WATCH: The World’s Largest Storm Water Tank Is Hidden Under A Golf Course In Madrid

Image of rainwater going down a drain.

Image of rainwater going down a drain. Credit: YouTube- Canal de Isabel II

THE DANA that has been affecting most of Spain for the past weekend has wreaked havoc across parts of the country.

One of the worst-hit areas was the Community of Madrid, especially the municipalities of Aldea del Fresno, Villamanta, Villamantilla, Villanueva de Perales, El Álamo and Navalcarnero.

On Sunday, September 3, the conditions were so bad that José Luis Martínez-Almeida, the mayor of Madrid, urged residents not to venture outside unless it was absolutely necessary to do so.

AEMET, the State Meteorological Agency issued a red alert for the region due to extreme weather phenomena. In accordance with this, Level 1 of the Special Civil Protection Plan against the Risk of Floods (Inuncam) was activated by the City Council.

With such a large amount of rain falling, one might wonder what happens to all the water that is absorbed by the sewers. In most places, it ends up going to the local sewage treatment plants, however, in Madrid, there is an incredible system in place.

As explained by the Canal de Isabel II which manages the system, Madrid’s huge underground infrastructure allows: ‘rainwater to be retained in storm tanks before it reaches the treatment stations’.

What are these storm tanks for?

Madrid possesses one of the largest rainwater retention infrastructures in the world. ‘Storm tanks are enormous underground tanks created to store the first rainwater, which is also the most polluting (even more than sewage) because it carries away all the dirt accumulated on the streets and on the asphalt’, the company detailed.

The storage tanks in Arroyofresno and Butarque are capable of holding an astonishing capacity of 400,000 m³ of water and are some of the largest in the world.

They are designed to help prevent the treatment plants from exceeding the maximum allowed flow. According to the regional government, a total of 65 storm tanks are installed under the Community of Madrid.

This is the largest such infrastructure of its type anywhere in the world, with 36 of the tanks located in the capital itself. They not only prevent flooding but also serve to protect rivers like the Manzanares.

In total, these giant tanks can store a total capacity of 1.48 hectometres. Each of the tanks in Arroyofresno and Butarque: ‘Can store up to 400,000 m³ of water, eight times more than the El Retiro reservoir’, Canal de Isabel II explained.

How does rainwater get to the tanks?

When such intense rains occur, as they did this Sunday with the DANA, the water still filters through the sewers, but the entire amount cannot be treated immediately.

Therefore, these quantities of water are retained in tanks until the rains stop. ‘It is then that the water is gradually sent to the purification stations’, indicated Canal de Isabel II.

Thanks to this technique, the pollution of rivers is avoided and possible floods and damage that can be catastrophic are prevented.

The water travels through pipes seven metres in diameter

Pipes that can be up to seven metres in diameter pass the collected water supply through to the tanks. They added: ‘Before reaching the tanks, the water passes through a series of filters that allow it to retain solid contaminating elements such as plastic bottles or other types of objects’.

Once purified, its water can be discharged back into the rivers in a better condition without threatening the ecology of the water flow.

The Arroyofresno tank, located to the northeast of the capital, under the Villa de Madrid Country Club’s golf course, is the largest storm tank in the world.

It has a surface area of ​​35,000 m² and a depth of 22 metres. As explained by Madrid City Council: ‘The water collector was built with a tunnel boring machine, with an internal diameter of 6.70 m and 3 km in length’.

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Written by

Chris King

Originally from Wales, Chris spent years on the Costa del Sol before moving to the Algarve where he is a web reporter for The Euro Weekly News covering international and Spanish national news. Got a news story you want to share? Then get in touch at