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

Nanotechnology for the enhancement of algal cultivation and bioprocessing: Bridging gaps and unlocking potential

Algae cultivation and bioprocessing are important due to algae's potential to effectively tackle crucial environmental challenges like climate change, soil and water pollution, energy security, and food scarcity. To realize these benefits high algal biomass production and valuable compound extraction are necessary. Nanotechnology can significantly improve algal cultivation through enhanced nutrient uptake, catalysis, CO2 utilization, real-time monitoring, cost-effective harvesting, etc. Synthetic nanoparticles are extensively used due to ease of manufacturing and targeted application. Nonetheless, there is a growing interest in transitioning to environmentally friendly options like natural and 'green' nanoparticles which are produced from renewable/biological sources by using eco-friendly solvents. Presently, natural, and 'green' nanoparticles are predominantly utilized in algal harvesting, with limited application in other areas, the reasons for which remain unclear. This review aims to critically evaluate research on nanotechnology-based algae system enhancement, identify research gaps and propose solutions using natural and 'green' nanoparticles for a sustainable future.

Advanced wastewater treatment with microalgae-indigenous bacterial interactions

Microalgal-indigenous bacterial wastewater treatment (MBWT) emerges as a promising approach for the concurrent removal of nitrogen (N) and phosphorus (P). Despite its potential, the prevalent use of MBWT in batch systems limits its broader application. Furthermore, the success of MBWT critically depends on the stable self-adaptation and synergistic interactions between microalgae and indigenous bacteria, yet the underlying biological mechanisms are not fully understood. Here we explore the viability and microbial dynamics of a continuous flow microalgae-indigenous bacteria advanced wastewater treatment system (CFMBAWTS) in processing actual secondary effluent, with a focus on varying hydraulic retention times (HRTs). The research highlights a stable, mutually beneficial relationship between indigenous bacteria and microalgae. Microalgae and indigenous bacteria can create an optimal environment for each other by providing essential cofactors (like iron, vitamins, and indole-3-acetic acid), oxygen, dissolved organic matter, and tryptophan. This collaboration leads to effective microbial growth, enhanced N and P removal, and energy generation. The study also uncovers crucial metabolic pathways, functional genes, and patterns of microbial succession. Significantly, the effluent NH4+-N and P levels complied with the Chinese national Class-II, Class-V, Class-IA, and Class-IB wastewater discharge standards when the HRT was reduced from 15 to 6 h. Optimal results, including the highest rates of CO2 fixation (1.23 g L-1), total energy yield (32.35 kJ L-1), and the maximal lipid (33.91%) and carbohydrate (41.91%) content, were observed at an HRT of 15 h. Overall, this study not only confirms the feasibility of CFMBAWTS but also lays a crucial foundation for enhancing our understanding of this technology and propelling its practical application in wastewater treatment plants.

Exploring effective light spectral conversion techniques for enhanced production of Spirulina-derived blue pigment protein, c-phycocyanin

Spirulina is a promising feedstock for c-phycocyanin, a blue pigment-protein, commercially incorporated in many food products for its desirable bright blue attributes, exceptional bioavailability, and inherent therapeutic properties. Remarkably, enhancing c-phycocyanin synthesis in Spirulina would facilitate economic viability and sustainability at large-scale production, as the forecasted market value is $ 409.8 million by 2030. Notably, the lighting source plays a key role in enhancing c-phycocyanin in Spirulina, and thus, strategies to filter/concentrate the photons of respective wavelengths, influencing light spectra, are beneficial. Enveloping open raceway ponds and greenhouses by luminescent solar concentrators and light filtering sheets enables solar spectral conversion of the sunlight at desirable wavelengths, emerges as a promising strategy to enhance synthesis of c-phycocyanin in Spirulina. Nevertheless, the conduction of techno-economic assessments and evaluation of scalability at large-scale cultivation of Spirulina are essential for the real-time implementation of lighting strategies.

Unlocking the potential of microalgae bio-factories for carbon dioxide mitigation: A comprehensive exploration of recent advances, key challenges, and energy-economic insights

Microalgae are promising alternatives to mitigate atmospheric CO₂ owing to their fast growth rates, resilience in the face of adversity and ability to produce a wide range of products, including food, feed supplements, chemicals, and biofuels. However, to fully harness the potential of microalgae-based carbon capture technology, further advancements are required to overcome the associated challenges and limitations, particularly with regards to enhancing CO₂ solubility in the culture medium. This review provides an in-depth analysis of the biological carbon concentrating mechanism and highlights the current approaches, including species selection, optimization of hydrodynamics, and abiotic components, aimed at improving the efficacy of CO₂ solubility and biofixation. Moreover, cutting-edge strategies such as gene mutation, bubble dynamics and nanotechnology are systematically outlined to elevate the CO₂ biofixation capacity of microalgal cells. The review also evaluates the energy and economic feasibility of using microalgae for CO₂ bio-mitigation, including challenges and prospects for future development.

Integrating wind-driven agitating blade into a floating photobioreactor to enhance fluid mixing and microalgae growth

Aiming at optimizing the poor fluid mixing state in the traditional horizontal floating photobioreactors and reducing the high energy consumption and operational cost induced by electric-driven mixing, a novel floating photobioreactor with an embedded wind-driven agitating blade (WDAB-FPBR) was proposed in this study, which can effectively utilize both wind and wave energy for fluid mixing. The results show that the selected wind-driven agitating blade contributed to a decrement of 75.3% in mixing time and an increment of 87.5% in mass transfer coefficient, and meanwhile strengthened the fluid velocity along the light gradient. Owing to the enhanced fluid flow and mixing properties, an even distribution of algae cells was achieved in the WDAB floating photobioreactor, which resulted in an improvement of 140% in the photosynthesis efficiency of microalgae. From this, the biomass yield and carbon removal ratio showed an increment of 88.9% and 73.9%, respectively.

Recent advances on microalgae cultivation for simultaneous biomass production and removal of wastewater pollutants to achieve circular economy

Microalgae are known for containing high value compounds and its significant role in sequestering carbon dioxide. This review mainly focuses on the emerging microalgae cultivation technologies such as nanomaterials technology that can improve light distribution during microalgae cultivation, attached cultivation and co-cultivation approaches that can improve growth and proliferation of algal cells, biomass yield and lipid accumulation in microalgal. This review includes a comprehensive discussion on the use of microbubbles technology to enhance aerated bubble capacity in photobioreactor to improve microalgal growth. This is followed by discussion on the role of microalgae as phycoremediation agent in removal of contaminants from wastewater, leading to better water quality and high productivity of shellfish. The review also includes techno-economic assessment of microalgae biorefinery technology, which is useful for scaling up the microalgal biofuel production system or integrated microalgae-shellfish cultivation system to support circular economy.

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.

Microalgae cultivation for antibiotic oxytetracycline wastewater treatment

Microalgae-based technology provides a potential approach to biologically treating oxytetracycline (OTC) wastewater due to its environmental friendliness, low cost, and high efficiency. However, the OTC degradation and transformation characteristics by microalgae are still unclear and need further exploration. This study used microalgae Chlorella sorokiniana MB-1 for OTC wastewater treatment. The OTC with an initial concentration less than 50 mg L^-1 promoted microalgae growth, while OTC with a concentration higher than 100 mg L^-1 inhibited microalgae growth significantly. More than 99% OTC was removed with the biomass productivity up to 1.8 g L^-1 when treated OTC with 10 mg L^-1 initial concentration for 7 days. Chlorophyll and total sugar contents decreased, while protein and lipid contents increased compared to the control without OTC. The malondialdehyde content firstly reduced but subsequently enhanced when increased OTC concentration, while superoxide dismutase content gradually enhanced, manifesting that traces of OTC stimulate microalgae antioxidant capacity, while the increasing OTC caused further oxidative damage to microalgae cells. The removal pathways of OTC mainly include photolysis (75.8%), biodegradation (17.8%), biosorption (3.6%), and hydrolysis (2.7%). Overall, removing OTC by microalgae was confirmed to be an excellent technology for treating antibiotics wastewater whilst accumulating microalgae biomass.

Oleaginous Microalga Coccomyxa subellipsoidea as a Highly Effective Cell Factory for CO2 Fixation and High-Protein Biomass Production by Optimal Supply of Inorganic Carbon and Nitrogen

Microalgae used for CO₂ biofixation can effectively relieve CO₂ emissions and produce high-value biomass to achieve “waste-to-treasure” bioconversion. However, the low CO₂ fixation efficiency and the restricted application of biomass are currently bottlenecks, limiting the economic viability of CO₂ biofixation by microalgae. To achieve high-efficient CO₂ fixation and high-protein biomass production, the oleaginous microalga Coccomyxa subellipsoidea (C. subellipsoidea) was cultivated autotrophically through optimizing inorganic carbon and nitrogen supply. 0.42 g L⁻¹ NaHCO₃ supplemented with 2% CO₂ as a hybrid carbon source resulted in high biomass concentration (3.89 g L⁻¹) and productivity (318.33) with CO₂ fixation rate 544.21 mg L⁻¹ d⁻¹ in shake flasks. Then, used in a 5-L photo-fermenter, the maximal protein content (60.93% DW) in batch 1, and the highest CO₂ fixation rate (1043.95 mg L⁻¹ d⁻¹) with protein content (58.48% DW) in batch 2 of repeated fed-batch cultures were achieved under 2.5 g L⁻¹ nitrate. The relative expression of key genes involved in photosynthesis, glycolysis, and protein synthesis showed significant upregulation. This study developed a promising approach for enhancing carbon allocation to protein synthesis in oleaginous microalga, facilitating the bioconversion of the fixed carbon into algal protein instead of oil in green manufacturing.

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.

Emerging biological wastewater treatment using microalgal-bacterial granules: A review

Aiming at deepening the understanding of the formation and evolution of emerging microalgal-bacterial granule (MBG)-based wastewater treatment systems, the recent advances regarding the formation processes, transfer phenomena, innovative bioreactors development and wastewater treatment performance of MBG-based systems are comprehensively reviewed in this work. Particularly, the successful establishments of MBG-based systems with various inocula are summarized. Besides, as the indispensable factors for biochemical reactions in MBGs, the light and substrates (organic matters, inorganic nutrients, etc) need to undergo complicated and multi-scale transfer processes before being assimilated by microorganisms within MBGs. Therefore, the involved transfer phenomena and mechanisms in MBG-based bioreactors are critically discussed. Subsequently, some recent advances of MBG-based bioreactors, the application of MBG-based systems in treating various synthetic and real wastewater, and the future development directions are discussed. In short, this review helps in promoting the development of MBG-based systems by presenting current research status and future perspectives.

Revealing the synergistic effects of cells, pigments, and light spectra on light transfer during microalgae growth: A comprehensive light attenuation model

As the sole energy for photosynthesis, light decrease rapidly with path due to absorption by pigments and scattering by cells in microalgal suspensions. By comprehensively considering cell concentrations, pigment components, and light spectra, a modified Cornet model for light transmission in microalgal suspensions is established. The developed model better fits experimental data with a higher adjusted R², which is 5% higher than the model that is based only on cell concentration. The attenuation of blue light is the most severe, followed by red and green light. Among the three main pigments, total carotenoids contribute the most to the absorption of blue and green light (with contribution coefficients of 89.26 ± 4.53% and 46.04 ± 3.77%, respectively), and chlorophyll a contributes the most to the absorption of red light (with a contribution coefficient of 75.33 ± 5.08%). This study provides a better understanding and prediction of light transmission during microalgal cultivation.

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.

Enhancing CO2 photo-biochemical conversion in a newly-designed attached photobioreactor characterized by stacked horizontal planar waveguide modules

Aiming at alleviating the adverse effects on attached microalgae biofilm growth caused by heterogeneous spatial light distributions within the attached cultivation photobioreactors (PBRs), an innovative PBR integrated with stacked horizontal planar waveguide modules (SHPW-PBR) was proposed in this work. Different from the conventional PBR, the emergent light from the external LED light bars were guided and evenly redistributed within the SHPW-PBR by the planar waveguides and hence provided light energy for microalgae cells photoautotrophic growth. In comparison with the control PBR, the average light intensity illuminating the attached Chlorella vulgaris biofilm in the SHPW-PBR was elevated by 204.11% and contributed to a 145.20% improvement on areal C. vulgaris biofilm production. Thereafter, responses of attached C. vulgaris biofilm growth in the SHPW-PBR to various light intensities were evaluated and the maximum areal C. vulgaris biofilm density reached 90.43 g m^-2 under the light intensity of 136 μmol m^-2 s^-1 after 9 days cultivation. Furthermore, the SHPW-PBR can be easily scaled-up by increasing the quantity of the stacked planar waveguide modules and thus shows great potential in biofilm-based biomass production.

Comparison of Galdieria growth and photosynthetic activity in different culture systems

In the last years, the acidothermophilic red microalga Galdieria sulphuraria has been increasingly studied for industrial applications such as wastewater treatment, recovery of rare earth elements, production of phycobilins. However, even now it is not possible an industrial cultivation of this organism because biotechnological research on G. sulphuraria and allied species is relatively recent and fragmented. Having in mind a possible scale-up for commercial applications, we have compared the growth and photosynthetic performance of G. sulphuraria in four suspended systems (Inclined bubble column, Decanter Laboratory Flask, Tubular Bioreactor, Ultra-flat plate bioreactor) and one immobilized system (Twin Layer Sytem). The results showed that G. sulphuraria had the highest growth, productivity and photosynthetic performance, when grown on the immobilized system, which also offers some economics advantages.

Simultaneous improvements on nutrient and Mg recoveries of microalgal bioremediation for municipal wastewater and nickel laterite ore wastewater

Effects of different mixing ratios between synthetic municipal wastewater (MW) and magnesium (Mg²⁺)-enriched nickel laterite ore wastewater (NLOWW) on growth of Chlorella sorokiniana (C. sorokiniana), photosynthetic activities, cellular biocomposition, nutrient and Mg²⁺ removal were investigated in photobioreactors. In the culture without NLOWW, wrinkled cells were observed with low biomass production. The culture mixed with 0.13% NLOWW obtained 1.89-fold higher biomass yield, 3.77-fold enhanced photosynthetic activity (Fv/Fm value), and improved nutrient removal (nitrogen by 102.2%, phosphorus by 39.3%). However, excessive Mg²⁺ at 100% NLOWW produced highest reactive oxygen species suppressing microalgal growth. The Mg²⁺ removal capacity increased with NLOWW loading. Moreover, microalgal assimilation primarily contributed to nutrient removal while absorption was the dominant Mg²⁺ removal pathway. Carbohydrate content in biomass increased with Mg²⁺ loading. Finally, the approach for MW/NLOWW treatment was demonstrated as economically feasible with revenue of $75.6 per kilogram biomass through a comprehensive economic model.

State of the art and challenges of biohydrogen from microalgae

Biohydrogen from microalgae has attracted extensive attention owing to its promising features of abundance, renewable and self sustainability. Unlike other well-established biofuels like biodiesel and bioethanol, biohydrogen from microalgae is still in the preliminary stage of development. Criticisms in microalgal biohydrogen centered on its practicality and sustainability. Various laboratory- and pilot-scale microalgal systems have been developed, and some research initiatives have exhibited potential for commercial application. This work provides a review of the state of the art of biohydrogen from microalgae. Discussions include metabolic pathways of light-driven transformation and dark fermentation, reactor schemes and system designs encompassing reactor configurations and light manipulation. Challenges, knowledge gaps and the future directions in metabolic limitations, economic and energy assessments, and molecular engineering are also delineated. Current scientific and engineering challenges of microalgal biohydrogen need to be addressed for technology leapfrog or breakthrough.

Exergy analyses of biogas production from microalgae biomass via anaerobic digestion

Biogas production from microalgae biomass without pretreatment and with hydrothermal pretreatment involve the energy with different quality and quantity, which makes it complex to evaluate thermodynamic performance. In this paper, exergy analyses were conducted in biogas production from microalgae biomass without pretreatment, with hydrothermal pretreatment, and with solar-driven hydrothermal pretreatment. The results showed that the materials and energy flow affected exergy efficiency in biogas production from microalgae biomass. The biogas production from microalgae biomass with solar-driven hydrothermal pretreatment achieved the highest exergy efficiency (40.85%), compared with that without pretreatment (26.2%) and with hydrothermal pretreatment (35.98%). In addition, the maximum exergy loss was caused by biogas residue, which accounted for 60.58%, 38.54%, and 35.13% of overall exergy input in biogas production from microalgae biomass without pretreatment, with hydrothermal pretreatment, and with solar-driven hydrothermal pretreatment, respectively. Exergy analyses provide important theoretical guidance to improve the performance of biogas production from microalgae biomass.

A novel jet-aerated tangential swirling-flow plate photobioreactor generates microbubbles that enhance mass transfer and improve microalgal growth

To reduce bubble diameter and enhance mass transfer, a novel jet-aerated tangential swirling-flow plate photobioreactor was developed that improves the growth rate of microalgae. In this system, the circulating microalgal solution enters a jet aerator that takes up 15% CO₂ by vacuum suction and then injects into a plate photobioreactor through four centrally symmetric nozzles. Each jetflow is tangent to a tangential circle, driving vertical vortex movement of the surrounding microalgal solution, which markedly reduced the bubble diameter and enhanced mass transfer. The mass transfer coefficient was enhanced by decreasing the nozzle number (n) and increasing the ratio of tangential circle diameter to plate photobioreactor equivalent diameter (d/D). The average bubble diameter decreased by 80.2% to 0.37 mm and the mass transfer coefficient increased 4.6 times to 48.9 h^-1 when n was 4 and d/D was 0.34. Finally, the optimized system increased the biomass dry weight of microalgae by 49.4%.

Enhanced biomass and lipid production of Neochloris oleoabundans under high light conditions by anisotropic nature of light-splitting CaCO3 crystal

The aim of this work was to study the anisotropic effect of crystalline CaCO₃ nanoparticles (CN)-driven multiple refraction/scattering from the CN-coated agglomerated cells on the rate of photosynthesis and the product yield under high light conditions in the freshwater microalgae Neochloris oleoabundans. The CN-coating via biomineralization significantly improved the biomass and lipid production of N. oleoabundans during second stage of autotrophic induction by sustaining relatively high rate of photosynthesis at high irradiance using the multiple-splitting effect of the anisotropic polymorphism. The CN were successfully produced, adsorbed and grown on the external cells under conditions of mild alkalinity (pH 7.5-8.0), mild CaCl₂ concentration (0.05 M) and under nitrogen starvation with strong light (400 µE m^-2 s^-1). Consequently, lipid content and productivity of N. oleoabundans cells cultured with 0.05 M CaCl₂ increased by 18.4% and 31.5%, respectively, compared to the cells cultured with 0.05 M CaCl₂ and acetazolamide to inhibit calcification.

Enhanced methanogenic performance and metabolic pathway of high solid anaerobic digestion of chicken manure by Fe2+ and Ni2+ supplementation

High solid anaerobic digestion (AD) is a promising technology for the treatment of organic waste in terms of efficient bioenergy production and digestate volume reduction. However, the high solid AD of chicken manure has been a challenge due to the inhibition effects caused by high ammonia levels. In this study, the addition of 280 mg/L Fe²⁺ and 2 mg/L Ni²⁺ has been proven to enhance the microbial activity and to overcome inhibition under the stressed environment. The mesophilic digesters feeding with total solid around 150 g/kg operated at an organic loading rate of 4.8 g/(L d) based on volatile solid and a 20-day hydraulic retention time. The total ammonia-nitrogen reached around 6.8 g/L in control and Fe-Ni digesters. The results from the 147-day continuously-fed experiment provided an increment of 34% methane production and a 29% reduction of volatile fatty acids against control. The acetoclastic and hydrogenotrophic methanogenic activity was increased by 89% and 40% respectively. The high throughout sequencing results showed an increased percentage of Methanosarcina sp., which may have contributed to the shifting of the methanogenic pathway towards acetoclastic methanogenesis. The positive effects of Fe²⁺ and Ni²⁺ supplementation obtained in this study lay the foundation for its use in AD of nitrogen rich materials.

Biodegradable branched cationic starch with high C/N ratio for Chlorella vulgaris cells concentration: Regulating microalgae flocculation performance by pH

To improve the carbon to nitrogen (C/N) ratio of harvested microalgae biomass for better producing biogas by fermentation, biodegradable cationic starch with high C/N ratio were synthesized to harvest Chlorella vulgaris. The impact of pH was also studied as the zeta potential of both microalgae and cationic starch would change with pH. Results indicated the cationic starch can harvest above 99% of the microalgae and the C/N ratio can rise from 7.50 to 7.90. The zeta potential of microalgae always kept negative and presented a trend of descending firstly and then upgrade. The maximum microalgae biomass flocculation capacity of 1 g cationic starch was 8.62 g with the help of self-flocculation at pH 3. The concentration of flocs formed at pH 11 was 25.74 g L^-1 and the diameter was 0.553 mm which was much larger than the flocs formed at pH 3 (0.208 mm).

Enhancement of CO2 transfer and microalgae growth by perforated inverted arc trough internals in a flat-plate photobioreactor

Flat-plate photobioreactor (PBR) with perforated inverted arc trough (PIAT) internals was proposed to promote CO₂ bio-fixation by microalgae. The PIAT internals can enhance CO₂ transfer from gas to culture medium by prolonging CO₂ gas-liquid contact time and generate periodic aeration in the suspension upper side the PIAT providing suspension mixing. Experimental results showed gas-liquid contact time was prolonged from 0.448 s to 256 s and the CO₂ partial pressure inside the PIAT internals was about 15.5 kPa during microalgae cultivation. Consequently, the dissolved CO₂ concentration in the microalgae suspension of the proposed PBR was increased by 26.0% compared to that in the PBR without PIAT internals when 15% CO₂ (v/v) was aerated at a rate of 15 mL min^-1. The elevated CO₂ transfer contributed to a 20.9% increment in biomass concentration (3.35 g L^-1) and a 26.2% increment in CO₂ fixation rate (36.6 mg L^-1 h^-1).

Hydrothermal hydrolysis pretreatment of microalgae slurries in a continuous reactor under subcritical conditions for large-scale application

Biofuel production from microalgae biomass via fermentation is promising as a renewable technology. Hydrothermal hydrolysis pretreatment of microalgae slurry prior to fermentation is an efficient method to enhance the biogas production. Herein, we report, for the first time, the hydrothermal hydrolysis pretreatment of microalgae slurries in a continuous reactor under subcritical conditions (temperature <200 °C, pressure ∼2 MPa). The results show that the yield of carbohydrates from microalgae slurries first increased and then decreased with increasing average outlet temperature, which reached a maximum value at 160 °C for 10 min. This phenomenon was caused by the combined effects of the release of organics and biochemical reactions. Notably, the radial temperature distribution of microalgae slurries, influenced by the mass fraction and the flow rate during the heating process, had effects on the yield of organics (carbohydrates, proteins) in aqueous solution. This study provides useful information for the operating conditions in large-scale applications.

Generating cycle flow between dark and light zones with double paddlewheels to improve microalgal growth in a flat plate photo-bioreactor

Double paddlewheels were proposed to generate cycle flow for increasing horizontal fluid velocity between dark and light zones in a flat plate photo-bioreactor, which strengthened the mass transfer and the mixing effect to improve microalgal growth with 15% CO₂. Numerical fluid dynamics were used to simulate the cycle flow field with double paddlewheels. The local flow field measured with particle image velocimetry fitted well with the numerical simulation results. The horizontal fluid velocity in the photo-bioreactor was markedly increased from 5.8 × 10^-5 m/s to 0.45 m/s with the rotation of double paddlewheels, resulting in a decreased dark/light cycle period. Therefore, bubble formation time and diameter reduced by 24.4% and 27.4%, respectively. Meanwhile, solution mixing time reduced by 31.3% and mass transfer coefficient increased by 41.2%. The biomass yield of microalgae Nannochloropsis Oceanic increased by 127.1% with double paddlewheels under 15% CO₂ condition.

Hydrogen production from algal biomass - Advances, challenges and prospects

Extensive effort is being made to explore renewable energy in replacing fossil fuels. Biohydrogen is a promising future fuel because of its clean and high energy content. A challenging issue in establishing hydrogen economy is sustainability. Biohydrogen has the potential for renewable biofuel, and could replace current hydrogen production through fossil fuel thermo-chemical processes. A promising source of biohydrogen is conversion from algal biomass, which is abundant, clean and renewable. Unlike other well-developed biofuels such as bioethanol and biodiesel, production of hydrogen from algal biomass is still in the early stage of development. There are a variety of technologies for algal hydrogen production, and some laboratory- and pilot-scale systems have demonstrated a good potential for full-scale implementation. This work presents an elucidation on development in biohydrogen encompassing biological pathways, bioreactor designs and operation and techno-economic evaluation. Challenges and prospects of biohydrogen production are also outlined.

Multilateral approach on enhancing economic viability of lipid production from microalgae: A review

Microalgae have been rising as a feedstock for biofuel in response to the energy crisis. Due to a high lipid content, composed of fatty acids favorable for the biodiesel production, microalgae are still being investigated as an alternative to biodiesel. Environmental factors and process conditions can alternate the quality and the quantity of lipid produced by microalgae, which can be critical for the overall production of biodiesel. To maximize both the lipid content and the biomass productivity, it is necessary to start with robust algal strains and optimal physio-chemical properties of the culture environment in combination with a novel culture system. These accumulative approaches for cost reduction can take algal process one step closer in achieving the economic feasibility.

Application of growth-phase based light-feeding strategies to simultaneously enhance Chlorella vulgaris growth and lipid accumulation

Considering the variations of optimal light intensity required by microalgae cells along with growth phases, growth-phase light-feeding strategies were proposed and verified in this paper, aiming at boosting microalgae lipid productivity from the perspective of light conditions optimization. Experimental results demonstrate that under an identical time-averaged light intensity, the light-feeding strategies characterized by stepwise incremental light intensities showed a positive effect on biomass and lipid accumulation. The lipid productivity (235.49 mg L^-1 d^-1) attained under light-feeding strategy V (time-averaged light intensity: 225 μmol m^-2 s^-1) was 52.38% higher over that obtained under a constant light intensity of 225 μmol m^-2 s^-1. Subsequently, based on light-feeding strategy V, microalgae lipid productivity was further elevated to 312.92 mg L^-1 d^-1 employing a two-stage based light-feeding strategy V₅₆₀ (time-averaged light intensity: 360 μmol m^-2 s^-1), which was 79.63% higher relative to that achieved under a constant light intensity of 360 μmol m^-2 s^-1.

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.

Boosting Nannochloropsis oculata growth and lipid accumulation in a lab-scale open raceway pond characterized by improved light distributions employing built-in planar waveguide modules

Aiming at alleviating the adverse effect of poor light penetrability on microalgae growth, planar waveguide modules functioned as diluting and redistributing the intense incident light within microalgae culture more homogeneously were introduced into a lab-scale open raceway pond (ORP) for Nannochloropsis oculata cultivation. As compared to the conventional ORP, the illumination surface area to volume ratio and effective illuminated volume percentage in the proposed ORP were respectively improved by 5.53 times and 19.68-172.72%. Consequently, the superior light distribution characteristics in the proposed ORP contributed to 193.33% and 443.71% increase in biomass concentration and lipid yield relative to those obtained in conventional ORP, respectively. Subsequently, the maximum biomass concentration (2.31 g L^-1) and lipid yield (1258.65 mg L^-1) was obtained when the interval between adjacent planar waveguide modules was 18 mm. The biodiesel produced in PWM-ORPs showed better properties than conventional ORP due to higher MUFA and C18:1 components proportions.

Rheological properties of microalgae slurry for application in hydrothermal pretreatment systems

Herein, the rheological properties of microalgae slurries (Chlorella pyrenoidosa) under different mass fractions and temperatures (293-373 K) were reported, which can significantly affect the energy requirement of hydrothermal pretreatment process and hence the performance of fermentation processes. The experiment results showed that the microalgae slurry had a shear-thinning phenomenon, suggesting that microalgae slurry is a non-Newtonian fluid. For the first time, the phenomenon of variable flow behavior index under varying shear rates was discovered. In addition, the viscosity and yield stress of the microalgae slurries decreased from 293 to 343 K; above 343 K, they increased with the temperature. Scanning electron microscopy analyses showed that the surface of the microalgae cells became rougher when the temperature increased from 343 to 373 K. Finally, the equations of the viscosity of the microalgae slurry, based on the temperatures and shear rates, were obtained.

Impact of the accumulation and adhesion of released oxygen during Scenedesmus obliquus photosynthesis on biofilm formation and growth

Microalgae cells release O₂ during photosynthesis. The gas can accumulate and adhere in form of bubbles, which affect the transport of nutrients in the biofilm and the biofilm microstructure. To investigate the reasons for the adhesion of these oxygen bubbles and their impact on biofilm, polytetrafluoroethylene (PTFE) emulsion was sprayed onto glass surface to change the parameters for gas accumulation and adhesion. The results indicated gas could aggregate into bubbles and adhere to hydrophobic and rough surface. The bubble behaviors caused the biofilm to be porous (with a microporosity of 9.43-20.94%). The biomass concentration of the more porous biofilm increased by 9.26% to 22.42gm^-2 on 1% PTFE-treated surface compared to that on an untreated surface. However, with an increase in PTFE concentration, the amount of adhered bubbles increased. More microalgae cells in biofilms were carried up by bubbles. The biofilm concentration on 5% PTFE-treated surface decreased by 15.30%.

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.

Photo-fermentative hydrogen production from crop residue: A mini review

Photofermentative hydrogen production from crop residues, if feasible, can lead to complete conversion of organic substances to hydrogen (and carbon dioxide). This mini review lists the studies on photofermentative hydrogen production using crop residues as feedstock. Pretreatment methods, substrate structure, mechanism of photosynthetic bacteria growth and metabolism were discussed. Photofermentative hydrogen production from pure culture, consortia and mutants, and the geometry, light sources, mass transfer resistances and the operational strategies of the photo-bioreactor were herein reviewed. Future studies of regulation mechanism of photosynthetic bacteria, such as highly-efficient strain breeding and gene reconstruction, and development of new-generation photo-bioreactor were suggested.