Aircraft Life Support Systems Part 2: Water and Waste System

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In the aircraft industry, several systems are designed to provide safety and comfort for the crew and passengers.

Regarding comfort, the water and waste system is designed to provide water for galleys and lavatories. Fresh water is stored and distributed while a different system deals with wastewater. That system includes a thoughtful engineering method to dispose of the different wastes that could occur during the flight.


Water must be supplied to different parts of the plane during flight. This water is kept in a tank in the compartment aft of the bulk cargo compartment. The whole system is made up of a passenger water system that stores, delivers, monitors and controls drinkable (potable) water for the galley units and lavatory sink basins.

In this blog, we are going to focus more specifically on the 737-classic model from Boeing.

Figure 1-Representation of different parts of the water and waste system
Figure 1-Representation of different parts of the water and waste system

The 3 main achievements of the water and waste system are the following:

  • Filling the water tank on land
  • Providing water during the flight
  • Storing toilet waste


The water and waste system is made up of:

  • Potable water system aims to deliver fresh water to every needed part in the plane (including every component between the water tank and sinks)
  • Water tank pressurization system focuses on the pressurization of the water tank and air dealing with the tank (including air compressor, pressure regulator filter, pressure relief valve)
  • Wastewater system focuses on water related to lavatory and sinks / galleys wastewater (including drain masts)
  • Toilet system includes components related to flushing and toilet water (including waste tank)


The water tank has a capacity of 34 gallons (about 0.15 m3). The water system in the plane needs to be pressurized for altitude just like the cabin, so it gets pressurized by an air inlet (linked to the pneumatic system). Therefore, the water quantity should not exceed 30 gallons (about 0.13 m3).

A smart fill/overflow valve is used to prevent an overflow. First, the fill channel is used to fill the water tank. Then, if the tank is filled with more than 30 gallons then the overflow part of the smart valve gets activated to get rid of the excess quantity.

Figure 2-Water and sinks waste system
Figure 2-Water and sinks waste system

In AxSTREAM NET™, such system can be modeled as seen in Figure 3.

Figure 3-Fresh water network modeled in AxSTREAM NET
Figure 3-Fresh water network modeled in AxSTREAM NET

For more comfort during the flight, some systems should be included, for example, to provide hot water.

In Figure 7, a cruise condition (at 35,000ft) is simulated.

That kind of simulation allows us to study the fluid flow behavior in various flight conditions (here the static pressure at the outlet is around 3.5psi).

Figure 4 - Water Supply
Figure 4 – Water Supply


Every person who has been on a flight has likely asked themselves where the waste goes. A popular rumor says that it is thrown overboard, hoping it will not land on somebody taking a morning stroll. In reality, there are two types of waste: waste coming from the lavatories and waste from the sinks/galleys.

During the flight, the wastewater coming from the sinks is disposed overboard, but the one coming from the lavatories is moved to a waste tank designed for this purpose.

To provide a system for the lavatories waste which fits reality, several parameters need to be taken into account. There is a draining system and a cleaning system for this purpose.  The draining system is a part of the cleaning process which includes the waste service panel, and one waste drain valve assembly to remove all the waste collected during the flight when back on land. On the other hand, there is also the waste tank rinse fitting assembly that allows rinsing/cleaning the waste tank to prevent bacteria.

Figure 5-Toilet waste system
Figure 5-Toilet waste system

After settings the right parameters in AxSTREAM NET™, we simulated:

  • Waste drain valve assembly to remove all the waste collected during the flight, we specified a pressure about 10psi.
  • Waste tank rinse fitting assembly that allows us to rinse the waste tank, the pressure here is set to 20psi
  • The pressure in the tank waste which is about 14.5 psi



The system is triggered when a toilet is flushed. Fresh water coming from the system previously described enters the toilet through a modeled valve to control the flow. The contaminated water then flows through a series of pipes and flows into the waste tank. The wastewater is sequestered in the waste tank until the plane lands.

Once the plane is on land, the next step is to rinse the waste tank. The waste service panel is made up of a waste drain valve assembly and a waste tank rinse fitting assembly.

The first step is to get rid of the waste in the tank by connecting a large hose which is connected to a waste collector (with a suction pressure) via the waste service panel assembly. The second step is to to clean out the empty waste tank using a mixture of cleaning product and water in. This cleaning solution is injected from the waste tank rinse fitting assembly, and the fluid mix goes into a cleansing nozzle in order to rinse the waste tank.­

In order to improve these systems, different options are provided in the software. Engineers can therefore adjust any parameter being modeled to reach the most desirable configuration.

Upon successfully running the simulation results can be reviewed, like those shown in Figure 6.

Figure 6 – Velocity contour of the waste system
Figure 6 – Velocity contour of the waste system

Looking at the results, the velocity contour is shown and matches the expectations based on published literature. The rinse system is expected to be at a higher velocity in order to carry away all the wastewater held in the tank. The velocity of the filling system is a bit higher than the whole process (aside from the rinse system for reasons noted).

To conclude, AxSTREAM® allows us to successfully model different situations linked to water and waste management systems, on land as well as during cruising altitudes in addition to other aircraft-related topics. Unsteady state simulation can also be performed in AxSTREAM NET™ to look at the flow behavior under transient conditions.