Improvement on light penetrability and microalgae biomass production by periodically pre-harvesting Chlorella vulgaris cells with culture medium recycling

Spirulina (Arthrospira) cultivation in photobioreactors: From biochemistry and physiology to scale up engineering

Spirulina (Arthrospira) has been extensively applied in CO2 biofixation, wastewater purification, and value-added bioproducts preparation. Light availability plays a pivotal role in Spirulina photoautotrophic cultivation, which is primary determined by characteristics of incident light and distribution of light within photobioreactors (PBRs). To clarify the role of light in Spirulina photoautotrophic cultivation, this review first analyzes the processes of light delivery and conversion in suspended PBRs. Then, effects of key light characteristics, including light intensity, spectrum, and photoperiod, on Spirulina growth and intracellular biochemical components synthesis are comprehensively summarized. Recent advancements in innovative PBR designs aimed at enhancing light utilization efficiency and promoting Spirulina growth are also highlighted. Finally, potential future research directions in the field of Spirulina photoautotrophic cultivation are outlined. Overall, this work provides a theoretical foundation and technical guidance for improving Spirulina production and specific target products synthesis from prespectives of light conditions regulation and PBRs design.

Performance and feasibility analysis of an integrated airlift microalgae photobioreactor for cultivation and pre-harvesting

Microalgae technology is highly attractive in the realm of wastewater treatment and CO2 removal. However, during the cultivation of microalgae, the phenomenon of light attenuation intensifies with the increasing cell concentration, resulting in a decrement in microalgal growth rate. To maintain high light transmittance and growth rate of microalgae, this study introduces a two-step pre-harvesting process involving flocculation and filtration within an airlift photobioreactor. In this way, the growth performance of microalgae was bolstered by an improvement in light availability, which was increased 1.59 times by harvesting 30% of the microalgae biomass. Additionally, the microalgae productivity was increased 12.3%. By diluting the cationic starch concentration to 3 g L-1, the harvesting efficiency approached levels comparable to those achieved through magnetic stirring, while filtration resulted in a 1.9-fold increase in final biomass concentration. Moreover, a comparative life-cycle assessment of three harvesting methods revealed that the flocculation-filtration method exhibited the lowest global warming potential of -0.09 CO2 eq, positioning it as a viable and low-carbon alternative for microalgae harvesting.

Photobioreactor configurations in cultivating microalgae biomass for biorefinery

Microalgae, highly prized for their protein, lipid, carbohydrate, phycocyanin, and carotenoid-rich biomass, have garnered significant industrial attention in the context of third-generation (3G) biorefineries, seeking sustainable alternatives to non-renewable resources. Two primarily cultivation methods, open ponds and closed photobioreactors systems, have emerged. Open ponds, favored for their cost-effectiveness in large-scale industrial production, although lacking precise environmental control, contrast with closed photobioreactors, offering controlled conditions and enhanced biomass production at the laboratory scale. However, their high operational costs challenge large-scale deployment. This review comprehensively examines the strength, weakness, and typical designs of both outdoor and indoor microalgae cultivation systems, with an emphasis on their application in terms of biorefinery concept. Additionally, it incorporates techno-economic analyses, providing insights into the financial aspects of microalgae biomass production. These multifaceted insights, encompassing both technological and economic dimensions, are important as the global interest in harnessing microalgae's valuable resources continue to grow.

Realization process of microalgal biorefinery: The optional approach toward carbon net-zero emission

Increasing carbon dioxide (CO2) emission has already become a dire threat to the human race and Earth's ecology. Microalgae are recommended to be engineered as CO2 fixers in biorefinery, which play crucial roles in responding climate change and accelerating the transition to a sustainable future. This review sorted through each segment of microalgal biorefinery to explore the potential for its practical implementation and commercialization, offering valuable insights into research trends and identifies challenges that needed to be addressed in the development process. Firstly, the known mechanisms of microalgal photosynthetic CO2 fixation and the approaches for strain improvement were summarized. The significance of process regulation for strengthening fixation efficiency and augmenting competitiveness was emphasized, with a specific focus on CO2 and light optimization strategies. Thereafter, the massive potential of microalgal refineries for various bioresource production was discussed in detail, and the integration with contaminant reclamation was mentioned for economic and ecological benefits. Subsequently, economic and environmental impacts of microalgal biorefinery were evaluated via life cycle assessment (LCA) and techno-economic analysis (TEA) to lit up commercial feasibility. Finally, the current obstacles and future perspectives were discussed objectively to offer an impartial reference for future researchers and investors.

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.

Bioenergy, Biofuels, Lipids and Pigments—Research Trends in the Use of Microalgae Grown in Photobioreactors

This scientometric review and bibliometric analysis aimed to characterize trends in scientific research related to algae, photobioreactors and astaxanthin. Scientific articles published between 1995 and 2020 in the Web of Science and Scopus bibliographic databases were analyzed. The article presents the number of scientific articles in particular years and according to the publication type (e.g., articles, reviews and books). The most productive authors were selected in terms of the number of publications, the number of citations, the impact factor, affiliated research units and individual countries. Based on the number of keyword occurrences and a content analysis of 367 publications, seven leading areas of scientific interest (clusters) were identified: (1) techno-economic profitability of biofuels, bioenergy and pigment production in microalgae biorefineries, (2) the impact of the construction of photobioreactors and process parameters on the efficiency of microalgae cultivation, (3) strategies for increasing the amount of obtained lipids and obtaining biodiesel in Chlorella microalgae cultivation, (4) the production of astaxanthin on an industrial scale using Haematococcus microalgae, (5) the productivity of biomass and the use of alternative carbon sources in microalgae culture, (6) the effect of light and carbon dioxide conversion on biomass yield and (7) heterotrophy. Analysis revealed that topics closely related to bioenergy production and biofuels played a dominant role in scientific research. This publication indicates the directions and topics for future scientific research that should be carried out to successfully implement economically viable technology based on microalgae on an industrial scale.

Improving Microalgal Biomass Productivity Using Weather-Forecast-Informed Operations

The operation of microalgal cultivation systems, such as culture dilution associated with harvests, affects biomass productivity. However, the constantly changing incident light and ambient temperature in the outdoor environment make it difficult to determine the operational parameters that result in optimal biomass growth. To address this problem, we present a pond operation optimization tool that predicts biomass growth based on future weather conditions to identify the optimal dilution rate that maximizes biomass productivity. The concept was tested by comparing the biomass productivities of three dilution scenarios: standard batch cultivation (no dilution), fixed-rate dilution (harvest 60% of the culture every three days), and weather-forecast-informed dilution. In the weather-forecast-informed case, the culture was diluted daily, and the dilution ratio was optimized by the operation optimization tool according to the future 24 h weather condition. The results show that the weather-forecast-informed dilution improved the biomass productivity by 47% over the standard batch cultivation and 20% over the fixed-rate dilution case. These results demonstrate that the pond operation optimization tool could help pond operators to make decisions that maximize biomass growth in the field under ever-changing weather conditions.

A Simulation Analysis of a Microalgal-Production Plant for the Transformation of Inland-Fisheries Wastewater in Sustainable Feed

The present research evaluates the simulation of a system for transforming inland-fisheries wastewater into sustainable fish feed using Designer® software. The data required were obtained from the experimental cultivation of Chlorella sp. in wastewater supplemented with N and P. According to the results, it is possible to produce up to 11,875 kg/year (31.3 kg/d) with a production cost of up to 18 (USD/kg) for dry biomass and 0.19 (USD/bottle) for concentrated biomass. Similarly, it was possible to establish the kinetics of growth of substrate-dependent biomass with a maximum production of 1.25 g/L after 15 days and 98% removal of available N coupled with 20% of P. It is essential to note the final production efficiency may vary depending on uncontrollable variables such as climate and quality of wastewater, among others.

Boosting photo-biochemical conversion and carbon dioxide bio-fixation of Chlorella vulgaris in an optimized photobioreactor with airfoil-shaped deflectors

Aiming at ameliorating the poor hydrodynamic regimes and uneven light distribution in the conventional airlift flat-plate photobioreactor (AFP-PBR), a novel PBR with static airfoil-shaped deflectors (ASD-PBR) is proposed in this study to boost the microalgal biomass manipulation and hence the photo-biochemical conversion. The ASD module accelerated the circulation of microalgal suspension from the center to two sides with the help of bubbling so that the microalgal cells got more opportunities to access the light source. Compared with the control PBR, the solution velocity along the incident light direction increased by 114.8% in the newly-proposed ASD-PBR. Furthermore, the ASD module also served as a static mixer, which resulted in an increment of 11.5% in mass transfer coefficient and a decrement of 21.4% in mixing time. The amended hydrodynamic characteristics eventually contributed to an improvement of 18.3% and 10.9% in the maximum algal biomass yield and CO₂ fixation rate, respectively.

Treatment of clean in place (CIP) wastewater using microalgae: Nutrient upcycling and value-added byproducts production

CIP wastewater is one of the major wastewater streams from the food industry, and its treatment is generally expensive, requiring a large effort to reduce its typically high nitrogen (N), and phosphorus (P) contents. Microalgae-based wastewater treatment is increasingly explored as a more sustainable alternative to the conventional methods, due to the added benefit of nutrient upcycling and value-added biomass production. For the first time, four microalgae species were used to treat CIP wastewater high in N (565.5 mg NO₃^--N/l) and P (98.0 mg PO₄^3--P/l). An intermittent biomass harvesting strategy was adopted in this study to enhance the purification of CIP water and redirection of nutrients into algal biomass. Over 93 days operation, N removal efficiency was 52.1 ± 2.9%, 54.8 ± 2.5%, 50.0 ± 2.3% and 48.3 ± 0.5%, and P removal efficiency was 65.5 ± 10.0%, 79.4 ± 6.1%, 61.8 ± 2.5% and 69.1 ± 7.7% for Chlamydomonas reinhardtii, Chlorella vulgaris, Scenedesmus obliquus and wastewater borne microalgae, respectively. After the first (acclimatization) and second growth cycles, cell growth and nutrient removal slowed down but increased again after adding trace nutrients, indicating the lack of trace elements after the first two growth cycles. In the fourth and fifth batch runs, both algal growth rate and nutrient removal rate decreased despite adding trace nutrients and/or increasing light intensity, this being a consequence of the excreted soluble algal products accumulating during long-term operation. S. obliquus had the highest protein concentration of 44.5 ± 9.8% DW, while C. vulgaris accumulated the highest total lipid content (15.6 ± 0.9%, DW). In this proof-of-concept study, the cultivation of microalgae in CIP wastewater with an intermittent harvest of the accumulated algal biomass is demonstrated and it outlines the potential of microalgae to sustainably treat effluents with extremely high nutrients concentration while producing the food-grade algae biomass.

Recent Advances in Carbon Dioxide Conversion: A Circular Bioeconomy Perspective

Managing the concentration of atmospheric CO2 requires a multifaceted engineering strategy, which remains a highly challenging task. Reducing atmospheric CO2 (CO2R) by converting it to value-added chemicals in a carbon neutral footprint manner must be the ultimate goal. The latest progress in CO2R through either abiotic (artificial catalysts) or biotic (natural enzymes) processes is reviewed herein. Abiotic CO2R can be conducted in the aqueous phase that usually leads to the formation of a mixture of CO, formic acid, and hydrogen. By contrast, a wide spectrum of hydrocarbon species is often observed by abiotic CO2R in the gaseous phase. On the other hand, biotic CO2R is often conducted in the aqueous phase and a wide spectrum of value-added chemicals are obtained. Key to the success of the abiotic process is understanding the surface chemistry of catalysts, which significantly governs the reactivity and selectivity of CO2R. However, in biotic CO2R, operation conditions and reactor design are crucial to reaching a neutral carbon footprint. Future research needs to look toward neutral or even negative carbon footprint CO2R processes. Having a deep insight into the scientific and technological aspect of both abiotic and biotic CO2R would advance in designing efficient catalysts and microalgae farming systems. Integrating the abiotic and biotic CO2R such as microbial fuel cells further diversifies the spectrum of CO2R.

Various potential techniques to reduce the water footprint of microalgal biomass production for biofuel-A review

Due to their rapid growth rates, high lipid productivity, and ability to synthesize value-added products, microalgae are considered as the potential biofuel feedstocks. However, among the several bottlenecks that are hindering the commercialization of microalgal biofuel synthesis, the issue of high water consumption is the least explored. This analysis, therefore, examines the factors that decide water use for the production of microalgae biofuel. Microalgae biodiesel water footprint varies from 3.5 to 3726 kg of water per kg of biodiesel. The study further investigates the cause for large variability in the estimation of the water footprint for microalgae fuel. Various strategies, including the reuse of harvested water, the use of high density cultivation that could be adopted for low water consumption in microalgal biofuel production are discussed. Specifically, the review identified a reciprocal relationship between biomass productivity and water footprint. On the basis of which the review emphasizes the significance of high density cultivation, which can be inexpensive and feasible relative to other water-saving techniques. With the setback of water scarcity due to the rapid industrialization in developing countries, the implementation of the cultivation system with a focus on minimizing the water consumption is inevitable for a successful large scale microalgal biofuel production.

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.

Microalgal bioenergy production under zero-waste biorefinery approach: Recent advances and future perspectives

In view of the globalization and energy consumption, an economic and sustainable biorefinery model is essential to address the energy security and climate change. From this perspective, renewable biofuel production from microalgae along with a wide range of value-added co-products define its potential as a biorefinery feedstock. However, economic viability of microalgal biorefinery at its current state is not considered sustainable. Reduce, recycle, and reuse of waste derived from algal bioenergy conversion process will lead to an energy efficient and sustainable zero-waste microalgal biorefinery. This review focuses on three major aspects of zero-waste microalgal biorefinery approach; (1) recent advances on microalgal bioenergy conversion processes (chemical, biochemical and thermochemical); (2) mitigation and transformation of liquid and solid waste and (3) techno-economic analysis (TEA) and lifecycle assessment (LCA). In addition, the study also focuses on the challenges and future perspectives for an advanced microalgal biorefinery model.

Optimizing light distribution and controlling biomass concentration by continuously pre-harvesting Spirulina platensis for improving the microalgae production

To improve the microalgae production in batch cultivation, a cultivation mode that continuously pre-harvesting Spirulina platensis from photobioreactor (PBR) with culture medium recycling was proposed. For realizing the continuously pre-harvesting cultivation mode, a Spirulina platensis culture column PBR with overflowing device was designed, which could adjust pre-harvesting rate through the overflowing device. By adjusting the pre-harvesting rate, the biomass concentration could be kept when biomass accumulation and pre-harvesting biomass were equal. Hence, the meridional light attenuation could be reduced by controlling biomass concentration in PBR. The maximum microalgae production were 44.6%, 10.98% higher in total production than that cultivated in batch cultivation without pre-harvesting and periodically pre-harvesting cultivation mode respectively, which was realized in pre-harvesting rate 0.228 mL min^-1 and biomass concentration 1.8 g L^-1. Besides, a model was built by mass balance and polynomial fitting for evaluating the continuously pre-harvesting cultivation mode.

The kinetics of the polyacrylic superabsorbent polymers swelling in microalgae suspension to concentrate cells density

Different from current harvesting methods, the aim of this study was to concentrate microalgae by removing the medium with polyacrylic superabsorbent polymers (PSAPs). This method can concentrate freshwater microalgae Chlorella sp. at a relatively high biomass concentration of 90.23 g L^-1. Without further dewatering, the concentrated microalgae can be directly used to produce biofuels by oil extraction or fermentation. The kinetic characteristics of PSAPs swelling in different solutions were investigated. The results indicate that the negative influence on absorbency caused by ionic strength was greater than microalgae concentration. Compared with the diffusion part, water absorbed by the relaxation of PSAPs was dominant and accounted for over 97%. Equilibrium absorbed water equations under different microalgae concentration were fitted and could provide guide to quantifiably concentrate microalgae. Increasing temperature decreased the absorbency of PSAPs, while, the absorption and desorption rate were increased. Moreover, the absorbency remained at 91.45% after recycling three times.

Physiological-phased kinetic characteristics of microalgae Chlorella vulgaris growth and lipid synthesis considering synergistic effects of light, carbon and nutrients

To comprehensively understand kinetic characteristics of microalgae growth and lipid synthesis in different phases, a phase-feeding strategy was proposed to simultaneously regulate light, carbon and nutrients in adaption, growth and stationary phases of microalgae cultivation. Physiological-phased kinetic characteristics of microalgae Chlorella vulgaris growth and lipid synthesis under synergistic effects of light, carbon and nutrients were investigated, and supply-demand relationships of electrons and energy between light and dark reactions of photosynthesis process were discussed. Finally, the optimized cultivation strategy for microalgae in various phases were obtained, under which the lipid productivity was significantly improved from 130.11 mg/L/d to 163.42 mg/L/d. The study provided some important guidance for the large-scale production of biofuels from microalgae.

High-value biomass from microalgae production platforms: strategies and progress based on carbon metabolism and energy conversion

Microalgae are capable of producing sustainable bioproducts and biofuels by using carbon dioxide or other carbon substances in various cultivation modes. It is of great significance to exploit microalgae for the economical viability of biofuels and the revenues from high-value bioproducts. However, the industrial performance of microalgae is still challenged with potential conflict between cost of microalgae cultivation and revenues from them, which is mainly ascribed to the lack of comprehensive understanding of carbon metabolism and energy conversion. In this review, we provide an overview of the recent advances in carbon and energy fluxes of light-dependent reaction, Calvin-Benson-Bassham cycle, tricarboxylic acid cycle, glycolysis pathway and processes of product biosynthesis in microalgae, with focus on the increased photosynthetic and carbon efficiencies. Recent strategies for the enhanced production of bioproducts and biofuels from microalgae are discussed in detail. Approaches to alter microbial physiology by controlling light, nutrient and other environmental conditions have the advantages of increasing biomass concentration and product yield through the efficient carbon conversion. Engineering strategies by regulating carbon partitioning and energy route are capable of improving the efficiencies of photosynthesis and carbon conversion, which consequently realize high-value biomass. The coordination of carbon and energy fluxes is emerging as the potential strategy to increase efficiency of carbon fixation and product biosynthesis. To achieve more desirable high-value products, coordination of multi-stage cultivation with engineering and stress-based strategies occupies significant positions in a long term.

Cultivation of Chlorella vulgaris on anaerobically digested swine manure with daily recycling of the post-harvest culture broth

In this work, a cultivation system with daily recycling of the post-harvest culture broth was set up and performed in order to reuse the water and nutrients in pretreated anaerobically digested swine manure, which was used as media to cultivate Chlorella vulgaris (UTEX 2714) at different recycling ratios. Results showed that the alga grew well in the system with an accumulative algal biomass and productivity of 1.68-3.47g/L and 234.1-532.2mg/L/d, respectively, at the end of the cultivation. Additionally, chemical compositions in this alga varied with the change of recycling ratios, and the highest productivities of carbohydrate, protein and lipids (76.4, 257.2 and 183.7mg/L/d, respectively) were obtained in the system with a recycling ratio of 1/4 or 1/6. Fatty acid profiles indicated that this alga could be used as a good-quality biodiesel feedstock with a biodiesel productivity of 9.65-40.1mg/L/d.

Simultaneous enhancement of Chlorella vulgaris growth and lipid accumulation through the synergy effect between light and nitrate in a planar waveguide flat-plate photobioreactor

Interval between adjacent planar waveguides and light intensity emitted from waveguide surface were the primary two factors affecting light distribution characteristics in the planar waveguide flat-plate photobioreactor (PW-PBR). In this paper, the synergy effect between light and nitrate in the PW-PBR was realized to simultaneously enhance microalgae growth and lipid accumulation. Under an interval of 10mm between adjacent planar waveguides, 100% of microalgae cells in regions between adjacent waveguides could be illuminated. Chlorella vulgaris growth and lipid accumulation were synchronously elevated as light intensities emitted from planar waveguide surface increasing. With an identical initial nitrate concentration of 18mM, the maximum lipid content (41.66% in dry biomass) and lipid yield (2200.25mgL^-1) were attained under 560μmolm^-2s^-1, which were 86.82% and 133.56% higher relative to those obtained under 160μmolm^-2s^-1, respectively. The PW-PBR provides a promising way for microalgae lipid production.

Integrating planar waveguides doped with light scattering nanoparticles into a flat-plate photobioreactor to improve light distribution and microalgae growth

Industrially manufactured planar waveguides doped with light scattering nanoparticles, which can dilute and redistribute the intense incident light within microalgae suspension more uniformly, were introduced into a flat-plate photobioreactor (PBR) with a width of 25cm to alleviate the adverse effect of poor light penetrability on microalgae growth. Compared with the flat-plate PBR without waveguides, the illumination surface area per unit volume in the proposed PBR was increased by 10.3 times. During the whole cultivation period, the illuminated volume fractions in the proposed PBR were 21.4-410% higher than those in the flat-plate PBR without waveguides. Consequently, attributed to the optimized light distribution in the proposed PBR, a 220% improvement in biomass production was obtained relative to that in the flat-plate PBR without waveguides. Furthermore, higher light output intensities emitted from the planar waveguide surfaces and increased microalgae growth rates were achieved by decreasing the length of planar waveguides.