Photosynthetic membrane-less microbial fuel cells to enhance microalgal biomass concentration

Microalgae as tools for bio-circular-green economy: Zero-waste approaches for sustainable production and biorefineries of microalgal biomass

Microalgae are promising organisms that are rapidly gaining much attention due to their numerous advantages and applications, especially in biorefineries for various bioenergy and biochemicals. This review focuses on the microalgae contributions to Bio-Circular-Green (BCG) economy, in which zero-waste approaches for sustainable production and biorefineries of microalgal biomass are introduced and their possible integration is discussed. Firstly, overviews of wastewater upcycling and greenhouse gas capture by microalgae are given. Then, a variety of valuable products from microalgal biomass, e.g., pigments, vitamins, proteins/peptides, carbohydrates, lipids, polyunsaturated fatty acids, and exopolysaccharides, are summarized to emphasize their biorefinery potential. Techno-economic and environmental analyses have been used to evaluate sustainability of microalgal biomass production systems. Finally, key issues, future perspectives, and challenges for zero-waste microalgal biorefineries, e.g., cost-effective techniques and innovative integrations with other viable processes, are discussed. These strategies not only make microalgae-based industries commercially feasible and sustainable but also reduce environmental impacts.

Simultaneous dairy wastewater treatment and bioelectricity production in a new microbial fuel cell using photosynthetic Synechococcus

Photosynthetic microbial fuel cell (PMFC) is a novel technology, which employs organic pollutants and organisms to produce electrons and biomass and capture CO2 by bio-reactions. In this study, a new PMFC was developed based on Synechococcus sp. as a biocathode, and dairy wastewater was used in the anode chamber. Different experiments including batch feed mode, semi-continuous feed mode, Synechococcus feedstock to the anode chamber, Synechococcus-Chlorella mixed system, the feedstock of treated wastewater to the cathode chamber, and use of extra nutrients in the anodic chamber were performed to investigate the behavior of the PMFC system. The results indicated that the PMFC with a semi-continuous feed mode is more effective than a batch mode for electricity generation and pollutant removal. Herein, maximum power density, chemical oxygen demand removal, and Coulombic efficiency were 6.95 mW/m2 (450 Ω internal resistance), 62.94, and 43.16%, respectively, through mixing Synechococcus sp. and Chlorella algae in the batch-fed mode. The maximum nitrate and orthophosphate removal rates were 98.83 and 68.5%, respectively, wherein treated wastewater in the anode was added to the cathode. No significant difference in Synechococcus growth rate was found between the cathodic chamber of PMFC and the control cultivation cell. The heating value of the biocathode biomass at maximum Synechococcus growth rate (adding glucose into the anode chamber) was 0.2235 MJ/Kg, indicating the cell's high ability for carbon dioxide recovery. This study investigated not only simultaneous bioelectricity production and dairy wastewater in a new PMFC using Synechococcus sp. but also studied several operational parameters and presented useful information about their effect on PMFC performance.

Microalgal Biorefinery Concepts’ Developments for Biofuel and Bioproducts: Current Perspective and Bottlenecks

Microalgae have received much interest as a biofuel feedstock. However, the economic feasibility of biofuel production from microalgae does not satisfy capital investors. Apart from the biofuels, it is necessary to produce high-value co-products from microalgae fraction to satisfy the economic aspects of microalgae biorefinery. In addition, microalgae-based wastewater treatment is considered as an alternative for the conventional wastewater treatment in terms of energy consumption, which is suitable for microalgae biorefinery approaches. The energy consumption of a microalgae wastewater treatment system (0.2 kW/h/m³) was reduced 10 times when compared to the conventional wastewater treatment system (to 2 kW/h/m³). Microalgae are rich in various biomolecules such as carbohydrates, proteins, lipids, pigments, vitamins, and antioxidants; all these valuable products can be utilized by nutritional, pharmaceutical, and cosmetic industries. There are several bottlenecks associated with microalgae biorefinery. Hence, it is essential to promote the sustainability of microalgal biorefinery with innovative ideas to produce biofuel with high-value products. This review attempted to bring out the trends and promising solutions to realize microalgal production of multiple products at an industrial scale. New perspectives and current challenges are discussed for the development of algal biorefinery concepts.

Enhanced treatment of landfill leachate with cathodic algal biofilm and oxygen-consuming unit in a hybrid microbial fuel cell system

An innovative cathodic algal biofilm microbial fuel cell equipped with a bioactive oxygen consuming unit (AB-OCU-MFC) was proposed for enhancing the leachate treatment containing biorefractory organic matters and high strength of ammonium nitrogen. The proposed AB-OCU-MFC performed better with regard to COD, NH₄⁺-N, TN removals and algal biomass yield than standalone algal biofilm-MFC and control reactors. AB-OCU-MFC with OCU of 2 cm thickness removed more than 86% of COD, 89.4% of NH₄⁺-N, 76.7% of TN and produced a maximum voltage of 0.39 V and biomass productivity of 1.23 g·L^-1·d^-1. The High-throughput sequencing of DNA showed a significant change in microbial community of reactors implemented with OCU, in which the ratio of exoelectrogenic bacteria of anode and denitrifying bacteria on cathode were significantly increased. The results obtained by cathodic algal biofilm MFC with low cost and bioactive barrier of OCU, would provide a new sight for practical application of MFC.

Potential applications of algae in the cathode of microbial fuel cells for enhanced electricity generation with simultaneous nutrient removal and algae biorefinery: Current status and future perspectives

Production of biofuels and other value-added products from wastewater along with quality treatment is an uttermost necessity to achieve environmental sustainability and promote bio-circular economy. Algae-Microbial fuel cell (A-MFC) with algae in cathode chamber offers several advantages e.g. photosynthetic oxygenation for electricity recovery, CO₂-fixation, wastewater treatment, etc. However, performance of A-MFC depends on several operational parameters and also on electrode materials types; therefore, enormous collective efforts have been made by researchers for finding optimal conditions in order to enhance A-MFC performance. The present review is a comprehensive snapshot of the recent advances in A-MFCs, dealing two major parts: 1) the power generation, which exclusively outlines the effect of different parameters and development of cutting edge cathode materials and 2) wastewater treatment at cathode of A-MFC. This review provides fundamental knowledge, critical constraints, current status and some insights for making A-MFC technology a reality at commercial scale operation.

Carbon dioxide sequestration accompanied by bioenergy generation using a bubbling-type photosynthetic algae microbial fuel cell

This study developed a bubbling-type photosynthetic algae microbial fuel cell (B-PAMFC) to treat synthetic wastewater and capture CO₂ using Chlorella vulgaris with simultaneous power production. The performance of B-PAMFC in CO₂ fixation and bioenergy production was compared with the photosynthetic algae microbial fuel cell (PAMFC) and bubbling photobioreactor. Different nitrogen sources for C. vulgaris growth, namely sodium nitrate, urea, ammonium acetate and acetamide were studied. The maximum CO₂ fixation rate in B-PAMFC with 2.8 g L^-1 urea reached 605.3 mg L^-1 d^-1, 3.86-fold higher than that in PAMFC. Urea also enhanced the solution absorption of CO₂. Furthermore, the B-PAMFC reached a high lipid productivity of 105.9 mg L^-1 d^-1. An energy balance analysis indicated that B-PAMFC had a maximum net energy of 1.824 kWh m^-3, making it a lab-scale energy-positive system. The B-PAMFC with urea as nitrogen source would provide an attractive strategy for simultaneous CO₂ sequestration and bioenergy production.

A photosynthetic algal microbial fuel cell for treating swine wastewater

A photosynthetic algal (Chlorella vulgaris) microbial fuel cell (PAMFC) with double chambers was adopted for power production and removal of carbon and nitrogen in swine sewerage that could provide nutrients for the growth of C. vulgaris. C. vulgaris was expected to utilize carbon dioxide (CO₂) delivered from the anode chamber and generate oxygen as an electron acceptor by photosynthesis. PAMFC presented a maximum voltage output of 0.747 V and a maximum power density of 3720 mW/m³ at 240 h, much higher than that of the standalone MFC. 85.6%, 70.2%, and 93.9% removal of ammonia nitrogen, total nitrogen (TN), and total organic carbon (TOC), respectively, were obtained in the anode chamber of the PAMFC system, while the corresponding removal in MFC was 83.1%, 56.0%, and 87.2%, respectively. PAMFC also presented a much higher removal of ammonia nitrogen (68.7%) in the cathode chamber than MFC (47.5%). The results indicated the superiority of the PAMFC device for carbon and nitrogen removal.

The effect of different light intensities and light/dark regimes on the performance of photosynthetic microalgae microbial fuel cell

This study develops a photosynthetic microalgae microbial fuel cell (PMMFC) engaged Chlorella vulgaris microalgae to investigate effect of light intensities and illumination regimes on simultaneous production of bioelectricity, biomass and wastewater treatment. The performance of the system under different light intensity (3500, 5000, 7000 and 10,000 lx) and light/dark regimes (24/00, 12/12, 16/8 h) was investigated. The optimum light intensity and light/dark regimes for achieving maximum yield of PMMFC were obtained. The maximum power density of 126 mW m^-3, the coulombic efficiency of 78% and COD removal of 5.47% were achieved. The maximum biomass concentration of 4 g l^-1 (or biomass yield of 0.44 g l^-1 day^-1) was obtained in continuous light intensity of 10,000 lx. The comparison of the PMMFC performance with air-cathode and abiotic-cathode MFCs shows that the maximum power density of air-cathode MFC was only 13% higher than PMMFC.

Microalgae biorefinery: High value products perspectives

Microalgae have received much interest as a biofuel feedstock in response to the uprising energy crisis, climate change and depletion of natural sources. Development of microalgal biofuels from microalgae does not satisfy the economic feasibility of overwhelming capital investments and operations. Hence, high-value co-products have been produced through the extraction of a fraction of algae to improve the economics of a microalgae biorefinery. Examples of these high-value products are pigments, proteins, lipids, carbohydrates, vitamins and anti-oxidants, with applications in cosmetics, nutritional and pharmaceuticals industries. To promote the sustainability of this process, an innovative microalgae biorefinery structure is implemented through the production of multiple products in the form of high value products and biofuel. This review presents the current challenges in the extraction of high value products from microalgae and its integration in the biorefinery. The economic potential assessment of microalgae biorefinery was evaluated to highlight the feasibility of the process.