Vacuum Toilets – How they Work?




How it works:

Liquid flows from the aeration tank of an Aerobic Sewage Tank System (AST) to a coarse impeller centrifugal pump. The Pump delivers the liquid under pressure via an ejector back to the tank. The ejector throat is connected to the sewage manifold line. The drop in pressure at the ejector throat reduces the pressure in the sewage system line to a set point after which the pump stops. The Vacuum in the line creates a suction at the closet when the system is activated. The black water gets sucked into the pipeline due to the vacuum. The vacuum drops and at a pre-determined set point, the pump starts and creates further vacuum.

Ship’s sewage system is segregated into three or more sections with the help of isolation valves. The toilets are connected to the system via a vacuum operated foot valve. In addition to this, vacuum timers are fitted to measure the flushing water quantity.


Very little flushing water is required and the volume of blackwater dealt with can be much reduced with the downsizing of relevant equipments resulting  in saving cost.

This made the vacuum toilet system popular amongst the cruise industry and now in the merchant ships. It should be stated that even in aircrafts vacuum systems are very popular.

Lloyd’s regulations state that the capacity of a sewage system for flushing water with conventional plant is 115 litres/ person/ day whereas  a vacuum system uses only 15 litres.


The main disadvantage of the vacuum system is blockage due to drying and crystallization of urea. Over a period of time this can be so severe that it can block the entire piping system.

To prevent such dangerous occurrence..

  • Proper and periodic chemical treatment as per maker’s schedule should be adhered to.
  • A class approved failsafe system must be in place to prevent dangerous gasses passing back into the accommodation in case of vacuum failure.


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