This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: After heavy rain, Spanish cities often awaken to refreshed air and clean streets, while the rainwater itself vanishes into storm drains along with everything it swept away. Underground, however, a very different story unfolds.
Once rainwater enters the sewer network, it mixes with domestic wastewater: everything that goes down shower, kitchen and bathroom drains and toilets. This water carries fecal matter and bacteria, along with toilet paper, cleaning products and many other things. If everything works as it should, this water flows through sewer pipes into treatment plants.
The wastewater leaves these plants much cleaner than it came in and is discharged into rivers and seas. But in the words of Valencian singer Raimon: "In my land, the rain doesn't know how to rain. It rains too much, or too little.
If it rains too little: drought. If it rains too much: catastrophe." He has been proven right several times in recent years, from Barcelona's Nova Icària beach to the Valencian coast after the devastating floods of October 2024. While the Valencia floods were exceptionally powerful and deadly, extreme rainfall is becoming increasingly common in Spain.
Wastewater treatment plants are designed to handle the flow rate of a certain number of residents. They cannot handle much higher loads without a risk of overflow or the treatment system ceasing to function properly. To prevent them from becoming overwhelmed, sewer networks have relief points, known as outfalls, located in rivers, streams or the sea.
In Barcelona, for example, most outfalls drain into the sea, but some also discharge into the Besòs River, which flows along the city's northern edge. Rainwater mixed with untreated wastewater is discharged into the environment via these points, carrying away the organic matter, fecal bacteria and waste flushed down the surrounding area's toilets. But it also carries urban pollutants such as heavy metals, oils and fuel from vehicles.
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