Help! I've been transported back in time to London in 1303 AD. After saying hi to the flock of other worldbuilders who have inexplicably travel back in time to Medieval Europe, I decide to set out to get something to drink. Luckily, there are plenty of things to drink in the middle ages.

Three days later, I'm fed up with the fact that all of those things are beer. I'd like to obtain some clean water for drinking and cooking, but the city I'm in is filthy and nobody has any idea why feces and drinking water should be kept far apart. With the rest of the worldbuilders off trying to prevent the black death from happening or giving leaders modern military hardware in order to sway the outcomes of pivotal battles, I'm left to solve my water worries on my own.

What's the best way for me to obtain fresh, safe drinkable water in a medieval city? What sort of technology or resources should I gather to make sure the act of drinking doesn't lead to my horrific demise to one of the many, horrible, diseases that are doing their best to kill my new neighbors?

I'm also a fairly empathetic person, so bonus points of the solution is scalable to the whole city.

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    $\begingroup$ Is merely boiling the water insufficient for making it potable? Also, people in medieval times knew to avoid mixing water and feces. If it happened, it was usually due to the richer folk who were up-river from them. $\endgroup$ – Nicol Bolas Apr 13 '16 at 1:30
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    $\begingroup$ Mix with alcohol; fermenting anything at all that has sugars, even grass, can kill most of the germs. $\endgroup$ – Mikey Apr 13 '16 at 2:39
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    $\begingroup$ Rainwater catchment. Does it rain in your city? $\endgroup$ – Mikey Apr 13 '16 at 17:42
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    $\begingroup$ It's a myth that people drank mostly small beer and kept shitting and pissing in their water sources. In fact, London even had the Great Conduit built in 1245. The best way to get clean water would be to drink what everybody else is drinking. $\endgroup$ – isanae Apr 14 '16 at 4:30
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    $\begingroup$ @JackAidley not necessarily - the problem with milk is more one of containment, if you put it in a bucket and leave it for a while, or the udders are not clean, then you get the contaniments that are the main cause of illness from it. The latter is more of a problem than many people realise. $\endgroup$ – gbjbaanb Apr 14 '16 at 7:46

11 Answers 11


While distilling leaves you with absolutely clean water, it is very costly. The constant need of burning material is a real problem if a bigger amount of water is required.

But fortunately, there is another method that is way cheaper.


Every citizen can build his own water filter, with gravel, sand, charcoal and some cloth:

Small water filter

This filter clears out most of the dirt and leaves you with clean water. On a bigger scale this might look like this:

Larger water filter with rainwater intake

To use it, simply pour your water through a few times. The water you get will be very clear, though still contain bacteria. To get rid of those, boil it for about three to five minutes. While you still need burning fuel for this, it is much more cheaper to boil it for a few moments than condensating it completely. Another advantage of boiling collected water instead of full distillation is, that the minerals that the water has collected won't get lost during the process.

Bonus: Rain-water

If using rainwater like in the second image, the water should be relatively bacteria-free and drinkable from tap. Rain-water is pretty similar to distilled water. After condensating as clouds, collected rainwater has had its only opportunity to get dirty while falling down to earth, picking up whatever is in the air. Back in the day, where chemical air pollution wasn't a big thing (usage of coal has been prohibited in 1273), that is no problem for you.

Finished concept

So, the final plan for your city-wide water supply would be to provide every household with a big water barrel that collects and filters rain-water collected from the rooftops. For the case in which the collected rain-water does not suffice all your needs, you can filter and boil water you collected from wells or streams.

Also, as T.J. Crowder mentioned (thank you!):

After 2-3 weeks once the hypogeal layer has formed, you probably don't need to boil the result. Note that it's necessary to renew that layer periodically as it gets too thick (I don't know how often; not all that often is my impression).

As he mentioned, after a while a biofilm will form.

The surface biofilm is the layer that provides the effective purification in potable water treatment, the underlying sand providing the support medium for this biological treatment layer. As water passes through the hypogeal layer, particles of foreign matter are trapped in the mucilaginous matrix and soluble organic material is adsorbed. The contaminants are metabolised by the bacteria, fungi and protozoa.

So you are not even required to boil your water anymore. But be careful, as you need to renew the layer every now and so often, as

Slow sand filters slowly lose their performance as the biofilm thickens and thereby reduce[…] the rate of flow through the filter.

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    $\begingroup$ My grand-parents used the second kind of filter (filling it with layers of sand and charcoal) as their main source of water, the second source being a well, until the late 70's. Rainwater passed through it and was stored in a water tank with a tap. (Sorry I couldn't resist adding a personnal story). $\endgroup$ – Babika Babaka Apr 13 '16 at 7:50
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    $\begingroup$ Thanks for sharing! This is not just a personal story, but a proof that this method is perfectly applicable in real world use $\endgroup$ – T3 H40 Apr 13 '16 at 7:53
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    $\begingroup$ I thought about rain water and filtering, but unfortunately the question specifies a city, which means there is pollution. Everyone was constantly burning something and then there are normal issues of roofs, bird shit and wind swept dirt... As for filtering, charcoal would be available, but the soil in the cities would have been contaminated, so a visit to the countryside would be required. Everything would be so much simpler, if the character would just leave the city... Not like medieval cities were that large and extensive. Maybe I should put leave the city as answer? $\endgroup$ – Ville Niemi Apr 13 '16 at 11:18
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    $\begingroup$ @VilleNiemi most of Bird-shit and the wind-swept earth should be filtered by the soil/charcoal/cloth... The bits that remain after that should not be much of a problem. If you want do be on the safe side (this also applies to the contaminated city soil), boiling the resulting water will kill any bacteria left. $\endgroup$ – T3 H40 Apr 13 '16 at 11:53
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    $\begingroup$ After 2-3 weeks once the hypogeal layer has formed, you probably don't need to boil the result. Note that it's necessary to renew that layer periodically as it gets too thick (I don't know how often; not all that often is my impression). $\endgroup$ – T.J. Crowder Apr 15 '16 at 7:08

A fire heated distiller like Ville Niemi's answer is a good idea, but if the fuel and constantly tending the fire aren't practical, you could try a solar distiller. Shouldn't need any space-age materials, just some glass & metal (avoid lead solder!) or wood containers to hold water in.

Solar still imageThe Solar Water Still Challenge

The guy in the videos on this page got 3.13 cups of water per day from 3.3 sq.ft of "glass area," or about a cup of water per day per sq.ft of glass.

Actually, if you're worried that glass is too expensive or scarce, you can make an old-fashioned solar oven with some shiny metal. The above design might work with a dirty/dark piece of metal on top, it would get hot and indirectly transfer heat to the water, but probably not as efficiently.

A solar oven would be perfect to put a regular distiller-type pot of water in, with the tube leading out to the "distilled water" container (just like a regular stovetop distiller, but no fuel or fire. Similar to this:

solar oven distiller

Even a basic box with a shiny interior & lid may be good enough: small solar oven

Or a more basic "hole in the ground" still (forum link) like this may work too, you may not have access to clear plastic sheets, but a clean dark cloth or tarp may work also (and you don't have to worry about space-age plastic chemicals leeching into the water either).

enter image description here

If there's enough space around people's homes in the city (no high-rises then, but I'm not sure a medieval roof would support a tank of water) they may each be able to have their own solar still & make their own water, no fuel or fires required.

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    $\begingroup$ Getting sufficient quality of glass is going to be very difficult for this idea. $\endgroup$ – Jack Aidley Apr 13 '16 at 16:05
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    $\begingroup$ @JackAidley maybe so, I'm not sure on the cost of glass in 1300, plate glass windows definitely existed, and "The 11th century saw... new ways of making sheet glass by blowing spheres." and Wikipedia on Anglo-Saxon Window Glass says glass windows existed from the early 7th century on and "Hundreds of window glass fragments have been found" at sites around England & Scottland. I suppose if you just wanted something cheap & plentiful & relatively safe to drink, you'd drink whatever the cheapest alcoholic beverage is available. $\endgroup$ – Xen2050 Apr 13 '16 at 16:33
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    $\begingroup$ @Xen2050, sheet glass (and glass in general) was pretty much exclusively the province of the wealthy. $\endgroup$ – Mark Apr 13 '16 at 20:26
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    $\begingroup$ @Mark Who time travels without bringing some money/valuables? ;-) Anyway, I'll update with some non-glass solar oven distiller info, shiny metal should be in no short supply $\endgroup$ – Xen2050 Apr 13 '16 at 21:49
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    $\begingroup$ Keep in mind that the question's premise is in London. Particularly during the winter season, it is quite cloudy and rainy over there. A solar distiller might go through long stretches of inactivity, which would be a problem if it's the only source of clean water you're relying on. $\endgroup$ – Ellesedil Apr 13 '16 at 23:48


London is built on clay. Clay is highly resistant to water so flow occurs between seams in clearly defined aquifers. As a result soil filtration is limited.

To rectify this situation, around the well plant Chrysopogon zizanioides also known as Vetiver in concentric rings at 15cm intervals and 15cm (6") between rings. Allow two years for the fine dense roots to descend 5m (16') and overlap. The more rings you plant, the better the filtration.

This on its own will substantially protect the well from not only from fecal pathogens but also heavy metal and other inorganic contaminations. However, it is still far from a modern notion of potable.


Build a settling tank with four compartments, each overflowing down a pipe to the bottom of the next stage. The bottom of each compartment should steeply taper to a pipe with a dump valve for quick and easy flushing of sediment.


Build a sand filter. Feed the top from near the top of the final stage of the settling tank, tap the bottom. Water exiting the sand filter will have residual turbidity. A charcoal stage would resolve this but would be an expensive nuisance to maintain.


There are various options.

  • Centrifuge. On a large scale this could be powered by waterwheel. The centrifuge chamber should have a spiral rill to transport sediment and to capture it when spun down. Output will be less murky and fit for bathing and washing clothes. Goes some way to removing larger pathogens provided the tap is correctly positioned.
  • Fines. Certain clays will clarify. Look up "fines" in the context of wine-making. Not a large scale solution. Does nothing for pathogens.
  • Distil. Expensive but very effective. Use for actual drinking water, not for washing or bathing.
  • Microfiltration.
    • Charcoal and muslin. Expensive, high maintenance, low throughput, not good enough for pathogens. Does clarify.
    • Filter through a raised sealed garden bed again containing vetiver. This should be long and narrow, to maximise travel through the root mat. Water emerging from 2m (6') of this will be clear and "sweet" and certainly a lot safer to drink than anything else short of distillation or brewing. Very suitable for washing and bathing. For drinking purposes either brew or boil small quantities on demand.

Vetiver foliage is dense and sharp and will form an effective hedge around your well. It will reach 2m but can be neatly hedged and the clippings used for vermin repellent thatch or mulch.

There are detailed documents on the application of C.zizanioides to well protection as well as riverine and slope stabilisation and phytoremediation (use of plants to rehabilitate contaminated soil or water). Modern applications of well protection can be found in Haiti. I'm pretty sure this is also done in Tonga but damned if I can find anything about it online. It's also widely used to contain contaminants on mine sites.

In an earlier revision of this answer I incorrectly attributed documentation to the UN. In fact it was sponsored by World Bank (see p12).


C.zizanioides doesn't like frost. It may be necessary to find another plant with similar root characteristics but more suited to lower temperatures.

  • $\begingroup$ Great answer! Can you link the reference to UN? $\endgroup$ – JDługosz Apr 14 '16 at 5:59
  • $\begingroup$ Clay is not impervious to water. Even some forms of rock are not impervious to water. In both cases water flow through them, just much more slowly than through sand or gravel. $\endgroup$ – Mark Ripley Apr 14 '16 at 15:01
  • $\begingroup$ "C.zizanioides doesn't like frost" -- and the questioner is ideally placed to establish exactly when the Little Ice Age started. $\endgroup$ – Steve Jessop Apr 14 '16 at 17:38
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    $\begingroup$ While adding references I came across a study contradicting this - apparently its more tolerant of cool conditions than I knew. However, I suspect with more planning you could use poplar which also has a dense fibrous deep root system but is happy in cold climates. It would give a whole new meaning to sacred grove. $\endgroup$ – Peter Wone Apr 14 '16 at 20:44

You could try sinking wells and just boil all the water you draw. Even in the middle ages people didn't typically excrete in their wells

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    $\begingroup$ It's true that they didn't, but wells would occasionally become contaminated due to poorly built cesspits and spread disease anyway. $\endgroup$ – Tacroy Apr 14 '16 at 2:40

You can build an aqueduct and bring fresh clean water to the city. The romans did it and were very successful.

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    $\begingroup$ From which source? $\endgroup$ – Revetahw Apr 15 '16 at 5:56
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    $\begingroup$ To clarify Fiksdal's excellent rhetorical question, Rome was downhill from springs in the Anio valley and surrounds. There is no such high altitude source of clean water for London. $\endgroup$ – Peter Wone Apr 15 '16 at 10:51
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    $\begingroup$ @PeterWone The Romans constructed their aqueducts with very low gradient. The Pond du Guard aqueduct is 50km long and descends in height only 17 meters. I don't know the sources of clean water around London, but surely there would be one in 50km radius? The Thames? One of it tributaries, like the Lea river maybe? $\endgroup$ – ventsyv Apr 15 '16 at 21:33

If it has to be in a city, distillation. It wastes wood or coal so it might get expensive but otherwise it is simple enough with water. Just boil (unclean) water and gather the steam with something made of metal in a way that makes condensed water run into a separate (clean) water container.

You can maybe pay the fuel costs by distillation of alcohol. While it was known in Europe at this point, it probably was not yet exploited commercially. So there might be a business opportunity. And the alcohol can be used as disinfectant, which might help with your disease issue somewhat.

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    $\begingroup$ You don't need distillation. Just boil it for a few minutes. Takes much less fuel. $\endgroup$ – WhatRoughBeast Apr 13 '16 at 2:23
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    $\begingroup$ @WhatRoughBeast Boiling may kill off the pathogens, but it does nothing for the unpleasant taste caused by mixing the water with feces. At least, I'm assuming the taste is unpleasant; I have no personal experience with the matter and I'm not keen to conduct an experiment. $\endgroup$ – Mego Apr 13 '16 at 8:43
  • $\begingroup$ @WhatRoughBeast What Mego said, boiling will make it probably safe to drink (few minutes might not be enough for some pathogens and it doesn't help with chemical contamination which might be an issue in a city), but it doesn't produce clean water which was what was asked. Filtering and boiling might qualify, though. $\endgroup$ – Ville Niemi Apr 13 '16 at 11:06

Go for tea or coffee, both of which require water to be boiled. As well as the benefits of caffeine, you also kill off the pathogens.

Your mediaeval friends haven't discovered tea or coffee yet? Time to go anachronistic and kickstart them early. Coffee is probably easiest - the Crusades have only just finished, and even at their height there was still substantial trade between the Arab and Christian worlds. This will be expensive, mind you, but it should be achievable.

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    $\begingroup$ Herbal tea was known since ancient Egypt, and much easier to come by than coffee. You can grow the herbs on a balcony, even. $\endgroup$ – DevSolar Apr 13 '16 at 11:44
  • $\begingroup$ Mediaeval Europe made fairly extensive use of herbal teas too, but strictly for medicinal reasons. None of them had the pick-me-up effect of actual tea either. $\endgroup$ – Graham Apr 13 '16 at 13:53
  • $\begingroup$ Apart from the coffee concept; I agree. Various pleasant herbal teas are easily made and some - e.g. sage tea - have a stimulatory effect. $\endgroup$ – Jack Aidley Apr 13 '16 at 16:03
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    $\begingroup$ @J.D.Ray Nope, it originally came from Africa. The Arab/Ottoman world first discovered it, and it spread from there across Europe. You might be confusing this with chocolatl, which was a South American drink made from cocoa beans. North America didn't give us anything interesting to drink, I'm afraid. (A tradition which Budweiser and Coors have continued in spades. ;) $\endgroup$ – Graham Apr 14 '16 at 10:10
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    $\begingroup$ @Graham: sorry, it's off-topic, but I can't let that lie: COLA. Say what you will about its nutrition, but it appears to be the leading class of non-alcoholic beverage in the U.S., if not the world. North America's contribution to obes- I mean, drinks! $\endgroup$ – user243 Apr 14 '16 at 17:13

The Romans solved this problem by building aqueducts to pipe in potable water from uncontaminated sources. It's a major civil engineering project, but by the 1300s European engineering technology was well up to the challenge.

The Great Conduit was built in London in 1245 so it seems obvious that these kinds of major civic works were being undertaken and that the importance of uncontaminated drinking water was well-understood.

The biggest public health improvement a time traveler to 1303 could do is introduce germ theory ~250 years early. Being able to build a microscope would help enormously in overturning the fallacious medical practices of the day. Convincing people that disease is caused by microorganisms and not an imbalance of humors and/or evil spirits would be a significant challenge.

  • $\begingroup$ Convincing people... Seeing is believing $\endgroup$ – Juha Untinen Apr 15 '16 at 12:24

Well, it's sort of similar to the distillation suggestion, but it's not quite the same - a dehumidifier (or condenser).

Warm air holds more water, and when it cools, water is displaced - and as a result, it'll be nearly clean. At a basic level, all you need to is blow warm (and damp) air at a cold surface.

Natural air flow can do that, and all you need do is provide the 'cooling' surface - something dark coloured in shade will radiate heat and have a cooling effect - and if you do it right, you can actually use the temperature differential to generate a convection airflow - cold air will fall, displace it's moisture, and cause 'warm' air to be drawn in from above.

Or if you want something a bit more high tech, there's:


You might manage to accomplish something similar with medieval tech.

Or perhaps:



You could make a dripstone 'vase' from limestone like this one: http://sydneylivingmuseums.com.au/stories/elizabeth-farm-dripstone. Contaminated water is poured in and fresh clean bacteria-free water slowly seeps out just the way it does in caves. A pan is placed below to catch the dripping water.


Could you drill a really deep well? i.e. a tube well or borehole well. You would have to develop the drilling technology from scratch. But what you know, that the locals did not, is that London sits atop an Artesian basin. Drill down through the clay strata (about 400 ft from memory) and you will hit porous chalk, out of which will gush large amounts of pure, highly calcified fossil water. Clay is soft and easy to drill.

Note: The mediaevals could dig really deep wells but had to stop at whatever depth they first hit water. That will happen before you reach the chalk. You'll have to drill.

Oh, and your technology will then be spread to other parts if the country where they may bring water naturally contaminated with Arsenic to the surface and poison themselves with it. You'll warn them that only water out of chalk is guaranteed safe to drink, but they won't listen.


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