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Various futuristic novels and movies have posited the existence of the mega-city; a city in the clouds built up hundreds if not thousands of meters above the surface area below. Examples are Coruscant from Star Wars or New York City from The Fifth Element.

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When your city is miles high, how do you get running water? Where does the sewage go?

Considerations

  • Assume that replicators don't exist.
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    $\begingroup$ @AlexP You should really answer in an answer. $\endgroup$ – kingledion Jan 26 '18 at 6:31
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    $\begingroup$ I would if I could. Plumbing is one field where my English vocabulary is manifestly inadequate, and internet resources quite scarce. Try to put plumbing high-rise "vertical zones" into a decent search engine. $\endgroup$ – AlexP Jan 26 '18 at 8:20
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    $\begingroup$ @AlexP your English in the comment was perfectly fine for an answer. The content of the comment should go in the answer box. $\endgroup$ – Tim Jan 26 '18 at 13:07
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    $\begingroup$ For a civilization capable of interstellar voyage like we travel by airlines communication and power generators are compact, safe, cheap, reliable and extremely powerfull. If megabuildings have similar size to ships why it cannot have a power generator itself (if it needs only a fraction of a same size ship since it don't needs to move) $\endgroup$ – jean Jan 26 '18 at 18:45
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    $\begingroup$ A minor note, your phrasing makes it sound like Coruscant was in both Star Wars and the Fifth Element $\endgroup$ – Azor Ahai Jan 26 '18 at 19:57
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Sewers still work the way they always did

The DWV (drain-waste-vent) system in a high-rise building is actually quite similar to that found in a house -- it's simply bigger! The principles (gravity flow, mostly) that make a building's DWV system work scale quite beautifully with the size of the building.

As to the sewers under the streets, you'll have separated sewers for sure in a city like this -- you simply have too much sanitary flow to afford spilling raw sewage due to rain-induced sewer overflow events. Your buildings will have roof drain systems that simply tie into downpipes to the storm drain system, while their DWV system ties into the sanitary sewers. Both of these systems will be underground rivers, basically -- at its most extreme, you get a "river atop a river" effect, with the storm drain system built atop the sanitary sewer system, and your streets built over the storm drains, with utility ducts/tunnels flanking them for other utilities.

Pump up the pressure to get water to go where you want

In order to get water to the tops of such high buildings, high-performance pumps are mandatory, both for fire suppression and domestic service. These superbuildings will have redundant fire pumps (due to the criticality of the service involved) feeding combination wet head sprinkler/wet standpipe systems (there is no other way to control a fire in an Earth-sized high-rise, never mind a Coruscant-sized one -- One Meridian Plaza and First Interstate Bank taught us that already), in addition to a pump dedicated to providing enough domestic water pressure to reach the top of the building. Pressure-reducing valves will be needed on each floor to prevent the high standpipe/vertical main pressures from reaching fixtures and damaging them, while the pumps themselves will need to achieve pressures likely into the thousands of PSI (such pressures are more typical for motive hydraulics than water service), and you will need break tanks every so often to keep the pump pressures from becoming totally unreasonable. A good break tank setup would pump up to the tank and then gravity feed down, by the way -- this way, redundant pumps can be used easily, and even if all the pumps fail, a limited supply of water will be available.

Evening out hot and cold

The square-cube law works in our favor here -- enlarging a building is beneficial from a thermal performance standpoint as the marginal heat loss goes down as you further increase size. However, the heating and cooling loads are going to be high, still. A high-performance, lightly glazed envelope (vs. the glazing-everywhere postmodern high-rise aesthetic) is going to be a necessity in these superbuildings, and they will likely be forced to rely on distributed ventilation in order to allow the structural design to prevent stack effects by placing air barriers between floors, with either mechanical floors feeding transfer media (steam, water, refrigerant) to air handlers in each compartment, or complete HVAC on a per-compartment basis. Domestic water heating will be handled the same way -- either by indirect tanks off the HVAC heat loops, or by per-compartment hot water heaters.

Big power means big problems

Last but not least, we have the electrical and communications infrastructure needed in such a megabuilding -- a medium voltage "trunk" with accompanying fiber optics will be run in a heavily firestopped vertical shaft in the building core along with the other building services, with dry-type transformers on each floor to provide low voltages for lighting, receptacles, and appliances. The fiber optics will feed distribution nodes on each floor (similar to a cable-TV hybrid fiber coaxial node, or a passive optical network switch for that matter, with primary voice service being provided via some type of Voice over IP setup).

Redundant trunking may be provided for both power and communications in order to prevent a single failure from knocking out power or voice/data services to the whole building, while secondary systems may also be present for power (such as a low voltage auxiliary communications power system to play the role of the 48V central office batteries in a POTS system) and communications (a firefighter's telephone system will be needed as handheld radios are no good for a fire crew working in a highrise). In addition, key services (fire/life safety) will have their own backups for power and the likes (including dedicated generators or engine drives).

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    $\begingroup$ Excellent answer. One bit on electrical: as you get larger megabuildings, you have more ways for the building to generate its own electricity. For example, a turbine that spins when the sewage waste goes down the drain, surface area for solar panels, wind turbines on building corners, etc. A number of modern buildings are experimenting with this and I'm told that as the building gets larger, these seem to be more successful at supplementing the power of trunk lines, thus decreasing the need for as much external power (but not eliminating it). $\endgroup$ – SRM Jan 26 '18 at 7:07
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    $\begingroup$ Rather than maintain huge presures couldn't you just have a chain of pumps and tanks to get the water up over several steps $\endgroup$ – Richard Tingle Jan 26 '18 at 7:44
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    $\begingroup$ @RichardTingle -- while sectionalizing domestic water like that is possible, it does change your maintenance requirements some. I doubt that's legal to do for fire mains, though... $\endgroup$ – Shalvenay Jan 26 '18 at 12:46
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    $\begingroup$ For a building miles high, I think extracting water from the sewage (recycling it for at least grey water usage, but probably all use) before returning it to ground level likely makes economic and practical sense. It would significantly reduce pumping requirements and reduce sewage disposal requirements. $\endgroup$ – Jack Aidley Jan 26 '18 at 15:22
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    $\begingroup$ Note how every now and then you have bridges connecting everything. These could be (more or less) independent levels of pressure/energy. $\endgroup$ – PlasmaHH Jan 26 '18 at 16:35
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Regular domiciles have 3 pipes: water, sewer, gas... and 2-3 wires: electrical and comms (phone and/or cable TV). For a skyscraper, the wires are simply more and bigger, and a superskyscraper won't have gas.

All the adventure will be in fresh water

In Earth gravity, a column of water 2 feet high weighs 1 pound per square inch. The bottom of that column will be at about 1 pound per square inch (PSI) of pressure. That's British PSI, so that should be a galactic unit, via the same mechanism that aliens speak English.

So a column of water 100' high will generate 50 PSI of head. Now, your household plumbing is happy at around 35-70 PSI, and water towers provide that passively.

The Burj Khalifa's top floor is 1918 feet, or about 1000 psi to push water up there. 2000 psi pipe is readily available (not cheap).

How do you push water to the top of a 20,000 foot high Coruscant skyscraper? Not by developing 10,000 psi of pressure - any leak would become a waterknife that could slice through the structure of the building like a lightsaber. Instead you have reservoirs every 1000' up the building, and push from pool to pool - only 500 psi needed.

Have distribution reservoirs every 70 feet or so, each reservoir serving the customers below the next reservoir, so everyone gets 35-70 psi passively.

Sewer - don't let it fill up

Pressure only develops if a pipe entirely fills. Usually a sewer pipe is just an air-filled chute, and does not fill up solid except at the very bottom, and only develops just enough pressure to move things along out the sewer lateral to the street. So a 20,000 foot tall sewer stack is not an engineering problem... unless city services have a problem. Then you do too.

Oh. Pressurization.

On Earth, a 20,000 foot skyscraper would have one serious problem. Much above 10,000 feet above sea level, people couldn't breathe. Realistically you would have to pressurize the upper floors (to about 8000' elevation, as jetliners do), or lease them to species who prefer those altitudes on this planet.

The pressurization will play havoc on how the sewers operate. You'll probably need to have a mechanism at the end of each pressurization zone to deal with that, to prevent all the pressurization from blowing out the sewer pipe. Simply having their own pipe down to their pressurization altitude should suffice.

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I could see miles high buildings with tens of thousands of occupants being largely self contained.

As has been stated, sewage and water systems would remain the same just larger. But instead of going to a centralized location in the city, which risks spilling tens of thousands of tonnes of raw sewage if there is a leak, it would go to a sewage plant in the basement.

The sewage could be treated normally, with the water being reclaimed for later use as it is today. The sludge would be separated, with phosphorous compounds sent to nearby farms for fertilizer, while the methane gas and whatever sludge remains would be burned and turned into electricity for the building. This extra power would be used to support the main power system also in the lower levels.

This system would help reduce the amount of water and energy coming from the city to a minimum. Another benefit is that a break in the grid would not shut the city down, but merely the one building. With backup generators and a large septic system, the building could keep functioning until repairs were made.

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    $\begingroup$ I concur except you can distribute the facilities throughout the building, no need to waste building space (bottom floor would need pipes supporting all floors above) and energy (moving things vertically) sending it all to the basement. $\endgroup$ – DHa Jan 26 '18 at 14:52
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    $\begingroup$ I was thinking that having it all localized would be more efficient overall and any major leaks happening in the basement would keep it from badly affecting the rest of the building. $\endgroup$ – Dan Clarke Jan 26 '18 at 15:41
  • $\begingroup$ If you extract too much water, would not harden into a solid block like cement? I am also concern that not all 100% will be burnable, and even 10-20% left could become a management nightmare. Think of needing 100 trucks/week to haul it away, as without water content it may just sit there and not go down the sewers. $\endgroup$ – cybernard Jan 27 '18 at 14:40
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    $\begingroup$ With todays technology at waste treatment plants, 94% of sewage waste is water and is extracted. the rest forms a sludge at the bottom. From there the sludge is removed to a vat, the gases, primarily methane is taken away as it's produced. Then certain processes happen and the sludge is divided into useful phosphorous and the rest. The rest is then burned, with the ashes put into a chemical slurry to extract more phosphorous. The little bit of ash remaining can be shipped away from the city. It's a really interesting process and when fully up and running properly very clean. $\endgroup$ – Dan Clarke Jan 27 '18 at 15:43
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Assuming energy is cheap, you can always beam it.

For waste, again if energy is practical free ( like unlimited fusion power ), just incinerate it with a small electric arc furnace.

You would still need water lines. But you could store rain water on various levels of your buildings, recycling it on the way down. Letting gravity do it's thing. Or pump it to the upper levels in stages with holding tanks between. Clean water goes up, bad water goes down. Living on the bottom floors would be a real joy in that case....

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    $\begingroup$ You still have to vent incinerated stuff into the atmosphere. No future society with such a huge ecological footprint already would let you do that. Maybe a super-recycling machine that can split matter down into components would be more realistic. $\endgroup$ – Scott Whitlock Jan 26 '18 at 14:24
  • $\begingroup$ If you have unlimited power that is relatively clean, you can just filter the planets atmosphere and remove all those toxic elements. If you can produce unlimited power by fusion, then you can do just about anything (in a sci-fi setting) I know in the real, that even fusion will wind up producing secondary radioactive isotopes in the containment vessel -via- neutron bombardment and "what not" $\endgroup$ – ArtisticPhoenix Jan 26 '18 at 22:51
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Make recycling local.

In the sequence that frame comes from the action shortly visits some lower unused areas, that might be called the bowels of the city. I suggest that's exactly what they would be.

Transportation is pretty clearly close to capacity, anything you don't have to ship any distance would be a win. Water, air, power are all totally fungible and while they do benefit from scale the volumes required might make transportation costs cancel any benefits beyond a certain point.

If ten thousand people live in a tower you probably save on pipes and pumps by putting a moderately sized water reclamation plant in it than pumping stuff out to a huge plant miles away and then back in.

Especially if co-ordinating infrastructure is hard, like say the if service to the next tower over needed to be disrupted to connect your line, or you would have to pay to add the needed capacity to the municipal processing center when building a tower it might make sense to make them more or less self sufficient.

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The sky does not depend on the ground!

If you change your perspective there is another potential solution.

How do you manage a city that is miles WIDE... you don't put all your resources one one end and pipe them all from that location to every building on the grid, you distribute them THROUGHOUT your city.

A city that is miles high can extend that paradigm to the third dimension...

  • Every n stories you have a wastewater processing plant... why send it all the way back down?
  • Every few floors you have water collection, why wait for the rain to get to the ground?
  • Electricity is generated (wind, nuclear, maybe hydroelectric from falling gray water)
  • Telecom is distributed more or less this way even in moderately smaller buildings already.
  • Agriculuture is distributed on new modern buildings, at the "miles high" scale this could become farming.

This opens up fantastic opportunities for cultural and social differences as well.

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    $\begingroup$ ... except for structural support, of course. We don't want to piss off those groundlubbers too much. $\endgroup$ – immibis Jan 27 '18 at 0:10
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The Buildings Are Self-Contained

For an in-depth discussion I recommend the Isaac Arthur episode on arcologies - self-sufficient structures with all their utilities, and most of their own economy, built in.

In essence, an arcology is a space habitat constructed on the ground. Another way to think of it is a city in a can. Waste is recycled internally so water supply is not an issue, food is often grown internally (this only needs fusion power or sufficiently advanced bioengineering, not magic tech), and everything that can be provided inside the building is. You don't need Star Trek style replicators - just living space for the people who work there, a reasonable supply of commonly needed technicians, tools, and spare parts, and the equivalent of an Amazon fulfillment center in there somewhere.

Compared to a space habitat, an arcology doesn't have to be quite so self-sufficient, and at least it doesn't have to be airtight. But much of the design is similar, because you don't want too many people or goods coming and going. Just as you can live your entire life inside a city today without really having to leave, you would be able to live your entire life inside an arcology if you wanted to.

The essential problem with traditional skyscrapers at extremely large scale is that the consumption requirements of the building increase with the building's volume, but the ability to get material in and out of the building increases only with the building's surface area. Forget the view from the top floor - the entire outer shell of the building has to be a loading dock or a garage! In concepts that don't involve ubiquitous flying vehicles, everything going in and out of the building has to pass through not only the edge of the building but the ground floor, so the available rate of material flow is essentially fixed, and buildings can't get much bigger than they are now. We already have trouble with traffic getting in and out of skyscrapers during rush hour periods. The design of elevator shafts for supertall buildings is already a serious problem and designers are inventing new ways to deal with it. So you keep everything inside the building that you can.

A facility for disposal of waste heat becomes the main external utility that an arcology must connect to, especially one that has its own fusion power plant. Even present-day skyscrapers have to run their air conditioners in the dead of winter, and city centers are hotter than the surrounding environments, in part because of heat generated in large buildings. For arcologies, air-cooled heat pumps aren't going to cut it. Something like a molten sodium cooling loop would be more useful.

In some ways the construction architecture of arcologies parallels computer architecture. Early computers (say 1970s to mid-1980s) handled relatively little data and the CPU could communicate with memory and peripherals at its own speed. CPUs got faster and RAM cache had to be invented so the CPU didn't spend most of its time waiting on system RAM. Later CPUs got more and more levels of RAM cache of varying speeds because the cache needed its own cache. Eventually RAM and peripherals started to move directly onto the CPU itself and we now have integrated systems-on-chips including everything but the power and human interface components. Most of the actual space inside the chip is used just for storage or moving signals from place to place, not actual computations. And the essential limiting factor on performance is power in and heat out.

The development of arcologies can be evolutionary, not revolutionary. One example of something that's on the path to an arcology is the MIT campus. It's possible to travel almost anywhere on campus without going outside, like most universities it provides basic living needs to the students, and it even has its own power plant. Of course, it's not really self sufficient - they still need to bring in almost everything from outside - but it probably feels self-sufficient to the students that live there. Arcology-like architecture has also been proposed for the upcoming Amazon HQ2 - even if it's not contained in a single tall building.

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