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Concrete BioSand Filters (CAWST Design)

BioSand Water Filter

The BioSand Filter (BSF) is an adaptation of traditional slow sand filtration designed for intermittent household use, pioneered by Dr. David Manz and standardized globally by CAWST (Centre for Affordable Water and Sanitation Technology).

Anatomy & Operation

           [ Untreated Water Poured In Top ]
                         │
                         ▼
           ┌───────────────────────────┐
           │ Diffuser Plate (PVC)      │ ◄── Prevents disturbing biological layer
           ├───────────────────────────┤
           │ Standing Water (5 cm)     │ ◄── Keeps oxygen diffusing into biofilm
           ├───────────────────────────┤
           │ Schmutzdecke Bio-Layer    │ ◄── Microorganisms consume pathogens
           ├───────────────────────────┤
           │ Fine Filter Sand (0.15mm) │ ◄── Mechanical straining & adsorption (40-50cm)
           ├───────────────────────────┤
           │ Separation Gravel (G1)    │ ◄── 5 cm medium gravel
           ├───────────────────────────┤
           │ Underdrain Gravel (G2)    │ ◄── 5 cm coarse gravel around outlet pipe
           └─────────────┬─────────────┘
                         │
                         ▼ Clean Water Outlet Pipe

Biological & Physical Mechanisms

  1. Predation (Schmutzdecke): A biologically active biofilm develops in the top 1–2 cm of sand over 2–3 weeks. Microorganisms prey on incoming bacteria and viruses.
  2. Mechanical Straining: Fine sand grains trap suspended sediments, parasites, and helminth eggs.
  3. Adsorption: Electrostatic forces bind viruses and iron/manganese particles to sand grains.
  4. Natural Die-off: Lack of oxygen and food deeper in the sand bed causes remaining organisms to die.

Specifications

  • Lifespan: 20 to 35+ Years (concrete casing does not degrade).
  • Cost: ~$80 – $150 USD constructed with local sand, gravel, and steel molds.
  • Flow Rate: 0.4 to 0.6 liters per minute when freshly poured; standing water depth strictly controlled to 5 cm.

Official CAWST Engineering Manuals & Training Toolkits

All resources below are open-access, licensed under Creative Commons, and directly downloadable for field implementation:

1. Construction & Sizing Blueprints

  • 📄 CAWST BioSand Filter Construction Manual: Download Construction Guide — Complete engineering blueprint covering concrete steel mold fabrication, aggregate washing, sand sieving specifications ($d_{10} = 0.15\text{–}0.20\text{ mm}$, uniformity coefficient $UC < 2.5$), and underdrain gravel placement.
  • 📄 CAWST Sand Sieve Analysis Job Aid: Download Sieve Sizing Tool — Practical field method for testing local river sand using calibrated wire mesh screens.

2. Operation & Maintenance Protocols

  • 📄 BioSand Filter Maintenance Sheet: Download Swirl & Dump Guide — Step-by-step pictorial protocol for performing the "swirl-and-dump" cleaning technique without destroying the deeper biological sand bed.
  • 📄 Household Water Treatment (HWTS) Implementation Brief: Download HWTS Multi-Barrier Guide — Training community mobilizers on user compliance, habit formation, and preventing post-filtration recontamination.

3. Video Demonstrations & Case Studies

  • 📺 CAWST Open Video Series: CAWST YouTube Channel — Visual demonstrations of BioSand filter operation, hydraulic pause periods, and safe water storage practices.
  • 📺 Field Case Study (Colombia BSF Implementation): BioSand Filter Field Evaluation — Real-world evaluation of community adoption and long-term water quality outcomes.