For a brief change of pace, septic systems!

One fifth of all residential housing units in the United States are served by individual, onsite septic systems, including 22% of all housing units less than four years old. In 2007, 50% of all housing units served by such septic systems were in rural areas, 47% in suburban areas and 3% were in central cities according to the U.S. EPA.

Geographic reference: United States
Year: 1985 and 2007
Market size: Number of housing units respectively: 24.6 million and 26.1 million
Source: Septic Systems Fact Sheet, October 2008, available online here
Original source: United States Environmental Protection Agency, Office of Wastewater Management