What methods might an ancient civilization use to desalinate sea water?

Question

Following on from my previous question here concerning the ancient civilization I have called the Androy and their struggle to survive terrible drought conditions on their planet.

After years of struggling with having to dig deeper into an aquifer to extract water from underground the Androy encounter an even bigger problem. During routine deepening of the wells they encounter granite. They are at the bottom of the aquifer and within a few years will have to find water from some other source.

They are relatively close to the ocean. What techniques can they employ using ancient technology, to extract sufficient fresh water from the ocean to meet their needs? Can they extract as much from the sea than they could from the aquifer? Is it even practical?

The geography / geology of the coastline may be configured as you wish.

Answer 1

You can build a solar still using nothing but pottery tubes, sealed with asphalt or similar. A single unit (different design) can produce as much as five liters per day.

For continuous production, the main "unit" is a large tube with two tubes inside, the upper one being half open. The upper tube is filled with slowly moving salt water, that evaporates and supplies humid air. The smaller lower tube contains much faster-flowing salt water, and the humid air condenses on its surface (it needs to be glazed), collecting in the low part of the outer tube. This would work a lot better if the upper part of the outer tube was made of glass, but with sufficient surface to dedicate to the project fired clay should work too.

A fully-fledged solar still can get you around five liters per square meter per day; assuming the clay is one tenth as efficient, a roof's worth should still (pun not intended) produce water enough for the inhabitants of a two-story building... in the summer. In winter, things might not go so well.

Agriculture would require much more water, and I'm not sure it is doable with solar power alone. You might use a slightly different scheme to allow both concentrating sunlight with reflectors and optionally (being careful not to crack the pipes) fire to increase the evaporation rate. To further improve thermal insulation the freshwater pipe could run underground.

Answer 2

Reverse osmosis.

from Aristotle, Meteorilogica.

There is more evidence to prove that saltness is due to the admixture of some substance, besides that which we have adduced. Make a vessel of wax and put it in the sea, fastening its mouth in such a way as to prevent any water getting in. Then the water that percolates through the wax sides of the vessel is sweet, the earthy stuff, the admixture of which makes the water salt, being separated off as it were by a filter. It is this stuff which make salt water heavy (it weighs more than fresh water) and thick. The difference in consistency is such that ships with the same cargo very nearly sink in a river when they are quite fit to navigate in the sea. This circumstance has before now caused loss to shippers freighting their ships in a river.

It looks as thought subsequent authors dispute whether this vessel should be wax, or earthenware, or some combination. It also looks as though it was difficult to make this work up until very recently, with modern ceramic technology. But is this necessarily so?

The principle laid out by Aristotle is sound: reverse osmosis involves a filter small enough to exclude dissolved ions (here, salt) and pressure enough to drive the fresh water through. Ceramic reverse osmosis membranes exist. Usually ceramic filters are unglazed ceramic.

My scheme for these fictional people:

1. Unglazed ceramic filter pipes. They make these using techniques similar to those invented by the ancient Chinese for making porcelain, also a very fine grained ceramic.
   
2. Hollow wooden cores (or cores of more robust and more permeable ceramic, like terra cotta) are threaded inside, to buttress the ceramic filter pipes against pressure at depth.
   
3. Filter pipes are lowered to a depth where water pressure will drive seawater through the filter, producing fresh water on the interior via reverse osmosis. This should be in open water - an area with circulating salt water to avoid buildup of concentrated salt brine, which would require higher pressures to desalinate. Perhaps at the end of a jetty?
   
4. Water is pumped up through these pipes as though from a well.
   
5. Pipes are periodically raised and scrubbed, to expose new filter surface and reduce fouling.
   
   

For any interested in back reading on the history of desalination I found this excellent and exhaustive source: A short history of water desalination

Answer 3

You mention that there is an ocean. Therefore, the air has humidity, at least in areas around the coast. Doesn't matter that there's no rainfall - you have humidity. And with a day/night cycle, you get fog and dew.

So, I give you fog catchers. Using nothing more than fabric mesh, which even a prehistoric civilisation could master, they'll catch water droplets from the air. No need to dig wells, no need for fancy desalination plants, just weave fine-mesh nets.

Your civilisation also needs to look at their agricultural practises. Mulches will radically reduce the need for water. If they can handle deep digging, they can easily enough engineer deep trenches (like several metres deep) to grow plants in, where the trench itself acts as a dew collector and temperature moderator, as with the Forestiere Underground Gardens.

This can potentially give you a unique landscape. Above-ground would be a riot of fabric, most likely with each village using their own colour schemes - but of course no houses above-ground. The trenches will form the "streets" for the villages, and homes will be dug into the rock in the walls of the trenches. The walls of the trenches will be covered with any kind of creeping plant which can provide food, and the floors will contain any other plants which need more space. A few precious locations above-ground may be planted with grain, but the people will mostly have to live off other staple goods - cactus, fruit and root vegetables will probably be their major foods. Trees will be particularly useful, because deep root structures have better access to water.

Answer 4

Plain old distillation.

Now, distillation is expensive. It requires huge amounts of energy. So how much they can produce will depend on just how rich they are, and specifically how much energy they can command. If they have oodles and oodles of easily accessible fossil fuels, they might very well be able to produce just as much as they got from an aquifer.

The simplest set up would be a solar still. A well-designed industrial-sized solar still, with concentrator mirrors, will produce much more drinkable water than passive evaporation, but you'd need a lot of them, over a very wide area to capture enough solar power to make enough water for your whole civilization. The mirrors don't need to be particularly high-tech; in particular, they don't need to be image-forming. Polished brass would be fine. You'd just have to make sure it's well-maintained. Alternatively, you could ditch the mirrors and just spread out the water over a much larger surface area--but that requires building a much larger still, with a much larger transparent roof (basically a greenhouse for water) for letting light in, holding in the heat, and preventing the escape of the vapor. Which may very well end up being a more difficult proposition, not less, especially considering the expense of glass in the ancient world and the technical difficulty of producing large sheets of it. It doesn't need to be optical quality, but still, that's asking a lot.

The other option is fire-powered stills. If they have access to fossil fuels, or a large supply of rapidly-regrowing vegetative fuel sources (like, say, a really big bamboo forest), solar concentrators can be replaced with furnaces to heat water for distillation. In theory, such a still could be built entirely out of ceramic pottery, but any amount of metal or glassworking technology they have would be helpful.

In any case, note that one of the waste products of the distillation process will be highly concentrated brine--and that's not necessarily a bad thing! Ideally, you'd want to keep up a good flow through your distillery so that you don't build up salt depositions inside (or so that you minimize build up--cleaning the works will be a necessary regular maintenance task regardless), and you can pipe the concentrated brine elsewhere. Don't throw it away, though! Put it into passive evaporation pools, and start manufacturing salt! Salt is an incredibly valuable trade good for ancient civilizations, and they could then start using it to buy supplies fro other civilizations--including fuel, and possibly even aqueduct access to other people's water, if necessary.

On top of all that, there are things they can do to reduce their dependence on fresh water. You can, for example, use salt water just fine for evaporative cooling. Additionally, they could try developing saltwater agriculture, as mentioned in Ash's answer, and aquaculture. Sea plants obviously already grow in saltwater just fine, so farming and using seaweeds as a large part of their own diet, as animal feed, or even as fuel for the distillery furnaces, would cut down on how much water they need to distill, as well as eating fish and other seafood to avoid having to spend water on as many domestic land animals.

Answer 5

I will elaborate on distillation with another solution: spray desalination

First, get some hot air, above 100°C but not too much. You can do this with concentrated solar power using flat mirrors. However a parabolic trough would be a much better option, because one person (ie, child labor) is enough to keep it aligned. The parabola was discovered a very long time ago, mirrors can be polished bronze, and the structure can be built out of wood. The theory to build it would be a bit advanced for the time, but not the construction materials.

Now you need to blow some air into the system and heat it up: if this is a coastal area there will be wind, so you can use a sail to funnel the wind into a pipe, or you can use a donkey-powered "windmill" as a blowfan.

Then, you would need seawater with enough pressure, so another donkey-powered water wheel, or archimedes' screw to hoist the water into a tank on top of a water tower. Height creates pressure...

Now use a spray nozzle to spray the water into the hot air flow.

The neat thing about spray evaporation is that a large quantity of droplets have a huge surface area to exchange heat with the air, thus they evaporate very fast. If the water and air flow are just right, you get salt crystals which you can collect and use, and very humid air.

The humid air can be cooled with seawater, which condenses the moisture and yields desalinated water.

This process is more efficient than plain distillation, although more complex to use, but you also get salt.

I'm not sure when spray nozzles were invented, though...

Answer 6

Saltgrass has not been mentioned yet. It grows in salty water, and excretes solid salt from it's leaves. Additionally, livestock can graze on it. After much experimentation, the culture follows the following recipe:

1. Excavate a large flat basin that isn't too deep and holds water after a high tide goes back out.
2. Run a flat channel from the basin inland towards the town.
3. Plant saltgrass in the estuary and the channel.
4. Harvest salt from the salt grass.
5. Take sweet water from the point furthest from the ocean.
6. Occasionally harvest and replant the saltgrass and feed livestock with it.
7. Every few years, remove silt from the estuary, re-grade the basin and dredge the channel. The soil from these endeavors will be high in organic content and likely valuable.

Answer 7

Evaporation, but the yields will be sod all at that level of technology. Alternately they can use salt-water agriculture (there's a better example in either Libya or Sudan but I can't find a link) in which salt hardy species like Mangrove are used for biomass fuel production and fodder species with low embodied salt like salt-marsh-grass hay can be used in place of traditional animal feed, some of these species, like Glasswort have a high water content and could serve a duel role (food and water) for some browsing species. I would also suggest that salt hardy animals like the Camargue breeds, which can survive year-round without access to truly fresh drinking water, would be a necessity in this situation if people were going to continue to keep livestock.

Or you could go big, really really big, landscape engineering scale, the basic technology is not that complex but the large-scale application has never to my knowledge been attempted. It uses the principles of the solar chimney but on a grand scale, put a large array of such chimneys along a coastal ridge to suck moist air in off the sea and use equally large wind harvesters to drain the water from that artificial air current, either using a mesh based condenser similar to a fog collector or allowing thermal expansion to do the precipitating within a cooling tower of some kind. The Chileans achieved average yields, from natural fog, of 15,000 litres per day from 94 capture meshes. Creating artificial winds on this scale will not have predictable outcomes so I'm not going to venture an estimate but it could be sufficient to sustain a population big enough to exploit and maintain the system, especially where integrated greenhouse space is used.

Answer 8

Does the night time temperature fall below freezing at least part of the year? They can freeze distill their water.

Get a bunch of salt water. Let it sit out in freezing temperatures. You will then have less salty ice floating in very salty brine. Pull the ice out, drain the brine off somewhere else, melt the ice, repeat. Each time the part that froze will be less salty.

Answer 9

For potable water ancient civilizations may have relied on filtration - it removes the salt, but leaves the water hard. The "technology" is still used in many Mediterranean countries - usually a large local rock hung in the courtyard, seawater is poured on top in a slight depression, the water percolates through the rock and is drinkable. I'm not sure on the volumes, but it's something I have used.

Distillation was also very common on ships, The Resolution built in 1770 had "the latest apparatus for distilling fresh water from sea water" – captaincooksociety.com

The ‘Ballochmyle’ water filter, circa 1830 a rare colonial Georgian water filter drip stone, made of sandstone and wrought iron framework in superb original condition with reservoir at base.

(source)

Answer 10

It depends on orography..

They may be able to build a wind trap.

General principle is to, somehow, drive hot and humid wind through a relatively cooler cave (possibly rocky). This will cause part of the humidity to precipitate on walls to be brought to underground cistern.

A cliff near the sea, possibly facing South, with some shore to build shallow pools would be a great place to build.

It might (depending on specifics of prevailing winds) to use something (windmills?) to force air to traverse cave in the right direction.

Answer 11

A water-cooled greenhouse uses evaporative cooling which, at the same time, provides humidity in the greenhouse. This reduces water consumption. The cooling works equally well with sea water. The humid air may cool enough at night to create condensation. The brine exiting the coolers may be used to produce salt and serve as a raw material for the fertilizer industry.

This model has been proposed in the Sahara forest project

Answer 12

How much water do we actually need? Modern household in industrial countries use ~150 l/d and person. This is only the smallest part of actual water usage. A more complete number for water usage must include water needed for all industrial processes, watering plants, supplying animals etc. So we look at virtual water data and the individual water footprint. The lower bound appears to be China with 1000m³/a person. I'll go out on a limb and say an ancient civilization needs about a third of that - no industry and I assume reasonably adapted agricultural practices. Also this rounds to 1 m³/d person for everything.

Seawater desalination needs stupendous amounts of energy. The most efficient thermal technology, multi-stage flash distillation, needs 23–27 kWh/m³ of distilled water. Reverse Osmosis used to be half that, but top of class technologies are said (wikipedia, I have my doubts) to be as low as 3 kWh/m³ (this includes energy recovery from brine etc.).

I'm not convinced that an ancient civilization can hop to approach these efficiencies, so I'd assume the energy demand to be worse by an order of magnitude. Note that the efficiencies stated above stem from decades of dedicate engineering, on the back of centuries of general engineering. So for the purpose of this answer, I will simply assume that "technological" desalination will not be implemented at scale or as only or main water source. What's left?

Your civilization cultures large, artificial salt marshes. The plants growing there are harvested as fodder, but mostly for their fresh water content. There's at least one plant that grows in salty habitats and can be used as fodder, so it's not totally impossible.

How to turn grass into water?

• Large presses
• feed it to milk animals and drink milk
• feed it to any animals and drink blood
• plant edible, water rich vegetables like (salt toelrant) cucumber. Not that the more water a plant stores, the less yield it will show in a salty environment as plants have to do the work for desalination too!

The marshes will be managed inetnesly, they will need to be flushed with lots of excess salty water to remove excess salt. They are also huge: at 1m³/d water means ~1.5 t/d total mass of grass! If we assume 25t/ha a yield (which is unreasonably high, this is grass in modern agriculture (and temperate climate)) every person would need 15ha marshland! At least the working animals won't need extra fodder.

This still doesn't look viable to me but is may become viable if ...

• you tweak the numbers for water need - maybe find actual usage for desert dwelling people or ancient civs. But make sure to include irrigation & water for animals!
• postulate diverse plants that are salt tolerant and have additional uses. Maybe your civ has salt tolerant rice, or can make fibers from salt tolerant cotton or cattails? Cattails also have edible, high starch roots that could double as potatoes?

Answer 13

They could harvest the juice from some plant, just as we harvest coconut milk and cactus juice.

They could drink the milk or blood of some animal that can drink salt water.

They could eat the flesh of sea creatures such as mollusks and get moisture that way. Or they could squeeze them in a press if they were plentiful enough.

They could do all of these things.

In short, they could exploit already-existing biological filters. It would be reasonable to expect that the native flora and fauna had adapted to the conditions and could successfully extract water from the environment.

Answer 14

What about a large dugout bowl made from the ground just near the edge of a precipice that gravity filters naturally through the soil layers that seeps out the side of the precipice into a collected pool. I was part of a biofiltration project once that prevented toxic asphalt buildup raining off into the vegetation. Our team combated this by layering a series of certain minerals, natural filtration membranes, and arrays of certain toxin-absorbing plants along the natural rain flow channels that fed into the groundwater. The idea behind this being that it filtered heavy metals from the runoff. Perhaps this could be done with salt too, naturally.

Answer 15

The two most used methods of desalination of ocean water are: *distillation *reverse osmosis Either is possible on a large scale where there is a commitment to resources needed. reverse osmosis would likely require support from a fishing industry to provide osmosis membrane materials until research develops suitable replacements. wave pumps established offshore could drastically reduce the pumping energy requirements to make either approach feasible. Parabolic troughs constructed with buoys and supplemented with wave pumps could provide the means for the supply of both energy and unprocessed water.

This sort of project might be taken on with little more than a large workforce, determination, and some skill with pottery, ceramics, and glazes.