Tennessee Wastewater Operator Practice Exam

What are the expected values for pH and dissolved oxygen late in the afternoon before dark in a stabilization pond?

1. Lowest values of the day

2. Stable values

3. Median values of the day

4. Highest values of the day

The correct answer is: Highest values of the day

In a stabilization pond, the levels of pH and dissolved oxygen typically reach their highest values late in the afternoon before dark. This pattern occurs due to the process of photosynthesis carried out by the algae and aquatic plants in the pond. During daylight hours, these organisms use sunlight to convert carbon dioxide and water into organic matter, releasing oxygen as a byproduct. As the sun reaches its peak and continues throughout the afternoon, photosynthesis is at its maximum, leading to increased levels of dissolved oxygen. Likewise, the pH tends to rise during this time because photosynthesis consumes carbon dioxide, which in turn reduces carbonic acid levels in the water, making it less acidic and raising the pH. Therefore, late in the afternoon, both the pH and dissolved oxygen levels in a stabilization pond are expected to be at their highest, supported by the activity of photosynthetic organisms during daylight. This is in contrast to the low values often recorded in the early morning before dawn, when photosynthetic activity has not yet begun for the day.