Why does oxygen drop after sewage enters water?

• A. Bacteria break down the waste and use oxygen during decomposition.
• B. Oxygen levels increase because water becomes aerated.
• C. Algae begin to produce oxygen rapidly.
• D. Salinity increases and binds oxygen.

Answer: (A)

When organic matter from sewage enters water, microbes — bacteria and other decomposers — feed on it and respire, using dissolved oxygen in the process. This rapid microbial activity increases the biological oxygen demand, so the amount of oxygen in the water gets consumed faster than it can be replenished. That drop in dissolved oxygen is the main reason oxygen levels fall after sewage enters a body of water. Over time, water can re-aerate from the atmosphere and the microbial population may level off, but the immediate effect of sewage is a decrease in oxygen due to decomposition.

Oxygen wouldn’t rise from sewage simply by aeration, and while algae can produce oxygen through photosynthesis, that happens during daylight and often isn’t enough to offset the immediate consumption. Salinity doesn’t meaningfully bind or remove oxygen in this context.