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 the rainwater , the water is purified using a purification device . a retaining element is disposed behind the purification device in the direction of flow . this element is at least partially detachable connected to a base surface and can be used to release the through - flow in the region of the base surface of the purification device in an intermittent manner . the retaining element is pivotable around an axis .