Waste water treatment: Students should be able to comment on the relative ease of obtaining potable water from waste, ground and salt water.

The idea

Potable water is water that is safe to drink, with suitably low levels of dissolved salts and microbes. The ease of obtaining it depends on the starting material. Ground water from aquifers is usually easiest: it is often filtered naturally through rock, so treatment may only need screening, aeration and chlorination. Waste water is harder because it carries organic matter, microbes and contaminants; it needs screening, sedimentation, biological digestion of sludge and sterilisation. Salt water is hardest because dissolved ions must be removed by distillation or reverse osmosis, both energy-intensive.

What earns the marks

• States that potable water must have low levels of dissolved salts and microbes, so it is safe to drink.
• Explains that ground water is relatively easy to treat because natural filtration through rock removes many insoluble particles, leaving screening, aeration and chlorination.
• Describes waste water treatment as involving screening, sedimentation, biological digestion of sludge and sterilisation, making it more complex than ground water.
• Explains that salt water requires distillation or reverse osmosis to remove dissolved ions, which uses large amounts of energy and is therefore the least easy source.
• Compares the three sources in order of increasing difficulty, linking each to the treatment steps and energy demand.
• Uses correct terminology such as potable, aquifer, sedimentation, chlorination, distillation and reverse osmosis.

Common mistakes and how to put them right

• Confusing potable water with pure water.

  Put it right: Define potable water by safety limits, not by purity.

• Thinking all water sources need the same treatment.

  Put it right: Match treatment to the source, for example ground water may need little more than chlorination while salt water needs desalination.

• Assuming boiling removes dissolved salt.

  Put it right: Distillation requires evaporation then condensation, and simple boiling alone does not produce potable water from salt water.

Exam tips

• Use comparative language such as easier, harder, more energy-intensive and more costly to show the ranking clearly.
• Link each source to named processes rather than saying it is cleaned; for example, waste water undergoes sedimentation and biological digestion.
• When asked to comment, give a balanced comparison of at least two sources, including one reason based on energy or cost.