Immobilized oleaginous microalgae as effective two-phase purify unit for biogas and anaerobic digester effluent coupling with lipid production

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.

Enhanced biovalorization of palm biomass wastes as biodiesel feedstocks through integrated solid-state and submerged fermentations by fungal co-cultures

Palm empty fruit bunches (EFB) were valorized into fungal lipids by oleaginous fungus Aspergillus tubingensis TSIP9 under solid-state fermentation (SSF) and submerged fermentation (SmF). An integrated SSF-SmF process increased lipid production from 116.2 ± 0.1 mg/g-EFB under SSF and 60.1 ± 0.2 under SmF up to 124.9 ± 0.5 mg/g-EFB, possibly due to the combined benefits of dispersed mycelia forming during SSF and better mass transfer during SmF. As A. tubingensis lacks sufficient β-glucosidase, it was co-cultured with high β-glucosidase-producing Trichoderma reesei QM 9414. The co-cultures improved overall lipid yields likely due to synergistic interaction of the two fungi. After inoculum size was optimized and the co-cultures were performed in bioreactors, the lipid yield was increased up to 205.1 ± 1.1 mg/g-EFB. The fatty acid composition of fungal lipids indicated their potential use as biodiesel feedstocks. The fungal fermentation of EFB also provided cellulose pulp residues. These strategies could be practical options for low-cost biovalorization of biomass wastes.

Chitosan-coated oleaginous microalgae-fungal pellets for improved bioremediation of non-sterile secondary effluent and application in carbon dioxide sequestration in bubble column photobioreactors

Oleaginous microalga Scenedesmus sp. SPP was rapidly immobilized in oleaginous fungal pellets by their opposite-surface-charges. Microalgae-fungal (MF) pellets were more effective in bioremediation of non-sterile secondary effluent than mono-culture. The optimal hydraulic retention time for dual bioremediation in semi-continuous mode was 72 h. The MF pellets coated with 0.4 %-chitosan improved removal efficiencies of COD, total nitrogen (TN), and total phosphorus (TP) up to 96.2±0.0 %, 88.2±2.8 % and 71.5±0.7 %, respectively, likely because of better cell retention and more nutrient adsorption and assimilation. Dual bioremediation by coated MF pellets was also successfully scaled up in 30-L bubble-column photobioreactors with improved COD, TN, and TP removal efficiencies of 98.5±0.0 %, 90.2±0.0 % and 79.5±2.1 %, respectively. This system also effectively removed CO₂ from simulated flue gas at 71.2±0.4 % and produced biomass with high lipid content. These results highlight the effectiveness of bio-immobilization by fungal pellets; chitosan coating; and their practical applications in bioremediation and CO₂ sequestration.

Advancement in algal bioremediation for organic, inorganic, and emerging pollutants

Rapidly changing bioremediation prospects are key drive to develop sustainable options that can offer extra benefits rather than only environmental remediation. Algal remediating is gaining utmost attention due to its mesmerising sustainable features, removing odour and toxicity, co-remediating numerous common and emerging inorganic and organic pollutants from gaseous and aqueous environments, and yielding biomass for a range of valuable products refining. Moreover, it also improves carbon footprint via carbon-capturing offers a better option than any other non-algal process for several high CO₂-emitting industries. Bio-uptake, bioadsorption, photodegradation, and biodegradation are the main mechanisms to remediate a range of common and emerging pollutants by various algae species. Bioadsorption was a dominant remediation mechanism among others implicating surface properties of pollutants and algal cell walls. Photodegradable pollutants were photodegraded by microalgae by adsorbing photons on the surface and intracellularly via stepwise photodissociation and breakdown. Biodegradation involves the transportation of selective pollutants intracellularly, and enzymes help to convert them into simpler non-toxic forms. Robust models are from the green microalgae group and are dominated by Chlorella species. This article compiles the advancements in microalgae-assisted pollutants remediation and value-addition under sustainable biorefinery prospects. Moreover, filling the knowledge gaps, and recommendations for developing an effective platform for emerging pollutants remediation and realization of commercial-scale algal bioremediation.

Microalgal cultivation for the upgraded biogas by removing CO2, coupled with the treatment of slurry from anaerobic digestion: A review

Biogas is the gaseous by product generated from anaerobic digestion (AD), which is mainly composed of methane and CO2. Numerous independent studies have suggested that microalgae cultivation could achieve high efficiency for nutrient uptake or CO2 capture from AD, respectively. However, there is no comprehensive review on the purifying slurry from AD and simultaneously upgrading biogas via microalgal cultivation technology. This paper aims to fill this gap by presenting and discussing an information integration system based on microalgal technology. Furthermore, the review elaborates the mechanisms, configurations, and influencing factors of integrated system and analyzes the possible challenges for practical engineering applications and provides some feasibility suggestions eventually. There is hope that this review will offer a worthwhile and practical guideline to researchers, authorities and potential stakeholders, to promote this industry for sustainable development.

Mixotrophic Cultivation of Microalgae Using Biogas as the Substrate

Biogas utilization through biotechnology represents a potential and novel technology. We propose the microalgal mixotrophic cultivation to convert biogas to microalgae-based biodiesel, in which methanotroph was co-cultured to convert CH₄ to organic intermediate (and CO₂) for microalgal mixotrophic growth. This study constructed a co-culture of Methylocystis bryophila (methanotroph) and Scenedesmus obliquus (microalgae) with biogas feeding. Compared with the single culture of S. obliquus, higher microalgal biomass but a lower chlorophyll concentration was observed. The organic metabolism-related genes were upregulated, verifying microalgal mixotrophic growth. The stoichiometric calculation of M. bryophila culture shows that M. bryophila tends to release organic matter rather than grow under a low O₂ content. M. bryophila rarely grew under five different light intensities, indicating that M. bryophila acts as a biocatalyst in the co-culture. The organic intermediate released by methanotroph increased the maximum biomass of microalgal culture, accelerated nitrogen absorption, accumulated more monounsaturated fatty acids, and improved the adaptation to light. The co-culture of microalgae and methanotroph may provide new opportunities for microalgae-based biodiesel production using biogas as a substrate.

Enhanced production of microalgal biomass and lipid as an environmentally friendly biodiesel feedstock through actinomycete co-culture in biogas digestate effluent

In this study, an innovative approach to enhance the production of microalgal biomass and lipid as a promising sustainable feedstock for biodiesel was proposed using an actinomycetes co-culture with microalgae in the biogas digestate effluent (BDE) that can be employed as an environmentally friendly and cost-effective strategy. Among tested actinomycete isolates, Piscicocus intestinalis WA3 produced indole-3-acetic acid and siderophores as algal growth promoting agents and showed effective lipid accumulation with satisfying fatty acids composition. During co-cultivation of P. intestinalis WA3 with microalga Tetradesmus obliquus AARL G022 in the BDE, biomass production, chlorophyll a content, and lipid productivity were significantly increased by 1.30 folds, 1.39 folds, and 1.55 folds, respectively, compared to microalgae monoculture. The accumulated lipids contained long-chain fatty acids with better fuel properties that could potentially be used as biodiesel feedstock. The overall results evidenced that actinomycete co-culture would contribute greatly to the cost-effective production of environmental-friendly microbial-based biofuel.

Immobilized Microalgae-Based Photobioreactor for CO2 Capture (IMC-CO2PBR): Efficiency Estimation, Technological Parameters, and Prototype Concept

Microalgae-mediated CO2 sequestration has been a subject of numerous research works and has become one of the most promising strategies to mitigate carbon dioxide emissions. However, feeding flue and exhaust gas into algae-based systems has been shown to destroy chloroplasts, as well as disrupt photosynthesis and other metabolic processes in microalgae, which directly limits CO2 uptake. CO2 biosequestration in existing photobioreactors (PBRs) is also limited by the low biomass concentration in the growth medium. Therefore, there is a real need to seek alternative solutions that would be competitive in terms of performance and cost-effectiveness. The present paper reports the results of experiments aimed to develop an innovative trickle bed reactor that uses immobilized algae to capture CO2 from flue and exhaust gas (IMC-CO2PBR). In the experiment, ambient air enriched with technical-grade CO2 to a CO2 concentration of 25% v/v was used. The microalgae immobilization technology employed in the experiment produced biomass yields approximating 100 g DM/dm3. A relationship was found between CO2 removal rates and gas volume flux: almost 40% of CO2 was removed at a feed of 25 dm3 of gas per hour, whereas in the 200 dm3/h group, the removal efficiency amounted to 5.9%. The work includes a determination of basic process parameters, presentation of a developed functional model and optimized lighting system, proposals for components to be used in the system, and recommendations for an automation and control system for a full-scale implementation.

Biohydrogen production from microalgae-Major bottlenecks and future research perspectives

The imprudent use of fossil fuels has resulted in high greenhouse gas (GHG) emissions, leading to climate change and global warming. Reduction in GHG emissions and energy insecurity imposed by the depleting fossil fuel reserves led to the search for alternative sustainable fuels. Hydrogen is a potential alternative energy carrier and is of particular interest because hydrogen combustion releases only water. Hydrogen is also an important industrial feedstock. As an alternative energy carrier, hydrogen can be used in fuel cells for power generation. Current hydrogen production mainly relies on fossil fuels and is usually energy and CO₂ -emission intensive, thus the use of fossil fuel-derived hydrogen as a carbon-free fuel source is fallacious. Biohydrogen production can be achieved via microbial methods, and the use of microalgae for hydrogen production is outstanding due to the carbon mitigating effects and the utilization of solar energy as an energy source by microalgae. This review provides comprehensive information on the mechanisms of hydrogen production by microalgae and the enzymes involved. The major challenges in the commercialization of microalgae-based photobiological hydrogen production are critically analyzed and future research perspectives are discussed. Life cycle analysis and economic assessment of hydrogen production by microalgae are also presented.

Volatile Fatty Acids (VFAs) Generated by Anaerobic Digestion Serve as Feedstock for Freshwater and Marine Oleaginous Microorganisms to Produce Biodiesel and Added-Value Compounds

Given an increasing focus on environmental sustainability, microbial oils have been suggested as an alternative to petroleum-based products. However, microbial oil production relies on the use of costly sugar-based feedstocks. Substrate limitation, elevated costs, and risk of contamination have sparked the search for alternatives to sugar-based platforms. Volatile fatty acids are generated during anaerobic digestion of organic waste and are considered a promising substrate for microbial oil production. In the present study, two freshwater and one marine microalga along with two thraustochytrids were evaluated for their potential to produce lipids when cultivated on volatile fatty acids generated from food waste via anaerobic digestion using a membrane bioreactor. Freshwater microalgae Auxenochlorella protothecoides and Chlorella sorokiniana synthesized lipids rich in palmitic acid (C16:0), stearic acid (C18:0), oleic acid (C18:1), and linoleic acid (C18:2). This composition corresponds to that of soybean and jatropha oils, which are used as biodiesel feedstock. Production of added-value polyunsaturated fatty acids (PUFA) mainly omega-3 fatty acids was examined in three different marine strains: Aurantiochytrium sp. T66, Schizochytrium limacinum SR21, and Crypthecodinium cohnii. Only Aurantiochytrium sp. T66 seemed promising, generating 43.19% docosahexaenoic acid (DHA) and 13.56% docosapentaenoic acid (DPA) in total lipids. In summary, we show that A. protothecoides, C. sorokiniana, and Aurantiochytrium sp. T66 can be used for microbial oil production from food waste material.

Valorization of volatile fatty acids derived from low-cost organic waste for lipogenesis in oleaginous microorganisms-A review

To meet environmental sustainability goals, microbial oils have been suggested as an alternative to petroleum-based products. At present, microbial fermentation for oil production relies on pure sugar-based feedstocks. However, these feedstocks are expensive and are in limited supply. Volatile fatty acids, which are generated as intermediates during anaerobic digestion of organic waste have emerged as a renewable feedstock that has the potential to replace conventional sugar sources for microbial oil production. They comprise short-chain (C2 to C6) organic acids and are employed as building blocks in the chemical industry. The present review discusses the use of oleaginous microorganisms for the production of biofuels and added-value products starting from volatile fatty acids as feedstocks. The review describes the metabolic pathways enabling lipogenesis from volatile fatty acids, and focuses on strategies to enhance lipid accumulation in oleaginous microorganisms by tuning the ratios of volatile fatty acids generated via anaerobic fermentation.

Development of integrated culture systems and harvesting methods for improved algal biomass productivity and wastewater resource recovery - A review

Microalgae biomass has been considered as a potential feedstock for the production of renewable chemicals and biofuels. Microalgae culture combined with wastewater treatment is a promising approach to improve the sustainability of the business model. However, algae culture and harvest account for the majority of the high costs, hindering the development of the microalgae-based wastewater utilization. Cost-effective culture systems and harvesting methods for enhancing biomass yield and reducing the cost of resource recovery have become extremely urgent and important. In this review, different commonly used culture systems for microalgae are discussed; the current harvesting methods with different culture systems have also been evaluated. Also, the inherent characteristics of inefficiency in algae wastewater treatment are elaborated. Current literature collectively supports that a biofilm type device is a system designed for higher biomass productivity, and offers ease of harvesting, in small-scale algae cultivation. Additionally, bio-flocculation, which uses one kind of flocculated microalgae to concentrate on another kind of non-flocculated microalgae is a low-cost and energy-saving alternative harvesting method. These findings provide insight into a comprehensive understanding of integrated culture systems and harvesting methods for microalgae-based wastewater treatment.

Bioenergy from biofuel residues and waste

This article is a review of the scientific literature published in 2019 on topics relating to bioenergy from biofuel residues and waste. This literature review is divided into the following sections: Feedstocks, Biodiesel, Bioethanol, Hydrogen, Biohydrogen, Biofuel Residues, Microalgae, and Lignocelluloses.

Removal of pollutants from biogas slurry and CO2 capture in biogas by microalgae-based technology: a systematic review

Recent research interest has focused on microalgae cultivation for biogas slurry purification and biogas upgrading due to the requirement of high efficiency for nutrient uptake and CO₂ capture, with economic feasibility and environmental benefits. Numerous studies have suggested that biogas slurry purification and biogas upgrading can occur simultaneously via microalgae-based technology. However, there is no comprehensive review on this technology with respect to the nutrient removal from biogas slurry and biogas upgrading. This article summarizes microalgal cultivation with biogas slurry and biogas from anaerobic digestion. The parameters, techniques, and modes of microalgae cultivation have been discussed in detail to achieve high efficiency in biogas slurry purification and biogas upgrading. In addition, the evaluation of energy efficiency and safety has also been explored. Compared with mono-cultivation of microalgae and co-cultivation of microalgae and bacteria, microalgae-fungi symbiosis has demonstrated greater development prospect and higher energy efficiency and the energy consumption for pollutants and CO₂ removal were 14.2-39.0% · USD^-1 and 19.9-23.3% · USD^-1, respectively. Further, a sustainable recycling scheme is proposed for the purification of biogas slurry from anaerobic digestion process and biogas upgrading via microalgae-based technology.

Immobilizing Microcystis aeruginosa and powdered activated carbon for the anaerobic digestate effluent treatment

The environment pollution caused by livestock anaerobic digestate effluent (ADE) is becoming increasingly severe recently. In this study, immobilized technology, embedding Microcystis aeruginosa (MA) and powdered activated carbon (PAC) with sodium alginate (SA), was employed to investigate the removal performance of nitrogen (N), phosphorus (P) and dissolved organic matter (DOM) in the treatment of ADE solution. Initially, orthogonal experiment was carried out to achieve the optimal conditions of the beads fabrication with the concentration of imbedding agents (PAC-SA) of 5% (w/w) and the ratio of microalgae and imbedding agents was 1:1 (v/v). The results indicated that the total nitrogen (TN), total phosphorus (TP) and total organic carbon (TOC) can be efficiently removed under the optimal operation conditions, with average removals of 91.88 ± 2.91% in TN, 98.24 ± 0.12 in TP and 78.31 ± 1.57% in TOC, respectively. Moreover, the fluorescence excitation-mission matrix (EEM) results illustrated that IMA-PAC beads joined system can efficiently diminish the concentrations of protein-like compounds and humic substances. Therefore, the organic contaminants and nutrients (i.e. N and P) can be efficiently removed in IMA-PAC beads joined system, which would contribute to developing new strategies for the treatment of ADE solution and nutrient recycle.

Emerging prospects of mixotrophic microalgae: Way forward to sustainable bioprocess for environmental remediation and cost-effective biofuels

Algal bioremediation becoming most fascinating to produce biomass as biofuels feedstock while remediating wastes, also improving carbon-footprint through carbon capturing and utilization (CCU) technology. Non-algae process however offers effective treatment but metabolic CO₂ emission is major drawback towards sustainable bioprocess. Mixotrophic cultivation strategy (MCS) enables to treat organic and inorganic wastes which broadly extend microalgae application towards cleaner and sustainable bioeconomy. Latest focus of global think-tanks to encourage bioprocess holding promise of sustainability via CCU ability as important trait. Several high CO₂ emitting industries forced to improve their carbon-footprints. MCS driven microalgae treatment could be best solution for those industries. This review covers recent updates on MCS applications for waste-to-value (biofuels) and environment remediation. Moreover, recommendations to fill knowledge gaps, and commercial algal biofuel could be cost-effectiveness and sustainable technology for biocircular economy if fuelled by waste streams from other industries.

Integration of anaerobic digestion and microalgal cultivation for digestate bioremediation and biogas upgrading

Biogas is the gaseous byproduct obtained during anaerobic digestion which is rich in methane, along with a significant amount of other gases like CO₂. The removal of CO₂ is essential to upgrade the biogas to biomethane (>95% methane content). High CO₂ tolerant microalgae can be employed as a biological CO₂ scrubbing agent for biogas upgrading. Many microalgal strains tolerant to the levels of CO₂ and CH₄ seen in biogas have been reported. A CO₂ removal efficiency of 50-99% can be attained based on the microalgae used and the cultivation conditions applied. Nutrient-rich liquid digestate obtained from anaerobic digestion can also be used as the cultivation medium for microalgae, performing biogas upgrading and digestate bioremediation simultaneously. Mixotrophic cultivation enables microalgae to utilize the organic carbon present in the liquid digestate along with nitrogen and phosphorus. Microalgae appears to be a potential biological CO₂ scrubbing agent for efficient biogas upgrading.