Virginia Wastewater Class 4 Practice Exam 2025 – Complete All-in-One Guide for Exam Success

Dissolved oxygen (DO) levels in stabilization ponds are influenced by various factors, including photosynthesis and the respiration rates of microorganisms and plants. During the daytime, especially in the mid-afternoon, sunlight is at its peak, which enhances photosynthesis in aquatic plants and algae. This process generates oxygen as a byproduct, leading to elevated levels of dissolved oxygen in the water.

In earlier hours, such as early morning, DO levels may still be relatively low due to the night-time respiration of microorganisms and a lack of photosynthesis. Late evening is also not ideal for high DO levels, as photosynthetic activity significantly decreases and organisms continue to respire. Mid-morning may show an increase from early morning, but it typically doesn’t match the peak reached around mid-afternoon when these natural processes are in full swing.

Thus, mid-afternoon is when you can expect the highest dissolved oxygen levels in stabilization ponds, due to the combination of robust photosynthesis and lower respiration rates as the day progresses.