On 9 October 2026, a Phys.org feature by Elisenda Ballesté of The Conversation laid out how Spain's combined sewer systems are discharging untreated wastewater mixed with stormwater into rivers, streams and the sea after heavy rainfall, with the country's aging networks unable to keep pace with increasingly intense storms Phys.org.
Key takeaways
- Spanish cities route stormwater and sewage through the same pipes, so relief outfalls spill the mixture into rivers and the sea once treatment plants are overwhelmed Phys.org.
- Beach classifications rely on E. coli and enterococci counts, and a red flag after storms typically means a temporary swimming ban until contamination drops Phys.org.
- Barcelona runs 11 storm tanks that hold back around 900 tonnes of suspended solids a year; Seville, Madrid and Alicante have built similar retention assets Phys.org.
- Water quality is tracked in real time through the National Bathing Water Information System and the Ministry of Health's annual reports, though monitoring stops outside the summer season Phys.org.
- The new European Directive on Urban Waste Water will require discharge management plans, event-by-event monitoring and stormwater retention measures Phys.org.
What happened: how Spain's sewers behave in a storm
The article traces the chain from street to sea. Once rainwater enters the sewer network it mixes with domestic wastewater carrying fecal matter, bacteria, toilet paper and cleaning products, then flows to treatment plants designed for a fixed population equivalent Phys.org. When flow exceeds that design capacity, the network opens relief points or outfalls into rivers, streams or the sea to stop the plant from being overwhelmed Phys.org. In Barcelona, most outfalls discharge to the Mediterranean, but some also release into the Besòs River along the city's northern edge Phys.org.
The runoff does not only carry sewage. It also flushes urban pollutants such as heavy metals, oils and fuel from vehicles straight into receiving waters Phys.org. Spanish bathing water quality is regulated by law and classified as excellent, good, satisfactory or poor based on E. coli and enterococci counts, which is why a red flag appears on many beaches after heavy rain Phys.org. The National Bathing Water Information System publishes the data in real time and the Ministry of Health publishes annual reports, but the system is not monitored outside the summer swimming season and does not cover pollutants such as plastics or chemical compounds with longer-term effects Phys.org.
Valencian singer Raimon summed up the country's pattern: "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" Phys.org. That pattern has produced repeated spill events, from Barcelona's Nova Icària beach to the Valencian coast after the devastating floods of October 2024, with extreme rainfall now a more common trigger Phys.org.
What is being built to hold the spills back
Local authorities are attacking the problem on two fronts. In new housing developments, separate sewer systems for wastewater and rainwater are now common practice, though retrofitting established networks is difficult Phys.org. At the city scale, Barcelona, Seville, Madrid and Alicante have built rainwater retention reservoirs or storm tanks that temporarily store stormwater until the treatment plant can process it; Barcelona alone runs 11 of these tanks, keeping around 900 tonnes of suspended solids out of the sea each year Phys.org. Sustainable urban drainage systems, including floodable gardens, green roofs, permeable paving and infiltration trenches, are also being layered in to reduce the volume of water entering the sewer in the first place and to recharge aquifers or supply irrigation and street cleaning Phys.org.
Looking ahead, the new European Directive on Urban Waste Water will require management plans and discharge monitoring so that spills can be tracked by event and volume, alongside structural measures such as stormwater retention basins, grates and screens, and urban improvements to cut surface runoff Phys.org. The piece closes with a simple design philosophy: "for decades, we have tried to drain rainwater away as quickly as possible. Today we know that it is often better to retain it, allow it to filter into the ground and make use of it" Phys.org.
Specification read
The reported scale is best read in population-equivalent terms. A combined sewer overflow event is triggered when stormwater inflow exceeds the hydraulic capacity of a wastewater treatment plant sized for its design population equivalent; the Phys.org piece does not publish a flow rate, a treatment capacity or a spill volume for any specific site, so the only buyer-relevant number supplied is the 900 tonnes of suspended solids per year kept out of the sea by Barcelona's 11 storm tanks Phys.org. A facility facing this class of problem typically runs a screening to primary clarification to biological stage (often A2O, MBR, MBBR, SBR or UASB depending on footprint and effluent targets) followed by tertiary treatment and disinfection, with DAF flotation slotted in upstream of the biological stage when oil and grease loads rise, as they do in urban runoff carrying fuel from vehicles Phys.org. This story bears directly on the head-of-works and primary stages, where hydraulic surges and grit, oil and solids loads spike first, and on the screening and grit removal stage that protects the downstream biological train. If your plant is a combined sewer catchment serving a Spanish urban population with storm-driven flow swings, periodic E. coli and enterococci exceedances on receiving beaches and tightening expectations under the revised European Directive on Urban Waste Water, the practical lever is stormwater storage, fine screening and upstream solids reduction before the biological stage. Buyers scoping that envelope will compare Daf Flotation Systems for primary solids and oil removal, and reference How to Size a Containerized MBR STP in Madrid, Spain (2026 Guide) when planning biological capacity for a Spanish coastal site.
FAQ
How does Spain decide when a beach is closed after a storm?
Beaches are graded excellent, good, satisfactory or poor based on E. coli and enterococci counts under Spanish law, and a red flag means a temporary swimming ban until contamination levels fall Phys.org.
What infrastructure is Spain using to cut combined sewer overflows?
Cities including Barcelona, Seville, Madrid and Alicante have built storm tanks to hold back peak flows, and Barcelona's 11 tanks keep around 900 tonnes of suspended solids a year out of the sea Phys.org.
Will the new European directive change monitoring requirements?
The revised European Directive on Urban Waste Water will require management plans and event-by-event monitoring of discharges, including volume tracking, plus measures such as retention basins and screens Phys.org.
What should a plant manager scoping a Spanish urban site prioritise first?
The first priority is head-of-works capacity to absorb storm peaks, since outfalls are the relief points that spill when treatment plants are overwhelmed Phys.org; the Phys.org feature does not give a target capacity, so any flow figure should be derived from local catchment data rather than a generic range.
