In waste water installations, the amount of falling rainwater which increases as a result of heavy precipitations is diverted into lakes and waterways in order to relieve the sewage treatment plant. In order to remove the impurities carried along with said rainwater, the water is purified using a purification device. A retaining element is disposed behind the purification device in the direction of flow. Said element is at least partially detachably connected to a base surface (15) and can be used to release the through-flow in the region of the base surface (15) of the purification device in an intermittent manner. Preferably, the retaining element (3) is pivotable around an axis (7).