Owing to the accelerated growth of Zurich's agglomeration during the 20th century and the insufficiency of the purification plants built in the 1960s, it has been strongly polluted; from 1994 to 2002, the sewage system was restructured by driving a tunnel between the Glatt Valley and the Limmat Valley.
As a result of Zurich's metropolitan region experiencing rapid growth during the 20th century, coupled with insufficient sewage infrastructure built in the 1960s, the area has become heavily polluted. Between 1994 and 2002, the city undertook major renovations to its sewer system by building a tunnel that linked the Glatt Valley and Limmat Valley.