculation stream), both effective toward light flashing, have been found and characterized: a recirculation-induced frequency of 3.7 Hz and straight flow-induced frequency of 5.6 Hz were estimated. If the channel slope is increased, the recirculation area becomes less stable while the recirculation frequency is nearly constant with flow rate. The validated CFD model is a mighty tool that could be reliably used to further increase the flashing frequency by optimizing the design, dimensions, installation, and operational parameters of the sloping wavy photobioreactor.

A critical review on designs and applications of microalgae-based photobioreactors for pollutants treatment

The development of the photobioreactors (PBs) is recently noticeable as cutting-edge technology while the correlation of PBs' engineered elements such as modellings, configurations, biomass yields, operating conditions and pollutants removal efficiency still remains complex and unclear. A systematic understanding of PBs is therefore essential. This critical review study is to: (1) describe the modelling approaches and differentiate the outcomes; (2) review and update the novel technical issues of PBs' types; (3) study microalgae growth and control determined by PBs types with comparison made; (4) progress and compare the efficiencies of contaminants removal given by PBs' types and (5) identify the future perspectives of PBs. It is found that Monod model's shortcoming in internal substrate utilization is well fixed by modified Droop model. The corroborated data also remarks an array of PBs' types consisting of flat plate, column, tubular, soft-frame and hybrid configuration in which soft-frame and hybrid are the latest versions with higher flexibility, performance and smaller foot-print. Flat plate PBs is observed with biomass yield being 5 to 20 times higher than other PBs types while soft-frame and membrane PBs can also remove pharmaceutical and personal care products (PPCPs) up to 100%. Looking at an opportunity for PBs in sustainable development, the flat plate PBs are applicable in PB-based architectures and infrastructures indicating an encouraging revenue-raising potential.