CAWST Biosand Filter (v10)¶
BioSand & Media Filters • Est. CapEx: $22.50 USD • Lifespan: 20 Years (Standardized)
Technical Overview¶
A household-scale, intermittently operated slow sand filter housed in concrete or sturdy plastic, using a layered bed of sand and gravel to turn unsafe water into safe drinking water. Over 2 million units have been deployed worldwide based on CAWST’s freely available open-source design, reaching deep into remote regions such as the Amazon delta.
Key Specifications & Engineering Parameters¶
| Parameter | Specification |
|---|---|
| Category | BioSand & Media Filters |
| Capital Cost (CapEx) | $22.50 USD |
| Power / Energy | 100% Gravity / Zero Power |
| Target Scale | Household (5–10 people) |
| Standard Lifespan | 20 Years (Standardized) |
| Technology Maturity | Commercially Available |
| Primary Mechanism | Water poured into the filter slowly percolates downward through a biologically active ‘schmutzdecke’ layer at the fine sand surface and subsequent sand bed. Mechanical straining, predation, adsorption, and natural biological processes in the established biofilm work together to reduce bacteria, protozoa, and turbidity using only gravity. |
Engineering Trade-Offs¶
- Zero Operating Cost & 100% Local Materials: Uses locally sourced concrete, graded river sand, and gravel. Zero replacement cartridges or electricity.
- Long Engineering Lifespan: Concrete casing and sand beds do not rot or rust; can operate for 15–20+ years.
- High Flow Capacity: Produces 20–40 liters per hour of clarified water via simple gravity flow.
- Fragile Biological Layer (Schmutzdecke): Inactivation of pathogens relies on a delicate biological ecosystem living in the top 1–2 cm of sand. If the 5 cm standing water layer dries out, the biolayer dies immediately.
- 2–3 Week Ripening Delay: Takes 14–21 days of daily feeding with raw water for a new biolayer to mature. During this period, microbial pathogen removal is poor.
- High User Error Rate: Pouring water aggressively without a diffuser plate, moving the heavy filter, pouring in chlorinated water, or leaving it unused for a week destroys the biolayer.
- Cleaning Resets Biological Protection: The "swirl and dump" wet-cleaning method scrapes off clogging sediment but also strips away the predatory biolayer, resetting the ripening clock.
Cross-Reference & Interactive Tools¶
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