If an operator monitors DO and pH levels in a facultative lagoon hourly, what trend should they expect to observe?

In a facultative lagoon, the dynamics of dissolved oxygen (DO) levels and pH are influenced heavily by the daily cycle of photosynthesis and respiration. During daylight hours, particularly from morning to afternoon, photosynthetic activity by algae increases significantly. This process produces oxygen, leading to a rise in DO levels as sunlight enhances photosynthesis.

As the day progresses, the operator would typically observe a clear trend where DO levels rise from the morning to the afternoon. However, as evening approaches and photosynthetic activity declines, respiration dominates, causing a gradual decrease in DO levels as oxygen is consumed by microorganisms and organic matter decomposition. This pattern aligns with the natural diurnal cycle observed in aquatic systems, especially in shallow lagoons where light penetration supports algal growth.

Thus, the correct observation is a rise in DO levels from morning to afternoon, followed by a gradual fall as night falls, illustrating the overall biological activity within the lagoon. Understanding this pattern helps operators manage and monitor the health and stability of wastewater treatment processes effectively.