Remember the fundamental rule of rocket design: Every Gram Counts.
The spacecraft will have to lug along inconveniently large masses of air, food, and water ("consumables") so that the astronauts can live. And if the ship runs out while in a remote location, the crew will be reduced to a castaways in a lifeboat situation drawing straws to see who dies. With the added constraint that castaways in a lifeboat at least have unlimited access to breathable air. The fact that consumables run out at all will limit the duration of any given mission.
Which explains NASA's burning interest in Closed Ecological Life Support Systems (CELSS). In theory the only input such a system needs is energy, either sunlight or some power source to run grow lights. The advantages are:
- The astronauts will have air, food, and water forever (or until the equipment breaks down or the energy input stops)
- After a certain mission duration, it will be cheaper (in terms of mass) to use a CELSS instead of transporting consumables. With a primitive CELSS this happens at about 145 days, increasing the efficiency will bring the break-even duration point lower. The mass of the CELSS is constant regardless of mission duration, the mass of consumables goes up with mission duration.
The main functions of a CELSS are:
- Turn astronaut's exhaled carbon dioxide into oxygen
- Turn astronaut poop and table scraps into food
- Turn astronaut pee and washing wastewater into drinkable water
The current lines of research focus on doing this the same way Terra's ecosystem does: by using plants. In order to make the CELSS hyper-efficient they have to use hyper-efficient plants. Which explains the focus on algae.
CELSS technology would be a game changer if we had it. Alas, it has proven to be a much more difficult problem to crack than they thought back in the 1960s. As I write this it is 2021 and they still haven't managed to do it. Currently the state of the art is nowhere near achieving a 100% efficient CELSS.
But an efficiency of over 75% or so would be a huge help. Sometimes a sub-100% system is called a Controlled Ecological Life Support Systems. The system might also be partial, such as a system which can 100% recycle carbon dioxide into oxygen, but cannot recycle food at all.
In NASA jargon, a closed environment life support system based on algae is called a "yoghurt box", one based on hydroponic leafy plants is called a "salad machine", and one based on a fish farm is called a "sushi maker".
Problems with creating and maintaining a balanced CELSS include carefully controlling the amount of plants consuming carbon dioxide (so they don't gobble more CO2 than the astronauts can produce, resulting in plant death from asphyxiation), and small-closed-loop-ecology buffering problems. The latter means that the smaller your CELSS system is, the more rapid and violent the results from tiny changes. With a closed-loop ecology the size of Terra, tiny changes can take months to years to show any effects, and those will be mild. With a small spacecraft CELSS, tiny changes can cause immediate and drastic effects.
Since maintaining the balance of a CELSS is so tricky, it will be a major undertaking to keep things stable if the number of crew members changes. If the number goes down (by a group on a landing mission the the Martian surface, or by crew casualties) the amount of plants will have to be cut back. Adding new crew is more of a problem. Leafy plants take time to grow to useful size when increasing square meters of cultivation. Algae is less of a problem since single celled plants multiply at a speed that puts rabbits to shame. It will probably be only a few hours to grow the biomass of algae enough to accommodate the new crew.
Another concern is the shipboard supply of phosphorus and nitrogen, since these are biological bottlenecks.
Of course there is the problem of recycling disgust, but that has to be fixed by psychologists, not engineers.
Terra's ecosystem fundamentally works by plants feeding animals while the animals feed the plants. Pretty much all spacecraft CELSS system try to utilize plants to feed the astronauts. Replacing plants with something else seems like a silly attempt to re-invent the wheel. Why do all that work when Mother Nature has already done it for you, for free?
The plants ingest water, carbon dioxide, sunlight, and some trace elements. Using Mother Nature's photosynthesis process, the plants output carbohydrates and oxygen. The astronauts consume plant carbohydrates and oxygen. Using human digestion, the human body is nourished, while producing water, carbon dioxide, and some trace elements. If you can balance the system, it will keep spinning as long as there is sunlight available.
Photosynthesis essentially splits water into hydrogen and oxygen, spits out the oxygen, and fuses hydrogen and carbon dioxide to form carbohydrates.
According to Chris Wolfe, NASA's best estimate is that the amount of leafy plants needed to handle one astronaut's carbon dioxide exhalation will produce only half of that astronaut's food. This is a problem. If the CELSS is producing all of the crew's food, it will have twice as many plants as needed, which will rapidly deplete the atmosphere of carbon dioxide, which will suffocate all the plants. It will also overproduce oxygen, but that can be extracted from the air and put into tanks for latter breathing, for rocket oxidizer, or exported and sold.
The solution is to recover the carbon trapped in the non-edible parts of the plants harvested for food.
The easiest way is to burn those parts and feed the generated carbon dioxide to hydroponics. The ash will contain other nutrients. Another solution is to feed the leftovers to a supercritical water oxidation unit and let it generate the carbon dioxide.
For CELSS uses, plants are generally grown by using some species of soil-less cultivation. Using actual soil to grow plants, with all of its active cultures and other messy ingredients, is far to unreliable to use in a life-or-death system.
Plants require light in order to perform photosynthesis, but using direct sunlight is a problem. First off you'll have to filter out ultraviolet and other frequencies harmful to plants. A transparent window allowing direct sunlight to illuminate the plants will also allow deadly solar storm radiation to fry them to a crisp. Granted plants tolerate up to 1 Sievert per year, but lets be reasonable here. There ain't no such thing as transparent radiation shielding (as yet).
In theory a shielded shutter will protect the plants from solar storms, but it is yet another possible point of failure. This is unacceptable if you are relying upon your plants to allow the crew to keep on breathing. If the storm detector fails, if the shutter actuators fail, if the crew forget to close manual shutters, all of these mean death by asphyxiation.
A fiber optic pipe fed with filtered sunlight and containing numerous bends to defeat radiation might work. But then if the spacecraft moves further away from Sol than Terra's orbit, the intensity of sunlight drops off rather alarmingly due to the inverse square law. That moron Freeman Lowell took far too long to figure this out in the movie Silent Running.
When you figure in that plants can only use certain wavelengths of sunlight, you might as well give up and use artificial grow-lights. LEDs are best, due to their relatively low waste heat. Feed the LEDs with electricity from the power generator of your choice.
The wavelengths used by photosynthesis are called photosynthetically-active radiation (PAR). They lie in a band from 400 to 700 nanometers, more or less the visible-light spectrum. Chlorophyll, the most abundant plant pigment, is most efficient in capturing red and blue light. This is why plant leaves appear yellowish-green to our eyesight, the chlorophyll doesn't eat those wavelengths so they spit them out. PAR is usually measured in bizarre units of "µmol photons m−2s−1" because photosynthesis is a quantum mechanical process (i.e., a bizarre process).
According to Chris Wolfe, most plants flourish under 26 mol PAR per square meter per day (26,000,000 µmol PAR/m2/d). Assuming that 12 hours (43,200 seconds) of the day are daylight and 12 hours are nightime, this means the plants are enjoying about 601 µmol PAR per square meter per second (µmol m−2s−1, the negative exponents are a cute way of saying "per" or "divided by").
LEDs can produce about 1.7 µmol PAR per watt-second or 6,120 µmol PAR per watt-hour. Supplying 601 µmol m−2s−1 will require about 354 watts (601/1.7 = 354) per square meter.
In other words, your hydroponics LED lights are going to need about 354 watts of electricity per square meter of hydroponic plants, for 12 hours out of every day. Certain plants require different amounts of mols PAR per square meter, and different numbers of illumated hours per day, but this is a good back-of-the-envelope value. Lettuce and spinach want about 250 µmol m−2s−1 (not 601), while tomatoes and cucumbers can get by on only 100 µmol m−2s−1
If I am reading the reports properly, algaculture requires a wee bit more. Chlorella algae wants 450 µmol PAR/m2, and Spirulina's optimal value is about 120 µmol PAR/m2.
So doing the math, Chlorella's 450 µmol PAR/m2 will need about 265 watts/m2 of LED electricity, while Spirulina needs 71 watts/m2. Since algae is typically cultivated in tubes or tanks, I am unsure how to translate the square meters of illumination into volumes of algae culture. The thickness will not be much, because algae is so good at harvesting photons that is is practically opaque.
- Hydroponics: growing plants without live bacterial-infested soil ("salad machine")
- Solution Culture: no substrate, just nutrient water
- Static solution culture: plants in jars of still nutrient water
- Continuous-flow solution culture: flows of nutrient water passing over plant roots
- Aeroponics: plant roots are in the air, misted with a fog of nutrient water
- Algaculture: microalgae cells cultivated in either static or continuous flow solutions cultures ("yoghurt box")
- Medium Culture: using a sterile solid substrate bathed in nutrient water
- Aquaponics: solution or medium cultures in association with tanks of fish and other sea food ("sushi maker")
Growing leafy plants for food is not as efficient as growing algae or other single-cell plant. But it is quite a bit easier. The basic idea is to grow food plants not in soil, but instead in nutrient filled water or in an inert material bathed in nutrient filled water.
Medium cultures can use all sorts of substrates: rock wool, baked clay pellets, glass waste growstones, coco peat, parboiled rice husks, Perlite, Vermiculite, pumice, sand, gravel, wood fibre, sheep wool, brick shards, and polystyrene packing peanuts have all been used.
Algae is cultivated in photobioreactors. These try to hold the algae cultures in thin layers because the little greenies are so good at absorbing the light that any algae that is too deep will get no light at all. The concentration of algae is typically something like 5×108 algae cells per mililiter of water.
In 1965, the Russian CELSS experiment BIOS-3 determined that 8 m2 of exposed Chlorella could remove carbon dioxide and replace oxygen within the sealed environment for a single human (I am assuming this is in a very shallow tray). The official figure for Chlorella oxygen production is 25 to 400 femtoMol O2/cell/hour. If am I doing the math correctly, at a concentration of 5×108 cells/ml, this translates into about 0.0032 kg of O2 produced per hour per liter, and 0.768 kg of O2 produced per day per liter. Since astronauts require 0.835 kg of O2 per day, this implies they would need 1.09 liters of chlorella culture.
The threat of contamination of the algaeculture with algae producing microcyctines, anatoxin, or other deadly substances means the life support officer had better monitor the algae closely, since the crew is going to eat that crap. If it pops up, all the algae gets thrown into the supercritical Water Oxidation to be disintegrated into oxides, the algaculture system gets flushed and sterilized, and restarted with a fresh packet of algae spores. The crew has to live on emergency rations while the algaculture grows.
This might be a good reason for some redundancy, like two separate and independent algaeculture systems. Hopefully only one would go bad at a time.
A person who goes by the internet handle of Tom I. Bystanderson noted that apparently the synthetic pathway for various algae toxins are well understood, so with a little genetic engineering some strains of toxin-safe algae could be produced (scientists are not quite why algae produce such toxins, they might be needed for algae metabolism).
SF writers with an evil turn of mind will see some interesting plot possibilites in these facts. The ship's food supply could become contaminated by an incompetent repair of the algae system utilizing lead pipes, an algae culture supplier with poor quality control, or deliberate sabotage.
Genetically engineered algae guaranteed to be anatoxin-free is of course going to be quite a bit more expensive that garden-variety common blue-green algae. A tramp freighter spaceship captain might decided to economize by using cheap algae, and live to regret it. One would think that it would be easy for one ship-captain to ask another if they could borrow a cup of engineered algae, but that would expose them to a patent infringement lawsuit on the part of the algae company. In the real world the company Monsanto has pursued about a hundred lawsuits for seed patent violations and/or breach of contract when they caught farmers who didn't purchase any of Monsanto's genetically engineered seeds but had some growing in the farmer's field. A science fiction writer can imagine agents of NoAnatox Algae Inc. doing suprise spot-checks of spacecraft algaecultures at spaceports, trying to catch violators.
Tom I. Bystanderson observed that suppliers might try to enforce their intellectual property rights by making their custom algae dependent on a slowly-consumed, hard-to-synthesize licensing molecule.
And you'd better keep the algae tanks far from the atomic drive. The last thing you want is for the little green darlings to mutate into something you can't eat. Or worse: something that is really inefficient at producing oxygen.
Christopher Huff begs to differ:
In THE MILLENNIAL PROJECT, Marshall Savage sings the praises of Spirulina algae. However, you'd best take the following with a grain of salt. There is often a long distance between the ideal and the real.
Anyway, Spirulina is apparently almost the perfect food, nutritional wise. A pity it tastes like green slime (though Savage maintains that genetic engineering can change the flavor). Spirulina is highly digestible since it contains no cellulose. It is 65% protein by weight and contains all eight essential amino acids in quantities equivalent to meat and milk. It also has almost all the vitamins, with the glaring exception of vitamin C (I guess rocketmen will become "limeys" again). It is also a little sparse on carbohydrates. Savage calculates that it will be possible to achieve production rates of 100 grams (dry weight) of algae per liter of water per day. It breaks down 6 liters of algae water per person, supplying both food and oxygen, while consuming sunlight (or grow-lights), CO2 and sewage. 6 liters of algae water will produce 600 grams of "food" (540 grams is 2500 calories, an average daily food requirement), 600 liters of oxygen, and consume 720 liters of CO2 and an unspecified amount of nutrient salts extracted from sewage. Since food is generally 75% water, 600 grams of dry food will convert into about 2.4 kg of moist food, which compares favorably with the 2.3 kg on the USS Wyoming.
600 liters of oxygen is about 0.8574 kg of oxygen, which is above the NASA requirement of 0.835 kg of oxygen per astronaut per day.
NASA commissioned a study back in 1988 to determine how difficult it would be to cultivate Spirulina as part of a closed ecological life support system.
The advantage of algae is that it can theoretically form a closed ecological cycle. This means that 6 liters of algae water, one human, some equipment, and sunlight can keep the human supplied with food and oxygen forever. Theoretically, of course. 0.006 m3 per person compared to 90 m3 per person is a strong argument for lots of green slime dinners for enlisted Solar Guard rocketmen. Of course the Biosphere II fiasco shows how far we are from actually achieving a closed ecological cycle. Don't forget the 0.25 liters of water per person per day to make up for reclamation losses.
There were some figures in a report on a cruder life-support set up written in 1953. This used Chlorella algae, which isn't quite as good as Spirulina since it has an indigestible cellulose cell wall. The figures assume a Chlorella culture density of 55 grams per liter of water and a daily yield of 2.5 grams per liter. Savage's 100 grams per liter sounds a little optimistic, and 2.5 sounds a little pessimistic. The truth is probably somewhere in between.
At a yield of 2.5 g/l, to provide one rocketeer with 500 grams of food (instead of Savage's 600 grams) will require 200 liters of algae culture.
Urine is passed through an absorption tube to remove excess salt (which would kill the algae) but retaining urea and other nitrogen compounds the algae needs. Faeces are irradiated with ultraviolet to kill all bacteria and added to the urine. This is fed to the main algae tank along with pressurized carbon dioxide (previously removed from the air with calcium oxide). A pump sends a flow of algae culture to the growth trays under filtered sunlight. The culture then passes through a centrifugal separator on its way back to the main tank. The separator performs two functions:  removing excess gas to maintain a pressure equilibrium with the carbon dioxide injection and  periodically harvesting algae for food. Harvest will occur once a day, extracting 500 grams of algae from nine liters of culture per person. The pump will be controlled such that the algae on the average will experience two minutes of sunlight then three minutes in the darkness of the main tank before it starts the cycle anew.
A fresh batch of urine and faeces is added immediately after algae harvest, to give the algae twenty four hours to consume it. So by next harvest there is no human excretions contaminating the food (you hope).
Now for the answer you've been waiting for. Dr. Bowman estimates that the equipment will mass approximately 50 kg, plus 200 kg per man for algae culture. Since the equipment is such a small fraction of the total, mass savings depend upon getting the algae yield higher than 2.5 g/l. Such as Savage's 100 g/l Spirulina with 6 kg per man of algae culture.
Dr. Bowman points out that when one compares an algae system with merely stocking crates of food, the break-even point occurs at a mission of 145 days (about five months). Below this time it takes less mass to bring crates of food, as the mission duration rises above 145 days the algae tanks get more and more attractive.
This is from Water Walls Life Support Architecture: 2012 NIAC Phase I Final Report (2012)
The idea here is to make a environmental control life support system (ECLSS) with a higher redundancy and reliability by making it passive, instead of active. Meaning instead of needing a blasted electrically-powered water-pump moving vital fluids around, use special membranes so that the vital fluids automatically seep in the proper direction. Fewer points of failure, fewer moving parts, no electricity needed, much more reliable.
The system harnesses the power of Forward Osmosis (FO), which mother nature has been using for the last 3.5 billion years since the first single-celled organism. Each unit has two compartments A and B, which share a wall made out of what they call a "semi-permeable membrane".
Compartment A contains contaminated water. Compartment B contains a solution (the "draw solution") which attracts water like a magnet using osmotic pressure. The contaminated water gets sucked through the semi-permeable membrane but leaves the contaminants behind (because the membrane won't let them through). The pure water (or purer water) winds up in compartment B with the draw solution and the contaminants remain in compartment A.
Since osmotic pressure is used there is no need for an electrical-powered water pump. It happens naturally just like a ball rolling downhill.
The research team noted that there already exists a commercial example of this: the X-Pack Water Filter System by Hydration Technology Innovations. You put nasty river water full of toxins and pathogens in compartment A and add a special sports-drink syrup into compartment B as draw solution. In about 12 hours compartment B will be filled with a refreshing sterile non-toxic sports-drink and all the horrible crap will be left behind in A.
So the research team realized that they could make a full ECLSS if they could develop some different types of forward osmosis bags and connect them together. They need bags that can do CO2 removal and O2 production (via algae), waste treatment for urine, waste treatment for wash water (graywater), waste treatment for solid wastes (blackwater), climate control, and contaminant control.
As a bonus cherry on top of the sundae, since all these will basically be bags of water, they can do double duty as habitat module radiation shielding.
The reliability comes from using lots of independent inexpensive disposable bags. The current system depends on driving an electromechanical water pump until it fails, then frantically trying to repair the blasted thing before all the toilets back up. Because the FO bags are cheap and low mass, they can be considered disposable, the spacecraft brings along crates of them with the other life support consumables. Because each bag uses forward osmosis as a built-in pump, there is no single point of failure. When one bag or cluster of bags, or integrated module of bags uses up their capacity, you switch the water line to the next units in sequence. The used bags can be cleaned, filled, and reused. Alternatively they can be stuffed somewhere in the habitat module to augment the radiation shielding.
Other SF novels have suggested vats of yeast or tissue cultures of meat ( in vitro meat ) to supplement food supplies. H. Beam Piper's Terro-Human series had spaceships equipped with "carniculture" tanks. A. Bertram Chandler's Rim World stories featured spaceships with all sorts of food vats. Tissue-culture for meat, hydroponics for vegetables, algae and yeast for single-celled food.
But unless they can re-cycle wastes from the crew, it seems more efficient to just carry more boxed food.
Currently scientist can only grow tissue cultures as a single sheet of cells, making them thicker will require figuring out how to make them grow blood vessels to nourish all the cells ("vascularization"). But some technicians figure that they can grow lots of meat cell sheets, then laminate the sheet layers together to approximate a slab of meat.
There are researchers exploring several different strategies to make full-blown vascularization. But it ain't easy. Strategies include material functionalization, scaffold design, microfabrication, bioreactor development, endothelial cell seeding, modular assembly, and in vivo systems. See link for details.
The joke name for this process is "In Meatro"
If you are trying a closed cycle with tissue cultures, you will have to deal with the problem of the Food Chain. Typically each higher level of the pyramid has one-tenth the biomass of the one below, for reasons you can read about in the link. What this means is that you will have to feed ten meals worth of algae to the meat tissue culture in order to produce one meal worth of meat. Even on Terra, this is the reason why meat is more expensive than vegetables.
Obviously the food chain effect also applies to diverting some of the algae to fatten up some fish as a special meal.
At least the tissue culture helps increase the meat ratio. For instance, an entire cow is about 40% edible meat. The rest is bones, hooves, hide, and other inedible parts. Tissue cultures would theoretically turn that up to 100% edible meat. Granted the inedible parts can be recycled via supercritical Water Oxidation, but the inefficiency of wasting all that algae food energy on growing inedible bones kills this idea dead. Not that it would have been practical to bring a cow along on your spaceship in the first place.
As a side note, the idea of lab grown in vitro meat has caused some controversy among the vegetarian community. Any person who is vegetarian on the basis of avoiding animal cruelty, should have no objection to eating in vitro meat. But some vegetarians still maintain that one should not eat meat because of Reasons.
Of course things can become a real moral quagmire, as Sir Arthur C. Clarke points out in his disturbing short story The Food of the Gods.
There are quite a few edible insects that will happily eat algae. Since they are live, they make their own vascularization. They are very efficient at converting algae into insect meat. And a much higher percentage of insect body mass is edible meat.
Yes, most people from western cultures find the thought of eating bugs to be incredibly disgusting. However the astronauts are already drinking recycled urine so it just takes some training. Processing will help, a compressed-protein bar composed of finely ground insects will be easier to eat than a plate full of microwaved bugs with too many legs.
In the following table, the "Algae for 1 kg of animal" is how much algae an entire animal will need to eat in order to increase its weight by one kilogram. "Edible meat" is the percentage of the animal's mass that is edible. "Algae for 1 kg of meat" is how much algae the entire animal will need to eat in order to increase its edible meat mass by one kilogram (i.e., reciprocal of edible meat percent times algae for 1 kg of animal).
You can probably use the "Algae for 1 kg of animal" figure as a ballpark figure for a tissue culture.
1 kg of animal
|Edible meat||Algae for|
1 kg of meat
|Cow||10 kg||40%||25.0 kg|
|Pig||5 kg||55%||9.1 kg|
|Chicken||2.5 kg||55%||4.6 kg|
|Cricket||1.7 kg||80%||2.1 kg|
If you figure a beef tissue culture requires 10 kg of algae for each new kilogram of beef, the freaking live crickets are still more efficient.
At harvest time, insects are killed by freeze-drying, sun-drying or boiling (in space, exposing them to vacuum probably counts as freeze-drying). They can be processed and consumed in three ways: as whole insects; in ground or paste form; and as an extract of protein, fat or chitin for fortifying food and feed products. Insects are also fried live and consumed, but a deep-fat fryer in microgravity is insanely dangerous. Some species need to have their legs and wings removed before eating.
In practice, extracting insect protein is probably not worth the effort. Needs lots of exotic chemicals and equipment, and reduces the percentage of edible mass. It is easier just to grind them into powder or paste and make bug-burgers.
For more details than you really want to know, read the report.
Science fiction authors could use this as an interesting bit of historical detail. Old-timer spacers can tell tales about back in olden days when they had to eat bugs. You young whipper-snapper spacers have it easy nowadays, what with your vascularized filet mignon tissue cultures.
A shmoo is a fictional cartoon creature created by Al Capp, they first appeared in his classic comic strip Li'l Abner in 1948. Shmoos were prolific, required no food (only air), are delicious and nutritious, have no bones or other waste, and are eager to be eaten. (Ironically, they are the greatest menance to humanity ever known. Not because they are bad, but because they are good.)
Oddly enough, shmoos share many common traits with one-celled yeast. Yeast even looks a little like a shmoo. When a yeast cell senses the mating pheromone, it initiate polarized growth towards the mating partner, creating the characteristic outline of a shmoo. The process is called "shmooing", which shows that biologists have a sense of humor. As to the matter of the deliciousness of yeast, see the exerpt from Lucky Starr and the Oceans of Venus below.
In the real world, left-over brewers' yeast is used to create such foods as Marmite and Vegemite. Even in 1902 people realized that it was a criminal waste to just throw away the huge quantities of perfectly edible yeast protein that was a by-product of making beer. Marmite and Vegemite are still being sold today. Actually in Australia, Vegemite is more or less a food staple.
In science fiction, one occasionally encounters the term "dole yeast". In future societies that have some form of social welfare system for unemployed people, the food given is generally a portion of unpalatable raw yeast, since that is usually the cheapest food available. Single-cell protein is very inexpensive, especially if you grow it on minimally processed sewage.
"Aquaponics" is a way of raising both plants and meat in one tank. You use an over-sized deep hydroponic tank to grow the plants. Below the plants you raise fish. The fish are fed food pellets. The hydroponic nutrient media is supplemented by the waste the fish excrete. The plants consume the nutrients, purifying the water and keeping the fish healthy ("rhizofiltration"). The system is more stable than a standard hydroponic rig, since the larger tank will buffer and moderate any changes. The larger volume of water also means you can get away with a more dilute solution of nutrients.
Not just standard fish can be cultivated, the system can also be used for shellfish such as lobsters, shrimp, clams and oysters.
In NASA jargon an aquaculture system is called a "sushi maker".
If you really want to get back to basics, you can try to synthesize food in a laboratory, with no plants or animals involved. It is probably harder and less efficient than growing food, but might be the only thing left if there is an utter disaster in the CELSS. The crew will call this the horror of Food Pills, and they will be right.
NASA looked into this between the 1960s and 1970s.
Wastes have to be fed to the algae, or whatever. But it would be nice to turn the astronaut poop into sterile chemicals first instead of infecting the algae tanks with E. coli bacteria. And the problem of reducing to useable form plant stalks, fish bones, chicken feathers, and other tough scraps. Not to mention all the plastic bag bits.
Enter the Supercritical water oxidation (SCWO) unit.
By placing water at temperatures and pressures above the thermodynamic critical point, it turns into a fluid that combines the worst properties of a blast furnance and sulphuric acid. You feed anything into one of these hellfire-in-a-box thingies and nothing is going to come out the other end except water, oxidized chemicals, and mineral ash. This happens at about 374.1°C and 22.12 Mpa.
The only estimates I've managed to find (Parametric Model of a Lunar Base for Mass and Cost Estimates by Peter Eckart) for a SCWO unit are:
- Mass: 150 kg per person being supported
- Expendibles required: 10 kg per person per year
- Volume: 0.5 m3 per person
- Power required: 0.36 kilowatts per person
- Heat load: 0.09 thermal kilowatts per person
- Liquid waste input: 27.18 kg per person per day
- Solid waste input: 0.15 kg per person per day
Waste products from the astronaut's septic tanks and tablescraps are run through the SCWO. The appropriate output chemicals are fed to the Spirulina, which multiplies in meters of transparent tubes run under filtered sunlight. Filtered because raw sunlight in outer space is quite deadly to algae, and it isn't too healthy for humans either.
There is more information on SWO units here. The first reference describes a facility with a volume of just over 20 cubic metres that can process 7.5L per minute, more than enough for a crew of 300. (30L/person/day - 20 hours a day). Thanks to William Seney for these link.
General Atomics has some developed some SWO units for waste disposal.