A facultative lagoon primarily obtains its oxygen through which process?

In a facultative lagoon, the primary process for obtaining oxygen is photosynthesis. This type of lagoon is designed to treat wastewater in a shallow pond where both aerobic and anaerobic conditions can coexist.

During the day, when sunlight is available, aquatic plants and algae in the lagoon perform photosynthesis. They use sunlight, carbon dioxide, and water to produce oxygen as a byproduct. This oxygen is then utilized by aerobic bacteria to decompose organic matter in the wastewater. The presence of sunlight is crucial for this process, as it directly drives the rate of photosynthesis and consequently the level of oxygen produced in the water.

While diffusion is also a mechanism by which oxygen can enter water from the atmosphere, it is not the primary means in a facultative lagoon, especially in the presence of algae that actively produce oxygen through photosynthesis. Mechanical aeration is typically employed in other types of treatment systems, not in facultative lagoons, where natural processes are favored. Biological reactions involving aerobic bacteria indeed play a vital role in the treatment process, but these reactions depend on the oxygen provided primarily through photosynthesis during daylight hours.