A ceramic water filter is a system for purifying unsanitary water. This tutorial aims to show how a ceramic water filter works and how to build one on a semi-industrial scale.
In 1990, approximately 2.3 billion people do not have access to drinking water in the world (source: UNICEF - UN). Today in 2020, 750,000 people still drink unsanitary water, making it the leading cause of non-age-related death in the world.
Locally produced ceramics have been used to filter water for hundreds of years. The water is poured into a porous ceramic filter pot and is collected in another container after passing through the ceramic pot. This system also allows for safe storage until the water is used. Ceramic filters are usually made from clay mixed with a combustible material like sawdust or rice husks. Sometimes colloidal silver is added to the clay mixture before firing or it is applied to the fired ceramic pot. Colloidal silver is an antibacterial which helps inctivatae pathogens, while preventing the growth of bacteria in the filter itself.
Pathogens and suspended elements are removed from water by physical processes such as mechanical entrapment and adsorption. Quality control regarding the size of the combustible materials used in the clay mixture ensures that the pore size of the filter is small enough to prevent contaminants from passing through the filter. Colloidal silver facilitates the treatment by breaking the membrane of the cells of pathogens, causing their death.
This filter was developed in 1981 by Dr Fernando Mazariegos of the Industrial Research Institute of Central America (ICAITI) in Guatemala. The aim was to make water contaminated with bacteria safe for the poorest by developing an inexpensive filter that could be manufactured at community level. The professor decided to freely bequeath this knowledge to Humanity, and began to train potters around the world to produce these filters locally with the NGO Potters for Peace. There are now 61 factories working with this model in 39 countries around the world!
This tutorial show how the ceramic filter works and outlines the main stages of manufacturing. It is aimed mainly at entrepreneurs rather than at individuals. Don’t try to create this technology at home (you need an oven, you need to test materials, etc.). If you are interested in setting up a small factory like this, you will need more training. The Potters for Peace organization in partnership with CAWST and the company Ecofiltro (which we visited in Guatemala) offer this kind of training. All this knowledge is freely available in open-source form.
Clay is the base material for the water filter device. A clay pot allows for extremely slow movement of water through the natural pores that exist between the fired clay tablets. The size of these pores has been measured (using an electron microscope) and found to be between 0.6 and 3.0 microns (μm).
They are able to eliminate most bacteria, protozoa and helminths (Lantagne, 2001a), as well as dirt or sediment and organic matter.
The clay used to make classical pottery may be suitable for the production of water filters. However, hydraulic conductivity and pore size can vary widely depending on the type of clay, potentially to the point of not being suitable for flow rates and / or microbiological removal (Oyanedel-Craver and Smith, 2008, in Lantagne et al, 2010, ). A high content of sand or silt in the clay can reduce cross-linking of the clay and weaken the structure of the filter. On the other hand, an overly refined clay (smaller particles) has a greater water holding capacity and is therefore more prone to shrinkage and cracking during firing.
As the characteristics of clay are a critical factor in the success or failure of ceramic water filter production, it is recommended that you carefully study the sources and potential types of clay before committing significant resources. Potters for Peace has produced a document providing details of the clay test, listed in the "Reference Note" section 
"Combustible" organic materials, such as sawdust or ground rice husks, are added to the clay mixture. When exposed to the high temperatures of the kiln, the "combustible material" burns, leaving behind cavities in the fired clay. Water moves more easily through cavities than through pores in clay. Therefore, the presence of the cavities decreases the distance that water must travel through the clay substrate, and therefore increases the overall flow rate of the filter.
It is important to carry out tests on your materials. The ratio of clay to combustible material is important for establishing the flow rate and therefore the efficacy of the filters.
Colloidal silver is a solution made of suspended nanoparticles of silver and silver ions. It has been used as a natural disinfectant in medicine for many years. Although the exact mechanisms of bacterial destruction are not yet fully understood, it appears that colloidal silver breaks down the cell walls of bacteria and then binds to their proteins, thus disrupting their function  . Today it is mainly produced by electrolysis.
The silver applied to the inside and outside of the filter is absorbed into the pores of the clay. The silver ions are reduced to elemental silver and form colloids inside the walls of the filter. Silver acts as a biocide against bacteria when there is sufficient contact time (= not too large pores).
All the laboratory and field efficacy values are the result of independent trials. The details are given in the links in the "Notes and References" section
The main steps for producing a ceramic filter are listed in order below :
Clay powder and combustible material (sawdust, ground rice husks, etc. ) are mixed dry, then water is added evenly and mixed well to form a modulable and homogeneous paste.
It is wise to make sure density gradient is consistent throughout the clay mixture to minimise potential defects during the clay firing process (removal of air pockets etc.). The right mix and machinery is therefore crucial.
Proportion used by RDIC:
30 kg of clay powder + 8.9 - 10 kg of rice husks + 12.5 L of water
The wet clay mixture can be manually shaped into cubes before being pressed. But it is strongly advisable to use a machine to compress and extrude the clay mixture as cubes. The extruder is similar to those used for extruding clay bricks, but the outlet opening is larger to obtain the size of clay cube required for pressing.
The clay cubes we want should weigh approximately 8kg.
Cut a cube of the same length and put it in the press.
Using a hydraulic press significantly reduces the labour requirements of the process and greatly increases the efficacy and consistency of the product. The filters are squeezed between a male mould and a female mould which are covered with plastic bags to prevent sticking. The hydraulic press includes a fixed plate in the lower mould that pushes the pressed mould outward when the mould opens.
This press was originally developed and built by the Potters for Peace teams:
Minimum surface finishing is required after molding. It is done to ensure rim strength and surface uniformity. Filters are labelled to indicate pressing date, batch and filter number.
Drying the filters eliminates excess water before firing in the oven. If the water is not removed before firing, it will heat up, evaporate and expand, causing the filter to crack. At the end of the firing process, the filter elements will have lost more than 3 kg of water compared to the first time they are pressed.
Dehydration: filters are initially dried on air drying racks. Ideally in a warm place with good ventilation. After this initial drying period, the filters may retain their shape but are not solid and remain soluble in water.
You can use different types of ovens and different types of fuels (wood, gas, etc.). Potters for Peace has created two documents teaching you build a traditional clay oven "Mani Kiln"   
Flow control is an important step in quality assurance which indicates the rate at which water flows through the filter. Once the clay formula and production process has been established, the flow test is performed on EACH filter produced to ensure its viability.
A filter passes the control if its flow rate is within 1.5-3L per hour. Otherwise, it is downgraded and will have to be destroyed.
Silver is known for its ability to kill microorganisms. Colloidal silver has been used in hospitals and clinics as an antimicrobial agent for cuts, burns and to prevent eye infections in newborns (Lantagne, 2001) and to disinfect drinking water and swimming pools (Russell , 1994, in Lantagne, 2001). The silver is used by NASA to purify water from space flights (NASA CASI, 2007).
For example, the RDIC manual describes:
Each filter must go with a container fitted with a tap. The containers may be made of different materials (plastic, ceramic, glass, stainless steel). Make sure when packaging that the contents of the cardboard box will be protected during transport.
It is important to distribute these filters with good instruction leaflets on how to use, maintain and replace the filters.
If you are interested in this technology and wish to learn more about how to set up a factory locally, have a look at these case studies proposed by the CAWST:
Notes and references This tutorial was written by Guénolé Conrad following the visit of the Ecofiltro factory in Guatemala in November 2020, which was a stopover on the Nomade des Mers expedition.
This tutorial is largely based on the open-source documentation provided by RDIC, CAWST and Potters for Peace. Some photos from these tutorials have been used.
1. Lantagne, D., Klarman, M., Mayer, A., Preston, K., Napotnik, J., Jellison, K. (2010). Effect of production variables on microbiological removal in locally-produced ceramic filters for houshold water treatment. International Journal of environment Health Research.
2. Latagne, D. (2001) Investigation of the Potters for Peace Colloidal Silver Impregnated Ceramic Filter
Translations:Filtre à eau céramique/80/en 3. Effet de l'argent colloidal comme désinfectant: Ehdaie Beeta, Su Yi-Hsuan, Swami Nathan S., Smith James A., ; (2020) Protozoa and Virus Disinfection by Silver- and Copper-Embedded Ceramic Tablets for Water Purification
15. Removal of waterborne bacteria from surface water and groundwater by cost-effective household water treatment systems (HWTS): A sustainable solution for improving water quality in rural communities of Africa.