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A WWTP (Wastewater Treatment Plant) is a plant dedicated to the purification of wastewater whose basic function is to reduce, through a series of treatments, the pollution of domestic or industrial wastewater and return them to the sea, through a sea outfall, in an acceptable state of health.

Diagram of a Wastewater Treatment Plant (WWTP). Source: Ministry for the Ecological Transition and the Demographic Challenge.

The basic general scheme of a WWTP is composed of two or three main lines:


Water line

Treatments are usually grouped by levels, distinguishing between pretreatment, primary treatment, secondary and tertiary treatment.

  • Pre-treatment: The pre-treatment is located at the entrance of the facilities for the elimination of coarse fragments that cause damage.

  • Primary treatment: The objective is the removal of floating solids, suspended solids, sands, greases and oils.

  • Secondary treatment: The basis of biological purification processes is the elimination of water pollution through biocenosis.

  • Tertiary treatment: It is used to obtain an improvement of the treated effluent and / or improvement of the effluent for its subsequent reuse.

Muds line

Within the muds line, we have the following processes:

  • Thickening: Its objective is to reduce the volume of sludge by partially removing the water it contains.

  • Stabilization: It is the stage in which the destruction and/or transformation of the organic matter present in the sludge and responsible for its fermentation is achieved.

  • Dehydration: As much water as possible is removed from the mud, obtaining a greater consistency than its manageability.

  • Thermal drying: Process based on the contribution of thermal energy necessary to eliminate water reaching values close to 90% dryness.

Gas line

One of the great advantages of anaerobic digestion is the obtaining of gas that can be used as an energy source.

After applying all the treatments in the different lines of work of the treatment plant, so that the sea serves as a receiving medium for the treated effluent, it is necessary that its discharge is done at sufficient depth and that its exit to the sea facilitates rapid dilution.

Therefore, one of the most used techniques is the conduction of wastewater through microtunnels that, connected to a suitably designed diffuser system, transport the effluent to deep waters achieving adequate dilution.

The construction of a microtunnel as a technical solution, minimizes the environmental impact on the seabed and reduces the area of maritime works to the discharge area, where both the recovery of the tunnel boring machine and the connection of the diffuser with the outfall are carried out.


  • Alonso Martín, Tania. Environmental recovery, improvement of the purification and discharge of the city of A Coruña through the Bens Sea Outfall. TFM

  • ATHA. Reinforced Concrete Pipe Driving: Mompas Sea Outfall


  • Wastewater treatment plant (WWTP) - MTERD

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