The Biosand Filter

Presented to the WHO Stakeholder Forum on Household Water Management

January 30, 2007

Derek Baker dbaker@cawst.org

What is a Biosand Filter?It’s a slow sand filter adapted

for household use:1. Height of the water above the sand is 5 cm

This allows oxygen to reach the biofilm layer that forms near the top of the sand

2. Sand removal is not requiredA clean-in-place technique, called ‘swirl and dump’, allows the impurities to be removed without removing the sand

Flow Rate: ~ 30 litres per hour

Cost to Produce: 12 – 40 US$

Filters installed: Over 100,000 in 43 countries

Purpose: To help provide clean water for the 1.1 billion people without this basic need.

Diffusion Plate

Concrete Exterior

Filter Media:Fine Sand

Coarse Sand

Gravel

Water Out

PVC Tube

What’s it made of?

Concrete Concrete ExteriorExterior

Wooden LidWooden Lid

TubingTubing

SandSandBedBed

LargeLargeGravelGravel

FineFineGravelGravel

DiffusionDiffusionPlatePlate

How does it work?

4 Processes of Filtration

2. Predation2. Predation

4. Natural Death4. Natural Death3. Adsorption3. Adsorption

1. Mechanical 1. Mechanical TrappingTrapping

How does it happen?

1. Water is poured into the top of the Filter.

How does it happen?2. The Diffusion Plate slows the

force of the water.DiffusionDiffusion

PlatePlate

How does it happen?3. Water then travels slowly into the

biological layer at the top of the sand.

Biological Biological LayerLayer

How does it happen?

4. Water continues slowly through the sand bed.

SandSandBedBed

How does it happen?

5. After passing through both levels of gravel, the water is propelled up and out.

LargeLargeGravelGravel

FineFineGravelGravel

Household Water Treatment must meet the ‘Criteria of the Poor’

Effective in cleaning the water; including taste, smell and appearanceEasy to operate and maintainAffordable and durable with little or no recurring costsManufactured using local skills and knowledgeUses neither chemicals nor energy

Tools and materials to construct a Biosand Filter

Sustainable Use? A Field Study of 107 households in Haiti97% of the filters were still in use after an average 2.5 yearsFilters were found to be durable and well maintainedHigh level of satisfaction:

Taste, smell and appearanceEase of useSufficient quantity for family

95% believed that their health was better Major user problems:

Lack of knowledge in removing plugging material to restore flow rateSafe water storage practices

Effectiveness?Microbiological Contamination:

Bacteria Removal: 90 – 99%Virus Removal: 70 – 99%Protozoa: > 99.99%Worms: > 99.99%

Physical Contamination:Turbidity reduction: Water out is < 1 NTU

(meets municipal water standards)

Chemical Contamination:Arsenic: 90 – 93% (with adaptation)Iron: 90 – 95% (aesthetic issue)