Clay-Sawdust Water Filters




Introduction: Clay-Sawdust Water Filters

About: I'm just a lady who likes making stuff. I got my degree in engineering but also enjoy cooking, sewing, knitting, gardening and backpacking, among other things.

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.

Step 1: Raw Materials

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

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

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

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

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!

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.



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    35 Discussions

    A potter friend and I made the ceramic mold vessels, and would like to try making the filters from clay and sawdust. If someone could help me clarify a few things before I attempt this, it would be appreciated:
    1. I have access to Little Loafers clay, a low grog white clay, that I typically bisque fire to cone 06, glaze fire to cone 6, and wonder if I could use this fresh out of the box from our supplier and mix it with sawdust (and if so, about how many ounces of wet clay should I mix with how many ounces of saw dust for the proper 50:50 ratio equivalent to that of clay dust?)
    2. I also have access to a higher grog stoneware clay, also fresh from our supplier. Same firing temps, and same question as above if this higher grog clay is preferable to the lower grog.
    3. If I cannot use the wet clay or it is not recommended, I can certainly transition my wet clay into bone dry and then clay dust (that will be a first for me, I'm usually doing the opposite to recycle scraps into useable clay again!) but if you can advise me as to which of my two choices, if any, are suitable for this project.
    4. If my two existing clay bodies are not recommended for the filters, please provide a link to a place of purchase for the proper materials or a suggestion as to what I should search for in clay dust.
    5. I use an electric kiln, and would appreciate someone knowledgeable on the program I could use to successfully fire the filters. Would this work (?): slow bisque with a preheat of 3 hours to ^012 with an 8 hour hold then let the cool down happen with default settings of kiln until cool enough to open?
    Any suggestions are welcome!
    Thank you

    How long does the filtration take? Is it more of a single batch use, or could it be applied to a consistent flow, say, in industry?

    Do you happen to remember where you ordered your dry clay from?

    I am wondering if anyone has had experience using a moist clay and trying to incorporate the organic material into the moist clay vs. adding it to the dry?

    If so what ratio did you use?

    I am thinking this technique might be more challenging to achieve uniformity and therefore potentially cause less structurally sound regions of the filter.

    I just wanted to say how excited I am to read this. I went to Olin College's fall expo last year with some of my high school students and was completely blown away by so many of the different projects. I first learned about Potters for Peace at a Water as a Limited Resource seminar I took at Harvard years ago. I have always wanted to make these filters with my students and I just got a small grant to purchase supplies and pay for kiln firing to make water filters with my Natural Resource students at Norfolk Aggie. I will let you know how ours come out. I hope you are out in the world doing great things.


    9 months ago

    Can this be used to filter the oil (crude oil) from wastewater ?

    If done correctly there is no reason these cannot filter pathogens out of the water.

    1 reply

    Could this stuff be useful to obtain filttered milk by product whey?

    Is it possible to heat the filter to 1200 degrees Celsius rather than what you did in the fire kiln? How about 2500 degrees Celsius? Will the water still be filter

    Is there a kit you recommend I could order to make this for science fair project or places I can go to get the materials?

    If you used coloital silver instead of plain water wouldn't this inpregnat silver into your filter,and therefor create a anti bacterial filter?

    1 reply

    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!

    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?

    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.

    2 replies

    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.

    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