Potable water and Using the Earth's Resources - C10.1 - C10.2

water that is safe to drink is called potable water. it is not pure, however it is safe to drink because it has:

• the correct pH
• an acceptable amount of dissolved salts (for example NaCl)
• no harmful microbes (pathogens)

PURE WATER contains only H₂O, while POTABLE WATER can contain dissolved substances like non pathogenic microbes and minerals. to TEST for pure water, you can check the boiling point. pure water has a boiling point of exactly 100'C, while potable water's boiling point can be higher due to the dissolved substances.

making potable water

there are three sources of water that can be treated to make it potable (the one that is used depends on the available supplies of water and local conditions):

• fresh water (lakes, rivers and reservoirs)
• salt water (the sea)
• waste water (sewage and agriculture)

most potable water in the uk is produced from fresh water. it is obtained from rain that collects in the ground (ground water) and in lakes, rivers and reservoirs (surface water). although it contains low levels of dissolved solids, it still needs to be treated for the water to be potable.

step one - filtration

• the water first passes through a mesh to remove larger deposits.
• it then passes through a sand and gravel bed, which removes smaller deposits. 

step two - sterillisation

• treated with ozone, uv lights and chemicals like chlorine.
• this kills harmful microbes in the water.

when supplies of fresh water are limited, sea water can be used. the main salt in sea water is sodium chloride which must be removed. this process of removing salt from water is called desalination, and is expensive because it requires lots of energy.

method one - distillation

• first, the salt water is heated. because the boiling point of water is greater than that of the dissolved salts, the water evaporates, leaving the salts in the basin.
• the water vapour is then cooled, causing the pure water to condense into a different basin.

method two - reverse osmosis

• the salt water is passed through a membrane. only the pure H₂O remains, leaving us with pure water.
• lots of energy is required to push the water through the membrane.

after desalination, the salt water is filtered and sterilised in the same way that fresh water is.

water in the home, industry and agriculture may contain organic matter, harmful chemicals and harmful microbes. this waste water could pollute fresh water and pose health risks to humans and other organisms, so it must be treated before it is released into the environment. 

the waste water produced is passed into the sewer system and ends up in a sewage treatment centre, where 4 processes happen:

step 1: screening (filtration) - larger insoluble solids are removed, like baby wipes, toilet roll and twigs.

step 2: sedimentation - when the water is left to settle, the solids sink to the bottom (called sludge) and the effluent remains at the top.

step 3: digestion using microbes - microbes carry out aerobic digestion of organic matter in the effluent, while microbes carry out anaerobic respiration of organic matter in the sludge. H₄ is produced in the sludge due to a lack of oxygen.

step 4: sterilisation - the water is treated with clorine, ozone and UV light to sterilise it.

humans use the earth's resources to provide food/drink, shelter, transport, and warmth. 

natural resources are resources that are from the earth without human input. some examples include wood, stone, clay, and metals.

agriculture allows us to supplement the earth's resources by proving conditions that enhance them. examples of agricultural resources include cotton, wool, leather, rice, and sugar. 

synthetic alternatives to natural resources made through industrial processes:

• wood - plastics (e.g furniture, flooring, and window frames)
• rubber made from tree sap - polymers (e.g tyres)

finite - a resource that cannot be replaced quickly so will run out (e.g fossil fuels).

renewable - can be made at a similar pace or even faster than they are used (e.g wood).