Module 2 ยท How a Facultative Lagoon Works
Algae, Sunlight, and Oxygen
Algae are simple plant-like organisms that grow in lagoon water. Through photosynthesis, they use sunlight and carbon dioxide and give off oxygen. During the day, this oxygen supports bacteria in the upper part of a facultative lagoon. In return, the bacteria release carbon dioxide and nutrients that the algae use.
Photosynthesis stops at night, but organisms keep using oxygen. So dissolved oxygen and pH often rise during the day and fall overnight. Cloudy weather, short winter days, deep water, ice, and thick surface growth all reduce the light that reaches the water.
Sample timing matters. An afternoon DO reading does not show the low point that is often reached near sunrise. Follow the permit and approved monitoring plan for compliance samples. For process control, test at the same times each day so trends can be compared.
Exam Need to Know
- Algae make oxygen by day; DO and pH drop overnight.
- DO is often lowest near sunrise, not in the afternoon.
Exam area: Treatment Process