A alcantarilla (diminutive of alcántara, in turn from Spanish Arabic alqántara, in turn from classical Arabic الْقَنْطَرَة al-qantarah, perhaps coming from the Greek κέντρον kentron, "center of circle"; according to another hypothesis, it was first used as a diminutive of "bridge" and then to name the small aqueduct that passes underneath [1]​) is an underground aqueduct intended to evacuate domestic wastewater or other types of used water. It is part of urban sanitation systems. The set of sewers in a town or neighborhood is called sewage system.

The Romans excelled in this type of construction, which they gave the name sewers "Cloaca (construction)") under which name they continued to be known in some Spanish towns until many centuries later. Sewers were also found in the ruins of Pompeii that were used to collect water from various streets and direct it through an aqueduct out of the city. In Spain there is no shortage of examples of Roman sewers, among which those of Astorga can be mentioned, which are preserved almost in their entirety.

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First sewers

The oldest type of culvert was actually shaped like a small bridge: a barrel vault, also made of brick, was supported on two brick walls. At the bottom a channel was formed that, when the sewer was small, went from wall to wall and when it was larger, it left steps on one or both sides of the channel. In any case, the smaller size of the sewer allowed a person to pass through, even if it was necessary to adopt uncomfortable positions to work.

When instead of a vault it was covered with a stone or concrete slab (generally in small sizes) it took the name of drainage, a name that is currently also given to other types of conduits.

Current sewers

They are now generally made from precast concrete ducts, with different types of cross section.

For large flows, sections with the shape of the old sewer can be used, although sometimes instead of being vaulted, they have a lintel top. In its lower part, it has a channel similar to the one described above.

The sewers form a network, the sewage system, which gathers used water through branches to large pipes called collectors.

Open sewers

Although it is no longer used, when the dirty water went to the open sky, without a cover, the pipeline was called esgueva&action=edit&redlink=1 "Esgueva (construction) (not yet written)").