The primary purpose of a facultative lagoon is to:

The primary purpose of a facultative lagoon is to treat wastewater. These lagoons are designed to utilize both aerobic and anaerobic processes to break down organic matter present in the wastewater. In a facultative lagoon, aeration occurs naturally due to the action of wind and the presence of algae, which produce oxygen through photosynthesis. This environment supports microbial communities that effectively degrade pollutants and reduce the biochemical oxygen demand (BOD) and suspended solids in the wastewater.

While sludge storage, fostering aquatic life, and transferring effluent may occur in or around facultative lagoons, these are not their primary functions. The focus is fundamentally on the biological treatment of wastewater, making the treatment of wastewater the main goal of these systems. This distinction is crucial for understanding the role that facultative lagoons play in wastewater management and their design specifications.