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While worldbuilding my merfolk society I've been thinking about what method of waste disposal they would use. I've come up with a few ones that I will describe, and ask how practical they are, and if there are any problems with them that could need fixing.

The merfolk can only stay above water for around half a day, and they have some contact and trade with humans who are technologically around classical antiquity.

One method is one used by merfolk commoners, who might use a large emptied gastropod shell as a "chamber pot" and another seashell as a cap to prevent the contents from leaking out. The shell would be emptied into a landfill where fish and various invertebrates eat the waste, or poured onto cultivated sea plants as fertilizer.

The other method is used by royalty who live in a massive underwater palace that has been constructed both using magic and items imported from humans. One room in the palace serves as a "restroom", the toilet is a hole connected to a tunnel that also leads into a waste pit where the waste is eaten by sealife.

Are there anything that needs to be taken into consideration with my methods of waste disposal?

edit: the merfolk could easily live near an ocean current they could utilize to carry waste away.

Biologically, my merfolk are closer to large carnivorous fish like sharks than mammals. But from videos I've seen, dolphins and sharks both release their waste in a misty cloud so it doesnt seem to be a strong difference.

Also, could the merfolk utilize both ocean currents and domesticated bottom feeders, such as currents carrying waste someplace where bottom feeders can feed on it, thus solving the problem with toxins polluting the environment?

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    $\begingroup$ Is the basic premise that merfolk leave the water to go to the bathroom? If not, you seem to assume that waste always sinks. $\endgroup$
    – JonSG
    Aug 9 at 14:48
  • $\begingroup$ No. The merfolk cant leave water for long and it wouldnt be practical. $\endgroup$ Aug 9 at 14:49
  • $\begingroup$ Please succinctly describe their level of magical attainment. Basically, what can magic do for them and how difficult or finnicky is the magic? I.e., could they ensorcell that little tunnel to create a slight current that would draw poo away from the toilet room? How secure is their magic and how often would a merman get his cloaca stuck to the tunnel opening if the magic goes a little awry? $\endgroup$
    – elemtilas
    Aug 9 at 16:34
  • $\begingroup$ My merfolk do have some magical power, but their magic is specialitet in manipulating coral, causing it to grow at a much faster rate as well as control its shape. But your idea of ensorcelling a current is interesting. $\endgroup$ Aug 9 at 17:04
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    $\begingroup$ One more question: Is merfolk biology closer to fish or to humans? Excretion is different for humans and fish. Please edit your query to include all clarifications and additional details. Comments tend to disappear and not all people read all of them. $\endgroup$
    – Otkin
    Aug 9 at 19:30
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The biggest issue I see is waste buoyancy.

Urine can be buoyant neutral and can hang around in a cloud for a while. Anyone with a pool and young kids can confirm that :-P

Feces can sink or float or also be neutral. Some people tend to have sinkers some floaters and my fish seem to squirt out a more neutral stew.

The concept of a "landfill" or gravity assisted waste management will not work underwater (I think).

What might work though is merfolk civilization based around strong ocean currents where waste might be discharged. The rich and powerful being up-current from the fertile sea beds inhabited and tended by the "less fortunate".

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  • $\begingroup$ Yeah its true, but I really want it to work, since the idea of keeping sealife for waste disposal seems pretty ingenious to me, as well as using the waste as fertilizer. $\endgroup$ Aug 9 at 15:18
  • $\begingroup$ "down flow" from a settlement could still be "nutrient rich" :-) $\endgroup$
    – JonSG
    Aug 9 at 15:19
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    $\begingroup$ Yes, just as some human discharge points can be teaming with sea life. While others (like the Mississippi River) can be over fertilized resulting in death zones due to rampant plant overgrowth leading to hypoxic conditions for creatures. $\endgroup$
    – JonSG
    Aug 9 at 16:01
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    $\begingroup$ @Otkin I think it will be an issue if one wants to pee and poop into "something" (gastropod shell) and then have that something empty into a "landfill". Both suggest that buoyancy and consistency of waste are rather important. Unless the merfolk catheterize themselves to go. $\endgroup$
    – JonSG
    Aug 9 at 18:44
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    $\begingroup$ @JonSG I said that buoyancy is not the main problem (does not mean that this is completely unimportant, though, just not the biggest problem) because trapping debris is rather trivial (walls or sponges will work just fine if there is current). I see toxicity as the biggest problem because civilisations distort natural ecosystems. IMO, urban environments will require supplementary filtration systems to avoid toxic buildup. Currents do help. But without proper planning cities will have 'dead spots' where all kinds of waste accumulate. $\endgroup$
    – Otkin
    Aug 9 at 19:06
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Scale is likely to be a really important aspect of this. If the population of this merfolk society is on the large tribes range, say 30-150 members a sustainable solution would not be too complicated. However if we are talking modern city's density of 100-200k or more then a lot more serious infrastructure is going to be needed. Likely in the latter cases waste would be used and recycled, and not just dumped in a landfill/stream equivalent.

A simple practical solution could be a variation on real life building air ventilation combined with water supply/disposal. The most basic form of a toilet would effectively be a shower under water. A small room would have a grate on the floor. A slow but steady flow of water would get sucked down this and piped away. A more advanced version might have a source of fresh water at the top of this "shower" instead of using the surrounding water.

With any degree of large scale the waste water would be treated and reused, extracted solids used as fertilizer or food for edible fish.

If the scale is small the water processing part could likely be skipped and shower/toilet water would just be discharged from the pipe some place away from people.

Depending on how it would fit in your story this system can be expanded to be used in the same way our modern buildings ventilation system work, but with water. Fresh clean water is piped in to a living space and another vent to extract stale water. If water oxygenation is a issues in a large city this system would help with that also.

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Waste Utilization:

If you have mer-agriculture, then you can have mer-fertilizer. Don't worry about collecting solids; your merfolk probably look like they have diarrhea. Somewhat the opposite solution for many of these answers, you co-mingle agriculture and cities. A column of plants rising to the surface in a current-protected cylinder (down-current to carry away leakage) is a daily destination for your well-fed merfolk. Humans do the same thing ("night soil"), but it's easier for us since we can collect it in buckets. It might very well be a social function as well, as everyone gathers in a few spots to regularly do their business.

If everyone does it at the same time, the waste would have a chance to be absorbed/settle/wash away during the rest of the day. Occasional flushing (of the fertilized area with opened current) will keep down the growth of undesirable algae and microbes. I'm still guessing down-stream is not going to be valuable real estate, but it might be useful for agriculture requiring fewer nutrients, and also attract animals, which could be a useful side-effect.

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None of the solutions that we use on land will be applicable for merfolk in a described setting (classical antiquity technology, 100-200m below sea level). Waste management should be based on water filtration to prevent the spread of toxic products of waste decay. Most importantly, all solutions should be 3-dimensional because in aquatic conditions waste and products of decay may stay in the water column.

All waste that a merfolk settlement will most likely produce can be roughly divided into 3 categories:

  • inorganic matter that does not dissolve in water (or does not dissolve quickly enough to make rubbish a non-issue);
  • organic waste (all organic matter excluding excrements);
  • excrements.

1. Inorganic matter

These include things like pottery, stone, and metal. Some of them will eventually degrade (some metals) due to saltwater corrosion, some can last for centuries (stone).

This type of waste can be buried in a marine equivalent of a landfill. However, it is a bit too wasteful. A better solution would be to break things into smaller pieces and use them for filtration (more about it below). Porous stone is one of the best natural filtration media as it has a large surface area where beneficial bacteria can live.

Metal is a bit harder to deal with. Perhaps, merfolk can collect scrap metal and sell it to humans. If this is not an option, just bury it somewhere. Please note that copper is highly toxic to some aquatic animals, for example, crustaceans (shrimp). If bronze corrosion can lead to increased concentrations of copper it should be considered in your urban and waste management planning. Most likely it is not a problem, but I would suggest verifying this point (I am not a chemist, please ask someone more qualified).

2. Organic waste

Organic waste becomes a problem only when merfolk settlements grow too big and natural filtration mechanisms fail to keep up.

Organic waste in water can become a toxic nightmare, especially when water flow and biological filtration are not sufficient. Cities must be planned to avoid 'dead spots' where water hardly moves and debris accumulates. All organic matter decays underwater and produces toxic compounds. Ammonia is one of the most dangerous for aquatic life.

Some of the approaches to managing organic waste would be:

  • build homes with good water flow (this also might be a necessity if merfolk biology is sufficiently close to sharks and they have to sleep within currents to keep breathing);
  • plant live corals on all walls (corals receive nutrients from the water column);
  • keep bottom-dwelling fish and crustaceans as waste cleaners (can be done in a form of a farm or housepets);
  • build filter chambers filled with edible algae and small aquatic animals (imagine a waste bin with walls made of coarse filter material [remember those stones and pottery?]; water should flow through the walls for filtration to work);
  • plant fast-growing aquatic plants inside settlements and around them (most aquatic plants can receive nutrients from the water column; you need to research what plants can thrive at your desired depth).

For public buildings which produce more waste, it might be necessary to supplement the abovementioned options with off-site facilities. Something like a big coral garden (perhaps, inside a cave with complicated currents) with fish and small animals would work wonders. Alternatively, waste can be used to fertilise farms (think 3D, make sure you have mechanical filtration around farms).

It is very important to remove organic waste promptly. Organic matter decays very fast (in a matter of hours) and turns into a slimy substance that is hard to transport or clean. In aquaria, this decayed matter is usually removed with the help of siphons. But I do not think that it is a plausible solution in your conditions.

3. Excrements

First of all, you need to decide whether merfolk share human sensitivities when it comes to urination and defecation. Humans perceive excretion as a dirty activity that should be done privately. This is not necessarily the case for other beings. It is also not clear if merfolk are capable of regulating the excretory system the way humans do. In other words, I am not sure if merfolk can hold long enough for a trip to the toilet.

If merfolk need privacy, the best solution would be fish gardens in small chambers or caves. Shark faeces are semi-liquid and contain some half-digested food. Fish will eat this food. The rest is a perfect fertiliser for corals.

I did not find a lot of information about shark urine (because different shark species manage urination slightly differently; and yes, most sharks urinate), but most marine fish produce highly concentrated urine that is highly beneficial for corals, algae, and other tiny aquatic organisms.

It might not be very acceptable from a human point of view, but the smartest solution for merfolk would be having 'vegetable gardens' instead of toilets. Excrement collection is not a very feasible option, and filtration would waste a lot of nutrients. So the best option is to fertilise your own garden.


This answer is already very long, but it still may be missing some details. Feel free to ask in the comments if you need me to elaborate on something or to supply additional information.

I would also encourage learning about aquarium filtration and natural aquatic ecosystems to have a better understanding of various processes happening in aquatic habitats.

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  • $\begingroup$ Thanks! I think your solutions are the most practical and make the most sense. However, I don't quite understand where the filter Chambers would be located and how they could work. $\endgroup$ Aug 12 at 16:23
  • $\begingroup$ @BoaHancocklover Filter chamber can be a small closet in smaller households or something like a shed for bigger ones. It must have current passing through it for filtration to occur. Take a look at sponge filters to understand the principle. The ones used in aquaria are air-driven, which is not accomplishable in your setting. However, if you can make water flow through them you will still have the filtration effect, just at lower efficiency. That's why they should be bigger to increase the surface area for beneficial bacteria. Add shrimp to avoid algae problem. $\endgroup$
    – Otkin
    Aug 12 at 20:16
  • $\begingroup$ The walls of the filtration chamber should be made of crushed stone and pottery that are held together with a net or something similar. They can be glued together as well. But if you choose this approach you still need to make sure that filtration media is not compressed and permits water flow. These walls will work as sponges in sponge filters. Walls will provide both mechanical (filter out debris) and biological (remove toxins) filtration. $\endgroup$
    – Otkin
    Aug 12 at 20:20
  • $\begingroup$ I think my merfolk could easily weave nets made out of kelp, but not sure if it would be strong enough to hold together the building materials, and I have some problem visualizing how it would look/be built. Would the crushed stones and debris that make up the wall be held between two nets that are like a tennis net? $\endgroup$ Aug 12 at 20:33
  • $\begingroup$ @BoaHancocklover Yes, something like that. Kelp will work only if it is strong enough and alive (but it is a feasible option). Dead kelp will decay fast. I imagine this chamber as an artificial cave with a small opening on one side which also serves as an entrance for water current. The same opening is used to dispose of organic waste. $\endgroup$
    – Otkin
    Aug 12 at 20:38
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Copied from my other answer

Many sponges, otoh, already look like tubes or barrels - why not use them? Dead, they’re a bit too brittle (although there may be suitable species) and alive, the micro-organisms in their walls produce a constant water flow through the walls and out of the mouth of the tube - spitting out anything you put inside: not ideal! However, if you carefully grow/train your sponges to point away from your settlement, then pile up your waste just outside them, they will actively filter it through their walls (eating up anything worth metabolising) and then conveniently funnel it out into the deep. Assuming your merfolk’s waste is pretty juicy, I can imagine you’d be able to cultivate and sustain abnormally long sponges that act almost like pipes.

They may select different species and configurations for different types of waste (high-nitrogen organic waste may be routed towards pasture/agricultural land, inert waste from industry funneled into ravines). They may have public latrines feeding into the “pipes” while industries with noxious runoff may be restricted to downstream areas with stronger currents.

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