The frenetic and uncontrolled growth of the urbanized areas of many cities in recent decades combined with climate change and the increase in extreme weather events, both in terms of frequency and intensity, has led to a significant increase in hydraulic risk and the consequent need to seek solutions capable of addressing these critical issues.

Overflow interventions are included in this context as measures to mitigate the growing phenomena of urban flooding. The overflow tanks are temporary accumulation works that allow for the storage of large volumes of water, when major meteoric events occur, and to release them gradually over time, with the aim of controlling discharges from the drainage system.

The reference legislation on this is Regional Regulation of 29 March 2019, no. 6 on the regulation of discharges and Regional Regulation of 23 November 2017, no. 7 on the principles of hydraulic and hydrogeological invariance.

The tanks are generally divided into overflow tanks, first rainwater tanks and dispersion tanks and are distinguished by the purpose with which they are designed.

The aptly named overflow tanks, are designed to overcome the problems of hydraulic risk of a quantitative nature. The goal is therefore to reduce the volumes of water passing through the network thanks to the accumulation of surplus water from the sewer network. Examples of such infrastructures are the two Parabiago tanks, currently being designed, which have been considered a priority due to the risk of flooding of some roads and underpasses in the municipality, or the Solaro tank which receives excess water from the municipal sewer system sewer overflows.

The first rainwater tanks have the function of solving a problem of a qualitative nature. Located at the sewer overflows of the sewerage system in a surface water body (rivers and streams), they are used to store the first part of the meteoric event which is normally the most pollutant laden. At the end of the event, the accumulated volumes of water are gradually emptied into the sewer system and sent for purification. The first rainwater tanks can be built with nature-based solutions using phyto-purification systems

The Novate Milanese tank, currently being designed, is an example of a first rainwater tank with an accumulation volume of 5,500 cubic metres and is served by the sewer overflow of the sewerage network that currently delivers the water into the Pudiga stream.

Lastly, the dispersion tank lightens the sewer network. Unlike the overflow tank, this one accumulates the overflow water from the network and gradually disperses it into the ground thanks to the permeable material with which it is designed. This type of tank lends itself to becoming a natural type of work, such as a green infrastructure or nature-based solution.

Examples of dispersion tanks are those of San Giorgio su Legnano and Cantalupo which have been the subject of improvements to increase the volumetric and dispersing capacity of the infrastructure and further lighten the load on the sewer system. The Sedriano tank is an example of a complex infrastructure that combines a system of tanks consisting of a first rainwater tank, an overflow tank and a dispersion tank in order to solve the problem of frequent flooding in the southern area of the municipality in compliance with the quantitative and qualitative discharge limits.

From 1 January 2021, the CAP Group is also taking over some so-called white tanks at the exclusive service of the meteoric network.