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Clay-Sawdust Water Filters

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In this instructable we will show the process for creating clay-sawdust ceramic filters while explaining the science behind them.

This type of filter is a common solution to the problem of obtaining clean water in many parts of the world, especially rural parts of developing nations. It is a fairly simple process that involves only materials that are available in most parts of the world.

A friend of ours is away in Kenya working on various projects having to do with sustainability including filtering water for drinking. Where he is now, "water guard" tablets are used to kill bacteria, but it does nothing to improve the clarity or taste of the water so he has been experimenting with different filtering techniques.  He has a local source of both clay and sawdust, making this sort of filter a viable option.

Note that this filter can only filter particles and color out of the water. It cannot filter bacteria or viruses. Water filtered through this filter is not meant for drinking, and should be boiled before being consumed.

This instructable is the culmination of a project for the Spring 2011 Stuff of History class at the Olin College of Engineering in Needham, MA.
 
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Step 1: Raw Materials

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The raw materials needed are:

Dry clay. Clay dust is probably easiest. (Warning! Clay dust is extremely harmful to your lungs! Only use outside, under a hood, or in another well ventilated area!)

Fine Sawdust. Best obtained by sieving sawdust. A #30 (600 Micon) size sieve works best, although other sizes can work. Smaller is not advised, as it will be a slow process to sieve and will slow the filtration rate. Larger can be used, up to the size of a window screen, although the filtration quality will suffer some.

Water. The purity of the water should not matter much.

A mold. The clay-sawdust mixture lacks plasticity, so using a mold to shape the pots is best.

Kiln/Furnace. We used an electric programmable furnace, although anything that can reach the required temperatures at a controllable rate will work.

Step 2: Mix the Clay and Sawdust

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Proportions are key at this stage. Most sources use a 50/50 ratio by volume of clay to sawdust; however, we determined a ratio up to 20/80 clay  to sawdust can be used and still effectively filter.

If the sawdust is well sieved with at least a #30 sieve, more sawdust will increase the speed while not negatively impacting the filtration significantly up to about 20% clay and 80% sawdust by volume. If the sawdust is not well sieved or is sieved by a mesh larger than #30, more sawdust will negatively affect the results of the filter, and so a slower ratio of 50/50 clay to sawdust by volume is recommended.

The other main factors when determining the proportions of clay and sawdust is the plasticity of the clay during working and the strength of the pot after firing. More sawdust will reduce both plasticity and strength. At 50/50, the clay is not too difficult to work with and will come out of the kiln strong enough to be easily handled, though still not nearly as strong as clay alone. At 20/80, the clay cracks significantly during even simple working, which can lead to air bubbles in the clay that may impact filtration, and will come out weak and possibly even crumbly.

In the end, a 50/50 or 40/60 proportion by volume of clay to sawdust is probably best for most applications, coming out generally fast and strong enough to be useful. Higher proportions of sawdust can be useful if speed is an issue, effective sieving is possible, and strength is not an issue, although this is not recommended otherwise.

Step 3: Just Add Water

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After the clay and sawdust have been mixed, add water and mix in small amounts until the clay clumps together completely and is soft and workable. Be careful not to add too much water and make the clay into a sludge.

Once the clay is workable, wedge it to further mix the clay and remove bubbles from the inside of the clay. This may be difficult with less plastic clays and at higher sawdust proportions, however try your best. Some smaller bubbles in the clay should not significantly affect the filtration.

Step 4: Mold

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Form your workable clay into/around your mold. We used small plastic balls for our molds, although larger ones can be used. Adding water to the clay at this point may be necessary to increase its plasticity and to help reform cracks that may occur. Wall width should be around 0.5 - 1 cm.

Once you have the clay in the proper shape, let it dry until it can be safely removed. Then remove it and let dry until bone dry. Larger pieces may need to be covered in plastic or another waterproof material, allowing them to dry slowly and evenly.

Step 5: Firing

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Once the pieces are bone dry, they are ready for firing. We used an electric programmable furnace, although a more traditional kiln and cones can certainly be used.

As with most any clays, the furnace should be brought slowly up to about 100 or 120 degrees Celsius to allow the pore water in the clay to boil off and escape. We used a time of 2 hours to reach 100 degrees.

Then the furnace should be brought up to about 890 degrees Celsius, or ^012, in about an hour. It should be then held there for 8 to 9 hours, then allowed to slowly cool off down to room temperature in 2 hours or more.

There is a lot that happens to the clay between 120 and 890 degrees Celsius. Understanding the way this firing process affects the microstructure, and thus the porosity, is crucial to understanding the filter.

The most significant process as the piece fires is the ceramic change. This begins at about 350 degrees, peaks at 600 degrees, and ends by 700 degrees Celsius. At this point, the water chemically bound to the clay is driven off, converting the clay into a hard ceramic. At this point the clay is no longer held together by water, but by a process that occurs called sintering, where small points of contact between clay crystals are welded together.

Thus by 700 degrees the clay is extremely porous, with crystals being held together only by small welds at points of contact. While good for the purpose of filtration, this makes the ceramic extremely weak.

At 700 degrees the burning out process, where carbon, sulfur, and organic molecules combust, begins. This is crucial in the filter making process as that pores left over from the burnt off sawdust contributes significantly to the filtering process. The burning out reaches a peak around 800 degrees, and is pretty much completed by 900 degrees Celsius.

The final process to take place during firing is vitrification. Vitrification starts at 800 degrees C and involves a glass of metal oxides and silica filling in the pores between clay crystals, strengthening the ceramic but also reducing porosity.

Thus 890 degrees C is the optimal firing temperature because it creates a balance between porosity and strength. At that temperature almost all of the organic matter will be burnt off, leaving behind pores, and the vitrification process will have begun but will be be extensive, lending some added strength to the final ceramic but also leaving many of the pores. 

Step 6: Filtration!

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Once the ceramic pieces have been fired and have cooled down to room temperature, they are ready to do their job!

First of all, beware of cracks. If the firing was done correctly, it should not have resulted in any significant cracks, although there may be some cracks leftover from the forming of the clay. Cracks will make any filtering done by the ceramic useless.

The clay will need to be soaked in water before it can actually filter anything, otherwise your first attempts at filtering water through will just be soaked into the body of the ceramic. Once it has been soaked, pour the water to be filtered in and place the pot over something such that it can drip out of the bottom.

The filters we made in this project could filter color out of dirty pond water and could filter 20 micron clay particles out of water. SEM images of our ceramics show many of the pores to be less than 20 microns, while also many pores between 50 and 100 microns. The sawdust particles we added fell largely into the 50-100 micron regime, with some being in the 20 micron or less regime. Clay without sawdust added also had many of the 20 micron or less pores, but many fewer of the 50-100 micron pores. This suggests that the sawdust adds some to the active filtering pores, the smaller 20 micron ones, but mainly adds to the larger pores, which increase speed and don't negatively impact filtration, assuming there are enough of the smaller pores that no path exists that does not go through one.
Buster Hyman10 months ago
If you used coloital silver instead of plain water wouldn't this inpregnat silver into your filter,and therefor create a anti bacterial filter?
Maybe! What we were exploring with this project was to create a way to filter physical impurities from the water using materials that were readily available where our friend was living in Kenya and they already had a way to disinfect the water, but that would be an interesting thing to look into in the future!
jmccleve2 years ago
This is really interesting. How does this stack up to conventional water filtration systems? Have any studies or tests been done of the water that has been filtered this way?
submark2 years ago
Would It be possible to use powdered charcoal for the porosity filler?
Charcoal is easier to powder, sieve to size, and would produce less outgassing of non-carbon materials. This might help control cracking and voids.
I'm talking about real charcoal - not briquets, which are made up of a combo of mined coal and wood patricles This produces a more mineral rich ash than charcoal from wood.
It is messy, but then again we are talking about working outside with sawdust and mud anyway.
I know you're saying to use charcoal for a quicker filter, but had an idea for a combination of charcoal and clay? Would it maybe increase the filtration effect.

Also, an easy source of clay is unscented cat litter; it's 100% clay.
hm, probably would work.
I have done the mass production of such clay filter candles and have the patent done by CSIR a leading Indian research organization and me the licensee. Can supply whole filters having 15 litre capacity on the top and 15 litre at the bottom, with 3 candles of 100 mm dia fitted at the top portion.

Can supply to any NGOs, part of the world, ex-India, @ US $ 12 per filter. Candle costs approximately $ 0.7 US $ each, can be retrofitted in any filter

See www.cleanwater.co.in
wyrdlg2 years ago
Very interesting. How long does one filter work. They might block after some time, don't they. Can they be cleaned then?

A good way of disinfecting water is to fill it in clear pet plastic bottles and lay them in the bright sun. The ultraviolet light kills the germs.
http://en.wikipedia.org/wiki/Solar_water_disinfection
Time that filtration takes depends on the size of the sawdust used in the clay and sawdust mixture.
The main goal of this project was to make the water less turbid, since they already have a way of killing the bacteria and things in the water, but had no way of actually filtering particles out of the water.
This is a good idea!

If you have a kiln you can make a good drinking water filter
by adding some Silver solution to the filter production.

1) POTTERS FOR PEACE:
http://s189535770.onlinehome.us/pottersforpeace/?page_id=9

2) POTTERS WITHOUT BORDERS:
http://potterswithoutborders.com/wordpress/

Again, this is a good thing. Look at the above links and spread the word!
Llewner2 years ago
If done correctly there is no reason these cannot filter pathogens out of the water.
http://info.anu.edu.au/ovc/media/Media_Releases/2005/January/190105filters
http://s189535770.onlinehome.us/pottersforpeace/?page_id=9
check out potters for peace,

you can kill viruses and bacteria by soaking the filter in colloidal silver solution for 45 seconds and let dry.

Potters for peace build potteries in south america to be operated by the local population, they have full explanationof how to build an operate a comercial press
operation and all the specs for making these filters on a large basis.

per the water issue- the purity of the water is important, do not use clorinated water, it will casue high talc clays to fail to fire correctly- fire to cone 010 (1860f)
use a mid fire or high fire high iron clay. the sawdust needs to be only wood, no plywood or melanite, as it will cause no ends of problems when it fires (carbon entraptment) ratio should be aorund 60 % sawdust by weight, mixed with 40% (by weight) of dry clay and mix with as little water as possible. a pugmill is better than a blunger in this instance, though you can hand mix in a wheel barrow with a shovel (before adding the water)

www.pottersforpeace.com

this clay needs to be pressed to ensure crack prevention.
jstkatz2 years ago
There is a very similar design that can filter for water born illnesses as well, it's open source from a group called potters for peace, it does make use of a press however, I would guess uniformity of pore size may be more important when trying to kill bacteria

It uses colloidal silver applied after firing to disinfect, though this plus the solar disinfection technique requires even less
hogey742 years ago
Hey I've seen a doco here in Australia that talked about using old coffee grounds instead of sawdust - same idea I think - when you fire the clay the coffee burns out and leaves microscopic holes. With the right coffee grounds however, it can filter enough bugs out of the water (bacteria only, not viruses) to be useful without further treatment. Might be worth looking at?

Great instructable mate.
Tony Flynn from ANU came up with the idea of using organic material (like coffee grounds) - see the press release at ANU: http://info.anu.edu.au/ovc/media/Media_Releases/2005/January/190105filters

and there is a wikipedia article!
http://en.wikipedia.org/wiki/Clay_pot_filter
Nice! Just checking that out now. I think I must have seen it on Catalyst or one of those shows. cheers John
bluenevus2 years ago
Depending how it's made, it can filter out bacteria like E. coli:
"Tests with the deadly E-coli bacterium have seen the filters remove 96.4 to 99.8 per cent of the pathogen — well within safe levels. Using only one filter, a litre of clean water can be produced in just two hours."
source: http://info.anu.edu.au/ovc/media/Media_Releases/2005/January/190105filters
SHIFT!2 years ago
Holy cow you are on a roll today Danger! Congrats on getting featured for your third time this week!

I noticed that you're using Dry Clay as the clay base in this project. Is this terra cotta or is "Dry Clay" actually a specific type? Also, will this project be waterproof when you're finished?
I'm pretty sure we used terra cotta dry clay. The project is finished and most of the pots we made are in pieces since we had to break them to examine them.
Also how would we waterproof them? I would like to know. :]
XTL mahuirong2 years ago
Clay comes in several variants - generally separated into four regions, low fire (terracotta), medium fire (earthenware), high fire (stoneware) and Porcelains.
The higher you fire the clay - the more vitrified (glass like) the clay body becomes - and therefore less porous.

Terracotta clays (low fire) are the ones you see used in garden pots - have an orange cast and are porous to some extent. You should be using Terracotta or earthenware clays. You can use stoneware clays also - firing them to low temperatures will also make them porous. Porcelain clays are expensive and very weak at low temperatures and therefore unsuitable for this process.

The most important aspect is to not fire the clay to hot - else it will vitrify and no longer be porous.

Clay comes in bags - already wet, ready to use - or as a powder with all water removed (cheaper to post). Any dry clay can be made into clay by wetting it (slaking) and wedging (massaging) it into a mass. So if you have a lump of dry clay - just add water. If your clay is old and not completely dry then let it dry before adding water - its easier for dry clay to slake its thirst and absorb the water than partially dry clay.

Sawdust acts to make gaps in the clay body - which burn out in the firing - leaving voids, or holes, for water to flow through.

Another material with similar properties to sawdust is Paper. If you mush the paper up in water until it is almost dissolved (or leave it a few days in water) then add it to the clay - the clay will be easier to work than with sawdust. I suspect the availablility of materials, in this case, is what really counts.

More info on Paperclay can be found with a google search.

The electron microscope images are very cool. Congrats on an excellent instructable
am0053 SHIFT!2 years ago
I'm pretty sure the clay we used was #25, although most any clay should work. Also the dry clay we bought is normally only a part of the real, workable clays that are normally used. Often other materials or clays like kaolin are added to give it the best properties.

We tried to select a clay that would be porous on it's own, so some of this information may change depending on the type of clay.

Unfortunately, we didn't have time for this project to test different types of clay. It would be a really cool thing to look into for the future, however.
canida2 years ago
Nice! Thank you for sharing - this is potentially a very useful project.
I read an article a few years ago (might have been a copy of Make) where they mentioned doing this. However they cited using manure for the filtration mix on the premiss that it would have finer holes. The firing process would then remove it same as here.

Needless to say, I think I would prefer you approach if I found myself in that environment.
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