The humble flush toilet is a technological wonder that carries our waste safely away from our homes and workplaces. Yet roughly 2.5 billion people don’t have access to decent sanitation. And even for those who do, the toilet is an imperfect solution that often creates problems of its own. The World’s special five-part series “Toilet Tales: Water and Waste” examines efforts to solve those problems around the world, from China to India to Haiti to Cambridge, Massachusetts.