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
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
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
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
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
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!
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.