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This question, led me to wonder, how big are the tanks that hold poo. I had seen a BBC video, showing how they empty the tank on an A380 at Heathrow, can't remember the capacity.

Also, what about planes like Cessna etc, do they have toilets?

In an emergency can the tank be emptied, in case it overweight, i.e. above the Maximum Landing Weight?

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The A380 has a total waste tank capacity of 2096L or 554 US gal, according to Airbus.

B. Capacity There are four waste tanks, two upper deck tanks and two main deck tanks, see FIGURE 5-4-10-991-003-A.
(1) Upper Deck Waste-Tanks - 373 l (99 US gal).
Each tank is precharged with 35 l (9 US gal) of chemical fluid.
(2) Main Deck Waste-Tanks - 675 l (178 US gal). Each tank is precharged with 35 l (9 US gal) of chemical fluid.
(3) Total Waste Tank Capacity - 2096 l (554 US gal).

The size will vary for each aircraft type, the Boeing 747 had a capacity of only 1000L.

Smaller General Aviation aircraft do not have lavatories, this depends on aircraft size and purpose. The average Cessna will not have lavatories either, pilots have to plan their stops properly or rely on accessory such as the TravelJohn. Larger General Aviation aircraft such as smaller jets and aircraft designed for more than 4 passengers are more likely to have lavatories on board.

enter image description here
(Image Source: Siebert Aero)

There is no dumping system for waste tanks, mostly for hygienic and environmental reasons, as the waste tanks are not only filled with fecal and thus biological matter, but also with chemicals.

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    $\begingroup$ Also, there is no possibility of the aircraft becoming overweight during flight due to waste tanks filling up. Whatever goes into those tanks was on the plane at take off, whether in the galley, a carry-on, or a person. If it wasn't overweight at take-off, transferring the location of the weight won't make it become so during flight. $\endgroup$
    – FreeMan
    Mar 20 '15 at 12:13
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    $\begingroup$ @FreeMan An aircraft can be below MTOM/MTOW, but once airborne it can be above MLW. It cannot technically have more weight than before someone used the lavatory, there you are right... $\endgroup$ Mar 20 '15 at 12:17
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    $\begingroup$ Yes, a trip to the loo is a simple weight redistribution. $\endgroup$
    – FreeMan
    Mar 20 '15 at 12:38
  • $\begingroup$ Airlines also should provide travel john's along with the puke pouch, for cases of emergency.. $\endgroup$
    – Firee
    Mar 24 '15 at 6:21
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In modern airliners,the toilet seat sits directly on the holding tank It is covered by a shroud. Imagine a tank sitting on the floor of the bathroom and the shroud around it. That is the size of the tank. About the size of a large fishing cooler. The tank holds a mixture including formaldehyde. A contained pump with a screen continually recycles the fluid over and over.. The inlet has a screen in it to keep most of the solids out, the pump macerates any leftovers.
The tanks are evacuated and filled through plumbing that leads to ports on the aircraft exterior skin. The unit is modular and when empty surprisingly light.
Couple of side notes. When a plane is down and sitting in the sun the smell permeates the whole cabin. The shrouds around the toilet seat becomes extremely nasty yet was a favorite place for smuggling cocaine in the 80's.

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