BIOLOGICAL OXYGEN DEMAND
If untreated sewage is discharged into fresh water, it will be decomposed by bacteria in the water. These will multiply rapidly and use up all the oxygen present, so that fish and other animals will die. Biological oxygen demand (BOD) means the demand for oxygen created by the presence of organic material in water. Untreated sewage has a high BOD because it is rich in organic material. The organic waste from intensive farming, e.g. high density pig units, can be so concentrated that it overwhelms the capacity of sewage works, leading to effluent with high BOD.
EUTROPHICATION
This is the appearance of rich growths of algae, called algal blooms, which form on the surface of water. It can be caused by the presence of excess nitrates from neighboring over-fertilized fields, and phosphates from detergents. When these masses of algae die, they also create a high BOD and may cause the death of fish.
Water pollution
| Water pollutants | Source | Damage caused | Prevention |
| 1 Artificial fertilizers | Application of excessive inorganic fertilizers, especially nitrates to fields, causes run-off into rivers and lakes. | -Excess inorganic salts in the water may cause eutrophication and death of fish.-High nitrate levels in drinking water may cause stomach cancer or severe anemia in babies. | -Persuade farmers to use correct quantities. -Encourage use of organic fertilizers |
| 2 Heavy metals, e.g. mercury, lead | Effluent from industry. | -Poisons microorganisms, disrupting sewage treatment.-Can get into food chains, e.g. mercury dumped at sea accumulate in fish causing nervous diseases in humans. | -Compel factories to purify their own effluent before discharge into sewers. |
| 3 Oil | Leaked from tankers. | Clogs feathers of seabirds, fouls beaches. | -Improve safety at sea.-Develop effective methods for removing oil. |
| 4 Sewage | Human wastes. | -Raw sewage is discharged into rivers or lakes; may cause bacteria to multiply rapidly and use up all available oxygen so fish die.-Phosphates from detergents may cause eutrophication and death of fish. |
-Ensure all effluent has a low biological oxygen demand. -Encourage use of phosphate free detergents |
FOOD CHAINS AND POLLUTION
Pollutants often become concentrated in the bodies of animals that are at the top of a food chain. Top consumers such as carnivores (including humans) are therefore affected more than producers or lower consumers. For example, if radioactive rain falls over grassland, the radioactivity is absorbed into grass through its roots. If cows eat the grass, radioactive substances such as strontium become concentrated in their bodies. When humans drink milk from these cows, they take in radioactive strontium. This may accumulate in the bones in quantities high enough to cause cancer. The same applies to some insecticides, heavy metals and other pollutants. All of these may kill predators or cause eggs to be so thin-shelled that they do not hatch.
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