Storm tanks and ponds

What is a storm pond?

The storm pond, also known as a storm tank or spillway, is a large sewerage structure that collects and holds rainwater and "black" water (from households) for subsequent treatment in the wastewater treatment plant.

This control element of the sewage network is designed to limit the flow produced during rainfall, preventing possible flooding and the first rains, the most polluted, from being discharged directly into natural aquatic systems, such as rivers or streams.

How does it work?

During the dry season, wastewater is conveyed directly to the treatment plants. However, during the rainy season, the stations usually exceed their capacity, so the runoff water generated by rainfall, together with the wastewater, is conveyed by so-called "Discharges from Unit Systems" (DSU) to the storm ponds.

The storm ponds retain the DSU until it can be managed in the wastewater treatment plants. This minimizes the environmental problems caused by the first phase of the rainfall event, where most of the pollution is concentrated, since it is this water that washes streets, cars, or carries waste and even dead animals.

Parts of a storm tank

A storm tank consists of 4 parts:

Central chamber. An in-line tank normally located between the retention chamber and the relief chamber that conveys wastewater from the tank inlet to the inlet of the flow regulating element.

Holding chamber. Off-line tank where the first phase of the storm is stored once the capacity of the central chamber has been exceeded.

Relief chamber. It conducts the excesses of the storm to the receiving medium, therefore, it has a sill with a slight slope towards the outlet pipe of the tank.

Dry chamber. Flow regulating element.

Where and how are storm ponds located?

Storm ponds can be placed in series or parallel.

• Serial. Controlled waters in the pond join the uncontrolled wastewater, thus producing variable dilutions of the wastewater on its way to the treatment plant.
• In parallel. Dilution is constant and flow rates are controlled.

Did you know that Madrid has one of the largest storm ponds in the world?

The Arroyofresno storm pond, located in the Club de Campo, was built by the Madrid City Council to collect rainwater and sewage for subsequent treatment at the Viveros de la Villa wastewater treatment plant.

This infrastructure, built with the purpose of improving the waters of the Manzanares River, collecting the water that falls in the northwest area of the capital, had an investment of 105 million euros. And, after its completion in 2009, it became part of the Plan for the Improvement of the Water Quality of the Manzanares River, which is complemented by another 28 ponds. Thanks to these, the Madrid facilities treat around 1.3 million cubic meters of water a day.

An impressive structure not only for its functions and dimensions: 140 meters wide, 290 meters long, 22 meters deep and a capacity of 400,000 cubic meters, (eight times that of the pond of the retreat). But also for its reminiscence to the Arab cisterns, a feature that has made it the setting for some movies.

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