Slum-dwellers in Dar es Salaam pay the equivalent of £4 ($8, €5) for 1,000 litres of water, bought over time and by the canister. In the same Tanzanian city, wealthier households connected to the municipal supply receive that amount for just 17p. In the UK, the same volume of tap water costs 81p and in the US it is as low as 34p.

Figures from other countries confirm the evidence: it is generally the poorest who pay most for what is one of the most essential of all natural resources. Water is in short supply for a large proportion of the world’s people: about 1bn lack access to clean water and 2.6bn have no sanitation. An estimated 5,000 children die every day from water-related disease, according to WaterAid, the London-based charity.