I spend a lot of time worrying about poop.
Being a crew member on a long duration mission requires getting your hands dirty. Something around the habitat will always be getting used up, worn-out or broken, and the only people available to fix it are the crew. Because of this, I like to say that a synonym for “astronaut” should be “dirty space janitor.” As the engineering officer for our mission, most of the dirty and un-glamorous parts of the habitat fall under my responsibility. In general, I love it! Some tasks are straightforward like calculating our rate of water consumption and projecting the nightly power drain on the batteries. A lot of the things I do are the same as what you’d do in your own home like regularly changing water and air filters. But some jobs require a little extra fortitude, like removing pounds and pounds of human crap from the toilets each week. I’m lucky enough to share this task with all of my crew mates, but the health of the toilets is my ultimate responsibility, and I spend a lot of time thinking about them.
In the HI-SEAS habitat, we have two composting toilets. The toilets looks somewhat like a regular household unit on steroids. The toilet seat sits about two feet high and the entire toilet base is several feet in length. Each unit is a white plastic monolith dominating the bathroom floor space. Requiring a built-in step to mount it, the composting toilet gives you a different perspective of the world, both literally when you sit high atop it and figuratively when it’s your turn to empty them.
The toilets are a complex system requiring a lot of care. Theoretically, human waste goes in and compost comes out. When a person uses the toilet their waste falls through a trap door under the toilet seat into a horizontal barrel inside the toilet body. After each use we add a mixture of peat moss and wood chips to add organic dry mass to the compost pile. A few times a week we turn the barrel to mix the compost using a built-in crank handle at the front of the toilet. The barrel spins on its horizontal axis and mixes the compost with each rotation. Microbes living inside the barrel break down the mixture of waste and dry mass and turn it into a nutrient rich compost. When it’s time to empty the toilets, the crank handle turns the barrel the opposite direction and the trap door opens on the bottom of the barrel, allowing the compost to fall into a removable tray at the base of the toilet. The compost dries out in the tray until it is pulled out and dumped.
Or at least that’s how it’s supposed to work according to the manual. It turns out that when three adults use the same composting toilet for all their bathroom trips, you aren’t going to have time for a nice natural compost cycle. In fact, most of your solid waste isn’t going to break down at all. Repeat this for the other three crew members and the second toilet and you quickly have a shit-uation on your hands. I’d argue the toilet manufacturers are more than a little optimistic when rating each of these units for a family of 5.
The toilets are an entire ecosystem requiring careful balance of inputs: urine, poop, dry mass and microbes. The good news is that it’s easy to know when something is out of balance: the toilets will tell you. The bad news is that toilets only know one way to communicate: odor. While compost should be somewhat moist, the urine of just one or two people can quickly overload the toilet system, creating an ammonia cloud that will sting your eyes when you empty the compost tray. Overly damp compost will sit and stew in its own juices, filling the bathroom with a noxious sewage smell. If the liquid drain at the base of the toilet is blocked, liquefied poop will back up into the base of the toilet and if you aren’t quick to fix it, overflow onto the bathroom floor (this is a caution from past crews. May we never experience this particular horror). You can add more dry mass to balance the moisture, but the extra mass in the toilet means it will fill up sooner requiring more frequent emptying. This means more bags of waste sitting around stinking up the habitat until we can EVA to take them outside. Leaving the compost in the toilet longer encourages flies to breed in the waste, quickly turning into a championship level fight between species: 10 rounds, human vs. fly, no holds barred. It’s a relief when the bathroom just smells like dirt. That means things are working just fine.
It’s obviously not the most enjoyable job, but toilet maintenance is good for one thing: team-building. There’s no bonding experience quite like teaming up to cook a great meal and then shoveling it back out together later that week. And that’s really what HI-SEAS is all about – finding ways to work and live together cohesively as a team.
NASA spends a lot of time worrying about poop, too: https://www.nasa.gov/feature/winners-of-space-poop-challenge-receive-30000