Concrete BioSand Filters (CAWST Design)

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).

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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.