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Let's say a modern man is transported to a world that is roughly medieval and modeled very clearly off of medieval Europe. He manages to impress the son of a king and eventually gets the king to hear out his 'crazy' theories. The king is willing to experiment with sanitation, but is not certain he believes in it and is only willing to commit so much effort to it.

The modern time traveler explains the basics of germ theory and that most medieval diseases were transmitted via drinking water. In addition to explaining the advantage of boiling drinking water he suggests a sewage system. The problem is he doesn't know how to make one. He has a vague idea that modern sewers use filtration, chemicals, and UV light to clean our sewage, of those only the first seems remotely plausible given the tech available but he doesn't know much more about it.

So he suggests that a sewage system can be made to collect waste and drop it off down river of the city so at least they won't be drinking their own waste. He is far less confident about the ability to filter and clean out waste but he vaguely suggests he thinks they may have used sand and/or stones to filter waste before and perhaps they could figure out some way to do that.

Finally he happens to recall a bit about Miasma theory, the flawed theory that 'bad air' makes one sick, and that the first sewage systems were built based off of this theory. Thus while he does clarify that stench is more our bodies system of warning us of stuff that may make us sick rather then the source of the sickness he does suggest a general rule of thumb that if a sewer system keeps the stink of human waste away from the human nose it's probably protecting from disease.

Given this very vague and limited explanation the king pulls together his top experts and asks "think you can do something based off of all this nonsense the supposed future knowledge stranger claims will help? Also I'm only toying with this idea to humor my son's belief in the loon and the potential to maybe make our cities not reek so much, so how cheap could you make this whole sewer idea?"

My question is basically can an effective sewer be made given such limited description at a cost low enough that a king not entirely sold on the idea would be willing to sign off on it? If so can you give me a rough idea of how significant the cost would seem compared to the available budget of such a king and how much of a difference would it make in spread of disease to a city that built one? Would the results be noticeable enough to prove the efficacy of germ theory and/or sewers to the skeptical king?

Less relevant, but related, question would be where the fist sewer would be built. Ideally it would be in the capital, but would a modern already built congested capital city be able to fit in the new construction and sewage system? If not what type of city might be an easier place for a first experimental sewer to be built?

Please note I realize that pre-medieval sewers existed in our world, but this world isn't exactly identical to ours. Either sewers were not discovered in this part of the world or they were forgotten, much as roman sewers were neglected. The point is there is no historical example of a fully functional sewer available to draw ideas off of.

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    $\begingroup$ I do not understand this question at all. Obviously, at some point in history there were no sewerage systems. Equally obviously, at some later point in history there were sewerage systems. This means that at some point in history people figured it out all by themselves, with no need for a Great White Saviour to inspire them. Moreover, the point when people figured it out and build the first functional sewerage systems was a loooong time before the Middle Ages, with a lot more primitive technology. (No, don't believe the propaganda. Technology did progress during the Middle Ages.) $\endgroup$
    – AlexP
    Mar 8, 2023 at 20:02
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    $\begingroup$ P.S. The main problem with extensive sewerage systems is that the are expensive to build. Very expensive. Medieval people were not stupid; they were poor. For example, the oldest (quite short) underground sewer in Paris was built in 1370, which is definitely medieval; but expanding that to cover the entire city and connecting the houses to it (which is rather the entire point) was way beyond their financial capabilities. $\endgroup$
    – AlexP
    Mar 8, 2023 at 20:14
  • $\begingroup$ Hi @dsollen, welcome to WB SE. Please reduce this to one question or it will get closed, that's how we operate here. This question is good and interesting and it would be a shame if it got nuked. $\endgroup$
    – user86462
    Mar 8, 2023 at 21:12
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    $\begingroup$ @AlexP There's a good case that a dedicated, deliberately engineered sewage system could cut out many unnecessary centuries of sewage related diseases. My Nelson example shows what knowledge can do even in the face of very limited resources. $\endgroup$
    – user86462
    Mar 8, 2023 at 21:17
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    $\begingroup$ @AlexP Too expensive to build even today. 20% of the US and France, and 27.5% of Ireland use septic tanks, according to Wikipedia. The cost-benefit doesn't justify a sewer system, even in some suburban locations. $\endgroup$
    – user71659
    Mar 11, 2023 at 7:17

6 Answers 6

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Sewers were not invented to get rid of poo, they were invented to resist flooding. Getting rid of poo is just an add on.

So, depending on the geography of your city, there may already be a sewer system. Basically, if there is a likelihood of flooding, it doesn't even need to be common, it just needs to be a reasonable concern, than the water might sweep away the buildings that make up the city, and even if it doesn't, if the water has no place to go it will quickly spread disease. So, you build a place for the water to go! Hence the invention of the sewer.

Which actually gets into another point. People know that waste is bad.or at least dangerous. They don't necessarily know exactly why, but they know it is. Additionally, depending on the time period, if your character tries to explain sanitation he is more likely to be called a crazy reactionary than a mad revolutionary, because sanitation is intimately linked in most cultures to religious rights. It's likely that he will be trying to convince people to do things they already associate with priests and other ritual orders that are much older than the city itself, while new issues with disease are typically not so much a result of stagnation, as a result of unchecked growth. Like, the most devastating dusease out breaks in history were not caused by people not washing their hands (though that certainly made them worse) but by exploration and trade. On a less grim note, the river thanes didn't get the big stink until the industrial revolution filled London with far more people than the ancient city could support, and the ganges didn't get full of trash till the 19 and 20th century.

Basically, most of the suggestions the character makes might not be nearly as radical as you might think. And they will tend to seem less crazy the less "advanced" the civilization is

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    $\begingroup$ Maybe, although if the country is Christian then the religious aversion to waste is lessened. While I understand (and endorse) the Christian lack of concern with ceremonial cleanliness*, when it comes to sanitation it was a downside. (* = Christians have never had the same focus on external cleanliness as Jews or even Muslims, due to Jesus' famous rejection of certain cleanliness rituals. The exception would be in a medieval church, particularly during Mass, where ritual cleanliness really was a thing. But even that was framed as a contrast with the world.) Upvoted anyway, worth considering. $\endgroup$
    – user86462
    Mar 9, 2023 at 8:51
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    $\begingroup$ Prior to the invention of flushing toilets, and the availability of other options, poo was too valuable a fertilizer to simply wash away $\endgroup$ Mar 9, 2023 at 10:08
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    $\begingroup$ @AncientGiantPottedPlant true to an extent, but they still knew it was icky, and they knew it was icky for a reason, even if they didn't know what that reason was. $\endgroup$
    – user102593
    Mar 9, 2023 at 12:19
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    $\begingroup$ @JackAidley this is also true. Plants need nitrogen, and if you don't have chemistry than the best way to pipe nitrogen to your wheat is with either your waste, or animal waste. And this typically only because a water problem where getting very obviously too big or too close $\endgroup$
    – user102593
    Mar 9, 2023 at 12:21
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    $\begingroup$ The religious sanitation really nails it on the head. Many civilizations already developed very effective sanitation practices for thier regions long before germ theory. They knew most of the whats, just lacked information on the whys. $\endgroup$
    – Nosajimiki
    Mar 9, 2023 at 20:07
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They can surely dig a trench

In one New Zealand city that I know well, Nelson, they had a simple trench because they couldn't afford a sewer during a long lean post settlement era. It drained sewage to a river mouth, leaving clean water upstream.

The trench was notorious because for many years it was open and drunks would fall into it and drown or die of infection.

This was from 1840 until 1870 (?) or so! Compared to having sewage running down the streets, it was still a huge improvement.

Take a test village (or wealthy city street) with the correct geography that is administered by your enthusiastic prince, and install small pipes down to a big trench with wooden covers. Bring in water using an aqueduct or pipe from a higher elevation or just take it from an upstream point of a river. Dump the sewage in the sea or a lower elevation river or a downstream point of the same river they get water from.

Make soap plentiful and provide some warm baths too if you feel like it.

That part of Nelson has very expensive real estate now, FWIW.

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    $\begingroup$ And really that's probably the low end: trench, optional cover. For wealthier cities, see "Roman sewers". $\endgroup$
    – JamieB
    Mar 8, 2023 at 21:20
  • $\begingroup$ And even as recently as the1950's-60s in the first world, there was often no filtration or anything similar. All the sewer system did was ensure that the waste was a contained flow, separated from drinking supplies, that exited into a river or sea (prefferably well downstream from town). In medieval times, even if nobody knows how to build large diameter pipes, you could achieve most of this with trenches and open/covered drains. Not perfect but 'good-enough', $\endgroup$
    – Penguino
    Mar 9, 2023 at 0:49
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    $\begingroup$ All of New Zealand has very expensive real estate now. Worst in the OECD. $\endgroup$
    – user253751
    Mar 9, 2023 at 15:20
  • $\begingroup$ @user253751 Yes, it's unreal. All the same, the area where that trench was is now Nelson's best commercial zone. $\endgroup$
    – user86462
    Mar 9, 2023 at 18:32
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Open Sewer

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The simplest type of sewer is the famous Tudor style open sewer in the middle of the street and people throw the contents of their chamber pots out the window.

Make it sloped like modern sewer pipes you can drain the effluence into the outskirts of the city. Perhaps assisted by rain, the peepee and poopoo goes into big compost heaps and eventually fertilizes the nearby farms.

There is no need for UV treating or recycling waste water. There are fewer people around and more water from the river available. Though you might try to filter the water from the river somehow.

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    $\begingroup$ if possible, channeling some river/stream water to always flow through the open sewer would massively improve its "flushing" capabilities and reduce the stench enormously $\endgroup$
    – Hobbamok
    Mar 9, 2023 at 10:35
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If you want to get through to this king in that period you're better off starting with the army.

European kings tended to be at war with each other on a fairly steady basis and one of the great limiting factors of the army was how many died of disease while on campaign. You can start with the much simpler option of ensuring the army digs a "long drop" well away from their drinking water supply while in camp. Keeping the army healthier for longer will go a long way to getting you in vogue with a king.

Once you've successfully demonstrated the effectiveness of this, you might have some progress with getting the funding for a deliberately constructed sewer in a city, but remember that didn't really happen in Europe between the Romans and Bazalgette (the Indus civilisations had sewers 2000 years earlier than the Romans).

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As described by others, getting a pre-industrial sewer built is not a problem at all. Which leaves the question of filtering the sewer water, and this can also be done with pre-industrial technology.

What you do is run it through a wetland. If your city is on a river, it would already have wetlands in that river's flood plain which, due to being regularly flooded, is not built upon (or at least not by anybody important). On that floodplain you can construct a system of shallow ponds, and drain the sewer into them. The nutrient-rich water will support lavish plant life. Crucially for your local economy you would have reeds, useful for thatching and potentially forming an export industry if they are not used within the city itself, and regular algal blooms providing plenty of food for carp (it must be carp, because it can surface to breathe air; other fish would suffocate). You can also have a sluice system allowing each pond to be drained every few years; the muck from the bottom makes incredible fertiliser, and boosts local agriculture accordingly.

There is plenty of historical precedent for all of these. Wetlands very much like those described above have been built by many European cities, and even without sewers emptying into them, fish ponds were lucrative enough to be granted as privileges to monasteries and the like.

And on a last note, you would not need (or want) to discuss such matters with a prince. Why even bother when the city magistrate is more accessible, probably already in favor, and unlike a prince has the power to make such decision and the resources to carry it out?

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This question shows a common (and erroneous) belief about the middle ages

This belief stems from the the Victorian Era, and the "age of enlightenment" which was smellier, dirtier, sicker, and overall less enlightened then the "dark ages."

It is true that medieval people didn't understand germ theory, however they did not wallow around in puddles of excrement, and had come up with a fairly good theory of miasma (bad smells cause sickness, which is not wrong... Sewage, Rotting food, and many mold smell, and cause sickness). Many cities had sewers, or other waste disposal alternatives. Because people don't like the smell of shit. Smaller municipalities used latrine pits that were slowly moved around. And the waste products composted and eventually used for fertilizer.

There were open and closed sewers (Some of which as they aged started leaking sewage into the ground water, this was more prevalent in the Victorian era, then the medieval period). There were also jobs built around waste handling. Dung carts, bintman, rakers... Human excrement it turns out (And the excrement of other creatures) was valuable, It was turned into fertilizer. Used by tanners for curing leather.

So the answer to the question is:

Your man from the future doesn't need to teach those poor unenlightened savages about sewers... They know what sewers are, they build sewers, and maintain them. They have legal disputes, lawsuits, and fist fights between neighbors who have disagreements over the others improper disposal of waste. (Lots, and lots of cases).

It also shows another mistaken belief

"Most medieval diseases were transmitted thru water".

Nope, cholera, and dysentery were transmitted thru water. And don't get me wrong, those are pretty awful, but influenza, mumps, measles, rhubelia, avian flu, swine flu, the pox, smallpox, the common cold, and tuberculosis, are not waterborne diseases. Some of these can be transmitted thru water, but their primary mode of communication was thru contact with infected individuals, insect bites, unwashed clothes/utensiles, unwashed surfaces... etc.

These can be slowed down by better personnel hygiene (hand washing). But not stopped.

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    $\begingroup$ This overlooks the fact that there was a fairly massive decline in the quality of sewer systems (and just about every other form of civil engineering) after the peak of the Roman Empire. Roman-quality sanitation works did not reappear in Europe until the early 1800s. $\endgroup$
    – Mark
    Mar 9, 2023 at 23:13

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