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Imagine a self-contained city in space with a usable surface area of $50 sq. km.$ (we're talking about the distant future, of course). Over 20,000 residents, industries like agriculture and fisheries, and reasonably humid air to maintain comfort.

We're now faced with the task of plumbing this behemoth. My rough calculations estimate that around 50 million litres of water must be stored at all times to sustain the station for at least a month in the event of a failure of some sort.

During normal operation, the water would be constantly replenished, using techniques similar to those on the ISS. Moisture will be extracted from the air, waste, and other by-products, and be filtered and added to system again. Some water will also be manufactured on the station itself.

How should the plumbing be managed on a station like this? Should there be constant replenishment of the water supply through the means discussed above? Are my estimates for water consumption correct? How should drinking water be managed?

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    $\begingroup$ This question might be to broad to get answered, if you don't get an answer in a few days, trying narrowing the scope. $\endgroup$ – James Jenkins Nov 4 '15 at 19:32
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    $\begingroup$ A station that large would have many parallel, independent water recycling systems. A failure of one wouldn't threaten the station, so no need to keep vast amounts of water in reserve. You might want large water reserves so you can use them as shielding though. $\endgroup$ – Hobbes Nov 4 '15 at 19:50
  • $\begingroup$ We can only comment on your question "Are my estimates for water consumption correct?" if you give a bit more detail of your estimates. $\endgroup$ – Hobbes Nov 4 '15 at 19:51
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    $\begingroup$ Would recommend narrowing it down straight away. A megastation in space will have the same plumbing issues as a town back here on Earth, but with a vengeance. Setting your question in distant future doesn't help, either: we have no way of knowing the limits and advantages of future technologies. $\endgroup$ – Deer Hunter Nov 4 '15 at 19:52
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    $\begingroup$ You might find upcoming 9th Pumps & Pipes conference on December 7th of interest. They present innovations, high technology and challenging problems about, you guessed it, pumps & pipes. I watched 7th and 8th P&P conference live online and I expect they'll provide live webcast for the 9th too. They were quite interesting, even entertaining. And it's always a good idea to keep up to date on pumps & pipes when constructing a brand new Stanford torus. :D $\endgroup$ – TildalWave Nov 8 '15 at 0:01
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My rough calculations estimate that around 50 million litres of water must be stored at all times to sustain the station for at least a month in the event of a failure of some sort.

I believe that the "one month of storage" logic is not correct. A space city will be utterly reliant on its technology working properly. There are so many things which would require technology, water, atmosphere, day/night cycle, thermal management.

Here on Earth, if the machinery keeping our cities alive broke down people would be dying in droves. Water, electricity, gas, petrol, telecommunications, we are super reliant on them working smoothly and having excellent uptime.

Although I wouldn't dispute the usefulness of a buffer, and 50 million litres does seem reasonable or a bit on the generous side.

Redundancy and decentralization

On a space colony, the best way to manage the processing of atmosphere and water would be to do it with hundreds of parallel units all of which are capable of working independently, that way if a unit or three fail there is no danger to the colony.

Decentralization could be taken to the extreme by having each dwelling have its own independent water supply and recycling. This is not nearly as extreme as it may seem at first glance. These days Air Conditioning window units are a common sight on the sides of buildings. One function they can perform is dehumidification, which with the right engineering can produce drinkable water. An appropriately designed unit can easily produce 15 gallons (60L) of water per day, this is rather more than the minimum amount of water a person needs per day (according to UN standards) and is a reasonable daily usage for someone on a rainwater (rather than townwater) scheme and so practicing frugality in water user.

The idea of using dehumidifiers or other condensing methods to draw water from the atmosphere introduces the intriguing concept that a space colony might not even need plumbing for water, or another way of looking at it is can use the atmosphere for plumbing. It sure eliminates a whole lot of pipes and pumps and if any single unit breaks down that unit is very easily repaired or replaced. There is a reason the small window units are popular these days and it has a lot to do with it being the most economical approach - it's cheaper and easier to mass produce 20,000 small identical units than a single big custom built unit and it is by far the more robust solution.

Is using the atmosphere for water buffering and storage crazy? Not at all! The atmosphere of the Earth contains a lot of fresh water.

The atmosphere contains 12,900 km3 (3,000 cubic miles) of fresh water, composed of 98 percent water vapour and 2 percent condensed water (clouds): a figure comparable to the renewable liquid water resources of inhabited lands (12,500 km3).

The water cycle is vital to the functioning of Earth's biosphere, so the idea of using a mechanically-augmented water cycle rather than having pipes running everywhere is certainly not complete madness. In this case, the primary mechanical augmentation is using dehumidifiers in place of rainfall. To a certain extent humidity is self-balancing, evaporation is faster when humidity is lower, so a few lakes might help with passively maintaining balance, but it would be wise to invest in active central climate control which is responsible for either adding or removing water from the atmosphere.

How much water could the atmosphere hold? That depends how big it is of course. The area is stated at $50 sq. km.$ and a cubic km of air at 20C and 50% Humidity contains 10 million L of water, so if the height of the atmosphere was 200m ($10 cub. km.$ of air) the atmosphere would hold 100million L of water which would seem to be a pretty solid buffer.

Waste Recycling

Of all challenges, I think that waste recycling is one of the greatest. If we are going with an augmented water cycle approach which minimizes the need for plumbing, the obvious solution is to use evaporation extensively and return water to the atmosphere as quickly as possible, a common technique to reduce the volume of waste is to just evaporate off the water using heat and fans. This is quite a popular design for toilets in remote locations, the urine and excrement simply falls into a pit, where heat (passive solar and/or heat from decomposition) and a fan (solar powered) evaporate off the water, and the solids decompose. I'm not saying that exact design would be good, just that it scales well.

Removing the water isn't too hard, but the solid component of human waste is more problematic, the low tech solution is essentially a wetland, let microbes and plants recycle the waste back into water, CO2 and N2. Plants are great at pumping water into the atmosphere so this can double as an evaporator. The main problem with an approach like this is that it requires considerable space and plants are terribly inefficient at utilizing light, but you may want a wetland as part of the city for aesthetic reasons.

A related way to close the water cycle is to have waste water from residences go not to a wetland but instead to agriculture where it nourishes useful crops, and the plants return the water to the atmosphere. This isn't quite as easy as it sounds because it is also desirable to grow crop plants under tightly controlled conditions, but it would definitely be a feasible solution.

The high tech solution to waste disposal is to use an electric plasma arc to directly dissociate the molecules (plasma pyrolysis) then release the resulting gases to the atmosphere. Human waste is moderately energy rich so this process can generate rather than consume energy and might actually be the less energy intensive solution to disposal of solid waste. It's a sterile solution which would go well with intensive forms of agriculture which aren't compatible with organic farming styles.

Water Use Reduction

The most water-efficient way to grow plants is aeroponics (growing plants with their roots dangling in a mist of water and nutrients), any kind of plant can be grown using aeroponics and it works in microgravity too. Compared with conventional agriculture water use is dramatically reduced.

For animal protein, smaller animals are more efficient than large ones, at the extreme many insects are capable of growing using humidity in the air alone.

For personal hygiene, I've seen some interesting ideas in science fiction, including using ultrasound to clean skin by knocking off particles of dirt and dead skin. Ultrasound is already used in beauty care so it is a real thing. I suspect many people will prefer water even if only as a form of recreation/comfort, but as noted earlier the water usage for someone on a rainwater scheme is very reasonable and wouldn't be difficult to provide. There are toilets which don't rely on water, but a simple way to economize on water is to re-use shower water for toilet flushing.

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  • $\begingroup$ Absolutely perfect. This is highly detailed, thank you so much! $\endgroup$ – Vedant Chandra Nov 5 '15 at 18:09
  • $\begingroup$ @VedantChandra when I was thinking further, it might actually be absolutely necessary to have a strategy to continuously remove humidity from the air. If a space colony has a surface area measured in square kilometers, with lakes, streams, trees, crops and other plants increasing humidity by evapotranspiration, then unless humidity is removed either mechanically or by inducing rainfall/dew via climate manipulation the result would be 100% humidity and serious condensation problems. $\endgroup$ – Blake Walsh Nov 6 '15 at 13:35
  • $\begingroup$ You're right, that's what we're doing. We have a regulatory system in place that uses dehumidifying condensers to constantly remove moisture from the air. We therefore maintain a humidity of 60% (relative). Thank you again, this answer is fantastic. $\endgroup$ – Vedant Chandra Nov 6 '15 at 15:50
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It probably wouldn't be all that different from how things are on Earth. Any water storage would probably happen in some kind of lakes. For a space city of that size, I would assume that one would have sufficient land for some lakes, which would bring food, regulate the atmosphere, and otherwise help the station to thrive. As you mentioned, it would be captured from all sources, etc. Such a station will have multiple redundant facilities, but at least to some extent things will happen as on Earth. With artificial gravity, water will flow somewhat similarly as on Earth, but will have some slight difficulties due to the spinning of the station. Still, this will result in some slight correction depending on which way the water flows. The treatment will probably be similar to what is done on Earth, but to a higher extent.

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  • $\begingroup$ Broad answer for a broad question, thank you! $\endgroup$ – Vedant Chandra Nov 5 '15 at 10:33

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