This is part of my generation ship worldbuilding. I mentioned my water purification system in a different question, but I never directly asked about it. Here, I will directly ask about it. I think it will make more sense if I go step by step through the water purification process, starting at the input.


Here are all the things that are input into the system:

  • Wasted water
  • Poop
  • Urine
  • Diarrhea
  • Blood
  • Vomit

Stage 1: Solid filtering

The wasted water will just move on. The rest of it though goes through the solid filtering system first. Solid poop and any solid menstrual contents won't pass through. Everything else will, leaving behind:

  • Urine
  • Wasted water
  • Blood
  • Stomach acid
  • Diarrhea

Stage 2: Gradual filtering

At this point, the contents reach a series of microfilters and nanofilters of finer and finer mesh. To make sure that the water doesn't get blocked, the finest filter is no smaller mesh than a urea molecule. Here are the average sizes of what would be filtered out:

  • Blood cells: 6-8 micrometers for RBC's, 2.6-2.9 micrometers for platelets
  • Bacteria: .2-2.0 micrometers
  • Viruses: 20-200 nanometers
  • Protein: 53kDa
  • Urea: can't seem to find size for, but is definitely bigger than water

All that is left in this stage is stomach acid and water.

Stage 3: Acid Base Reaction

This is the stage that gets rid of the stomach acid. The acidic water goes into a pH correction tank. In there are a few wires, a valve, a sensor, and a dispenser. To avoid toxicity, I chose my base wisely. The sensor is calibrated for high acidity. When all that is left is acidic water and the acid is hydrochloric acid, the pH is anywhere from 1-2 depending on how much acid there is. This is way too low for some water to be drinkable.

When the sensor senses an increase in acidity, it electrically activates the base dispenser. To avoid acid corrosion, the wires are made out of either gold or platinum. Since there is no nitric acid present, there is no way the wires would dissolve, unlike how they would if they were made out of say copper. Gold is a way better electrical conductor than platinum, so the wires would most likely be made out of gold.

Here is the acid base reaction that occurs in the pH correction tank:

HCl + NaOH → H2O + NaCl(aq)

This is what the reaction looks like:


The valve is also unable to corrode in hydrocholoric acid, just like the wires. Once the pH gets to 7, the base dispenser is deactivated and the valve is opened. This leaves behind saltwater, very close to the final product.

Stage 4: Desalination

Now, to get rid of the salt, the water is repeatedly boiled and condensed. This leaves behind pure water and solid salt as byproducts. The salt is then stored and the water is ready to be used again. Since there is more water produced than was there at first after the acid base reaction, there should be no issues with the generation ship running out of water.

Will this water purification system work at both providing a constant supply of water and making sure that it is completely safe to drink? Or would the water get blocked up at the fine filters?

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    $\begingroup$ But it leaves the ship at risk of running out of sodium hydroxide, which is consumed at step 3. And waste water can also contain a lot of different stuff: alcohol, ink, soap, sugars... And usually salt is separated out via reverse osmosis through semi-permeable membranesm, with much lower energy requirements than boiling the water. $\endgroup$
    – AlexP
    Commented Aug 22, 2019 at 5:31
  • 1
    $\begingroup$ You forgot about ecological filtering. Swamps work because a combination of plants and other flora work with animal life to strain, separate, degrade and detoxify the effluents. Such ecological filtering systems have worked for millions of years. $\endgroup$ Commented Aug 22, 2019 at 13:50
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    $\begingroup$ If you're gonna go to all the trouble of distilling it (step 4), why not just have that be your only step? $\endgroup$
    – Joel Keene
    Commented Aug 22, 2019 at 16:54
  • $\begingroup$ There are several problems with distillation being the only step. 1) Protein from blood and urine, is going to froth up from the heat. 2) Hydrochloric acid will still be present, no matter how many times you distill it, because it lowers water's boiling point. A base is required to get rid of the hydrochloric acid. 3) Acid vapors might escape the system and cause everybody to get severe lung irritation. Some might even get "Acid pneumonia" because the irritated lungs leads to infection and the source is the acid, and they might die from boiling hot hydrochloric acid escaping the system. $\endgroup$
    – Caters
    Commented Aug 22, 2019 at 18:55
  • $\begingroup$ You should be able to use electrolysis to purify even heavily contaminated water consuming only electricity. $\endgroup$
    – Jim Fell
    Commented Aug 22, 2019 at 19:44

6 Answers 6


Do it like it's done on Earth

Using filters and machinery is useful when you need it fast or compact but on a generational ship, it's materials you can't spare.

You need garden/agricultural areas for oxygen and food. Human waste water is good for growing plants. Said plants absorb the water through the roots and expel as vapor through the leaves which then condenses on the walls and runs into water holding tanks.

Really you want the system to use as few moving parts as necessary as moving parts wear out and need to be replaced plus if you lose power, you want as much as possible not require it.

Generational ships would be more like giant farms and parks enter image description here

  • 2
    $\begingroup$ The answer is great, I gave you thumbs-up. And it's a pretty picture. But why would the buildings in this thing be made to resemble those on Earth? For example, why ever would you need a slanted roof? Are you planning to make it snow? Why would you have buildings at all? Directly under these buildings is ship. Which will have rooms, hallways, etc. Why use up space in the growing area to put that red barn? Photons falling on that roof are wasted. $\endgroup$
    – puppetsock
    Commented Aug 22, 2019 at 14:25
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    $\begingroup$ @puppetsock, you might be overlooking the psychological issues you need to deal with on a generational ship. It would do a lot of good to keep the "culture" of Earth alive with realistic buildings even if that's really their only purpose. Of course that's also pretty style/story based. $\endgroup$
    – JPhi1618
    Commented Aug 22, 2019 at 14:56
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    $\begingroup$ @puppetsock If the habitat is tall enough, rain will happen (without major dehumidifying). Larger aircraft carriers are known to have weather. Also, the image is more likely for a space station/orbiting habitat than a generational ship; more room and resources there. $\endgroup$
    – jaxad0127
    Commented Aug 22, 2019 at 17:32
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    $\begingroup$ Your colonists will presumably arrive one day on an undeveloped world that they intend to farm and occupy. It makes sense if their generational knowledge of farming translates as easily as possible to a planetary surface. This goes beyond just farming really - psychologically speaking, having common design patterns will help your first generation transition to shipboard living, and your last transition back to planetary life. $\endgroup$
    – Saiboogu
    Commented Aug 22, 2019 at 18:38
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    $\begingroup$ I like this answer, but I'm interested in scale and efficiency. Even on a generation ship space is going to be at a premium (if you don't need the space, the ship can just be smaller and easier to move)-- do you think the massive parks would be better than a series of reaction tanks where the same biological operations can be carried out by (largely) the same biological operators? This may be another WB question... $\endgroup$
    – Upper_Case
    Commented Aug 22, 2019 at 19:46

Filters will always eventually clog. The only question is how frequently you have to exchange them or clean them out.

You may want to look into how actual wastewater treatment and municipal water supply systems work. You may want to start with an initial coarse filter (just a wire mesh would be fine) to exclude big chunks of solid material, but after that you go to a setting tank. This is a large volume of relatively still water that allows non-solvated contaminants to either sink to the bottom or float to the top, where they can be skimmed off without the need for filters. At this stage, you may or may not choose to introduce bacteria to help process the waste, by predating other harmful microbes and either breaking down or accreting other chemicals in the water and helping them settle out.

After that, you could just distill water directly, with some pH balancing afterwards. The total quantity of acids in the wastewater, even if it's nothing but straight vomit to start with, won't likely be strong enough that you need to worry about dissolving your sensor apparatus. However, that is a very energy-intensive process, which no real large-scale water treatment facility actually uses. In practice, you would simply use a series of chemical treatments with coagulants to precipitate out the remaining solvated chemicals and suspended particles (alkalyzing the solution is usefulat this point), possibly combined with UV radiation treatment, and followed by another round of settling and then a final sand filtration. The final stage filters will need to be periodically regenerated, but that's relatively easy to do with a fine sand filter: you clean it simply by forcing air through it backwards to loosen the particles, followed by water to carry off the trapped contaminants.

  • 1
    $\begingroup$ One problems with distillation is that is doesn't address volatile compounds. Alcohols are the most obvious case of this, but you could consider soaps and shampoos with a small but nonzero vapor pressure as well. This is a good solution otherwise $\endgroup$
    – Aliden
    Commented Aug 23, 2019 at 14:11

You have a problem you are solving here: where will you put all the solid you are filtering out? You don't want to throw it into space, because it is useful organic material.

Do what sewer depurators do: set up an environment where bacteria and chemistry can do their work. Bacteria are very efficient at degrading any organic material suspended in water, just provide oxygen and the right pH. You will get water and organic substances you can use to fertilize your on ship agriculture, closing the loop.

  • 2
    $\begingroup$ Perhaps not "bacteria" per se, but some tailor-made microorganism that either is itself nutritious food for humans or is something that can feed what the humans eat. $\endgroup$ Commented Aug 22, 2019 at 20:48

I have a well and my drinking water goes through 5 sets of filters/treatments.

Actually, it's 6 sets. The well itself is the first. Rain water and other water that enters the water table under my property is filtered by the soil. Soil (which is living dirt, not just dirt) is perhaps the best filter there is.

My system, in order, is:

  1. Well water coming up from the groundwater.
  2. Large micron (30-50 microns depending on what I buy) filter to get dirt and other big things. My water is literally brown without it.
  3. Water softener (which also removes iron). My water is insanely hard. You almost certainly can skip this step.
  4. Carbon point of use filter (which has some particulate filters there too).
  5. Ultraviolet light (not needed for municipal water but a very good idea for untreated wells like mine; helps with bacteria, viruses, and cysts that might be in the water).
  6. Reverse osmosis. This creates a lot of waste water, which can be diverted for showers and etc. It's only necessary if you have stuff that needs to be filtered that way.

You also want to test your water periodically. This is really important no matter where in the universe you are.

Your suggested system is overkill. I don't desalinate my water and I add salt to it (that's what a water softener does). It's completely within normal limits. Remember, minerals in water are a good thing.

pH shouldn't be an issue, but you can test for it and correct it if needed. How much stomach acid are you expecting?

Particulate filters are fairly easy to make and you don't need more than a couple per water stream. Carbon filters can also be made by hand with purified charcoal. Use these after using particulate filters and before reverse osmosis (if you bother with that). They take out quite a lot of stuff.

My suggestion for your system is as follows:

  1. Remove large solids and compost them.
  2. Put on your farmland/gardens/etc. (only use more processed water for leafy greens, root vegetables, and crops you will be harvesting soon)
  3. Collect "ground water" for further processing.
  4. Run through 1-2 particulate filters (different sizes if you use more than one, perhaps 50 and 10 microns).
  5. Run through a really good carbon filter.
  6. Use ultraviolet light to zap baddies.
  7. If you wish, use reverse osmosis for extra filtration, send waste water back for general household use.

Test and monitor for bacteria, other microorganisms, pH, salinity, hardness, minerals, etc. Adjust if needed. If you need to treat with chlorine or something else to kill things, do it between stages 3 and 4.

  • $\begingroup$ I would be expecting quite a bit of stomach acid actually. First off, my starting population is 45,000. Secondly, if my starting sex ratio is 1:1 and I assume that 80% of the females become pregnant, that means that about 18,000 will get morning sickness, which can be anything from nausea to worse than any stomach virus. Thirdly, the acceleration of the ship can cause nausea and vomiting. Lastly, viral gastroenteritis, one of the most common illnesses, would be allowed on there along with other illnesses to keep the immune system in its prime. Thus, I expect quite a bit of stomach acid. $\endgroup$
    – Caters
    Commented Aug 23, 2019 at 0:05
  • $\begingroup$ @Caters So test for it it. pH is very simple to test for and very simple to control. Will you have baking soda? That's how one adds alkalinity in swimming pools, for example. $\endgroup$
    – Cyn
    Commented Aug 23, 2019 at 0:07
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    $\begingroup$ Also, the acidity from pee is probably more than from vomit. :-) $\endgroup$
    – Cyn
    Commented Aug 23, 2019 at 0:12

I agree with Thorne, we already do this stuff so let's not re-invent anything we don't have too.

That having been said, doing this in a space ship will have other issues not normally encountered on a planet.

From my perspective, doing Stage 4 first, will separate the liquids from the solids and have the additional bonus of killing any/all bacteria, parasites, and germs the are present in sewerage. As long as you have planned to ensure that no toxin's, heavy metals or micro-plastics enter the system then the remaining solid waste is essentially fertiliser.

From there you just need to separate the pure water from the other liquid wastes..... Possibly graduated cooling to isolate the various chemical formulas... not sure.

This should minimise any filter-clogging by greatly reducing the solids needing to be filtered out.


If you have enough energy available to do what you suggest you should also have enough energy to incinerate it all, filter and condensate and distill. You would get water, nitro oxides, carbon dioxide and metal oxides. You may have to reclaim some of the oxygen in the oxides to feed the incinerator if you have too much inorganic waste. If you are near a star you can use collimators (lenses) to incinerate and distill. Benefit is being a whole lot less maintenance dependent and way simpler to operate and design.


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