In a vaguely Medieval setting with some limited magic, how would an underwater city handle sewage?

Sanitation was one of the biggest limiting factors in the growth of human cities, and being under water is going to make the usual ways of removing sewage less feasible.

Some points:

  • These cities are built in areas of weak currents.
  • Magic exists but isn't all powerful.
  • Magically creating currents on a city scale would cost more then the rulers can/want to spend.
  • The inhabitants can breathe water and are mostly humanoid.
  • Technology is limited to both the time and the limitations of working with extremely limited metal usage.
  • This city has defences that would interfere with usual flow of water. There are other, hostile cities. Walls, wall-roofing and other defensive obstructions are present.
  • 6
    $\begingroup$ One imagines it would work exactly the same way fish currently "handle" their sewage. Or the way air-breathers "handle" their carbon dioxide output. One organism's output is another organism's input. Your city will be a bit hazy from the algae. $\endgroup$
    – user535733
    Commented May 21, 2019 at 13:40
  • 1
    $\begingroup$ How did actual medieval cities handle sewage? Let's say, for example, medieval London. $\endgroup$
    – AlexP
    Commented May 21, 2019 at 13:47
  • $\begingroup$ @AlexP as far as I understand, the sewage was dumped into the river by the way of the open and closed ditches. The river went out to the sea, it didn't bother population much after that. This system relies on strong currents and gravity assist. $\endgroup$
    – Cumehtar
    Commented May 21, 2019 at 13:58
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    $\begingroup$ (a) Are you asking only about sewage, or all forms of sanitation? (the latter might be too broad, so I'm hoping for the former). (b) How many people, on average, are in a city? (c) How far underwater are they? Please measure from water surface to the ground. (d) A weak current is mighty powerful when a lot of water is involved. What, exactly, is a "weak current?" (e) Most natural water isn't all that clean to begin with. After all, fish poop when they need to, current or not. How "clean" must the water be? Said another way, how much sewage/sanitation are we moving and why? $\endgroup$
    – JBH
    Commented May 21, 2019 at 16:38
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    $\begingroup$ Possible duplicate of Atlantis and Sewage $\endgroup$
    – Willk
    Commented May 22, 2019 at 21:12

3 Answers 3


Bottom feeders

A bottom feeder is an aquatic animal that feeds on or near the bottom of a body of water... In the aquarium, bottom feeders are popular as it is perceived that they will clean the algae that grows in the tank. Generally, they are only useful for consuming the extra (fresh) food left by overfed or clumsy livestock https://en.wikipedia.org/wiki/Bottom_feeder

Algae will consume excreta. Bottom feeders will consume the algae. They will also dispose of food waste. Humans can eat many of the bottom feeders but must of course maintain stocks.

  • $\begingroup$ So, farming and animal husbandry? ) $\endgroup$
    – Cumehtar
    Commented May 21, 2019 at 14:31
  • $\begingroup$ You actually mean detritivore, which is one specific type of bottom feeder. There are many bottom feeders that won't eat garbage, such as many of the "clean" fish that people like to eat: halibut, flounder, plaice, sole, cod, haddock, bass, grouper, carp, and bream (snapper). $\endgroup$ Commented May 23, 2019 at 13:42

Two factors are important here

  • one, there are consistent currents, strength is less important than consistency.
  • two, sea floors are not, generally, flat on a large scale.

Taken together this means that getting sewage downhill and/or down current away from a city should be possible and even practical making it "someone else's problem" which is what medieval cities traditionally did and to some extent what we still do to this day.

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    $\begingroup$ A gentle, consistent breeze will rapidly disperse even the most malodorous of smells, but occasional gusts in an otherwise dead calm will leave you pinching your nose. $\endgroup$
    – Joe Bloggs
    Commented May 21, 2019 at 15:20

Most people pee while swimming anyway, and from working at a pool that had 2000+ patrons most people do not not mind swimming in literal piss water. Poo on the other-hand tends to turn people off while swimming. Perhaps a siphon type system can be rigged up, no magic required, just poo into a constantly sucking pipe, or a pipe that requires a few actuation of a hand/ foot pump to create the suction. The pipes can lead out of the area, or into a deep sea crevice. Out of sight out of mind, especially for the medieval types.


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