New Mars Forums

Louis--

I applaud your optimism. On another planet, optimism will get a lot of people killed.
I'm a professional scientist by trade, and I have always gone with "just the facts." kbd512 has satisfied me with his basic math calculations. Sooner or later, Elon will also wake up to the need for nuclear power, in spite of selling solar panels for (Part of) his living.
There are lots of other people besides myself who abhor the idea of he entire landscape covered with solar panels, as the fantastic Mars landscapes are part of their reason for going there in the first place.

The rosiest energy usage projections that came from Mars One were 3.7kWh of continuous power, per person, per day, for ECLSS and replenishment of O2 / N2 from the Martian atmosphere and H2O from surface ice deposits, to support a crew of 4 colonists.  That means 3.7GWe of continuous electrical power for a million people.  That equates to 32.412TWh of power per year.  That presumes all food will be imported from Earth, which would be grossly impractical for a city of a million people.

32.412TWh / 0.65TWh per Bhadla Solar Park (10km^2 of solar panels) = 49.86 Bhadla Solar Parks to keep a million people from running out of air and water.  That's 500km^2 worth of solar panels, spread out over 2,850km^2 of land area (I confirmed the 57km^2 for the Earth-based Bhadla, which spaces out the solar arrays to prevent shadows being cast by the other panels of the array, onto adjacent solar panels, and for access roads / trails to erect the panels).  That would qualify as the absolute largest solar array that humanity has ever constructed, bar-none.  The next largest array on Earth isn't 1/10th of the size, even though those arrays are clearly visible from space.  That covers basic life support only.  The real power requirements are associated with food production, construction, mining, and manufacturing.  Bhadla was considered something of a record, but it required 8,500 personnel to construct it over 8 months.  We're going to need around half a million people to do nothing but erect solar panels for 8 months.  There won't be enough power to keep them alive, though, until after the array has been built.  Minimally, we need all that excess power while the city is being constructed, to supply the power to make Sulfacrete and to construct pressurized living spaces and indoor farming spaces.  Realistically, this city will take 10 to 20 years to build, and then we need to replace the solar panels with more panels imported from Earth or made on Mars from local resources and recycled panels.

Mars One noted that they couldn't supply enough power from their mass budget allocated to the solar array during the shortest days of the year, and would have to figure out how to reduce their energy requirements by prioritizing energy usage for the most critical life support functions.  Since insufficient power was available for life support, no power for construction, food production, or manufacturing would be available, either.  Their report was published in 2015.

NASA has had six years since to perfect CAMRAS (amine swing bed CO2 scrubber / atmospheric revitalization) and IWP (grey water to potable water filtration system), which is what Mars One was relying upon on the agency to develop.  Those systems are finally slated to fly as fully operational subsystems aboard Orion later this year or early next year.  They've spent hundreds of millions of dollars on "closing the loop" and reducing power requirements.  Anyone who thinks they're going to do significantly better should stop talking and start demonstrating their flight-ready hardware aboard ISS.  CAMRAS and IWP prototypes have already flown aboard ISS, and those systems are now there to stay because they worked so well during testing.

If we can't realistically construct an array 50 TIMES the size of Bhadla on Mars, then we have to admit that a city of a million people isn't possible with solar power alone, or we have to instead rely upon our one technologically feasible alternative, which is nuclear power.

Can anyone else here appreciate the incredible energy consumption associated with building and operating our farms and cities and factories and mines and educational system to educate our people to use the available technology?

It's a truly staggering amount of power, even in a place where the air and liquid water are "free".

It could be the case that scaled-up life support systems require less power on a per-person basis, but I find that assertion to not be credible because the power consumption is based upon pumping rates and electrical heating.  Electric motors / pumps circulate the atmosphere or water through the devices that scrub CO2 from the atmosphere or produce potable water from grey water.  Pump proportionally more atmosphere through the amine swing bed of CAMRAS, and you need more power.  Pump proportionally more grey water through the ionomer membranes of IWP, and you need more power.  Absent more power, there can be no increase in pressurized living space, nor the population size inhabiting the base / city.  A million people will need something along the lines of at least 26 NRG Stadiums to live in.

This entire endeavor appears to be utterly impractical with solar power alone, marginally feasible with nuclear power even though I still have my doubts about that, and probably only truly practical and economical with nuclear fusion, on account of the material input rates to sustain fusion vs fission vs photovoltaics.  You need at least several orders of magnitude more material input with solar, and about an order of magnitude less input with nuclear fusion vs nuclear fission.  If this was going to be practical to do with photovoltaics or nuclear fission, then we should ask why we haven't already established a city or even a tiny crewed base on the moon.  We had the technology to go to the moon about a decade before I was born.  Five decades later, there's still no moon base to test out all these ideas we have about what we can or can't do, what humans can or can't tolerate in terms of gravity, etc.

Micrometeoroids do not survive Mars atmosphere. On Earth they burn up 100km above the surface, on Mars 30km. It doesn't matter how many kilometres, they still burn up. The Moon has to worry about micrometeoroids, Mars has to worry about dust storms.

louis wrote:

I would agree that Bigelow-style inflatables won't become a permanent dominating feature of the Mars settlement but they will be v useful on Mission One I suspect. As for additional radiation shielding that can be provided by having a separate steel frame that can be assembled over the inflatable and filled with regolith or water (including along the sides) so that the inflatable has good protection from  radiation events.

Nice idea: a steel frame holding regolith bags. It seems good for radiation shielding but even for micro-meteoroid impact

We're back to the old question of "how many do we send" in the first expedition. In my estimation, the 4 person crew of the original Mars Direct plan is simply inadequate to do all that's necessary on the first visit to our new second home.
I've given this problem lots of thought, and after I state my number, I'll expand and expound my reasoning.

A hard and fast number is not possible because there will be lots of changes in planning what the first pre-colonization mission will want to accomplish, but I'm going a bit higher than the NASA plan of 7. I figure around 14 would be barely adequate, but 25 might strain the available resources sent in supplies. My number is 17.

There are many reasons to consider a larger mission crew, based on the workload.

Mainly, I'm considering the physical and psychological workloads on the crew. We're expecting these people to be there for over a year between Hohman transfer windows, and operating at a damned near 25/7 workweek. I'm considering fatigue and mental exhaustion will play a role. There will be differences of opinion between members that can often disrupt the desires of the chair warming planners. We're gonna need many different talents in order to "build stuff" for future use in addition to doing science. Housekeeping will become an issue. Equipment maintenance will become an issue. There will definitely need to be time set aside for simply "doing nothing," in order to get the personal batteries recharged. Illness could be an issue, although we need to quarantine everyone for about 3 weeks prior to departure. Injuries sustained through working and exploring need to be considered. We simply need internal backup through numbers

We need to have several teams within this number--teams of compatibles. Everyone will want to go exploring, but there will need to be limits on simply wandering around. This has to be a team effort, and I'm suggesting that no more than 6 be outside and exposed to GCR at one time, with a 6-8 hour outside time limit. Sleeping quarters need to be well protected from Solar flare radiation and from GCR; we need to limit exposure as much as possible; The entire hab should be covered with a decent layer of regolith, and in a lava tube or cave would be ideal (as we've discussed endlessly in the past).

My bottom line is: there will be too much workload for a skeleton crew to accomplish without going bonkers or becoming angry.

More to follow as time permits.

GW Johnson wrote:

Point 1 -- We do not yet know for sure what the life support power requirements will be.  We only know they are "large".

I have to challenge this point. I agree with your other points. But we know what humans need. Finding all supplies for permanent settlement is an issue. I still think the best plan is a relatively small exploration mission first; something the size of Mars Direct. Prove a greenhouse can grow food, with a mission that has enough stored food that crew are not dependant on it. Do experiments with producing brick from Mars dirt. Do various construction proof-of-concept experiments. Prospect for dirt that has no perchlorates. Prospect for ice. Prospect for other resources: hematite concretions as iron ore, anorthite as aluminum ore, white sand for glass.

Then build a relatively small permanent base; Mars Homestead Project designed for 12 crew. Then that would build habitats and life support for the first 100. Only then would a SpaceX Starship arrive with the first 100 settlers. Then they would build for the next 1,000 settlers. But Elon Musk wants to go massive from day one.

Ps. When I was part of Mars Homestead Project, I found technical details for life support equipment for ISS. That includes power requirements. I think that's a fair estimate. The manufacturer kept moving their web pages, as if they didn't want me to see. I think that's silly considering I was recommended we plant to buy life support equipment from them. They proved it works on ISS, so just buy more for Mars. Doesn't that mean I'm giving them free marketing? But that means I could give you those power requirements. And we can estimate heat requirements from JPL's work with rovers.

Noah,

I made some mistakes in that last post by using some figures about Bhadla that I didn't bounce off of multiple sources.  That's what I get for not checking.  I also thought that 34GWh/yr figure was actually 34TWh/yr, but that clearly wasn't the case (didn't even check, because this entire "solar power for a million people on Mars" absurdity turns out to be an even greater absurdity).  According to the "Interesting Engineering" article link below, Bhadla covers 10km^2 and produces 1.3TWh per year.  Maybe that's only solar panel surface area.  Wiki says it actually covers 45km^2; maybe that figure also includes area for the service roads around the arrays.  Since Mars is twice as far from the Sun than Earth is, let's presume we get half of that power, meaning 0.65TWh per 10km^2.  That equates to 140km^2 covered in photovoltaic panels.  I can't be sure if either source is correct.

One Farm to Rule Them All: The World's Largest Solar Farms

Here are the other stats associated with the Bhadla farm from the "Interesting Engineering" article:

Another claim to the title of the world's largest solar farm comes from India. The solar plant, located in Kamuthi, Tamil Nadu, Southern India is certainly enormous, but is it the world's largest?

The entire project cost an estimated $679 million and has a total peak capacity of 648MW. The installation covers an area of 3.8 sq mi (10 sq km) and kicks out 5.5 kWh/m² (1 m²=10.7 ft²) a day, and an annual generation of 1.3 TWh/yr is thought to be possible — impressive.

The farms consist of 576 inverters, 154 transformers, and almost 4,660 miles (7,500 km) of cables. All in all, the project consumed 30,000 tonnes of galvanized steel, and around 8,500 personnel worked to install around 11 MW a day, in order to set up the plant in the stipulated amount of time.

Amazingly, the facility was completed within just 8 months. It also includes its own robotic cleaning system. This cleaning system is charged using its own solar panel system.

* Note to Louis on your solar-powered Mars colony fantasy: Well, at least Bhadla has its own robotic cleaning solution, but no mention of how the panels are cleaned (but I'm guessing it's using water), how much all that stuff weighs, etc.

91TWh / 0.65TWh = 140 (meaning, 140 of those 10km^2 photovoltaic arrays that produce an equivalent amount of power from similar panels used on Mars; and of course, actual conversion efficiency won't suffer at all from having half as much input TSI or an atmosphere chock full of fine abrasive dust, because "muh green energy fantasy")

So, to build a similar array on Mars that provides 91TWh of electricity, we need 4,200,000t of steel (42,000 Starship flights), 1,050,000km of power cabling, 21,560 power transformers (no idea how big those are), and 80,640 power inverters.  Oh, and 1,190,000 people to complete the array in just 8 months, or maybe only 8,500 people to complete the array over a little over 93 years.  I turned 40 last year, so I don't think I'll live to see the end of the array construction if ONLY 8,500 workers are devoted to completing the arrays.  Whether or not we invoke robots, AI, CNT - 6,000 flights vs CFRP - 12,000 flights vs Aluminum - 24,000 flights vs steel - 42,000 flights, this only becomes slightly less utterly impractical.  Every 20 years, you're either replacing a substantial number of panels and recycling the materials or you're importing more from Earth if you don't have a solar panel factory on Mars that's sourcing its materials from Mars.

Elon Musk is selling a dream, not a practical engineering solution to do what he says he wants to do.

If "The Plan" is to use photovoltaics and batteries on Mars to supply power to a scratch-built city of a million people, THEN THERE IS NO PLAN!

This is impractical for much the same reasons that that "solar powered airliner" that Louis was fantasizing about is and always will be utterly impossible.  You can invoke, invoke, invoke "magic" until the cows come home, but in the real world the numbers don't change.  I invoked 100% efficient thin film 100g/m^2 solar panels and couldn't come close to making his science fiction fantasy work using his "flying Dorito" / lifting body made from CNT composite, in terms of Pavailable vs Prequired (to move at airliner speeds and carry airliner passenger loads).  I know engineering reality can be a real drag, but that's life.

Anyway, I'm done wasting my time on this unadulterated absolute nonsense.  Fantasize forever, I don't care.  If you want a practical engineering solution for supplying power to a city of a million people living on Mars, then that's called "nuclear power", and even that is at the very edge of feasibility.  Heck, I wouldn't drop a dime on any of this silliness until we know with absolute certainty that humans can survive in 0.38g, indefinitely, and figure out how to grow crops on Mars.  If Elon Musk or NASA wants to prove they're serious about going to Mars, then gravity simulation / human physiology evaluation is the first experiment I need to see.  Nobody's doing that because nobody's serious about going.  It's all wild fantasy, "selling the dream", such as it were.

I saw the videos that Spacex posted on their website.  The one camera view that showed the fire on the side of the engine powerhead was quite informative.  You could see the sparks from the wires shorting out as their insulation burned away. 

Of more interest,  and still unaddressed,  is the reason for the explosion that blew the vehicle apart.  Musk tweeted about a "hard start".  One has to wonder whether the extreme force transient of that "hard start" cracked the thrust puck open,  and in the process cracked open a fitting in the oxygen piping from the oxygen tank ahead of the fuel tank. 

Put oxygen into an ongoing fuel-air fire and you get a fast-deflagration explosion nearly every single time.  I'm talking about the kind of event that "disassembled" shuttle Challenger.  The loose booster poked a hole in the LOX tank,  that dumped LOX onto the chronic hydrogen-air base fire at the base of the tank.  In less than the blink of an eye,  the explosion ripped the center tank to shreds.  The shuttle turned broadside to the slipstream,  and was ripped to pieces.  In the cabin section,  the crew was still alive,  until they hit the sea.

It's not too hard to put together a similar (and very credible) scenario to explain what happened to SN-11.  I just did. 

GW

I thought it was about 100 metric tons per spacecraft. So 200 t on the first trip and another 200 t 26 months later when the crew arrives (total = 400 tons). 
Or did they update the design of the starship?

3D printers are so amazing. I just bought on a few weeks ago and it’s unbelievable how many possibilities you have with a 3D printer. Currently I’m building a small rocket with my friends and the 3D printer is a mercy.

I have seen some concepts where the EVA suits are stored outside and the suits themselves have the airlock built in. This is because some dust particles are so tiny that they cannot be filtered and would be deposited in the lungs.

Actually,  given plume impact/spreading,  the thrown rock hazard to the landing spacecraft is low,  except for the landing legs themselves.  The risk to anything else adjacent is very,  very high. 

The debris more-or-less follows a fanned-out "apron" of high velocity gases oriented not horizontally 360-degrees around,  but angled upward a few degrees.  The debris pieces won't be moving at full gas speed,  but at a fair fraction of it,  which is quite high.  It will arc out there a very long ways.  Measured in kilometers,  not tens of meters.

That does assume the skirt is high enough off the surface to let the gas fan out like that.  If there is no gap under the skirt,  all bets are off.  Stupid is as stupid does.

GW

Did you want answers? First life support. I see multiple life support systems, configured so you can mix-and-match components. That gives options should equipment fail. On a planet with no breathable atmosphere, a robust life support system with multiple backups is necessary.

Chemical/Mechanical based on ISS: electrolysis of water
Regenerable CO2 sorbent: That means a fan to blow cabin/habitat air through a sorbent, which will absorb/adsorb CO2. Periodically the sorbent is "baked out". The CO2 released will be compressed, stored in a pressure cylinder. Mercury/Gemini/Apollo used lithium hydroxide (LiOH) because it's very light. But that doesn't bake out fully, so it's consumed. Silver oxide is heavier, but does bake out fully. I have a paper from NASA from the 1990s about silver oxide granuals, regenerated (baked out) with a microwave oven. Would you believe a microwave oven is more energy efficient? Duh! ISS currently uses silver oxide for white EMU spacesuits, regenerated with an electro-resistive oven aka toaster oven. Silver oxide is configured as sheet metal. To use a microwave oven, it must be granuals. Navy nuclear submarines use a tank of liquid amine, bubble air through the tank to remove CO2, and bake out periodically. This has a problem in zero-G, the liquid will float away creating a breathing hazard. NASA developed an amine paste based on the Navy's amine. That paste is painted on styrofoam beads. When baking out, ensure you don't heat it so much that you melt the styrofoam. NASA developed an "extended mission life support pallet" for Space Shuttle that fit in the cargo hold. That pallet had styrofoam beads with amine, and extra tanks of oxygen. And the docking port with ISS had a connector so ISS could provide electricity to Shuttle. ISS has large solar arrays, so that provided unlimited power. Amine on styrofoam is lighter than silver oxide, but bulky, large volume. Good for a shuttle or station, but not a spacesuit. On Mars you could use any of these.

Oxygen generator, electrolysis of water: Using a semipermeable membrane allows the tank to work in zero-G. On Mars, that's not an issue, the planet has gravity. Oxygen bubbles will float to the surface.

Sabatier reactor: convert CO2 and H2 into methane and water. This requires high temperature, but it's exothermic. That means it produces heat, so once started it keeps going. Human metabolism converts carbohydrates + O2 into CO2 + water. If you work it out, oxygen generation by electrolysis of water alone only produces half the oxygen that humans need. If you produce oxygen that way, and just run it to produce enough oxygen, that will consume water. The Russian space station Mir just let that happen, shipped up large bags of water. After all, 88.8% of the mass of water is oxygen, and with this system they only had to provide enough water for half the oxygen cosmonauts breathe. The rest of the water came from water recycling: urine filtration and cabin dehumidifier. A Sabatier reactor combines all of the hydrogen from the water electrolysis tank with half of the CO2 removed from cabin air, producing methane and water. This greatly reduces the need for water from Earth. It almost but not quite closes recycling. There's still a need to replace some water. Methane and the other half of CO2 must be dumped in space, any moisture in those gasses are a water loss. Furthermore, moisture in solid human waste (feces) is another water loss.

When Mir was still in space, NASA asked why Russia doesn't just ship oxygen up? Yes, their life support system (without Sabatier) recycles half the O2, but still, why ship water? The answer is tank weight. Oxygen gas requires a heavy metal tank to hold pressurized gas. Water only requires a bag; they use a plastic bag with an outer fabric bag so the plastic doesn't tear. 88.8% of water is oxygen, water plus bag has lower mass than O2 plus metal tank.

Water recycling: urine processing and dehumidifier. Filter to produce potable water.

Direct CO2 electrolysis: convert 80% of CO2 into carbon monoxide (CO) and oxygen. Done over a thin zeolite catalyst; CO2 and CO stay inside the zeolite tube, oxygen passes through due to electricity to the outside. CO2 must be heated over +900°C. Between heat and electricity for electrolysis, this consumes 3 times as much power per unit mass of oxygen vs water electrolysis. Since it consumes 3 times as much power, it's something you want to minimize. Furthermore, this recovers 50% of the oxygen from 80% of the CO2, so total 40% oxygen recovery. Sabatier reactor with water electrolysis recovers 100% of the oxygen. However, this can be used on CO2 that would otherwise be dumped in space. So this can be used to replenish life support recovery losses. The mixture of CO with remaining CO2 dumped in space.

This one has a danger: a crack in the thin zeolite tube will allow CO get into cabin air. When humans breathe carbon monoxide (CO) it binds with hemoglobin. We use hemoglobin in blood to transport oxygen. But CO binds to hemoglobin, and never lets go. Once CO binds, that hemoglobin can no longer transport oxygen. There is no treatment to cure CO poisoning. But red blood cells only last 6 weeks. Our spleen breaks down old and damaged red blood cells, and bone marrow makes new ones. So once exposed to CO, it will take about 6 weeks to recover. A life support system that uses direct CO2 electrolysis must use several small zeolite tubes, each in a separate canister. Each canister must have a CO detector in the O2 outlet stream. If there's any CO at all, shut that unit down. It will have to be replaced.

Human metabolism:
Complex carbohydrates are polymerized sugar. Simple sugar, known as monosaccharide: C6H12O6. Every time two monosaccharids bind, one sugar loses a hydrogen atom, the other a hydroxyl group (OH), which combine to form water (H2O). The free bond of one monosaccharide binds to the other. The first step of human metabolism is to undo this. Humans have enzymes that break complex carbohydrates into simple sugar, adding water back.
Cellular respiration: 1 C6H12O6 + 6 O2 → 6 CO2 + 6 H2O
Plant photosynthesis: 6 CO2 + 6 H2O → 1 C6H12O6 + 6 O2
Water electrolysis: 2 H2O → 2 H2 + 1 O2
Sabatier: 4 H2 + 1 CO2 → 1 CH4 + 2 H2O

Toilet: A new toilet the Russians developed for Mir2 would use vacuum desiccation to extract moisture from feces. This module is now the Russia module Zarya. NASA thought plumbing for the new toilet was too complicated, so insisted they go back to the old toilet, the one for Mir. I imagine after the Columbia accident, NASA wished they had the new toilet.
And some years ago, Johnson Space Center did a study called Advanced Life Support Project. They used an incinerating toilet. This does not require combustion, it can be done with an electro-resistive oven (electric oven).

Backups:
1) When water electrolysis fails, use CO2 electrolysis. It will consume a lot of power, but will produce oxygen.
2) Extract CO2 from Mars atmosphere, use that for CO2 electrolysis.
3) Extract Mars ice, will probably be muddy and salty so filter for pure water, feed that to water electrolysis.
4) If water recycling fails, use CO2 electrolysis for O2, shut down water electrolysis. This means water humans produce metabolism will not be consumed. That assumes you still have some water recycling: eg processing condensate from cabin dehumidifier.

Biological:
I have argued strenuously for greenhouse(s) that use ambient light on Mars. The reason is all the above systems have a single point of failure: power. An ambient light greenhouse is the only life support system that will operate with absolutely no power. Of course a greenhouse will produce food (vegetables, grain, fruit), but also produces oxygen. In fact, NASA studies have shown that if a greenhouse produces all the food for astronauts, it will produce 3 times as much oxygen as they need. On a planet without a breathable atmosphere, excess oxygen is good! Plants will transpire moisture through their leaves, producing humidity in the greenhouse. That humidity will condense on cold windows. A trough along the bottom of windows can collect that water. This produces water that tastes more pure than any filtration system that NASA has devised. The question is how to recycle urine and feces into something that can be used as fertilizer for plants. Yes, you can process Mars dirt to become arable soil. But that still requires water of some kind.

This raises a couple questions. How to process Mars dirt to break down perchlorates. They're toxic, and plants grown in soil with perchlorates become contaminated with those perchlorates. You don't want vegetables contaminated with perchlorate, because that's toxic. Bacteria can break down perchlorate, but you want a faster process. That's one area of research.

Arable soil requires micronutrients, but Mars dirt has that. Fertilizer on Earth is normally K-P-N: potassium-phosphorus-nitrogen. Mars dirt has enough phosphorus, has some potassium but it's low, and no nitrogen at all. At least no nitrogen within detection threshold of instruments on Spirit & Opportunity. Curiosity had instruments 10 times as sensitive, I think Curiosity found a tiny bit of nitrogen in some samples. But not enough, and most samples by Curiosity still had none. We can make ammonium nitrate fertilizer from nitrogen extracted from Mars atmosphere. Yes, ammonium nitrate. That's the white granules that farmers have used for many decades, and people used to use on their lawn before the Oklahoma bombing. You can't be afraid of it, just do it. And potassium fertilizer. The usual source is potassium salt, found where an ancient saltwater sea dried up. It can be found in deposits beneath the Mediterranean Sea, beneath the Great Lakes, etc. There's also a deposit in western North Dakota / eastern Montana, and southern Saskatchewan. (A Canadian province that borders those two states.) And others. On Mars it should be at the bottom of the dried up ocean basin. Until such a deposit is found, we'll have to make do with Mars dirt.

Devising a toilet for a greenhouse on Mars is different than a toilet for a chemical/mechanical life support system. Instead of extracting moisture, you want to process urine and feces for something that can be used as fertilizer for plants. Two basic approaches: grey water sewage processing, or composting toilet. With a composting toilet, you have to ensure to collect urine separately from feces. Current toilet on ISS does that. Urine is left to sit for 6 months, bacteria break down urea to form nitrates that plants can use. Feces must not be contaminated with urine, because it changes which bacteria grow on it. Feces mixed with urine smells like an outhouse. Feces that has never been touched by urine smells like wet soil. Bacteria break down feces to produce compost that can be mixed with soil to produce rich black soil. Composted feces have a lot of potassium. Urine has a lot of nitrogen. Grey water sewage processing allows urine to be mixed with feces, basically a flush toilet. This is called black water. A septic tank uses bacteria to break it down. A carefully controlled series of bacteria breaks down black water into something suitable as fertilizer. That's called grey water. That grey water can be used to fertilize roots of crops, either water the soil or part of the hydroponic solution. Although you may not want to use either composted feces or grey water for root crops like potatoes or carrots. It is suitable for crops where produce grow above ground, like tomatoes, beans, peas, grain, strawberries, etc.