Lesson 6 of 9 / Organisms and their environment
Why sewage can remove oxygen from water
How can added organic matter harm fish?
In this lesson: Explain the effects of sewage using decomposition, respiration and dissolved oxygen.
About 5 min
The key ideaMicroorganisms breaking down sewage respire and use dissolved oxygen. If demand exceeds replacement, oxygen falls and aquatic organisms can be harmed.
Dissolved oxygen is oxygen present in water and available to aquatic organisms. Biological oxygen demand describes the oxygen used by microorganisms breaking down organic material under specified conditions.
Explore the idea
What changes downstream?
2. Below discharge: 3 mg/L. Added organic material supports decomposer respiration. Oxygen use can exceed replacement, lowering dissolved oxygen.
Original practice data, not a real river survey. Lines guide the eye between three samples; distance and time are not measured on the horizontal axis. Temperature, flow and further inputs can change the pattern.
Explanation
Untreated sewage introduces organic matter into water. Microorganisms feed on and decompose this material. As aerobic decomposers become more active and numerous, their respiration uses more dissolved oxygen.
If oxygen use is faster than replacement from air exchange and photosynthesis, dissolved oxygen concentration falls. Fish and other aerobic aquatic organisms may then be unable to obtain enough oxygen for respiration. The causal link is increased biological oxygen demand, not simply that sewage physically blocks all oxygen.
Sewage can also add mineral nutrients that stimulate algal growth. Dense growth can shade submerged plants. When algae and plants die, decomposers break them down, increasing oxygen demand further. This sequence is part of eutrophication; each stage should be connected rather than stated as a memorised list.
Treatment reduces organic loads and harmful contamination before discharge. Interpreting river data also requires care: oxygen can recover downstream as organic material is broken down and water is aerated, but recovery depends on flow, temperature and continued inputs. Not every river follows one identical curve.
Step by step
- 1
Start with the added material
Sewage adds organic matter and may add nutrients.
- 2
Name the biological response
Decomposers use organic matter and respire, increasing oxygen demand.
- 3
Connect oxygen to organisms
Lower dissolved oxygen limits aerobic respiration in fish and other aquatic organisms.
Worked example
An original downstream pattern
Dissolved oxygen is 8 mg/L upstream, 3 mg/L shortly below an organic sewage discharge, and 6 mg/L farther downstream. Explain the initial fall without assuming sewage directly poisons every fish.
One way to explain it
The added organic material supports increased microbial decomposition. Aerobic microbial respiration uses dissolved oxygen, causing concentration to fall when use exceeds replacement. Fish may suffer because less oxygen is available for their respiration. The later rise is consistent with reduced organic demand and re-aeration, but further data are needed to identify their contributions.
Why this answer works
- Connect the discharge to decomposers.
- Connect respiration to the oxygen fall.
- Treat downstream recovery as evidence to interpret, not proof of one cause.
Is this true? "Algae alone are the whole explanation for every sewage-related oxygen decline."
Decomposition of sewage organic matter can directly raise oxygen demand. Nutrient-driven algal growth and later decomposition can add another route to oxygen depletion.