Microalgae cultivation has gained significant attention in recent years due to its numerous applications in the fields of biofuels, bioproducts, pharmaceuticals, and nutrition. Among the various techniques used for microalgae cultivation, closed photobioreactors have emerged as a promising alternative to traditional open pond systems. Closed photobioreactors offer several advantages over open ponds, including better control of culture conditions, higher biomass productivity, reduced contamination risks, and lower land and water requirements. However, there are also challenges associated with the use of closed photobioreactors, such as higher capital and operating costs, light limitation, and difficulty in scaling up. This article provides an overview of closed photobioreactor technology and its application in microalgae cultivation.
Closed photobioreactors can be classified into different types based on their design and configuration, including tubular, flat-panel, and column systems. Tubular photobioreactors consist of transparent tubes arranged in various configurations (e.g., horizontal, vertical, or helical) through which the microalgae culture is circulated by means of pumps or airlift systems. Flat-panel photobioreactors are composed of transparent panels stacked vertically or horizontally, creating a thin layer of culture that allows for efficient light penetration. Column photobioreactors are cylindrical vessels filled with microalgae culture and illuminated externally or internally using artificial light sources.
One of the main advantages of closed photobioreactors is the ability to maintain optimal culture conditions for microalgae growth. Temperature, pH, nutrient concentrations, light intensity, and CO2 levels can be precisely controlled in closed systems, resulting in improved biomass productivity compared to open ponds. Moreover, closed photobioreactors can be operated continuously or semi-continuously, allowing for higher volumetric productivity and more efficient use of resources.
Another critical advantage of closed photobioreactors is the reduction in contamination risks. Open ponds are susceptible to contamination by unwanted microorganisms, such as bacteria, fungi, or other algae species, which can compete for nutrients and light, thereby reducing microalgae productivity. In contrast, closed photobioreactors provide a more sterile environment that minimizes the risk of contamination. Furthermore, closed systems can help prevent the escape of genetically modified microalgae into the environment, addressing biosafety concerns associated with genetic engineering.
Closed photobioreactors also have lower land and water requirements compared to open ponds. Due to their higher biomass productivity and smaller footprint, closed systems can produce more microalgae per unit area than open ponds. Additionally, closed photobioreactors can recirculate water within the system, reducing water consumption and evaporation losses.
Despite these advantages, there are several challenges associated with closed photobioreactor technology. One of the main issues is the high capital and operating costs due to the complex design, construction materials, and energy requirements for maintaining optimal culture conditions. Additionally, light limitation is a significant challenge in closed photobioreactors. As microalgae cultures become denser, light penetration decreases, leading to reduced photosynthetic efficiency and biomass productivity. Various strategies have been proposed to overcome this issue, such as optimizing reactor geometry, using artificial light sources or light-emitting diodes (LEDs), and implementing light/dark cycles.
Another challenge in closed photobioreactor technology is the difficulty in scaling up from laboratory-scale systems to large-scale production facilities. Scaling up involves not only increasing the reactor size but also maintaining efficient mass transfer (e.g., CO2 supply and oxygen removal) and heat exchange while minimizing energy consumption. Moreover, the harvesting of microalgae biomass from closed photobioreactors can be challenging due to the small cell size and low settling velocity of microalgae.
In conclusion, closed photobioreactors offer several advantages over traditional open pond systems for microalgae cultivation, including better control of culture conditions, higher biomass productivity, reduced contamination risks, and lower land and water requirements. However, challenges such as high capital and operating costs, light limitation, and difficulty in scaling up need to be addressed to fully realize the potential of closed photobioreactor technology in microalgae cultivation and its various applications.