Questions & Answers

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# Question Answer

Where can I find information on the wood mold?

The wood mold was created by OHorizons. Their website, which includes a manual in their resources section, can be found here:


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Where can I see the biosand filter videos?

The English version of the biosand filter videos can be seen on youtube:

To see other CAWST videos, such as webinars or links to videos on water, sanitation, or hygiene made by other organizations but which we like, visit CAWST's youtube channel here:
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Can I build a biosand filter to use in North America or Europe?

The biosand filter, in the versions supported by CAWST, is a technology addressing the great need for potable water in the developing world. The filter does not provide microbiological treatment up to the water quality standards of developed nations, nor does it remove most chemical contaminants.

The biosand filter is most appropriate for use in areas where people do not have access to piped, potable water to their home; pathogens in the water are making, or could make, people sick; there is limited or no access to electricity; and resources and access are so limited that people are unable to purchase or maintain more sophisticated home treatment technologies.

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Why does CAWST recommend that filters be ripened for 1 month, when other organizations recommend 2 weeks?

One of the ways by which the biosand filter removes bacteria and viruses is through biologically mediated mechanisms, such as predation and adsorption onto biofilms. In addition, some virus removal is thought to be due to attraction to positively charged ions which accumulate on the negatively charged sand particles. However, these processes take time to develop. A newly installed biosand filter typically removes only about 0.3 log, or about 50%, of E. coli. This increases over the following weeks as the biolayer develops and the filter “ripens”. However, the exact amount of time required to ripen a BSF is highly variable.

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What types of tubing can I use for the outlet tube in the Version 10 Biosand filter?

The tubing used in the Version 10 Biosand filter can be polyvinyl chloride, polyurethane or polyethylene – as long as it is safe (for drinking water), flexible (won’t give kinks) and has an inner diameter greater than or equal to 6mm, and an outer diameter less than or equal to 9mm.
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Can I use weaved mesh for sieving sand and gravel as oppsed to welded mesh?

Typically weaved mesh will have smaller openings so ensure that the one that you buy will give the right size sand particle (e.g <0.7mm for the filter medium). Weaved mesh will also tend to be heavier and more expensive. Other than that there shouldn’t be any problems
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Can I restart a biosand filter that was abandoned?

There are many things that are unknown about the biosand filter, and the effect restarting a filter that has not been in use for some time is one of them. This is an area that is currently being researched.

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I have seen small worms crawling on the top of the sand in my biosand filter. What should I do?

It isn’t so unusual to have worms living in the filter. It isn't necessarily a bad thing. The filters are complex aquatic ecosystems, and there are lots of creepy crawlies who can make their home in the filter. In conventional slow sand filters, little critters are even considered a good thing as they can ensure aeration. They generally stick to the surface so a complete re-install isn’t be necessary.

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Does the biosand filter add minerals to the water? Specifically, if the water has been through a reverse osmosis process can the biosand filter return some healthy minerals to the water?

To our knowledge, there has not been any research on the topic.
However, it is known that, for the first year at least, water that flows through the biosand filter increases in pH and total dissolved solids.  This is due to the water dissolving minerals from the sand (and concrete, in the case of a concrete filter). This effect wears off over time as the readily dissolvable minerals on the sand surface get used up.

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Where can I find a BSF mold manufacturer in country X ?

We maintain a list of Concrete Biosand Filter mold manufacturers here:  

If you know a reliable mold manufacturer that is not listed. You can submit their details using this form:

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