Using the bathroom on the Moon is more complicated than you might think. “The Moon is just Earth Jr, so if something is easy here, it’s even easier over there”, you might say. “On the moon the average person could jump over two meters in the air and lift their body weight over their heads, so everything’s a cake walk, right?” Wrong. You’re not thinking about aerosolized human waste spraying around your habitat like a golf ball hit by Alan Shepard.
The spray of contamination created by urinating into a pool of water or flushing a traditional toilet is impressive, as the studies “Lifting the lid on toilet plume aerosol” and “Aerosol Generation by Modern Flush Toilets” describe. On Earth, with easy access to disinfecting cleaners, fresh air, and emergency medical treatment, this isn’t so much of a risk. On a cramped lunar base hundreds of thousands of miles from the nearest hospital, sanitation becomes a substantial concern.
To begin with, lunar gravity is 1/6 of what we feel here on Earth. You tend not to have to worry about cleaning up a kitchen spill any more than a few feet from the spot where things landed. Our gravity means particles from a splash of water poured from waist height reach about 50″ from the initial point of impact. On the Moon, a stream of water coming down with the same force arcs about six times the distance, meaning that the total area contaminated becomes almost fifty times greater. The arc is also significantly taller, meaning biological material might end up on the ceiling of a lunar bathroom.
Incremental Architecture for Lunar Habitation
This article summarizes the research and design process for a low gravity toilet I developed at RISD as a student in Michael Lye’s course: “Design for Extreme Environments – Incremental Architecture for Lunar Habitation” back in 2005. The class’s purpose was to interface with designers at NASA, researching and prototyping architectures that could facilitate long-term lunar habitation. The class gave me some sincerely useful skills in large-scale prototyping, human/system integration, and the NASA design methodology.
I’ve expanded that research with what I’ve learned working in space technology in the ten years since.
The challenge with the design came down to creating something compact that could potentially last for years of continuous use. People have made zero-g toilets (such as the one currently on the ISS) but I was not able to find toilets specifically designed for the reduced gravity of the moon. NASA has done extensive research on maintaining hygiene while in an isolated, self-contained environment, but my investigation didn’t turn up anything for moon toilets, so I set out to design one myself.
Professor Lye recommended I read “The Bathroom: Criteria for Design” by Alexander Kira. It turned out to be an incredibly thorough examination of the human body, how we go to the bathroom, design patterns we use in bathrooms across the world, and suggestions on how to improve on those designs.
I started thinking about a lunar toilet as a standard piece of hardware – something everyone would use with as much regularity as a standard toilet on Earth. If it’s got to be durable, hygienic, and convenient as we expect from any other toilet, then it needs to facilitate the routines we’ve developed for them.
The toilet on the ISS is a sort of genital vacuum. It has apertures so you can line it up with your anatomy and excrete with suction assistance. Afterwards, you clean all of the components to try and mitigate cross-contamination. Given that there’s no source of fresh air, smells linger in the ISS until the multiple layers of scrubbers are able to remove them. The ISS stink has been described by several returning astronauts.
My concept was a hybrid of terrestrial and space toilets: a cistern for solid waste that could isolate and flush the material into a recycling system, and a vacuum assisted funnel that would minimize spray from urine without having to touch any one’s body. I wanted a system that would reduce the potential for splashing or releasing odors into the atmosphere.
Thinking on the Pot
“The Bathroom” has an axe to grind with western toilets. The author makes the case that bowel movements are intended to be taken in a squatting position. This position straightens the colon relative to the upright posture encouraged by western toilet designs. With the recent success of the Squatty Potty, more people are familiar with this general concept than when I was investigating it in 2005. Given that space missions are difficult enough, I incorporated this same intention into the lunar toilet design.
The book also highlighted weight distribution as an issue with toilet seat designs. Toilet seats have a wide aperture which doesn’t have any support underneath the femoral heads. This squashes people into the toilet bowl, but a minor redesign to add more support could help.
Given that a person on the Moon weighs 1/6 of what they do on Earth this benefit is marginal. However, the proposed solution from “The Bathroom” did give me an idea of how to cleanly break the waste removal system into to halves and still support good ergonomics.
Movements and Mechanisms
The proposed lunar toilet uses an evacuated tank to provide both power and a receptacle for waste. The solid waste portion of the system uses an oil layer to minimize contamination due to splashing and smells rising up out of the cistern. The flush mechanism involves a pair of valves to trap solid waste below the oil layer and empty it into the evacuated tank.
The front half of the system uses a funnel with an air assist to draw urine away and prevent particles from escaping. The separation between these two segments can be placed to also provide support underneath the femoral heads.
The flush mechanism for solid waste is designed to vent material to a container without a substantial volume of water per flush. Using the oil layer as a separator, it prevents odors from waste venting up into the environment. Waste is trapped between a pair of valves just past the cistern, and then a valve on the waste container pulls a vacuum on the cistern, venting its contents.
The form of the lunar toilet mimics a squat toilet with the division between the urinal and solid waste mechanisms situated to also provide support for body weight.
Issues with the Design
This design was the result of undergraduate research. There were gaps in the research that undercut the concept, though I feel there are elements worth salvaging. If I were presented with this design by one of my students today, I’d poke at a few different aspects to try and get a more robust design in place.
What is the actual splash radius?
- Lower gravity means lower acceleration. In this design, I’ve mixed the concepts of a plume created by a toilet flush, splashing from the impact of solid waste on the surface of a liquid, and the spray of urine. These all have similar negative consequences for sanitation, but their causes are all modified by the lower lunar gravity. Toilets can have lids you lower before flushing. Cisterns can be deep enough to prevent splashed liquid from reaching the user. In this case, is the oil layer serving a necessary purpose?
What is the buoyancy difference between a BM and oil?
- BM’s vary a lot. People in extreme environments can’t count on perfect movements every time, either. The mechanisms of the flush design rely on there being a clean separation between water and waste, unless it’s acceptable to have water that is always partially contaminated by waste. How does the system handle issues like loose stools?
- Engineers designing waste treatment plants have to know the buoyancy of human waste. Is there an oil that will, with a significant safety margin, cleanly separate all potential waste? This research really needed to be done before ideating on mechanisms.
- What is the method for cleaning up the system if the oil becomes contaminated?
- What happens to the rest of the system (waste water recovery, the cistern mechanisms, the fluid level sensor) if oil is accidentally flushed? Does there need to be a safety mechanism to prevent this?
- Does the oil need to be topped off periodically?
Does the cistern accommodate other bathroom waste like personal wipes or toilet paper?
- The ISS toilet does not dispose of toilet paper itself. There are sanitary bags for depositing wipes, etc. Is this also the case for the lunar toilet?
Is oil a reasonable choice in this scenario?
- Is this pool of standing oil a fire hazard? Can you guarantee that in low gravity it won’t evaporate into the air?
- Is there an oil that will remain sanitary while meeting the other specifications of the system?
- Will an oil layer really block odors, or will they permeate through it?
How important is avoiding contamination relative to mechanical complexity, system complexity, having a pool of oil hanging around?
- There needs to be an understanding of how this fits into a larger lunar habitat to measure whether the design achieves its overall goals of improving the safety and life quality of lunar inhabitants.
In summary, this isn’t a bad starting point for thinking about long-term habitation on the Moon. Sanitation, health, and hygiene are important anywhere people travel, and the proposed architecture could be prototyped at full-scale to optimize ergonomics, mechanical robustness, and integration with other systems like waste water recycling. There is a gigantic amount of work remaining to turn this into a genuinely robust system that could be considered for an actual lunar base, but the building blocks are there. If we continue to push for long-term habitation on other planets, prototyping durable systems on our own moon is the best place to start.