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. Under the ground, however, a very different story unfolds.

Once rainwater enters the sewerage network, it mixes with domestic waste water: 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, these waters flow through sewerage pipes into treatment centres. The wastewater leaves these centres 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.

Read more: Valencia floods: Our warming climate is making once-rare weather more common, and more destructive

Overloaded treatment stations

Wastewater treatment plants are designed to handle the flow rate of a........

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