The Purifier

Once it has been used, tap water loses the quality it had. Before it is returned to nature the potentially dangerous components have to be eliminated.

This is done at the Sewage Treatment Plant (the Spanish initials are ETAR).

La Coruna's ETAR is near the city, in Bens. The general purification method used at an ETAR is as follows:

Pretreatment: elimination of large particles with grilles and filters. Waste is then compressed.

Sand eliminator

Primary treatment: physical separation of dissolved solids, fats and oil. Even though this is a physical process, chemical products are used as coagulants to facilitate the process.

Secondary treatment: biodegradable organic material is eliminated with the collaboration of microorganisms capable of absorbing it and then it is relatively easy to eliminate by decanting as slime. Variables such as pH, oxygen concentration and temperature have to be kept under observation.

Tertiary treatment: any organic material that was not eliminated in the previous process, together with nitrogen and phosphorus compounds and dissolved inorganic salts are separated by various means, such as flocculation or filtering. This is the most expensive process due to its being highly specific.

Once water has been purified it is poured into the sea. A pipeline along the ocean bed at Suevos takes it several hundred metres into the sea.

The continuous evolution of the population and industries in the area has increased the amount of water to be treated over the last few years, although now it is quite stable. We are working on extending the ETAR with state financing in order to meet new needs.

Following is the data from the Bens ETAR in 2000.

Water treated year total  55,997,717 m3  monthly average  4,666,476 m3  daily average  153,418 m3

Waste summary  Total waste  1,156,174 Kg.  Sands  481,310 Kg = 41%  Desanding  312,100 Kg. = 27 %  Large particles  342,584 Kg. = 30 %  Fat  20,180 Kg. = 2 %

An analysis of the water that comes out of the sewage plant will give us an idea of the quality of water pumped into the sea.

The parameters referring to the following concepts are especially important
Electrical Conductivity: depends on the salts dissolved.
pH: measures acidity of water, it should be around neutral (pH=7).
DBO and DBQ: biological and chemical oxygen demand, reflecting the water's capacity to regenerate itself
Dissolved Solids: make water murky
Anions: chemical components such as cchlorides (measuring salinity), nitrogen compounds (organic contamination), cianurtaes (industrial waste), etc.
Heavy Metals: highly toxic, such as lead, cadmium or nickel.

Parameter on leaving EDAR  PH  7,41  Conductivity at 20 ºC  864 µS/cm  DBO5  239 mg/l  DQO  376 mg/l  Dissolved solids  82,0 mg/l  Chlorides  209 mg/l  Copper  0,078 mg/l  Cadmium  0,005 mg/l  Lead  0,058 mg/l