• Graphene sieve makes seawater drinkable

And there's also a pollution problem. Official figures from 2015 showed that 40% of Beijing's surface water was polluted to the point of not being useful even for agriculture or industrial use.

The Chinese authorities have tried to address the problem by creating massive water diversion projects. They have also introduced educational programmes, as well as price hikes for heavy business users.

4. Cairo

Once crucial to the establishment of one of the world's greatest civilisations, the River Nile is struggling in modern times.

It is the source of 97% of Egypt's water but also the destination of increasing amounts of untreated agricultural, and residential waste.

World Health Organization figures show that Egypt ranks high among lower middle-income countries in terms of the number of deaths related to water pollution.

The UN estimates critical shortages in the country by 2025.

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