Altes Wasserwerk, Gothic Revival architectural monument in Gronau, Bonn, Germany.

The Altes Wasserwerk is a building in the Gronau district that originally served water supply functions and was later repurposed. The structure displays pointed arches, steep rooflines, and decorative details reflecting 19th-century Gothic Revival design.

Built from 1873 by the Rheinische Wasserwerksgesellschaft and opened on 1 April 1875, the works supplied Bonn with filtered Rhine bank water. The city bought the site in 1900 and added a second well and the surviving pump house with two piston pumps and compound steam engines. Water supply ended in 1958; the pump house served as the Bundestag's replacement plenary hall from 1986 to 1992/93, where the reunification and capital-city decisions were taken.

The building's name reflects its origins as a water supply facility that once served Bonn, and it now demonstrates how industrial spaces can transform into modern office complexes. Its Gothic Revival features remind visitors of 19th-century craftsmanship and shape the character of the UN Campus today.

The site is part of the UN Campus and located at Hermann-Ehlers-Strasse 29, where it functions as office and conference space today. Visitors should note that it remains an active administrative building, so access may be limited.

Most of the technical equipment was removed in 1965 when the federal government bought the site, so the machinery is not intact. What survives is the pump house itself, rebuilt as the Bundestag's replacement plenary hall: at about 500 m² it was half the size of the real chamber, and after 1990 folding chairs had to be added for the new MPs.