In previous posts I’ve described plans for space habitats which include allowances and techniques for the closed system recycling we will need to establish self-sustaining life in space.
This post describes in some detail just what is needed, and where I found the information. Fundamentally, we will need to provide recycling for nearly everything in space.
Humans breathe oxygen which is plentiful in the form of oxides and silicates, but rare as the free element in space. We each need a bit less than a kilogram of oxygen per day (0.83kg on average, more under high work loads). Note that processes such as smelting metallic ores use heat and a reducing agents such as carbon to turn metal oxides into the metal plus oxides such as CO2. Green plants turn the CO2 into carbon (or carbohydrates), freeing the oxygen, and the carbon can be fed as raw material into the smelter. The only net products of smelting are metal (such as iron) and oxygen.
Humans consume water for drinking, food growth and preparation, cleaning, even entertainment. Since hydrogen is in short supply (until we can gather and use the resources of a comet, in which case we’ll have more than enough to throw away), we’ll need to carefully recycle water. Each human drinks or eats about 2.6 kg of water daily; more is needed for hygiene and growing crops, a lot more.
Working humans need to eat, on average, about 2500 Calories per day (some estimates suggest 2000, my wife’s diet suggests 1500). This can be provided by about 540 grams (dry weight) of food (50g protein, 70g fats, 420 carbohydrates).
The total consumption of a human is about 4 kilograms of oxygen, water, and food per day, and it should come as no surprise that that the same human produces 4 kilograms of waste (in the form of carbon dioxide, exhaled water, sweat, urine, and feces) per day.
Also unsurprising is that green plants (such as algae) can take those 4 kg of wastes and produce the needed food and oxygen with only the addition of energy in the form of light.
The complications arise from the need for a balanced, nutritious, and tasty diet containing all of the essential amino acids and fatty acids, and from the fact that we do not digest all portions of plants. Cellulose (fiber) is undigestible, and we don’t even attempt to eat most of the plant material of a crop (stems, roots, leaves, bark).
The ecosystem we create in space must be perfectly balanced. When plants produce food for us to eat, they simultaneously produce the exact amount of oxygen needed to metabolize that food. But they also produce those stems, roots, leaves, etc., and excess oxygen to match. All of that extra plant matter must be fed to some other animals (such as rabbits or goats), or to fungi, or to bacteria, or burned. The excess CO2 must be captured and fed back to the growing plants of the next crop, because we don’t exhale enough CO2 to feed the plants, only enough to grow the food we ate – a fraction of the total plant material.
Note that we can meet all of our dietary needs by growing a variety of algae such as blue-green algae including spirulina, and chlorella.
Somewhat surprising is how little water is needed to grow adequate volumes of algae – as little as 6 to 10 liters per person. This is due to the extremely high growth rates of alga under optimal conditions.
However, an algae diet is not only boring, it doesn’t taste good. It is likely to only be used for long space missions where space and payload is at a premium, and even then at least 4 varieties must be cultivated to meet our dietary requirements.
Whether we are growing alga or traditional crops, much of human waste is not readily usable as fertilizer. Portions are, and some bacteria excel at producing nitrates out of urea and ammonia. But much solid waste cannot be so easily processed. Luckily, a technique is available to solve the problem: a Supercritical Water Oxidizer applies high pressure, modest temperatures, and oxygen to burn the carbohydrates to water and CO2, freeing nitrates and mineral salts in the process. This is called the Zimmerman Process.
The process boils down to:
- feed CO2 and light to growing plants
- harvest human-edible feedstuffs
- feed much of the rest to animals such as rabbits, goats, and chickens, as well as vegetarian fish such as tilapia.
- burn the rest of the plant matter to produce CO2 and ash (which is fertilizer)
- feed food byproducts (and table scraps) to animals such as chickens or pigs (which when harvested produce still more byproducts)
- use that Supercritical Water Oxidizer on animal and human wastes to convert them back into CO2 and fertilizers for the plants.
- Condense water out of the air for drinking, and recycle irrigation water (which holds excess fertilizers) for plants.
Remember, as we are growing crops we need to feed them extra CO2 (much more than humans exhale), and store the excess oxygen they produce; we’ll restore the balance when we burn the crop residues and wastes. The “burning” doesn’t have to be an open fire. Feeding crop residues to goats counts as burning, as does using the plant matter as a reducing agent in the production of iron – both produce CO2 and free the water in the carbohydrates.
What crops should we grow? In general, dwarf varieties of grains, beans, and vegetables will satisfy most of our needs. We’ll have bread and pasta from wheat, rice, soybeans, oatmeal, lettuce, tomatoes, melons, potatoes, sweet potatoes, onions, herbs, etc.. I’m sure we’ll grow strawberries and other fruits, and eventually our parks will also serve as a source for nuts and fruits such as apples. I’m also quite certain we’ll grow grapes for wine, barley and hops for beer, coffee, and tea. Some human appetites insist on being satisfied.
That does leave the question of space. Just how big must our farm be? According to T.A. Heppenheimer’s excellent book Colonies in Space, the answer is derived from existing studies and experiments in high-intensity farming. Using dwarf varieties that have also been selected for short planting-to-harvest times, using interplanting (sowing the next crop before the current one is harvested), and optimizing CO2, water, light, and humidity, Heppenheimer calculates that 60 acres of farmland will support ten thousand people. This is only about 25 square meters per person.
I propose to average less than half that efficiency and allocate 64 square meters per person to include space for crop tending, to support a greater variety of foods, and to allow some extras to feed goats (for milk, cheese, and meat), rabbits (for meat), and chickens (for eggs and meat). Note that crops don’t require high ceilings; a single meter is good enough (on average), yielding a volume requirement of 64 cubic meters per person. This is less than the 100 cubic meters per person of living space I recommend in Designing a Space Habitat, where I also recommend about 33 cubic meters of workspace volume and an equivalent amount of overhead.
In that previous post, I assumed that 3 levels of living space would be allocated for the farms, but that may not be the best option. Rather, the farms are the primary source of waste heat. All that light energy ends up as heat and must be dissipated. The end caps of our cylinder expose a great deal of surface, so it makes the most sense to place our primary heat sources – the farms - adjacent to them. Using 8 meters along both end caps as our farms provides 64 cubic meters per person, independent of the size of our habitat (as long as we use the Kalpana geometry). Many plants need little gravity, indeed aquaculture (raising algae and fish) may require none, and these may be placed near the center. I expect that we’ll place livestock near the outer rim, as their needs for gravity are likely to mirror our own.
A future post will describe the lighting needs of the crops, and the technologies we’ll use to provide it.