Drop of Hope BioSand Water Filter project introducing Shona Grow and Go childrens curriculum into schools in Zimbabwe

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Drop of Hope - How the BSF works

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How does a BSF work?

1. Pour a bucket of dirty water in the top of the filter. Water will start to flow out of the tube. Put the lid back on the filter. The filter should be filled between 1 and 4 times every day.

2. The top of the filter is called the reservoir. It can hold 12 litres of water- about 1 bucket. Water coming out will flow fastest when the reservoir is full.

3. It usually takes at least 1 hour for the water to stop flowing.

4 After the water stops flowing, the filter must rest. The filter must rest for at least 1 hour before pouring more water in. This is called the Pause Period.

What Makes the BSF Special?

The Biolayer!

In a BSF, small microbes live in the top of the sand. This is called the BIOLAYER . The biolayer is very important for making the water safe to drink. The biolayer takes about 30 days to grow.

Day 1 Many microbes live in water. They are too small to see, but they are there! When you pour water into the filter, the microbes start living in the top of the sand.

Day 15 As you keep using the filter, more and more microbes start to live in the sand. The biolayer grows. The microbes get comfortable and start looking for food.

Day 30 After a few weeks, the microbes start to eat each other. Now every time you pour water in, the microbes living in the sand will eat the new microbes in the water, including the pathogens.

What Does Each Part Do?

Lid - The lid should be tight. It prevents contamination and keeps out unwanted pests.

Reservoir - The top where water is poured in is called the reservoir. It holds about 12 litres, or 1 bucket of water.

Diffuser - The diffuser catches the water poured into the BSF. It can be a box or a plate. It has small holes in it, so the water slowly drips through to the sand. The diffuser prevents disturbing the filtration sand and protects the biolayer from damage when the water is poured into the filter.

Standing Water - When the water stops flowing, there should be 5 cm of water on top of the sand. This layer of water protects the top of the sand and the biolayer from the force of the dripping water. It also keeps the biolayer wet, the biolayer will die if it dries out. The biolayer needs oxygen. Some oxygen can still get to the biolayer through 4 to 6 cm of water. But if there is more than 6 cm of water, the biolayer may die from lack of oxygen.

Filter Container - The container is made out of concreter. It is rectangular depending upon the mold. It holds the sand, gravel and water. It can be painted on the outside to make it look nice.

Filtration Sand - The sand inside the filter is the most important part. The sand removes almost all the pathogens and dirt from the water. The sand must be prepared correctly for the filter to work.

Biolayer - The biolayer is the top layer of sand (1-2 cm or 0.8" deep), where very small microbes live. You cannot see them - they are too small. They eat the pathogens in the water that make you sick.

Separation Gravel - The small gravel stops the sand from moving down and blocking the outlet tube.

Drainage Gravel - The large gravel stops the small gravel from moving and blocking the outlet tube. The large gravel is too big to get inside the outlet tube.

Outlet tube - Water that comes out of the outlet tube is safe to drink. The tube can be made out of plastic or copper.

Safe Storage - You must have a clean safe water storage container to collect the water as it flows out of the outlet tube.

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