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Ceramic water filters
Ceramic water filters are ceramics pots made from a mixture of clay and sawdust fired at a relatively low temperature. The most well known example of their use for filtering water in the developing world comes from Potters for Peace, who have a process for mass producing such filters, which are also impregnated with silver to kill off harmful bacteria and viruses. Such mass produced filters are certainly not the only instance or possible way of obtaining ceramic clay filters, as the resources needed, clay, sawdust, and water, are available in most parts of the world, and the manufacturing process is relatively simple.
Reasons for Use
Lack of access to clean water is one of the leading problems facing the world today, affecting over one billion people. Past attempts by the developed world to create centralized water purification and distribution systems in the developing world have been met with failure. This leads to a need for Point-Of-Use household water filtration as a transition technology at least, and possibly even a sustainable solution.
This availability of materials and relative ease of production makes ceramic clay filters ideal as an appropriate technology to fill this need in many communities. Instead of relying on outside organizations to provide outside solutions, communities can create filters using materials and techniques that are likely already used. This decreases reliance on charity, provides opportunities for local artisans to earn money, and generally leads to a sustainable method for water purification.
This is not an appropriate solution for all areas however. If resources like clay or fuel for firing are absent or hard to obtain, or if better options exist, ceramic clay filters may not work. In addition, ceramic filters can usually filter out sediment and particles, and occasionally bacteria, but without silver impregnation they cannot filter out viruses or some small bacteria. In this case, other purification methods like boiling or chlorine disinfecting additives like “water guard” must also be used.
How it's Made
The Potters for Peace organization has a relatively standard process for creating their filters, described here: http://pottersforpeace.org/wp-content/uploads/production-manual-iraq.pdf.
Their recipe involves a 50/50 mix of clay and sawdust by volume. While an effective proportion, slightly higher proportions of sawdust can also be used to create a slightly faster but still effective filter.
The dry clay and sawdust should be mixed thoroughly. Then water should be added as mixing is continued until the entire mixture can clump together. The clay should then be wedged thoroughly and formed into the filter shape, often through the use of a mold.
For firing, the pots should be heated to 100 or 120 degrees Celsius slowly, over the course of about two hours, to allow all of the pore water, the water from moisture in the air trapped in small holes in the clay, to escape without damaging the pots.
After this the pots can be brought up to the firing temperature of 890 degrees Celsius (cone 012) much faster. They should then be held at this temperature for 8 to 9 hours, then allowed to cool over the course of several hours.
How it Works
Ceramic clay filters work by filtering water through tiny pores small enough to stop many contaminants like clay, dirt, and possibly even bacteria. These pores are formed partially by the sawdust burning off during the firing process, and partially by the pores that form naturally in the clay due to the low firing temperature.
Most pathogens are attached to particles in the water, so removing these particles makes the water much safer to drink.
Clay pores alone would make an extremely slow filter, while a filter with too much sawdust would be ineffective and easily crushed. A mixture of 50/50 by volume and a firing temperature of 890 degrees creates an effective balance between speed, effectiveness, and durability.
Dung fired clay pot filters
- Note: more up-to-date information is needed for this section. The following information dates from approximately 2005.
An Australian materials scientist and potter, Tony Flynn, developed a very similar process for a World Vision project in Manatuto, East Timor, but using open dung fires. He describes the difficulty of accessing suitable kilns in rural East Timor. However he later saw a dung fire with a color that he recognized from his experience as a potter - the color indicating a temperature in the range needed for firing the simple hand-formed clay pots. He then confirmed through experiments that firing the pots on an open dung fire did create an effective water filter.
The initial filtrate (output water) is slightly cloudy, from residue from the firing. After a few times being used, the water should become clearer.
A proposed use is to create a series of three water filters, each feeding water to the next, so that the final output is much more pure.
This could be considered an intermediate technology, as the resulting water is not as pure as using a filter made in a more exacting process; however it is much better than is generally achieved achievable at the village-level, and potentially more achievable than mass-produced filters.
Notes and references
Potters for Peace 
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