Because of the limited space available for the wastewater treatment plant in Besós, located in an emblematic zone where the Forum of the Cultures was held during 2004, a compact study was required. Thus the decantation process was designed on two levels so it would occupy half the area it would normally take. There were only two cases when this had been done before, one in Singapore and the other in Boston. This was the first time such an arrangement was used in Europe.

Furthermore, in addition to being underground, this plant handles the largest volume of water treated by any other plant in Spain, over 500,000 m³/day. It was built underground freeing all the space on ground level above it for a large public square.