This document discusses photobioreactors, which are bioreactors that use light to cultivate phototrophic microorganisms. Photobioreactors allow for carefully controlled conditions that enable higher growth rates than what is possible in natural environments. They can potentially use nutrient-rich wastewater and carbon dioxide from flue gases to derive phototrophic biomass. The document describes open and closed photobioreactor systems. Open systems like ponds have limited productivity due to depth and light availability, while closed systems can achieve higher cell densities but require more advanced engineering. The document outlines the wastewater treatment process using a photobioreactor and notes their advantages in preventing contamination and better controlling conditions.