Photobioreactors

Photobioreactors are vessels designed to cultivate photosynthetic microorganisms under controlled illumination, gas exchange, and nutrient conditions. They are used to grow microalgae, cyanobacteria, and other photoautotrophs for biomass, pigments, nutraceuticals, and research. The aim is to supply sufficient light while maintaining CO2 availability and stable conditions to maximize growth and product yields.

Photobioreactors are generally divided into closed and open types. Closed systems include tubular, flat-panel, column, and bubble-column designs that confine cultures behind transparent walls and control gas flow. Open systems, such as raceway ponds, rely more on natural light and ambient air. Closed reactors offer tighter process control and higher predicted productivity, while open ponds tend to be cheaper to construct but more susceptible to contamination and environmental variation.

Operation focuses on balancing light delivery with mixing to minimize shading, ensuring efficient CO2 supply and O2 removal, and maintaining suitable temperatures, pH, and nutrients. Lighting may be natural sunlight or artificial sources such as LEDs, often with light-dark cycles. Gas exchange is achieved by sparging or diffuser membranes. Design challenges include avoiding light-dark zones, preventing biofouling of surfaces, managing heat buildup, and scaling up without losing productivity. The choice of reactor type depends on organism, product, and economics.

Applications span biofuels, animal feed, and specialty compounds like omega-3 fatty acids, pigments, and phycobiliproteins, as well as wastewater treatment and carbon capture in some configurations. Advantages over open ponds include higher productivity and process control; drawbacks include higher capital and operating costs and energy requirements for lighting and mixing, plus complexity of scale-up and contamination risk.