What happens during decomposition of wastewater in natural waters?

• A. Bacteria break down organic waste using oxygen, reducing dissolved oxygen levels
• B. Sediments settle out
• C. Bacteria add oxygen to water
• D. Algae remove nutrients

Answer: (A)

When wastewater enters natural waters, microorganisms break down the organic matter. In the presence of oxygen, bacteria oxidize this material, using dissolved oxygen in the water as the electron acceptor. This process reduces the dissolved oxygen available for other aquatic organisms, a drop known as biological oxygen demand. If the oxygen is consumed faster than it’s replenished, DO levels fall and can stress or harm aquatic life.

That’s why the best answer is that bacteria break down organic waste using oxygen, reducing dissolved oxygen levels. Sediments settling out is a physical separation process, not decomposition; bacteria don’t add oxygen to water; and while algae can take up nutrients, they aren’t describing the decomposition process itself.