Students should be able to comment on the relative ease of obtaining…

The idea

Potable water is water that is safe to drink, with low levels of dissolved salts and microorganisms. The ease of obtaining it depends on the source. Ground water, such as water from aquifers, is usually easiest because it is often naturally filtered through rock and may need only screening, aeration and chlorination. Waste water, including sewage and industrial effluent, is harder because it contains organic matter, harmful chemicals and microorganisms, so it needs multiple treatment stages. Salt water is the hardest because dissolved salts must be removed by distillation or reverse osmosis, both of which need significant energy and equipment. When comparing sources, consider the number of treatment steps, the energy required and the cost of the plant needed.

What earns the marks

• Ground water is generally the easiest source to treat because it is often already filtered by rock and may need only simple screening, aeration and disinfection.
• Waste water is harder to treat because it contains organic matter, harmful chemicals and microorganisms, so it requires several stages such as screening, sedimentation, biological treatment and disinfection.
• Salt water is the hardest source because dissolved salts must be removed by distillation or reverse osmosis, which require large amounts of energy and specialised equipment.
• The relative ease of obtaining potable water can be compared by considering the number of treatment stages, the energy demand and the cost of the process.
• All sources must be treated to reduce microorganisms and dissolved salts to safe levels before the water is considered potable.

Common mistakes and how to put them right

• Saying salt water is easy to treat because the sea is large.

  Put it right: The volume of water does not remove dissolved salts, so energy-intensive desalination is needed.

• Assuming waste water only needs filtering.

  Put it right: Waste water also requires biological and chemical treatment to remove organic matter, harmful chemicals and microorganisms.

• Confusing potable water with pure water.

  Put it right: Potable water is safe to drink and still contains some dissolved substances, whereas pure water contains no dissolved substances.

Exam tips

• Compare sources using clear criteria such as number of stages, energy use and cost, rather than saying one is simply better.
• Use the term potable water correctly and avoid calling it pure water.
• When describing desalination, name distillation or reverse osmosis and state that both require significant energy.