Abstract: A backwater tank system is provided for a building connected to a vacuum drain facility wherein the waste water of the building is drained off by gravity feed lines into a collector which is connected to a vacuum drain conduit by means of a pneumatically controlled check valve that opens automatically at a specific maximum water level, and wherein a ventilated backwater tank is connected to the gravity conduit or the collector at a level above the maximum water level in the collector, the capacity of the backwater tank being large as compared with that of the collector. The check valve and the pneumatic control devices therefor are disposed in or on the tank outside the main chamber thereof and preferably in an auxiliary chamber in the tank. The tank, check valve and control device form an integrated pre-assembled unit which includes an inlet connection adapted to be connected to the gravity feed line.