Vacuum Toilet

The use of vacuum toilets provides a similar level of comfort as traditional flush toilets, but they use much less water due to air sucked into the toilet when flushing, thereby producing a vacuum. This results in a minimal requirement of water (0.5 to 1.5 litres) for the transport of faeces and urine (WHO 2006; ROEVAC 2011). The system is completely closed. Is there a leak, the negative pressure in the pipes reduces the risk of raw sewage spill (see also vacuum sewers) (WHO 2006). Vacuum toilets can be installed in single households, hotels, or whole communities and are also adapted for train, ships or airplanes. Due to the fact that the effluent has a high organic matter content, vacuum toilets are specifically adapted for the use in combination with separate greywater and blackwater treatment; or aerobic digestion treatment for biogas production. To minimise the water consumption of a vacuum toilet even more, urine diversion bowls can be applied (learn more about it: urine diversion components).

Basically, there are two different designs for a vacuum toilet system: The constant vacuum system (CVS) and the vacuum on demand (VOD) system. Which one will be applied depends on specific framework conditions.

This is the better solution for larger buildings. Virtually, it is possible to add an unlimited number of toilets and the system can be expanded later on. If there is a permanent vacuum, the risk of a leakage is very low (JETSGROUP 2011). However, energy requirement are generally high in order to maintain the vacuum constantly. It depends on the manufacturer, but normally a constant vacuum system functions as follows (adapted from ROEVAC 2011):

Step 1: The user pushes the push-button, the interface valve is opened and the wastewater is evacuated. Air, which also helps to transport the waste, is also sucked into the system. At the same time, the clean water valve is opened and rinsing water is sprayed into the bowl.

Step 2: The vacuum valve is closed but the water valve remains open. A small amount of fresh water is sprayed into the bowl.

Step 3: The water valve is closed, a small volume of clean water is retained in the bowl and the toilet is ready for use again.

The collected blackwater can be discharged in different ways (JETSGROUP 2011):

• Semi-centralised treatment plant (e.g. free surface, horizontal or vertical constructed wetland)
• Biogas production (e.g. biogas settler)
• Sewage tank (which is emptied by a vacuum truck periodically)
• Public sewer system (e.g. conventional- or separate sewer)

Vacuum toilets systems are very favourable for source separation. In this case, greywater is collected in “greywater interface units” and is soaked away as soon as it is filled up.

(adapted from JETSGROUP 2005)

Vacuum on demand systems produce a vacuum only at the moment the toilet needs to be flushed. The system also uses air together with water for transport of sewage, as opposed to conventional flush toilets, which use water only. The VOD toilet evacuates air from the drainpipes automatically upon activation of the activator button. A valve opens in the toilet, and the difference in air pressure that results causes the sewage to be flushed.Due to lower energy requirement, small VOD systems can be solar powered.

At the end of the vacuum pipes, “biotanks” can be made available to compost the already shredded waste on-site. Flushed sewage is pumped through a particle filter into the composting tank. As soon as it is full, it can be emptied and put aside for further composting (see also small-scale or large-scale co-composting. The liquid, which is collected at the bottom of the tank, can be infiltrated or used as fertiliser (see also leach field, soak pit, fertigation) (JETSGROUP 2005). Depending on the local standards and restrictions a secondary treatment may be required before discharge or reuse (see also non-planted filters; vertical, horizontal or free surface-flow constructed wetlands.

As it is a high-tech system, it is expensive. But in comparison with a common flush-toilet system, it can be slightly cheaper, because piping costs are lower (the dimensions are smaller, 50 to 75 mm) and a on-site treatment system can be more easily installed. Also the installation can be cheaper because piping is independent from the building structure. Vacuum lines may be installed vertically into the suspended ceiling as waste can be lifted.

The in-house installations require little maintenance and are easy to clean. The suction effect increases hygiene and reduces odour (HEEB et al. 2007). If a composting tank is used, it has to be emptied and cleaned periodically to avoid an overflow. If there are any problems with the technical components, such as valves or pipes, the manufacturer has to be contacted.

A vacuum toilet system guarantees a high level of comfort and hygiene. As long as the toilet is kept clean, there are absolutely no hazards. When composting such products, special care should be taken as the associated health risk is higher than for common kitchen waste.