By the end of this lesson
- Describe what sewage adds to water.
- Explain how sewage pollution lowers oxygen concentration.
- Describe the consequences for aquatic organisms and human health.
BIG QUESTION
Why does adding food to a river kill the fish?
The organic matter in sewage feeds bacteria, and their respiration removes the oxygen fish need.
What sewage contains
sewageWaste water from homes, farms and industry, containing human and animal waste; untreated sewage in rivers raises the biochemical oxygen demand. is waste water from homes, farms and industry. It contains organic matter such as faeces and food waste, mineral ions such as nitrates and phosphates, and may contain pathogenA microorganism or infectious agent (including viruses) that causes disease, such as a bacterium, virus, fungus or protoctist. such as bacteria and viruses.
If sewage enters rivers or lakes without treatment, for example after storms overwhelm treatment works, it can cause serious pollution.
Oxygen depletion
Aerobic bacteriumA single-celled prokaryote with no nucleus, whose DNA lies free in the cytoplasm as a circular chromosome, often with plasmids. decompose the organic matter. Their populations increase rapidly and their respiration uses up dissolved oxygen in the water.
Fish and many invertebrates cannot obtain enough oxygen and die or move away. This is often measured as an increased biochemical oxygen demand (BOD)The amount of dissolved oxygen used by microorganisms to break down organic matter in water; a high BOD indicates pollution., which is extension terminology.
Indicator species and health risks
Organisms tolerant of low oxygen, such as sludge worms and rat-tailed maggots, become more common, while mayfly and stonefly larvae, which need high oxygen, disappear. These indicator speciesA species whose presence, absence or numbers show the level of pollution, such as lichens for air quality or mayfly larvae for clean water. can show water quality.
Pathogens in sewage can cause diseases such as cholera and typhoid if contaminated water is drunk, so sewage treatment and clean water supplies protect public health.
Effects downstream of a sewage outflow
Follow the river from clean water through the outflow and downstream.
| Position | Dissolved oxygen | Typical organisms |
|---|---|---|
| Upstream (clean) | High. | Mayfly and stonefly larvae, fish. |
| At the outflow | Falling as bacteria multiply. | Bacteria increase; fish leave or die. |
| Just downstream | Lowest. | Sludge worms, rat-tailed maggots. |
| Far downstream | Recovers as organic matter is used up. | Sensitive species gradually return. |
Language that earns precision
- sewage
- Waste water containing human and animal waste and other material.
- pathogen
- A microorganism or infectious agent (including viruses) that causes disease.
- indicator species
- An organism whose presence or absence shows the level of pollution.
Common misconceptions
Claim: Sewage kills fish because it is poisonous.
Correction: The main cause is oxygen depletion by decomposer bacteria respiring.
Claim: Bacteria in sewage use up oxygen by photosynthesis.
Correction: Bacteria use oxygen in aerobic respiration.
Claim: If water looks clear, it is free of pathogens.
Correction: Pathogens are microscopic, so clear water can still be contaminated.
Match the answer to the command word
Sequence: organic matter → bacteria multiply → bacterial respiration → less dissolved oxygen → fish die.
When given oxygen readings along a river, describe the change and identify the outflow position.
Recall, apply, then justify
Explain why fish may die after untreated sewage enters a river.
Hint
Bacteria and oxygen.
Suggested marking points
- Bacteria decompose the organic matter in sewage and multiply.
- Bacterial (aerobic) respiration uses up dissolved oxygen.
- Fish cannot get enough oxygen (for respiration) and die.
Suggest why stonefly larvae are absent just downstream of a sewage outflow.
Hint
What do they need?
Suggested marking points
- Stonefly larvae need high concentrations of oxygen.
- Dissolved oxygen is low just downstream of the outflow.
Explain why dissolved oxygen recovers further downstream from a sewage outflow.
Planning hint
Food for bacteria, mixing, photosynthesis.
What a strong design includes
- Bacteria gradually use up the organic matter.
- With less food, the bacterial population falls.
- Less oxygen is used in bacterial respiration.
- Oxygen dissolves from the air, especially in turbulent water.
- Algae and plants release oxygen by photosynthesis.
- So oxygen-sensitive organisms can return.