In single houses and villas that do not have a sewerage system connected to the public sewage network, the wastewater is eliminated through the pipes that discharge into a special tank inserted in the ground so that its upper end is at ground level.

These tanks are called "cesspools" or "septic tanks". Septic tanks contain a special purification device. They can be installed by anyone: before starting work, you must first request authorization from the technical office of the municipality of residence.

A black well is instead formed by an underground watertight reservoir, with a volume between 2 cubic meters and 20 cubic meters.

When the water and waste discharged into the well reach the so-called "guard level" (when the well is almost full) it is necessary to request the intervention of a company specialized in drains, which will empty the tank by sucking the contained with special pumps mounted on the service vehicle, and transferring the sewage into the appropriate landfills.

The frequency of bleeding depends on the capacity of the tank and the intensity of use of domestic exhaust systems.