Operation
Liquid flows from the aeration tank of an aerobic sewage tank to a coarse impeller centrifugal pump. This delivers the liquid under pressure via an eductor and back to the tank. The eductor reduces the pressure in the sewage system pipework to a set point after which the pump is stopped. When the pressure in the pipework rises above a set value it is restarted.
The pipework consists of a network of mainly pvc pipes connected into separate zones- typically by deck- and brought down to a common manifold via isolating valves. These valves allow work on sections of the system whilst still maintaining others in use.

The toilets are connect to the system via a vacuum operated foot valve. Vacuum timers are also fitted which allow measured quantities of flushing water to be applied.

Where toilets are connected in the same zone but exist at different heights non-returning valves may be fitted. In addition filter boxes may be fitted along with additional isolating valves to improve operation.

Advantages and disadvantages
Very little flushing water is required and the volume of sewage dealt with can be much reduced with the downsizing of relevant equipment and cost saving.This has made them very popular for passenger vessels.Lloyds regulations state that the capacity of a sewage system for flushing water with conventional plant is 115 litres/ person/ day and 15 litres for vacuum systems.
The main disadvantage is blockage due to drying and crystalisation of urea. Over a period of time this can be so severe as to completely close the pipes. Chemicals are on the market which can be added in very small doses which help remove and prevent this deposits but there success is not guarenteed.

In the event of vacuum failure a method must be in place to prevent dangerous gasses passing back into the accommodation.

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