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Introduction

All of you are familiar with the clay flower pots see at homes and nurseries for growing plants. In fact, this basic system can be modified to make low cost filtration system that are quite suitable for low income communities. Sawayra is working with interns at the University of Houston to develop, test and deploy a water purification solution based on micro-filtration, with Pakistan being the first country selected for field trials.

A low cost water purification filter

What is a ceramic water filter?

Ceramic water filter technology is an attractive choice for developing countries throughout the world, in many cases reducing diarrhoeal disease and viral contamination of water sources by up to 99%. A ceramic water filter is a large vessel that is made from a mixture of clay and combustible material, such as sawdust or rice husks. These materials are usually locally sourced, so that communities are able to produce the filters independently. A simple manual or electric press can be used to shape the filters into a mold. This is followed by firing in a kiln at 890℃ to burn away the combustible material, leaving tiny pores in the clay that give the filter its ability to purify. These microscopic pores are small enough to remove most bacteria, protozoa, sediments and organic matter from impure water. To enhance filtration, each filter is usually coated with a solution of silver, a natural biocide that inactivates bacteria and viruses on contact. The filter is usually placed inside a plastic bucket or receptacle, which collects the purified water after it passes through the filter. Flow rate of water through the filter is between 1.5 – 3 liters per hour – anything faster would signify an issue with the filter. Ceramic water filters can be used to treat rainwater, water from rivers, streams, or ponds, and groundwater. However, any water source that is high in heavy metals, arsenic, or nitrates will not be affected by this type of filter. Each filtration unit typically costs around $15-$25, depending on factory location and resources available.

How are ceramic water filters made? A step by step:

Materials

  • Clay: must be completely dry, plasticized, and free of chemical contaminants. A convenient clay source is unfired bricks from brick factories.
  • Combustible material: rice husks, sawdust, or coffee grounds depending on what is locally available.
  • Kiln fuel: can be wood, compressed rice husks, motor or palm oil.
  • Plastic bucket/receptacle: food grade HDPE plastic.
  • Hammer mill: reduces clay to fine powder. Only one of these is required, and can be purchased commercially for ~$250.
  • Clay mixer: an option only if there is electricity in the area. If not, mixing by hand is possible.
  • Kiln: constructed from brick and mortar.
  • Filter press: either hand or electricity operated.

Step 1: Producing the clay mixture

A hammer mill is used to grind dry clay into smaller particles, which are then screened through fine mesh to filter out particles that are too large. Sawdust (or other combustible material) is dried and then subjected to the same screening process. 60 lbs of clay is dry mixed with 12 lbs of sawdust by hand for ten minutes. 2.5 gallons of water are added to the dry ingredients until a homogeneous mixture is formed. This mixture can be separated into 16 lb balls.

Step 2: Shaping the filter

Each 16 lb ball is taken to the hydraulic press, where it is placed inside the mold. Molds are lined with plastic material so that the clay does not stick and the filter can be easily removed.

Shaping the clay
Shaping the clay prior to loading into kiln

Step 3: Firing

Filters must be dried for 4 – 21 days, depending on local weather conditions. After drying, they are fired in the kiln for 9 hours, at a temperature of 890℃. Temperature can be controlled using pyrometric cones – ceramic objects that bend differentially in response to different levels of heat. Fired filters are cooled down to room temperature, then submerged in water overnight to permeate the newly formed pores.

Cooling-and-Drying-the-filters
Cooling the filters coming of the kiln and then drying them

Step 4: Testing

Every filter must be tested before being put on the market or distributed within a community. Flow rates are measured, and must be within 1 – 3 liters per hour.

Step 5: Silver coating

Two milliliters of colloidal silver at 3.2% are added to 250 milliliters of water. When the filter is completely dry it is dipped into the solution. The solution can also be applied using a brush.

Step 6: Finalizing the product

Filters are placed inside the plastic receptacles, with the aid of fitting rings if needed. A plastic faucet may also be installed at the bottom of the receptacle for ease of use.

How the product works
How the final product works

For more information on this project, please contact the Farrukh Hamza who can be reached at farrukh@sawayra.org

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