The water and waste system of an aircraft serves a number of purposes. It provides potable water to the toilets and the galleys. It also stores toilet wastes and disposes of wastewater from the sinks in the lavatories and galleys.
The potable water system
The potable water in the aircraft is stored in water tanks that are normally placed under the cabin floor where the cargo holds are located. Depending on the size of the aircraft there can be more than one potable tank to ensure that water is available throughout the flight for all the passengers and crew alike.
As the water tanks are below the users, the water needs a mechanism by which it could be pumped up. In some aircraft, an electrical pump is used to pressurize the water and pump it up to the users. Most larger aircraft use bleed air to pressurize the water. On the ground, this bleed air comes from either the APU or an external air unit. When in the air, the bleed is provided by the engines.
When water is demanded from any of the users, a shut-off valve opens in the respective location, which allows water, under pressure to come out. For the lavatory washbasins, a water heater is provided, which allows the passengers or the crew to select either hot or cold water through the faucet controls. When hot water is selected, the heater operates, and hot water comes out of the faucet.
The potable water tank(s) can be serviced through a service panel located on the aircraft fuselage. Through this servicing panel, the water can be drained or be filled up. In modern aircraft, the amount of water in the tanks is measured by sensors which feed the data into the flight Attendant Panel (FAP) in the cabin. Through the FAP, the crew can determine and monitor the amount of water in the tank(s).
What happens to the wastewater?
The water from the lavatory sinks and the galleys are routed out of the aircraft through an air stop valve to the drain masts located under the belly of the aircraft. The air stop valve only opens when a certain amount of water is collected. This prevents air leaks from the cabin, which can affect cabin pressurization. Circling back to the drain masts – typically, there are two drain masts. One drains water from the forward toilets and forward galleys, and the other one drains water from the mid/aft toilets and aft galleys.
The drain masts are electrically anti-iced so that water does not freeze and cause a blockage.
On the ground, the water is drained with the help of gravity, and when in flight, it is discharged by the pressure differential between the aircraft cabin and outside atmosphere (when at high altitudes, the aircraft cabin is at a higher pressure than the atmosphere).
How does the toilet work?
The toilet in an aircraft works very differently from a normal toilet. Each toilet bowl has two valves. A water valve and the flush valve. The water valve provides water from the potable water tank for flushing the toilet waste and the flush valve drains the waste. When the flush button in the toilet is pressed, the flush control unit opens both the water and flush valves, allowing water to flush out of the toilet bowl into a waste tank.
On the ground, the pressure for flushing is provided by a vacuum generator. This generator is electrically powered and is connected to the waste tank through a pipeline. Whenever the flush button is pressed, the vacuum generator comes online, creating a vacuum in the waste tank, which allows the waste content from the toilets to fall into the tank. When in the air and above a certain altitude, the differential pressure between the cabin and the atmosphere is strong enough to pull the waste from the toilet bowl into the waste tank. Thus, the vacuum generator is no longer necessary.
If, for any reason, the flush valve fails to open, a manual flush valve under the toilet bowl can be manually operated to drain the waste material.
The flush control unit and the computers in the toilet system constantly monitor to check if things are working as they should. If a fault occurs, the information is sent to the centralized maintenance system of the aircraft, which can be accessed by the maintenance personnel for troubleshooting. The computers also measure the waste level in the waste tank and feed this data to the cabin attendant control panel.
The waste tank can be drained through the toilet servicing panel. In the panel, there is a connection for the drain and a separate connection for tank rinsing. To empty the waste tank, the drain hose of the servicing truck should be connected to the drain port and the drain valve opened. When the valve is opened, the waste flows out of the tank into the hose and then into the tank of the toilet servicing truck. After the waste is completely drained, the drain valve should be closed. Next, the flush fill hose of the truck must be connected to the tank rinsing port to clean the waste tank. This fluid must be then drained back by operating the drain valve a second time.
In some older and smaller aircraft, the toilet is of recirculation type. In this type of toilet system, a waste tank is not present. Instead, a holding tank with a flushing solution is used. This flushing solution contains disinfectants that give the water a blue color. In the holding tank, there is a pump and a filter.
When the flush button is operated, the pump runs, which draws flushing solution from the holding tank, through the filter, and sends it through the toilet bowl, flushing the waste and flushing fluid back to the holding tank. The filter prevents any large waste material in the holding tank from re-entering the flushing bowl.
The servicing of a recirculation toilet is similar to a vacuum toilet. The only difference is that once the holding tank is drained, it needs to be refilled or recharged with new flushing fluid as it requires this fluid for the flushing to work.
At times, these types of toilets can leak. This mostly happens due to improper toilet servicing or due to component failures. In flight, any wastewater that comes out of the toilet system freezes and this can leave a line of frozen flushing fluid on the aircraft fuselage. Sometimes, the frozen fluid can also fall off the aircraft. And if the toilet is serviced with blue disinfectant, the ice formed can be blue. This is sometimes called blue ice.
This ice can, at times, damage the aircraft. If the ice is large enough, it could hit the fuselage causing structural damage. In the past, toilet ice has also caused engine failures due to them being ingested by the engines. Most of these incidents involved Boeing 727s.
Source: simpleflying.com