A family inside their house in the favella of Vila Clara just next to Mil? Villela's communitarian association, Despertar. An estimated 15% of Sao Paulo's population lives in slums or favelas scattered around the city. The homes have very primitive plumbing systems and sewage is usually dumped directly into the rivers. Sao Paulo, with close to 20 million people, is one of the largest cities in the world and Brazil's most populous city.