WATER & WASTE SYSTEM
1. PORTABLE WATER SUPPLY
INTRODUCTION
In the water and waste unit you will learn about the following 3 subsystems. These are:
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the potable water system
the waste water system.
the toilet waste system.
The potable water system is the main water supply system on the aircraft and supplies water for
the lavatories and galleys.
The waste water system removes used water from the lavatory and galley areas and drains it out
of the aircraft.
The toilet waste system removes waste from the lavatories and stores it in tanks.
This lesson will concentrate on the potable water system. The system consists of water storage and
distribution, with pressurization and a quantity indication. The potable water needs to be serviced.
The potable water is stored in water tanks. They are made of glass fibre reinforced plastic. They
are located in the pressurized underfloor area of the fuselage. The number of tanks and their
capacity depend on aircraft size and operation.
The water tanks supply water to the user through supply lines and valves such as the fill and
overflow valve which is located near the tanks and the manual shut--off valves which are located,
one in each lavatory. In large aircraft you can also find electrical shutoff valves in the supply. The
2 electrical shut--off valves shown here are for isolating the forward or aft part of the water supply,
if there is for example a leakage. When you press the left push button the forward shut--off valve
closes and isolates the forward section of the aircraft. The blue light shows that the valve is closed.
When you press the right push button the aft shut--off valve closes and isolates
the aft section of the aircraft. The blue light shows that the valve is closed
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LAVATORY WATER SYSTEM
The lavatory water system has the following components:
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a header that distributes the water
a manual shut--off valve that is for isolating the lavatory from the water supply if there is
a leakage
a water heater that heats the water
a mixer valve for mixing warm and cold water
a washbasin with faucets.
Except for the washbasin you will find all these components inside the cabinet under the sink.
Water flows from the tanks through the manual shut--off valve and then to the header which acts
as a distribution point. The header distributes water to the toilet, to the water heater, to the mixer
valve and via the faucet into the washbasin. The mixer valve mixes the supply of cold water from
the header and hot water from the water heater. So that warm water can flow to the faucet in the
washbasin. With the temperature adjustment knob on the mixer valve you can select warm
water from 26 to 43 C. This corresponds to 80 to 110oF.
MECHANICAL FAUCET
In each lavatory compartment there is a faucet assembly which works either mechanically or is
infrared controlled. It gives a flow of hot water, or cold water ,or a mixture of hot and cold water
through a spout into the washbasin. There are different types of mechanical faucets on aircraft.
The type shown here has 2 internal cartridge assemblies. They are located in the body of the hot
and the cold faucet. You can control the flow of water from the mixer valve and cold water pipe
independently. If you press the hot or cold water control handle, a plunger valve is actuated in the
cartridge and this lets the water flow. The water flow stops when you release the control handle.
This type of mechanical faucet has only a single water inlet for warm water from the mixer valve.
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You must press the actuator button to start the water flow. When you release the actuator button
the water flow stops after a few seconds. This is done automatically by the cartridge.
INFRARED FAUCED
The infrared faucet is controlled by a reflective sensor unit with 2 lenses. When you need water
then the reflective sensor unit opens a solenoid--operated shut--off valve on the mixer valve.
One of the lenses on the reflective sensor unit sends out an infrared light beam. When you place
your hands near the reflective sensor unit, the beam is reflected back to the second lens, which is
the receiver. The receiver then sends a signal to an amplifier unit that opens a solenoid operated
shut—off valve. The water flow stops when you remove the hands from the infrared beam for a
certain time. You can adjust the length of time with the timer inside the sensor unit to between 3
to 15 sec.
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WATER HEATER
The water heater provides hot water to the washbasin in each lavatory. The heater is installed in
the water supply line to the mixer valve in each lavatory. It has a tank with electrical heating
elements that are controlled by a thermostat. The tank can hold approximately 3 pints or 1.5 ltr. of
water.
There is also a manual ON--OFF switch, an indicating light, a pressure relief valve, an overheat
switch and on some aircraft types there is also a temperature select switch. When you switch the
ON--OFF switch to ON, the heater elements are energized to heat up the water.
The thermostat switch controls the temperature in our example at 52oC. This is done by a cycling
switch operation. If you need further information about electrical heating please look at Lesson 3
of unit 30. The overheat switch opens when water temperature increases to about 90oC .
This can happen when the control thermostat is blocked in the close position. If an overheat
condition occurs then you must manually reset the overheat switch at the top of the heater.
There is a temperature select switch on the water heaters on some aircraft types. With this switch
you can select 3 temperature levels. In the electrical diagram you can see that this is done by
selecting 3 different control thermostats. When the switch is set to low, then the selected
temperature is about 38 oC. When the switch is set to the medium position then the selected
temperature is about 44 oC. When the switch is set to High, then the selected temperature is about
52 oC.
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We are now going to look at the last component of the water heater -- the pressure relief valve.
The pressure relief valve opens automatically when the pressure in the heater increases to more
than 140 oC. This can happen when the shut--off valve is closed and the water is heated. The valve
closes again automatically when the pressure decreases below 130 psi.
PRESSURIZATION
The potable water system needs pressurization to transport the water from the storage tanks to the
user. For water pressurization normally bleed air from the pneumatic system is used. When this
source is unavailable then an electrical air compressor is activated. There is an air filter in both
supply lines. They prevent that dirt from the air supply can enter the potable water system. The
check valves prevent a reverse flow of water and air to the air—supply when its pressure is lower
than the water tank pressure.
The electrical air compressor is activated by a pressure switch that is located between the pressure
relief valve and the 2 check valves. The pressure switch closes when pressure from the pneumatic
manifold decreases below 30 psi. The switch opens again when the pressure increases to more than
40 psi. This stops the compressor. A pressure relief valve is located between the pressure switch
and the water storage tanks. It opens when the pressure increases to 60 psi. This releases pressure
out of the water supply line to prevent damage. This can happen for example if the pressure switch
fails to open at about 40 psi and the compressor continues to operate. The pressure relief valve
closes again when the pressure decreases below 54 psi. If you must replace a component in the
potable water system be sure that the pneumatic system is depressurized.
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2. WASTE DISPOSAL SYSTEM
WASTE WATER SYSTEM
The waste disposal system has two sub--systems. One is the waste water system which removes
waste water from the washbasins and galleys. The other sub--system is the toilet waste system
which collects the toilet waste in waste tanks. The waste water system directs the waste water out
of the aircraft through drain lines and drain masts. The flow of waste water is generated by gravity.
In flight it is also generated by the differential pressure between the pressurized cabin and the
exhaust end of the drain mast.
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The drain masts and parts of the waste water lines are electrically heated to prevent water from
freezing in these lines. The heaters are active as long as electrical power is switched on. There is
more information about drain mast heating in the unit on ice and rain protection. Never touch any
heated parts when the heater is on or has just been switched off. Especially the drain mast which
is hot enough to burn you.
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FLUSH MOTOR TOILET
You may recall that the toilet waste system collects and stores waste from the toilets in the waste
tanks. There are 2 types of toilet waste system installed in aircraft. In modern aircraft like this one
you usually find a vacuum toilet system. Other aircraft have flush motor toilets. Let’s look at the
flush motor toilet first. Each lavatory has its own toilet unit with the following main components:
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Flushing equipment has a flush handle
a flush motor assembly.
a waste tank
the drain equipment
There is also the toilet shroud assembly including the commercial seat that usually covers the other
parts. The graphic on the left shows a cross sectional view of a flush motor toilet. The flush motor
assembly has, an impeller pump, which rotates in a filter basket when the flush motor is switched
on.
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The flush motor is a high power 3 phase electric motor. The motor is controlled by a timer. It
operates when you push or rotate the flush handle. When you operate the flush handle a switch
starts a 10 sec. timer circuit that activates the control relay. The control relay energizes and the
motor receives the 3 phase AC power and starts running. When you release the flush handle a latch
circuit keeps the relay energized until the timer circuit is expired. After 10 sec. the control relay
opens and the motor stops.
The motor drives an impeller that pumps the liquid from the tank through the filter basket to the
spray ring inside the toilet bowl. Approximately 2 gallons of liquid flushes the toilet bowl and
returns to the tank. The motor also rotates the filter basket with the filter drive pinion shaft. A fixed
wiper blade on the outer surface of the filter basket cleans the surface of the basket This prevents
clogging of the filter holes. The liquid in the waste tank is a mixture of water and a chemical. The
chemical is a mixture of disinfectant, dye and deodorant. This allows the toilet to be used about
100 times. After this you must drain the tank from an external service panel.
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VACUUM TOILET SYSTEM
The vacuum toilet system has 2 or more central waste tanks in the cargo compartment area.
The size and number depend on the aircraft type. This allows more flexibility in the cabin layout,
because the toilet location can easily be changed. In addition to the waste tanks the vacuum toilet
system has a vacuum system, the flushing components and the components for servicing. The
function of the flushing components is to transport the waste from the toilet to the tank. When you
press the flush switch the flush control unit controls the sequence and timing of the flush cycle.
The flush control unit opens the solenoid operated rinse water valve for about 1 sec. This sprays
water from the potable water system into the toilet bowl.
Then the motor operated flush valve opens for about 4 seconds and the waste goes to the tank.
Some aircraft also have a manual waste shutoff valve. You can close it if the flush valve fails in
the open position. When the rinse water valve opens then potable water goes to the spray ring,
also called the rinse water header, via the anti--siphon valve. The anti siphon valve is a check valve
that prevents back flow of water from the toilet into the potable water system. The valve opens
only if the water pressure is higher than the cabin air pressure.
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VACUUM SYSTEM
When you press the flush switch and the flush valve opens, than the waste is transported to the
tank by a differential pressure that is generated by the vacuum system. Above an altitude of 16000
ft the low ambient air pressure gives enough differential pressure for the system to operate
efficiently. Below 16000 ft an altitude pressure switch activates a vacuum blower. The blower
decreases the tank pressure when you press the flush switch.
The vacuum system is connected to the tank via a liquid or water separator. The liquid separator,
also named water separator, removes the moisture and waste particles from the waste tank air
before it is vented overboard by the vacuum blower. You can replace the filter during waste system
servicing.
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Each waste tank also has a level sensor. When the tank is full the sensor inhibits the flush circuits
of the connected toilets and activates the corresponding LAV INOP light on the attendants panel.
On many waste tanks you can find a second level sensor that does the ”Full” detection if the other
sensor fails. Or the waste tank has an independent waste level transmitter that controllers a level
indication on the attendants panel.
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