Explain why obtaining potable water from salt water is more challenging than from waste water.

Obtaining potable water from salt water is more challenging because it requires desalination processes, such as distillation or reverse osmosis, which are energy-intensive and costly. In contrast, waste water can often be treated using biological methods that are less demanding in terms of energy and resources.

A strong answer should directly address the approved learning objective to compare the relative ease of obtaining potable water from waste water, ground water and salt water. This question belongs to Waste water treatment within Using the Earth's resources and obtaining potable water, so the response should use that exact curriculum context rather than a generic statement. The answer is correct when it names the key idea, explains the link to waste water, and keeps the wording specific to AQA GCSE revision.