Household biosand water filters are proven effective and long lasting. Traditionally, the filters are built from concrete poured into a steel mold. Now, a non-profit called OHorizons has developed a new wooden mold that can be built for one-tenth of the cost using common household tools.

Biosand wood mold guide

What is a biosand filter?

These filters deploy mechanical and microbiological attacks against microbes in water. At their core, biosand filters are long columns of sand and gravel. Microbes and most of the grit and scum that make water look dirty get caught in the sand grains. At the top of the column, a thin layer of living organisms eats the pathogens in water as it passes through. This layer builds up naturally and lends the filter the “bio” prefix in its name.

Studies have shown that these filters strip away up to 90 percent of viruses, more than 99.9 percent of protozoa and helminthes, up to 98.5 percent of Escherichia coli, and up to 85 percent of turbidity. They are also sturdy. Some that were built in Haiti have been going strong for 12 years. For more please see OHorizons’ page on how biosand filters work. And visit the Centre for Affordable Water and Sanitation Technology’s site for information, technical consultation and a construction guide to a traditional steel mold.

The wooden mold advantage

Here, we show how to build a biosand filter mold in photos and videos.

The wooden mold can be made just about anywhere in the world using locally available materials and basic tools, says Dylan Lunney at OHorizons. And with their construction manual, no woodworking experience or even the ability to read are necessary. With this technology, OHorizons plans to provide safe drinking water for 1 million people worldwide within six years, Lunney says.

Skeptics have pointed out that wooden molds fall apart after producing 10-15 filters. But OHorizons’ design appears to be long lasting. Lab and field tests have shown that each mold can make at least 50 filters without breaking down, and more with high-quality plywood, Lunney says.

“Our design is significantly more durable due to the use of our patented collapsible inner core and 2” x 2” supports that hold the outer walls of the Mold together with bolts rather than screws that strip the wood every time it is de-molded,” Lunney says.

Let’s get started.

How to build the wooden mold

The basic steps:

  1. Construct and assemble the wooden mold
  2. Mix and pour concrete into the mold, let it sit overnight
  3. De-mold the bioSand filter and prepare filter media
  4. Install the filter media and operate the biosand filter
Biosand wood mold guide

Consult the construction manual for measurements.

biosand wood mold guide

Use the construction manual and appendix to troubleshoot.

biosand wood mold guide

The wooden mold has been made off-grid using locally available tools. No electricity is required.

How to make a biosand Filter using the wooden mold

biosand wood mold guide

To test your concrete for the proper consistency while mixing, take some in your hand (wear gloves) and make a ball. The ball should hold together rather easily, but also be dry enough that it doesn’t stick much to your glove.

biosand wood mold guide

OHorizons’ mold can make about 50 concrete bioSand filters (and more when using higher quality wood). Each filter can serve enough clean drinking water for a family of five. Fifty filters multiplied by five people per household equals 250 people served with each mold.

Biosand wood mold guide

Apply vegetable oil to the parts of the mold that will come in contact with the concrete. This will make the de-molding process easier and is especially important to prevent the concrete from sticking to the wood for the first couple of filters made.

Biosand wood mold guide

In SW Bangladesh, Sabina and Asit prepare the sand and gravel that will be used in the filters they’re building for their community. Three different sieve sizes help ensure that each layer within the filter will have the correct granular size for the filter to function properly.

Biosand wood mold guide

BioSand Filters are a household appliance placed within a recipient’s home. A safe water storage container is placed in front of the filter to allow more water to be filtered and to make sure the now clean water isn’t recontaminated.

Biosand wood mold guide

Studies have shown that these filters strip away up to 90 percent of viruses, more than 99.9 percent of protozoa and helminthes, up to 98.5 percent of Escherichia coli, and up to 85 percent of turbidity.

Biosand wood mold guide

This ratio is based on actual mold costs (including labor) in Bangladesh. The local cost of materials and labor will vary based on geography; in some places, the ratio of the OHorizons’ Wood Mold to steel molds is greater than 10:1.

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