GRIDFALLSurvival
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Water · 1 day + 3 weeks to mature · Advanced

Biosand Filter for a Family

A long-life filter where a living biological layer removes up to 99% of bacteria—serves a household for years.

This is for educational purposes only. Not professional medical, electrical, or safety advice.

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  1. 01

    Build the outlet

    Run a PVC tube from the bottom of the container up the side and out, ending 2 in. above the final sand level.

  2. 02

    Layer it

    Bottom up: 2 in. gravel, 2 in. coarse sand, ~20 in. fine washed sand.

  3. 03

    Keep water standing

    The outlet height keeps 2 in. of water above the sand at all times—this feeds the biolayer.

  4. 04

    Add a diffuser

    A perforated plate above the sand prevents pouring from disturbing the biolayer.

  5. 05

    Let it mature

    Pour water in daily for 3 weeks. Flow ~1 L/min. Disinfect output anyway for full safety.