Freixo WWTP - (Porto, Portugal)

Freixo WWTP is situated in Porto, Portugal, and has been designed to treat the wastewater from the the eastern part of the city, an average 35.900 m3/day (170.000 PE).

The flow is divided by two treatment lines, which include:

• Preliminary treatment;
• Primary sedimentation carried out in two rectangular settling tanks;
• Intermediate pumping station;
• Biological treatment by activated sludge, including nitrification and denitrification, in two tanks combining preanoxic and aerated stages;
• Secondary clarification in eight rectangular clarifiers (4/treatment line)
• Final UV disinfection.

The sludge treatment line consists of:

• Gravity thickening for primary sludge;
• Mechanical thickening of biological sludge by centrifuges;
• Buffer tank for mixing primary and biological sludge;
• Mesophilic anaerobic digestion in two stages (primary and secondary digesters);
• Digested sludge dewatering by centrifuges
• Sludge storage

The biogas produced in the anaerobic digestion is currently used for mixing and heating the digesters, by means of a boiler and heat exchangers.  The biogas line includes a gasholder and a flare.

Freixo WWTP process diagram:

Components

Freixo overview

Clarifiers biological reactors

Digester

Heat exchangers

Sludge dewatering

Photovoltaic digesters

Biological Sludge recirculation

MBR water reuse

Digeste top

Sludge storage flare