Basic Wastewater Operator Practice Exam

When organic wastes are discharged to receiving waters, what depletes oxygen?

1. Excessive flow

2. Bacteria

3. Color

4. High pH

The correct answer is: Bacteria

When organic wastes are discharged into receiving waters, bacteria play a crucial role in the process of decomposition. As bacteria break down these organic materials, they consume oxygen from the surrounding water in order to carry out their metabolic processes. This phenomenon is known as biochemical oxygen demand (BOD). An increase in organic waste leads to a higher population of bacteria that thrive on the waste, which, in turn, escalates the demand for oxygen. As oxygen levels drop, aquatic life—including fish and invertebrates that rely on dissolved oxygen for survival—can experience stress or even death due to hypoxic conditions. This depletion of oxygen is particularly significant in water bodies with limited circulation or low flow, where the replenishment of oxygen may be insufficient to meet the demands created by the bacteria. In contrast, factors such as excessive flow, color, or high pH do not directly contribute to oxygen depletion in the same manner as bacteria do. While these factors can influence overall water quality, it is the activity of bacteria that is primarily responsible for the reduction of dissolved oxygen levels in the presence of organic wastes.