Biosand Filter
A household slow-sand filter with a biological layer that removes pathogens and turbidity.
What is it?
The biosand filter is a concrete or plastic container of layered sand and gravel. A biological layer (“schmutzdecke”) develops at the sand surface and helps remove pathogens as water passes through.
Why does it matter?
It is durable (decades with care), needs no consumables, and handles turbid water, making it suited to long-term household use where replacement parts are scarce.
How does it work?
Water poured on top passes down through fine sand. Predatory microorganisms in the top biolayer consume pathogens, while mechanical straining and adsorption remove the rest.
Who benefits?
Households wanting a long-lived, low-running-cost option, and programs prioritising durability over portability.
Who may be disadvantaged?
Households that use it intermittently: the biolayer needs regular feeding (daily use) to stay effective, so sporadic users get weaker protection.
What evidence exists?
Studies report substantial reductions in bacteria and diarrhoeal illness, though performance rises over the first weeks as the biolayer matures.
What tradeoffs exist?
Heavy and immobile once filled; the maturation period means it is not instantly effective; virus removal is moderate.
Common misconceptions
The biolayer is a feature, not dirt to be scrubbed away — over-cleaning resets protection. It is not “finished” on day one.
What you can do next
See the rural filter-program case study, which deployed biosand filters, and the community water committee node on sustaining them.