Biosand filters purify dirty water so that it becomes safe to drink. They are very useful, both in rural and urban areas which lack safe piped water. Calgary University, Canada, developed an innovative low-cost design using concrete.
In Uvira, Democratic Republic of Congo, Tearfund has introduced biosand filters in two areas of the city where water-borne diseases, such as cholera, are a serious problem. Their objective is to encourage sustainability by providing the filters for sale, after first ensuring local people are aware of the benefits of the filters so they will want to buy them. A social enterprise, BushProof, trained technicians in the production and use of the filters.
Health centres in Uvira have been very pleased to receive sample filters. This means patients now have safe drinking water and the medical staff are able to promote the technique to patients and visitors.
This huge puppet is used to explain how the Biosand filter works!
The impact of biosand filters
These filters are really appreciated by the people in Uvira. They provide safe drinking water in a simple way. When correctly used they help to control nearly all water-borne diseases such as diarrhoea, cholera and typhoid. So far, 100 households in Uvira have bought, and are using, the filters after training.
Making a biosand filter
A strong metal mould is needed to produce the filters. This requires welding skills. The moulds should last for many years and most local metal workshops can make one.
Each filter contains six items (see drawing):
- The concrete outer shell, built using 1⁄2 sack of cement mixed with 2 sacks of gravel and 11⁄2 sack of sand
- A length of PVC pipe, 15mm diameter
- A diffuser plate full of small holes, made from metal or pottery
- A 40cm layer of clean, washed, medium-grade sand
- A 5cm layer of small gravel
- A 5cm layer of small stones or large gravel.
The cement mix is prepared and placed in the mould after putting the plastic hose into position. The mould is then closed up like a shell. After 2 days the mould is opened and the filter removed. Any holes can be filled to give a smooth surface. It is important to keep the concrete damp for 5 days so that it will not crack because of heat or dryness. The cement for one filter costs about US $6. The filters can be sold for between US $6 to $12, which ensures a profit for the producers but is still affordable for most households.
Installing the filter
This is done once the filter is in a permanent place – in the cooking or living area. Place a 5cm layer of small stones or large gravel at the base, followed by a 5cm layer of small gravel. Then fill the filter up with washed sand until it is exactly 5cm below the level of the diffuser plate. Rest the diffuser plate on the small ledge. Fill the filter with water. The diffuser plate should always be left in place when pouring in water.
However, the filter is not yet ready for use. A layer of what looks like dirt must first be allowed to develop on the surface of the sand. This is sometimes known as the schmutzdecke (which is a German word for dirty layer!). This layer is the most important part of the filter. It acts as a fine filter and actually ‘eats up’ some of the disease-causing microbes in the water. Lower levels in the sand continue this process.
Water must be poured into the filter every day. It takes two to three weeks for the schmutzdecke to develop fully. During this time the water is much improved, but not yet fully safe to drink. People should be encouraged to wait for three weeks before using the water directly for drinking.
Using the filter at home
After three weeks, filtered water will be safe to drink – tests show that around 99% of microbes and contaminants are removed. Water is simply poured in and collected from the spout in clean containers. The filter holds 20 litres of water.
After filling the filter, water will need to be collected in a clean jerrycan. Normally one litre of water is filtered every minute, so it will take 20 minutes for the contents of a 20 litre bucket to pass through the filter. The filter can be used as often as needed.
Maintaining the filter
Users must be given clear information about how to use and maintain the filters. Maintenance is very simple and free. There are just a few important points that people need to remember:
- If water is not poured into the filter every day the schmutzdecke can become less effective.
- Children and animals must not be allowed to touch the spout so that it remains clean.
- The filter must not be knocked or moved.
- The diffuser plate must always be in place when water is poured in to prevent damage to the schmutzdecke.
- Over time, the schmutzdecke may become very thick so that water takes a very long time to pass through the filter. If this happens there are two options:
GENTLE STIRRING Block the spout and fill the filter with water. Stir the water very gently and slowly with a clean hand. Don’t swirl too fast or the sand layers will be disturbed. Scoop the muddy water out with a cup, taking care not to touch the sand. You can repeat this a few times until the water is no longer very dirty during swirling. Unblock the spout and allow the water to pass through the filter as normal. It will be safe to drink almost immediately.
THOROUGH CLEANING Remove carefully the top 2–5cm of sand, wash it and replace it. Unless gentle stirring fails to restore a good flow rate, this method is not really recommended because it disturbs the schmutzdecke. It is therefore really important to wait three weeks before using the water again to ensure it is safe to drink.
Cleaning should be done only if the rate of water flow becomes too slow. Careful monitoring is recommended to ensure people are confident about how to do this. During the three weeks of waiting needed after thorough cleaning, other low-cost methods of making water safe for drinking can be used, such as boiling, SODIS (see Footsteps 51 and www.sodis.org) or using water from a neighbour’s filter.
Compiled with information from Nathalie Vezier, Disaster Management Team, South Kivu (Email: email@example.com) and Adriaan Mol (Email: info@biosandfi lter.org)
Photos: BushProof Illustration: Rod Mill
The Swiss-based NGO, Medair, trained technicians to make these filters in Machakos, Kenya, in the late 1990s. They educated people about their potential. Medair worked in the area for only a year, during which 400 filters were sold. A recent evaluation showed that over 2,000 filters were sold over four years, providing health benefits while generating income! The business of producing filters proved very successful and the technicians had set up businesses in new areas to meet the demand.
www.biosandfilter.org is a very useful website which contains detailed technical information on how to build the metal mould and how to produce the filters.
BushProof is an organisation that provides training in the production of the filters.