Substances in natural water

1. Water from natural sources, such as rivers, lakes, reservoirs and underground rocks, is never pure.
2. As water flows over land and through rocks and soil, it dissolves some substances and carries others along with it.
3. Water from natural sources may contain the following substances.
   1. Dissolved oxygen, which dissolves into the water from the air and is also released by water plants.
   2. Metal compounds, which dissolve out of the rocks and soil that the water flows through.
   3. Plastics, from litter and waste that are dropped or washed into rivers, lakes and seas.
   4. Sewage, from homes and farms, and from sewers that leak or overflow after heavy rain.
   5. Harmful microbes, such as bacteria, many of which come from sewage.
   6. Nitrates, which are washed off farmland that has been treated with fertilisers.
   7. Phosphates, which come from fertilisers and from the detergents in waste water from homes.
4. Some of these substances are beneficial to living things, while others are potentially harmful.

Beneficial substances

1. Dissolved oxygen is needed by aquatic life, because fish and other water animals take in oxygen from the water for respiration.
2. A river or lake with plenty of dissolved oxygen can support a large number and variety of living things.
3. Some metal compounds provide essential minerals for life.
4. For example, dissolved calcium compounds provide the calcium that people need for healthy bones and teeth, and magnesium compounds provide magnesium.
5. This is why the label on a bottle of mineral water lists the dissolved minerals that the water contains.

Harmful substances

1. Some metal compounds are toxic, such as compounds of lead and mercury, which poison people and aquatic life even at low concentrations.
2. Toxic metal compounds can build up in the bodies of fish over time, so they also harm the animals and people that eat the fish.
3. Some plastics harm aquatic life, because animals become trapped in plastic waste or swallow small pieces of plastic, which block their digestive systems.
4. Most plastics do not break down in water, so plastic waste keeps causing harm for many years.
5. Sewage contains harmful microbes, which cause diseases such as cholera and typhoid if the water is drunk without being treated.
6. Nitrates and phosphates lead to the deoxygenation of water, so the amount of dissolved oxygen in the water falls.
7. Without enough dissolved oxygen, fish and other aquatic organisms cannot respire, so they die and the aquatic life of the river or lake is damaged.

Treating the domestic water supply

1. Water taken from rivers and reservoirs must be treated before it is safe to supply to homes as drinking water.
2. Treatment happens in a set order of stages, and each stage removes or deals with a different type of substance.
3. Sedimentation is the first stage.
   1. The water is left to stand in large tanks, so that heavy insoluble particles, such as sand, grit and mud, sink to the bottom under gravity.
   2. The cleaner water is then drawn off from the top of the tanks.
4. Filtration is the second stage.
   1. The water passes slowly through beds of sand and gravel, which trap the smaller insoluble particles that did not settle out.
   2. Sedimentation and filtration together remove solids from the water.
5. Carbon is used in the third stage.
   1. The water passes through granules or powder of carbon, which removes the dissolved substances that cause unpleasant tastes and odours.
   2. The unwanted substances stick to the very large surface area of the tiny carbon particles, so they are held back while the water passes on.
6. Chlorination is the final stage.
   1. A small, carefully controlled amount of chlorine is added to the water to kill microbes, such as bacteria, that could cause disease.
   2. A little chlorine stays in the water as it travels through the pipes, so it keeps killing microbes until the water reaches the tap.
7. Treated tap water is safe to drink, but it is not pure, because dissolved salts pass through every stage of the treatment.