“Are we giving consumers potable water or not?” Mr. Basañes asked, noting that the countries of Latin America and the Caribbean region have invested a total of about $2.8 billion a year on improved water and sanitation since 1990. “Even if we are, is there a perception problem?”

With the move toward bottled water, families sometimes spend as much as 10 percent of their incomes on water, double what the development bank estimates they should. “Can you imagine a poor family paying their water bills — in some cases a fairly steep amount — and they are buying water on the side because they don’t trust the water they are getting?” Mr. Basañes asked.

Then there is the concern of whether the bottled water is really any better.

“We’ve never had any complaints,” said Maximiliano Santiago, who set up his own water purification business three years ago in a storefront at the edge of an Iztapalapa market.

He buys well water that is trucked in from outside of Mexico City rather than using the Iztapalapa tap water — “it would damage the filters,” he said — runs it through carbon and sand filters, and then purifies it using silver ionization. He said he calls a biologist from time to time to check the quality.

Mr. Santiago works seven days a week for a profit of about $15 a day. By midmorning, he stacks 40 five-gallon jugs on two three-wheeled cargo bicycles and pushes them through the neighborhood shouting “aguaaaa” along the way.

It is a business model that is emerging in megacities across the developing world. Rich people pay a premium for branded jugs that can be refilled from companies owned by multinational corporations like PepsiCo, Coca-Cola and Danone. In working-class neighborhoods, local entrepreneurs fill the demand.

“If you go to Mexico or Manila, you’ll see the same thing, but they have emerged independently,” said Ranjiv Khush, a founder of Aquaya, a nonprofit group that researches ways to get clean water to poor people.