The so-called "first rain water", or stormwater, is water that is quantified considering a height of 5 mm of rain on the surface area in question. The volume obtained is forwarded to the purification phase. This phase involves the use of one or more storage and sedimentation tanks and a process aimed at the removal of oils and hydrocarbons. Excess water is then sent directly to the plant exit via pipeline.
Stormwater treatment plants are primarily used for the treatment of run-off water from paved surfaces and, furthermore, may also involve a form of post-treatment; that is a process that make it possible to improve overall purification efficiency in relation to the type of discharge system and particular characteristics of the final delivery point.