Septic tank
In the United States, the 2007 American Housing Survey indicated that about 20 percent of all households rely on septic tanks, and that the overwhelming majority of systems are located in rural (50%) and suburban (47%) areas. Indianapolis is one example of a large city where many of the city's neighborhoods still rely on separate septic systems. In Europe, septic systems are generally limited to rural areas.
Centrifugal water–oil separator
With the interceptors the dirty water mixture enters the first tank of the interceptor where that tanks builds up hydrocarbons and other hazardous material in a layer. Then comes the second and third tanks of the interceptor that are all connected through pipes called "dip pipes" which don’t allow the hydrocarbons and other contaminants to pass through. The same process that happened in the first tank will happen in the second and third tanks just with less and less contaminants.