Background
Access to clean drinking water is a basic human need. Sadly, in too many places around the world, the water available for drinking is unfit for human consumption. In developing countries, the problem is particularly severe. For example, the World Health Organization (WHO) has estimated that 85 million people dont have access to safe drinking water in Pakistan. UNICEF study concluded that 20% to 40% of people admitted to hospital in Pakistan are there due to water borne diseases such as Hepatitis, Cholera, Typhoid and Dysentery, which are also responsible for almost a third of all deaths. Other developing countries in Asia and Africa have similar horrifying statistics. Sawayra is working on low cost water filter to remove bacteria, protozoa and turbidity from drinking water using sustainable methods based on local resources. The project is being done by a team on engineering interns who at NED University in Karachi, Pakistan. A second parallel effort is being planned for another location.
Water Quality Data
As a first step, the project team collected water from various parts of the city and submitted to the Pakistan Council of Research on Water Resources (PCRWR) for testing. Each water quality test costs about $28, and a blind test was also done to confirm repeatability of the test method. The results (shown below) indicate that five of the seven locations have public water that is unfit for drinking due to high bacteria content – as high as ten to sixteen times higher than safe drinking limit.. One location has water of marginal quality, while one final location has water that is safe.
Filter Design
Clay 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%. The 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. Clay 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 purified by this type of filter. Each filtration unit typically costs $1-$3 and would be fitted with a plastic container as shown below:
Materials and Manufacturing
- 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.
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.
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.
Project Status
The project team is currently working on a Taguchi design of experiment to finalize the configuration that will remove bacteria consistently. For more information on this project, please contact the project manager, Farrukh Hamza who can be reached at farrukh@sawayra.org
