Journal of Energy Bioscience 2024, Vol.15, No.3, 171-185 http://bioscipublisher.com/index.php/jeb 175 5 Bioreactor Design and Scale-Up 5.1 Types of bioreactors Open pond systems, such as raceway ponds, are one of the most common methods for cultivating microalgae. These systems are typically shallow and use paddle wheels to circulate the water, ensuring even distribution of nutrients and exposure to sunlight. Open ponds are cost-effective and easy to construct, making them a popular choice for large-scale microalgae cultivation (Narala et al., 2016). Closed photobioreactors (PBRs) provide a controlled environment for microalgae growth, which can lead to higher productivity and better-quality biomass. Types of closed PBRs include tubular, flat plate, and column bioreactors. These systems protect the culture from contamination and allow precise control over growth conditions such as light, temperature, pH, and nutrient supply (Gupta et al., 2015). 5.2 Advantages and limitations of each bioreactor type Low capital and operational costs, simple design, and ease of scalability. Suitable for large-scale biomass production. Limited control over environmental conditions, high risk of contamination, significant water loss through evaporation, and lower biomass productivity compared to closed systems (Singha et al., 2017). Better control over cultivation conditions, reduced contamination risk, higher biomass productivity, and efficient use of CO2 and nutrients. Suitable for producing high-value products such as pharmaceuticals and nutraceuticals. Higher capital and operational costs, more complex design and maintenance, and challenges in scaling up for large-scale production (Solimeno et al., 2017). 5.3 Scale-up challenges and solutions for commercial production Scaling up microalgae cultivation systems from laboratory to commercial scale involves several challenges. The high cost of photobioreactors and the associated infrastructure can be prohibitive. Solutions include optimizing the design and operation of bioreactors to reduce costs and developing hybrid systems that combine the advantages of both open ponds and closed PBRs (Sun, 2023). Open pond systems are highly susceptible to contamination from other microorganisms. Closed systems reduce this risk but are more expensive. Implementing semi-closed systems and using genetic engineering to create resistant algal strains can mitigate contamination issues (Loera-Quezada et al., 2016) (Figure 1). Maintaining optimal conditions (e.g., light, temperature, pH) is more challenging at larger scales. Advances in computational fluid dynamics (CFD) and real-time monitoring systems can help optimize environmental conditions and improve productivity (Hinterholz et al., 2019). Efficient nutrient and CO2 supply systems are essential for large-scale cultivation. Integrating waste streams, such as flue gases and wastewater, can provide a sustainable source of nutrients and CO2, reducing operational costs and environmental impact (Díez-Montero et al., 2020). Energy Consumption: High energy consumption for mixing, aeration, and temperature control can affect the economic viability of microalgae cultivation. Energy-efficient designs and renewable energy sources can help reduce the overall energy footprint (Kwon and Yeom, 2017). Figure 1 shows the growth of Chlamydomonas reinhardtii transgenic lines using phosphite as a phosphorus source. Panel (a) displays the positive PTXD transgenic lines (CrB-1, CrP-6, CrP-13, CrX-3, CrX-9) grown in Tris-Acetate (TA) media that either lacked phosphorus (-P) or was supplemented with 0.1 mM phosphate (Pi) or phosphite (Phi). Panels (b) and (c) illustrate the optical density (OD) at 680 nm over 6 days for P-starved and P-replete cells, respectively, with 0.1 mM phosphite as the phosphorus source. The experiments were conducted using a photobioreactor (Multi-Cultivator MC 1000) with a light intensity of 250 μmol photons/m²/s, at 28 ℃, and bubbled with air. The wild-type C. reinhardtii CC-125 (CrWT) strain served as the control. The results indicate that the transgenic lines show significant growth advantages under phosphite conditions.
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