Texas Class C Wastewater Treatment License Practice Exam

In facultative lagoons, the dissolved oxygen (DO) content typically varies with depth due to natural processes occurring in the lagoon. The upper layer of the lagoon, known as the epilimnion, has higher levels of dissolved oxygen as it is exposed to sunlight, which promotes photosynthesis by algae and aquatic plants. This photosynthetic activity produces oxygen, leading to a higher DO concentration near the surface.

Conversely, as you move deeper into the lagoon, particularly into the hypolimnion (the bottom layer), the dissolved oxygen levels can decrease significantly. This reduction occurs because the bottom layer is less exposed to light and does not facilitate photosynthesis, while organic matter decomposition by bacteria consumes oxygen without replenishment from the surface. Thus, the characteristic condition of having higher dissolved oxygen at the top and little to none at the bottom effectively represents the stratification of dissolved oxygen levels within the lagoon, aligning with the understanding of facultative lagoon operation and the factors influencing DO distribution.