There is nowadays a tendency in the south of Europe to recover the natural features of urban rivers as much as possible. The final part of the Besos River (Spain) flows inside a heavily industrialized area in greater Barcelona. Following a classical model in the 1970s, the river was canalized, theoretically to reduce the high flow impact but in practice to gain flat areas where building was easy. The result was many artificially created impervious areas which made infiltration difficult, and the invasion of the river-related high-flow zones by industrial facilities and urbanized areas. Additionally, for several decades, the Besos River was the most altered fluvial basin of Catalonia. Trying to improve the really bad conditions of that section of the river, a big conversion of part of its final section was undertaken in the 1990s. There, the low flow concrete channel was re-naturalized, bending it, and in the reclaimed bed surface an urban park and a tertiary wastewater treatment system were implemented. After treatment, the effluent directly enters the river, thus improving water quality. Water from the tertiary treatment, constructed wetlands, has the following percentage of removal if compared with the effluent of the activated sludge facility: 40% for suspended solids; 62% for COD; 20% for NH4+-N; 58% for P; and 1.1 log U/100 mL for fecal coliforms.