Build a DIY biosand filter from a 5-gallon bucket

A biosand filter uses layers of sand and gravel to remove 90-99% of bacteria and 100% of parasites from water. Developed for use in developing countries, it requires no power, no replacement parts, and costs under $20 to build.

Steps

1. Drill a hole near the bottom of a food-grade 5-gallon bucket. Insert a 1/2-inch PVC elbow and seal with silicone.
2. Layer the filter media from bottom to top:
3. 2 inches of coarse gravel (pea-sized)
4. 2 inches of fine gravel
5. 12-14 inches of clean, washed sand (construction-grade play sand works)
6. 1 inch of standing water above the sand (the "biolayer")
7. Season the filter: Run water through it daily for 2-3 weeks. A biological layer (schmutzdecke) forms on the sand surface that actively consumes pathogens.
8. Use: Pour rainwater in the top, collect filtered water from the spigot at the bottom. Flow rate: ~1 liter per minute.

Why It Works

The sand physically traps sediment and parasites. The biological layer on top consumes bacteria. Combined removal: 90-99% of bacteria, >99% of protozoa (Giardia, Cryptosporidium), and nearly all turbidity.

Tips

• The biolayer needs daily feeding (water passing through) to stay alive — do not let the filter dry out
• Biosand filters do NOT remove viruses reliably — follow with boiling or UV treatment for full safety
• When flow slows, gently stir the top 1 inch of sand and remove a thin layer ("swirl and dump") to restore flow