How do underground civilizations effectively manage waste sanitation?

Question

AKA what do underground civs do with their poop?

Vast underground networks of caverns, tunnels and caves are a staple adventurers' playground in many classic tabletop RPGs.

To make for entertaining games, these are populated with random monsters, eldrich beings and whole cities of creatures like dwarves, mindflayers, drow and other, more exotic, creatures. The cities and towns of the underground are often surprisingly populous and numerous (again to make for a better story I suspect).

That's a lot of potential poop to deal with right there.

Modern engineers might struggle in this environment, but D&D and other tabletop RPGs often assume that cities are pre-industrial. It goes without saying that without adequate sanitation: disease will spread, precious cave real estate will clog up, and dangerous predators and scavengers will be attracted to the stink.

Question: What is the most effective way for a pre-industrial city of approx 10 000 underground dwellers to manage their waste?

Assume the additional environmental conditions:

• Access to the surface world is often difficult or dangerous.
• Light sources are often weak: luminescent fungi, eldrich glow, almost certainly no sunlight.
• Travel through the dark and winding paths is also dangerous.
• Magic is insufficient for the clean-up task.
• Water is limited to underground rivers and lakes (replenished by rain from the sunlit world).
  • Dumping waste in water is possible, but if it doesn't wash into your own water supply it will almost certainly head into your neighbors' - which leads to strained relations.
  • Additionally, there is no rain underground, so waste stays where it's left or spread.

Answer 1

Most likely the waste would be used right away in growing food. Mushrooms grow well in climate controlled environments on manure, which an underground city would have. Once one has grown mushrooms, the left over waste is still valuable in terms of fertilizer.

An underground city would still need more food than what could be gotten from growing mushrooms, being low in calories, fats, and lacking essential amino acids and vitamins, so the fertilizer could either be traded to the surface, as biosolids or used for year round production of whatever plants can be grown in the underground. The problem with the underground being light levels rather than temperature (as the temperature would be nearly constant). With sufficient light it would be possible to grow tropical fruits year round in a non-tropical environment using the heat from composting waste. Without trading with the surface for grains or the creation of a low light growing grain I am not sure how an underground city would feed itself.

Now since we are talking about a pre-industrial society then all the many uses of urine come into play as a cleansing agent, dye, making leather, gunpowder (if they have that), and so forth. Pee from cities used to be quite valuable.

Answer 2

The underground city dwellers actually face some of the same issues as a spacefaring civilization. They live in a contained environment with limited resources and little ability to intake or expel things. Like a space colony, we can actually solve both problems to some extent by combining them.

Underground city dwellers use a system similar to ancient Japan (where lower class individuals would collect waste from what amount to outhouses and used it as fertilizer). Dung carriers collect the waste and bring it to special caverns set aside as fungus gardens. Species of fungi have been carefully selected and bred for the purpose of efficiently converting waste into organic matter. This fungus is then either processed directly or fed to an intermediate stage animal like fish, and then becomes a food source for the city.

In short, they eat their waste.

Answer 3

Reduce the amount of poop by not pooping in the first place

This question is directly tied to the question of what do underground dwellers eat. Well...what do they eat? Mushrooms? Bugs? Each other? A true underworld ecosystem like a Menzoberranzan won't have many plants; in-fact I think moss is the only such plant mentioned in Forgotten Realms, as food for a underground herd beast. In any case, you are looking at mostly fungus, bugs, and cave seafood in your diet.

That's not all bad. Removing plants from your diet removes the need to digest a variety of strange things that are found only in plants (like starch, for example). Fungus and animals are more closely related to each other than plants, so the GI tract of underworld dwelling creatures could specialize in things found in just fungus and animals. Also relevant, is that if you eat fungus, insects, and crustaceans, most of the 'hard' material that you have to digest is chitin...develop a way to digest that and you are in business.

If your GI tract is very efficient, then you might not need to eat that much, and you would poop even less. So in addition to the other answers of the poo being used to fertilize underground gardens, it is entirely possible that there won't be that much poo to take care of in the first place.

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Edit for doubters:

Here is more poop knowledge than you ever wanted. You poop 1 ounce per day for every 12 lbs of you. So lets say the average person is 120 lbs, and poops 10 ounces. Now cut that to 2 ounces, and multiply by a city of 10,000. That is a 1250 lb a day. That really isn't that much. One dude with a shovel and a wheel barrow can move that much easily. 10 dudes could clean up a whole city, and move poop to gardens. I argue that there is a very plausible explanation of reducing poop to quantities that are easy to manage.