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The Besòs wastewater and rainwater treatment plant, located in the Forum, is one of the largest covered treatment plants integrated into the urban fabric in the world and the one with the highest processing capacity in Catalonia with 525,000 m³/day . This treatment plant treats the water of more than half of the population of the metropolitan area in the municipalities of Badalona, Barcelona (three quarters), Montgat, Sant Adrià de Besòs, Santa Coloma de Gramenet, Tiana and part of Montcada and Reixac.

The treatment plant began operating in 1977 with physical and chemical treatments of the waste water and in 2005 it was completely remodeled to incorporate biological treatment and cover the plant to integrate it into the new urban space of the Forum. This facility, together with the underwater outfall for the return of purified water to the sea three kilometers from the coastline, have contributed to the recovery of the metropolitan coastline .

The harmonious integration of the installations and the highly sophisticated systems of air renewal and chemical washing of odors have been key to reducing the environmental impacts associated with purification in this urban space where there is a public square, hotels, offices and homes.