Circular Carbon Economy: The Role of Microalgae in Utilizing Industrial Emissions and Wastewater

The increasing urgency of climate change has necessitated the adoption of innovative strategies like the Circular Carbon Economy (CCE). The CCE framework integrates the principles of reducing, reusing, recycling, and removing carbon emissions to create a sustainable balance between industrial growth and environmental preservation. Unlike traditional linear systems, the CCE transforms carbon from an environmental liability into a valuable resource, aligning with global carbon neutrality goals and addressing the escalating levels of atmospheric CO₂ caused by industrialization and urbanization. In 2023, global carbon dioxide emissions from fossil fuels and industry totalled 37.01 billion metric tons (GtCO₂), with projections suggesting a 1.08% rise in 2024 to a record high of 37.41 GtCO₂ [1]. Without effective mitigation strategies, these trends pose a significant threat to climate stability.

Among various solutions, microalgae have emerged as a highly promising tool for integrating carbon sequestration with resource recovery. These organisms exhibit exceptional photosynthetic efficiency, fixing CO₂ at rates 10 to 50 times higher than terrestrial plants [2]. Marine phytoplankton alone contributes to global carbon balance by fixing approximately 50 GT of CO₂ annually [3]. Moreover, microalgae can thrive in environments such as industrial wastewater and flue gas streams, utilizing these byproducts for growth while simultaneously mitigating pollution. For instance, microalgae-bacteria consortia have demonstrated the ability to remove up to 99% of SO₂ and 87% of NOx from industrial flue gases, offering a dual benefit of carbon capture and pollutant remediation [4].

Despite their significant potential, critical research gaps remain, particularly regarding the scalability and economic feasibility of microalgal systems in industrial applications. Existing technologies often face inefficiencies in reactor designs, energy-intensive processes, and limited policy support, which collectively hinder their widespread adoption [5,6]. While numerous studies have explored individual capabilities of microalgae for CO₂ sequestration or wastewater treatment, integrated solutions that effectively combine these processes are still underdeveloped. Addressing this gap is crucial for achieving a cost-effective and cohesive strategy that aligns with CCE principles.

The research problem lies in overcoming the technical and economic barriers to scaling microalgae-based systems while optimizing their dual role in carbon sequestration and resource recovery. Current approaches are limited by high energy demands, suboptimal bioreactor efficiency, and a lack of coordinated policies to incentivize adoption. Without resolving these challenges, the full potential of microalgae in mitigating climate change and advancing sustainability remains unrealized.

The objective of this mini-review is to examine the transformative role of microalgae in the CCE by addressing their dual potential in CO₂ sequestration and wastewater treatment. It focuses on highlighting advancements in microalgal technologies for sustainable resource recovery, identifying critical gaps hindering large-scale implementation, and proposing strategies to overcome economic and technological challenges. By synthesizing recent developments and emerging trends, this mini-review underscores the potential of microalgae to advance global sustainability goals and align with the principles of the CCE.

Microalgae as a tool for circular carbon economy

Microalgae are emerging as a cornerstone of the CCE, offering unparalleled potential to integrate CO₂ sequestration with resource recovery. With an average CO₂ fixation efficiency of 10 to 50 times greater than terrestrial plants, microalgae are capable of addressing multiple environmental and industrial challenges simultaneously [3]. They exhibit unique physiological traits, including rapid growth, high adaptability to extreme environmental conditions, and efficient utilization of industrial waste streams such as flue gases and wastewater, making them indispensable in advancing sustainable carbon management practices [7].

Recent research demonstrates that microalgae species like Chlamydomonas reinhardtii can capture 113mg/L/day of CO₂ while producing 512mg/L of biomass, which can be converted into bioethanol at a yield of 0.46-0.49g/g under controlled conditions [8]. Similarly, Chlorella pyrenoidosa has achieved a CO₂ capture efficiency of 92%, generating 90.25g/day of biomass in reactors using industrial effluents, highlighting its applicability in industrial settings [9]. In addition to CO₂ sequestration, microalgae exhibit remarkable capacity for nutrient recovery. For example, Chlorella vulgaris demonstrated nutrient removal efficiencies exceeding 75% for nitrogen and phosphorus within five days, while concurrently achieving a CO₂ fixation rate of 187.65mg/L/day [10].

The biomass generated from microalgae cultivation serves as a versatile feedstock for producing biofuels, bioplastics, and fertilizers. Lipid-rich strains like Chlorella sp. have yielded up to 710.6mg/L of lipids in systems optimized with BG11 medium and hybrid technologies, emphasizing their potential in biodiesel production [11]. Furthermore, microalgae are increasingly recognized for their role in producing bioplastics. For instance, Spirulina platensis has shown promising results, with a lipid yield of 58.42mg/L and potential applications in compostable plastics [12].

Integration of microalgae into industrial processes not only mitigates carbon emissions but also facilitates waste valorization. Technologies such as Rotating Algae Biofilm Reactors (RABRs) have been pivotal in transforming wastewater nutrients into high-value products. RABRs optimized for municipal wastewater treatment have demonstrated phosphorus recovery rates exceeding 80%, while producing biomass suitable for biofuel and bioplastic production [12]. Additionally, the CO₂ absorption-microalgae conversion (CAMC) hybrid systems have shown carbon fixation efficiencies up to 93.7%, with lipid yields enhanced by 15.4% compared to conventional systems, showcasing the synergies achievable through integrated approaches [9].

The economic and environmental benefits of microalgaebased systems extend beyond carbon sequestration. Industrialscale applications of microalgae cultivation have demonstrated significant cost reductions in wastewater treatment, with simultaneous production of marketable bioproducts. For example, integrating Chlorella vulgaris into biorefineries processing flue gas and wastewater resulted in a 1.7-fold increase in biomass productivity and a 34% improvement in lipid accumulation under mixotrophic conditions [10]. Moreover, advanced genetic and adaptive laboratory evolution techniques are being employed to enhance microalgal performance under suboptimal environmental conditions, further improving their commercial viability [13].

Despite their immense potential, challenges such as high operational costs, energy-intensive harvesting, and scalability constraints remain barriers to widespread adoption. Addressing these issues requires multidisciplinary collaboration to develop cost-effective cultivation and bioprocessing technologies. Advances in photobioreactor design, genetic engineering, and hybrid system integration are critical to unlocking the full potential of microalgae in the CCE [11].

Utilization of flue gases in microalgal cultivation

Flue gases, a byproduct of industrial combustion processes, present a viable resource for microalgal cultivation due to their high carbon dioxide (CO₂) content, ranging from 10% to 15%, along with other components such as nitrogen oxides (NOx), sulphur oxides (SOx), and trace heavy metals [14,15]. However the presence of these impurities poses challenges, such as medium acidification and potential toxicity to microalgae. These challenges can be mitigated through strategies like pre-treatment of flue gases to remove contaminants or by selecting microalgal strains with higher tolerance to such impurities [16,17].

Microalgae-based carbon capture systems employ direct delivery of flue gases into photobioreactors or open raceway ponds, offering an efficient method for CO₂ sequestration and biomass production. For instance, studies have shown that strains like Chlorella vulgaris achieve CO₂ removal efficiencies of up to 64% when cultivated with flue gases, while also capturing 62% of SOx and 63% of NOx [17,18]. These results underscore the dual benefits of CO₂ capture and pollutant remediation. Additionally, the integration of flue gases into microalgal systems has demonstrated potential for nutrient recycling, particularly in municipal and industrial wastewater treatment, enhancing the sustainability of the cultivation process [15].

To optimize CO₂ absorption, flue gases are often bubbled into cultivation systems using gas diffusers, maintaining a balance between CO₂ availability and medium pH. For example, Scenedesmus obliquus cultivated in simulated flue gas conditions showed increased biomass productivity of 700mg/L/day, significantly higher than control conditions, due to the stimulatory effects of sulphur and other trace nutrients [18]. Furthermore, hybrid systems combining flue gases with wastewater have enhanced algal productivity by supplying both carbon and essential nutrients, demonstrating the synergistic potential of integrated approaches [14].

Wastewater as a nutrient source for microalgae

Microalgae cultivation in wastewater offers a sustainable approach to wastewater remediation and nutrient recovery while producing valuable biomass. Wastewater contains essential nutrients, including nitrogen (N), phosphorus (P), and trace elements, which are critical for the growth of microalgae. Studies have demonstrated the potential of municipal, agricultural, and industrial wastewaters as cultivation mediums, with nitrogen and phosphorus removal efficiencies often exceeding 90% under optimal conditions [19,20].

The nutrient composition of wastewater varies significantly based on its source. Municipal wastewater typically contains ammonium, nitrate, and phosphate, with concentrations conducive to microalgal growth. For example, centrate wastewater from sludge dewatering processes exhibits high nutrient levels, including nitrogen (130mg/L), phosphorus (200mg/L), and COD (2250mg/L), making it ideal for microalgae cultivation. Similarly, agricultural wastewater has been used successfully, although the high organic load requires pretreatment to avoid toxicity to microalgal cells [21,22].

The use of wastewater for microalgae cultivation provides dual benefits. First, it eliminates the need for synthetic fertilizers, reducing cultivation costs and environmental impacts. Second, it facilitates the treatment of wastewater by removing nutrients that would otherwise contribute to eutrophication in natural water bodies. Research has shown that microalgae-based systems can reduce total nitrogen (TN) and total phosphorus (TP) concentrations in wastewater by 97% and 98%, respectively, while achieving biomass yields of up to 1.27g/L [19,22].

Despite these advantages, challenges persist in the large-scale application of wastewater-fed microalgal systems. High levels of chemical oxygen demand (COD), turbidity, and ammonium can inhibit algal growth. Pretreatment methods such as electrooxidation and dilution have been implemented to mitigate these issues, enhancing the compatibility of wastewater with microalgal systems [20,23]. Additionally, environmental factors like light availability and temperature fluctuations in open raceway ponds can impact productivity, necessitating advanced reactor designs like photobioreactors for controlled conditions [21,22].

Another significant challenge is biomass harvesting, which accounts for up to 30% of the operational costs in wastewater-based microalgal systems. Innovative approaches, including flocculation and flotation, have been developed to improve cost efficiency while maintaining biomass quality. The harvested biomass can be further valorized into biofuels, fertilizers, and animal feed, adding economic value to the wastewater treatment process [21,24].

By integrating microalgae cultivation with wastewater treatment, industries can achieve resource recovery and environmental remediation simultaneously. This synergy not only aligns with circular economy principles but also addresses pressing global challenges like water pollution and nutrient depletion, making microalgae a cornerstone of sustainable wastewater management.

Integration of flue gases and wastewater in industrialscale systems

The integration of flue gases and wastewater into industrialscale microalgae cultivation systems presents a sustainable solution for carbon sequestration and nutrient recovery. Flue gases, typically rich in CO₂ (12-14%), NOx, and SOx, provide an abundant and low-cost carbon source for microalgal growth. However, the presence of impurities such as NOx and SOx can lead to medium acidification, inhibiting algal photosynthetic efficiency. Strategies like pretreatment of flue gases or the selection of robust strains, such as Chlorella vulgaris and Scenedesmus obliquus, have demonstrated significant improvements, with CO₂ removal efficiencies ranging from 60% to 65% and effective mitigation of NOx and SOx toxicity [4,17]. Wastewater, rich in essential nutrients such as nitrogen and phosphorus, offers a complementary medium for microalgal growth while simultaneously addressing eutrophication issues. Studies show nutrient removal efficiencies exceeding 90% in optimized systems, with biomass yields reaching 1.27g/L in municipal wastewater. Integrating flue gases into wastewaterbased cultivation systems has further enhanced productivity, with mixotrophic setups supporting higher lipid accumulation and biomass yields [22,25].

Hybrid systems combining flue gas delivery and wastewater have been implemented in open raceway ponds and photobioreactors (PBRs). Open ponds are cost-effective for large-scale applications but are vulnerable to contamination and environmental variability. PBRs, while more expensive, provide controlled environments that significantly improve productivity, achieving up to 30 times higher output compared to open systems. Innovations such as dual-stage systems, where initial growth occurs in PBRs followed by lipid accumulation in open ponds, have optimized resource use and reduced costs [26].

The role of automation and artificial intelligence (AI) in optimizing these systems is increasingly prominent. AI models, including neural networks and IoT-enabled devices, facilitate realtime monitoring of critical parameters such as pH, temperature, and CO₂ flow rates. AI-driven optimizations have reduced operational costs by up to 30% and improved productivity by 20%, making these systems more economically viable. IoT integration has also enhanced scalability, enabling continuous data collection and adaptive system adjustments to improve efficiency [25].

Valorization of microalgal biomass

Microalgal biomass offers immense potential for valorization into biofuels, bioproducts, and other value-added compounds, aligning with the principles of a circular bioeconomy. The high lipid, protein, and carbohydrate content of microalgae makes them a versatile feedstock for sustainable energy and bioproducts, contributing to environmental sustainability and economic viability.

Biofuels derived from microalgal biomass, such as biodiesel, bioethanol, and biogas, have gained significant attention as alternatives to fossil fuels. Microalgal species such as Chlorella vulgaris and Chlamydomonas reinhardtii have shown exceptional potential for biofuel production. For instance, some strains can produce lipid contents exceeding 80% of their dry weight under nitrogen-limited conditions, making them ideal candidates for biodiesel production. Anaerobic digestion of algal biomass has yielded biomethane with efficiencies as high as 356mL CH₄ g⁻¹ volatile solids added, highlighting the feasibility of biogas production from microalgae [11,27].

Beyond biofuels, microalgal biomass can be transformed into bioplastics, fertilizers, and animal feed. Bioplastics derived from algal polysaccharides offer a sustainable alternative to petroleum-based plastics, addressing environmental concerns related to plastic pollution. Additionally, the nutrient-rich residues from microalgal cultivation and processing can be repurposed as organic fertilizers, enhancing soil fertility and reducing reliance on synthetic fertilizers. For animal feed, the high protein content of microalgal biomass-ranging from 15% to 84% depending on species-provides a valuable resource for aquaculture and livestock industries [28,29].

The integration of microalgal biomass valorization into existing industrial systems offers economic benefits. For example, colocating algal biorefineries with wastewater treatment facilities or flue gas emission sources reduces operational costs while enhancing resource recovery. Innovative biorefinery technologies such as hydrothermal carbonization (HTC) has been developed to process wet algal biomass into hydrocar and bio-oil, maximizing energy recovery and minimizing waste. HTC can increase the carbon content of biomass by 40%, making it suitable for use as a soil amendment or as a co-firing material with coal [27].

Despite the promising potential, challenges remain in scaling up algal biomass valorization. High production costs associated with cultivation, harvesting, and downstream processing limit widespread adoption. Emerging technologies, such as AI-driven optimization and closed-loop biorefinery models, aim to address these challenges by improving efficiency and reducing costs. For instance, AI-based systems can optimize nutrient inputs and environmental conditions in real time, enhancing biomass yields and reducing resource use [29].

Environmental and economic perspectives

Microalgae-based systems offer a transformative approach to addressing pressing environmental challenges while advancing economic sustainability. These systems combine carbon sequestration, nutrient recovery, and waste remediation, aligning closely with global sustainability goals. By integrating microalgae cultivation into industrial operations, significant environmental and economic benefits can be realized, supporting the transition to a circular bioeconomy.

Microalgae provide an effective solution for mitigating industrial carbon emissions and reducing nutrient loads in wastewater. Their ability to sequester carbon dioxide at rates significantly higher than terrestrial plants makes them a valuable tool for reducing atmospheric greenhouse gas concentrations. Simultaneously, microalgae remove up to 90% of nitrogen and phosphorus from wastewater, addressing eutrophication concerns and improving water quality. For instance, microalgae cultivated in municipal and industrial wastewater demonstrated a reduction of over 75% in biochemical oxygen demand (BOD) and chemical oxygen demand (COD), further highlighting their utility as a sustainable wastewater treatment solution. Additionally, microalgae can assimilate heavy metals and toxins from industrial effluents, enhancing ecosystem health and contributing to clean water access, a critical Sustainable Development Goal [20,30].

From an economic perspective, microalgae cultivation offers a cost-effective alternative to traditional wastewater treatment and carbon capture methods. Conventional wastewater treatment processes are energy-intensive and often produce secondary pollutants, whereas microalgae systems not only treat wastewater but also generate valuable by-products such as biofuels, bioplastics, and animal feed. Research indicates that the cost of biomass production can be significantly reduced by utilizing nutrient-rich wastewater, with production costs ranging from $0.39 to $0.92 per kilogram, depending on the system design and operational efficiency. Furthermore, the incorporation of microalgae-based carbon capture technologies allows industries to meet regulatory standards while benefiting from carbon credits and other financial incentives [31,32].

Governmental policies and subsidies play a pivotal role in promoting the adoption of microalgae systems. Incentives such as grants for renewable energy projects, tax reductions for green technologies, and subsidies for wastewater treatment infrastructure can substantially reduce the capital and operational costs associated with implementing these systems. In regions with carbon taxation, microalgae technologies enable industries to achieve compliance while generating additional economic value through resource recovery [33].

Future directions and challenges

Microalgal systems have emerged as a promising solution for addressing global environmental challenges, yet significant advancements and strategies are needed to overcome barriers to their large-scale adoption. Advancements in genetic engineering and synthetic biology offer transformative opportunities for enhancing microalgal efficiency. CRISPR/Cas9 and TALENs, among other tools, have facilitated targeted modifications to metabolic pathways, improving carbon sequestration rates and biomass yields. For example, engineered microalgae with optimized Calvin cycle enzymes and enhanced carbon-concentrating mechanisms (CCMs) have achieved up to a 25% increase in CO₂ fixation efficiency [34,35]. However, challenges remain in scaling these advancements across diverse strains and industrial applications.

Scaling up microalgal systems presents significant technical and logistical hurdles. Pilot-scale studies highlight a decline in productivity when transitioning from controlled laboratory conditions to outdoor systems, primarily due to environmental fluctuations and contamination risks. Hybrid systems combining photobioreactors with open ponds have demonstrated improved performance, achieving 2.6 times higher biomass yields than traditional setups. However, further optimization is needed to reduce energy consumption and operational costs [36].

Economic barriers continue to limit the widespread adoption of microalgal technologies. High operational costs, especially in integrating flue gases and wastewater, necessitate innovative solutions. Artificial intelligence (AI) and machine learning (ML) algorithms have been instrumental in optimizing cultivation parameters, such as nutrient dosing and CO₂ delivery, significantly improving efficiency and reducing costs. Integrated biorefinery approaches, which co-produce biofuels, bioplastics, and high-value bioproducts, further enhance the economic feasibility of microalgal systems, aligning them with the principles of a circular economy [37,38].

The role of policy and interdisciplinary collaboration is crucial for advancing industrial adoption. Governments must incentivize the integration of microalgal systems through subsidies, tax credits, and funding for renewable energy initiatives. Collaborative efforts between biologists, engineers, and economists can address technical and economic challenges, facilitating the transition of microalgal technologies from research to large-scale implementation [34,35].

While microalgal systems hold immense potential for environmental remediation and sustainable resource management, addressing genetic, technical, economic, and policy-related challenges is essential. By fostering innovation and collaboration, these systems can contribute significantly to achieving global sustainability goals.

Microalgae offers a sustainable and versatile approach to mitigating climate change while advancing industrial efficiency and resource recovery. Their exceptional photosynthetic capabilities enable effective carbon sequestration and nutrient removal, positioning them as a cornerstone of the CCE. By integrating microalgal systems into industrial processes, significant environmental benefits, such as reduced greenhouse gas emissions and wastewater pollutants, can be realized. Additionally, the valorization of microalgal biomass into biofuels and bioproducts enhances their economic feasibility and aligns with global sustainability goals. Despite the promising potential, challenges such as scalability, high operational costs, and energy demands remain barriers to widespread adoption. Advancements in genetic engineering and AI-driven optimization are paving the way for more efficient and economically viable solutions. However, achieving large-scale implementation will require coordinated efforts in policy support, interdisciplinary research, and technological innovation. Microalgae represent a transformative pathway toward a carbon-neutral future, underscoring the importance of continued investment and collaboration to unlock their full potential.

Conflict of interest: None.

Funding: None.

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