Étape 1 - Step 1 – how it works - role of the different materials

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, [1]). 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 [18]

"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 [2] [3]. 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).

Étape 4 - Step 4 – Production - Preparation of raw materials

• Clay: Depending on its provenance, clay may need to be crushed, sieved and dried before it can be used.
• Combustible material: Depending what it is (sawdust, rice husks, etc.), the combustible material will need to be cut, sieved, dried and bagged.

Étape 5 - Step 5 – Production - Mixing of raw materials

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

Étape 7 - Step 7 - Production - Pressing the clay cubes to give them the shape of the filter

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:

• The plans are available in open-source form [19]
• This document describes how small producers can make a hydraulic press [17]

Étape 8 - Step 8 - Production - Finishing of the surface and marking/numbering each filter

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.

• Use a plastic scraper to smooth out the edges of the border.
• Mark each filter with a date, serial number and manufacturer's name using a metal "stamp". A database can then be used for filter tracking

Étape 9 - Step 9 - Production - Drying the filters

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.

Étape 10 - Step 10 – Production – firing filters in an oven

• The firing process is done initially at a low temperature of around 100ºC for 2 hours. This will remove excess water remaining.
• Finally, the temperature is gradually increased to high temperatures. At 866 ° C, clay vitrifies as the silica and alumina molecules melt and bind to form a new mineral with needle-like fibrous structures. Vitreous clay is hard, resistant to stress and does not change shape when water is added. After vitrification, clay has a new chemical structure and cannot be powdered and reused as clay dust.
• Fire at 900 ° C for 9 hours.

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" [20] [21][20] [21]

Étape 11 - Step 11 - Production - Flow testing of each filter

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.

• A high flow rate is an indicator of cracks or imperfections in the filter which could reduce the efficacy of the filtration and not remove bacteria, parasites and other necessary impurities. In addition, a high flow rate reduces the time the filtered water is exposed to the silver solution, thus reducing the ability to kill bacteria present in the water.
• Too low a flow rate may prove impractical for households who may choose to no longer use the filter and therefore waste their investment and put their health at risk.
• Fill each filter with water and measure the level of water after a set period of time.

Étape 13 - Step 13 – Production - Packing of filters

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.

Étape 14 - Step 14 - Use - Maintenance - Replacement

It is important to distribute these filters with good instruction leaflets on how to use, maintain and replace the filters.

• Operation and maintenance: the user pours water into the filter. The water moves slowly through the ceramic pot by gravity; it is then collected in a hygienic storage tank. Users have access to the treated water via a tap.
• Maintenance: the lower tank, tap and cover should be cleaned regularly. The ceramic pot should be cleaned every 6 months with a cloth or soft brush, taking care not to touch the bottom of the pot with anything that may be contaminated.
• Replacement: Ceramic pots should be replaced every 2-3 years, or sooner if visible cracks appear.