Moon Toilet: A closed-loop, water-based zero-gravity toilet, designed for NASA’s Lunar Loo Challenge.

Moon Toilet: Description of Operation

Overview

The Moon Toilet is a closed-loop, water-based toilet for zero gravity, designed for NASA’s Lunar Loo Challenge. Other zero-gravity toilets move waste with fast airflow. This one moves it with water, then cleans the water and uses it again.

To the astronaut it works like a bidet, which makes it a very hygienic system. Each astronaut has their own silicone personnel adaptor that clamps to the saddle. Inside, the system separates air, solids and liquid. The solids go into a disposable bag. The liquid is filtered, treated with a silver-based disinfectant, and stored ready for the next use. The treated water is not for human consumption.

Using the toilet

The enclosure (30) is a tall, rounded unit. The astronaut sits on the saddle (2) on top, fitted with their own silicone personnel adaptor (1), which clamps to the saddle. Two handles, the left (33) and the right (35), flank the saddle. Each carries a button. Water button (34), on the left handle: starts the water pump (6). It sends treated water along the water supply line (3) to the saddle, where it cleans the astronaut and carries the waste away. Air button (36), on the right handle starts the air supply (4), which injects a short puff of air to help push the waste down the waste discharge tube (5).

The controller (9) runs the pumps and valves.

Waste path and gas separation

Water, waste and air flow down the waste discharge tube (5) into the gas separator (7). In zero gravity, air does not rise out of water, and free gas would cause bubbles and air locks in the pumps and bladders. The gas separator keeps the injected air out of the rest of the system.

It does this in two ways. The separator spins, which throws the liquid and solids to the outside and leaves the air in a core along the axis. A gas-permeable membrane around that core lets the air pass and holds the water back. The air leaves through the odour filter (38).

The liquid and solids pass a check valve (8) into the waste water bladder (10), which holds about 1.5 L.

Solids separation

The waste water pump (11) moves the contents of the waste water bladder to the solids separator (12). This is a perforated-basket centrifuge designed to run at about 300 g. Liquid passes through the holes in the basket, and the solids stay behind as a cake on the inside wall.

A screw feed turns slightly slower than the basket. It scrapes the screen, which keeps the holes from clogging, and moves the solids along to the discharge. A magnetic clutch sets the speed difference, with encoders measuring the speeds of the basket and the screw.

An air injector forces the solids away from the bag coupler and into the disposable solids bag (13). The bag cap (14) seals the bag when it is full.

Water treatment, storage and excess water

The liquid from the solids separator passes a check valve (15) and the grey water filter (16), which is designed to last the whole mission. The disinfectant injector (17) then doses it. It is a screw feed that releases a metered amount of a silver-based disinfectant. The disinfected water goes to the treated water bladder (19), where it is stored for the next use.

Shut-off valves (18, 21) and the overflow valve (20) route the treated water back to the water pump (6). Because the water is recycled, the astronauts’ own water adds to the loop, and the excess leaves through the overflow valve to the spacecraft’s water recovery system. As an alternative, a high-velocity syringe, a piston that expels a slug of water at speed, could push it into space.

The treated water is not for human consumption.

Servicing and mission use

The toilet is primed as part of the pre-launch routine, so the pumps and the membrane start the mission in a known state. Unlike the space station toilet, it is refurbished after each mission, so its parts only need to last one trip.

A side door (31) gives access to the internal equipment, and the filter access panel (37) is on the top. The rear door (32) swings away from the back to expose the disposable solids bag (13). Full bags are changed during the mission and stored.

Parts list

Numbers match the labelled drawings.

1 – Personnel adaptor 2 – Saddle 3 – Water supply 4 – Air supply 5 – Waste discharge 6 – Water pump 7 – Gas separator 8 – Check valve 9 – Controller 10 – Waste water bladder 11 – Waste water pump 12 – Solids separator 13 – Disposable solids bag 14 – Bag cap 15 – Check valve 16 – Grey water filter 17 – Disinfectant injector 18 – Shut-off valve 19 – Treated water bladder 20 – Overflow valve 21 – Shut-off valve 30 – Enclosure 31 – Side door 32 – Rear door 33 – Left handle 34 – Water button 35 – Right handle 36 – Air button 37 – Filter access 38 – Odour filter