When is dissolved oxygen (DO) typically the highest in stabilization ponds?

Dissolved oxygen (DO) levels in stabilization ponds typically reach their highest levels during the mid-afternoon. This is largely due to the process of photosynthesis carried out by algae and aquatic plants in the ponds. During daylight hours, particularly when sunlight is at its peak, these organisms utilize sunlight to produce oxygen as a byproduct of photosynthesis.

In contrast, early morning and evening times usually see lower DO levels. In the early morning, oxygen levels may be lower because the oxygen produced during the previous day and night might have been consumed by aerobic respiration as well as by decomposition processes that occur continuously in the pond. Similarly, in the evening, as photosynthesis ceases with the diminishing light, the oxygen produced during the day is utilized, leading to lower DO levels.

Nighttime sees minimal photosynthetic activity, resulting in even lower levels of dissolved oxygen since organisms continue to respire, consuming the oxygen in the water. Thus, the combination of high sunlight intensity and active photosynthesis in the mid-afternoon results in elevated levels of dissolved oxygen, making it the optimal time for DO in stabilization ponds.