May 2010 Edition

2 Biosand Filter Overview
2.1 What is the Biosand Filter?
The biosand filter (BSF) is an adaptation of the traditional slow sand filter, which has
been used for community water treatment for almost 200 hundred years. The biosand
filter is smaller and adapted for intermittent use, making it suitable for households. The
filter container can be made of concrete or plastic and is filled with layers of specially
selected and prepared sand and gravel.
2.2 History of the Biosand Filter
Dr. David Manz developed the household biosand filter in the 1990s at the University of
Calgary, Canada. Dr Manz has trained many organizations on the design, construction,
installation, operation and maintenance of the biosand filter. He also co-founded CAWST
in 2001 to provide the professional services needed for the humanitarian distribution of
the filter in developing countries. As of June 2009, CAWST estimates that over 200,000
biosand filters have been implemented in more than 70 countries around the world.

1. Lid – Tightly fitting lid prevents
contamination and unwanted pests.
2. Diffuser – Prevents disturbing the
filtration sand layer and protects the
biolayer when water is poured into the
3. Filtration Sand Layer – Removes
pathogens and suspended solids.
4. Outlet Tube – Required to conduct water
from the base to the outside of the filter.
5. Filter Body – Holds the sand and gravel
6. Separating Gravel Layer – Supports the
filtration sand and prevents it from going into
the drainage layer and outlet tube.
7. Drainage Gravel Layer – Supports the
separating gravel layer and helps water to
flow into the outlet tube.

How Does the Biosand Filter Work?
The biosand filter has five distinct zones: 1) inlet reservoir zone, 2) standing water zone,
3) biological zone, 4) non-biological zone, and 5) gravel zone.
1. Inlet Reservoir Zone – Where water is poured
into the filter.
2. Standing Water Zone – This water keeps the
sand wet while letting oxygen pass to the
3. Biological Zone – Develops at the top 5-10 cm
(2-4”) of the sand surface. The filtration sand
removes pathogens, suspended particles and
other contaminants.
As in slow sand filters, a biological layer of
microorganisms (also known as the biolayer or
schmutzedecke) develops at the top 1-2 cm
(0.4-0.8”) of the sand surface.
4. Non-Biological Zone – Contains virtually no
living microorganisms due to the lack of nutrients
and oxygen.
5. Gravel Zone – Holds the sand in place and
protects the outlet tube from clogging.
Pathogens and suspended solids are removed through a combination of biological and
physical processes that take place in the biolayer and within the sand layer. These
processes include: mechanical trapping, predation, adsorption, and natural death.

Mechanical trapping. Suspended solids and pathogens are physically trapped in
the spaces between the sand grains.
Predation. Pathogens are consumed by other microorganisms in the biolayer.
Adsorption. Pathogens become attached to each other, suspended solids in the
water, and the sand grains.
Natural death. Pathogens finish their life cycle or die because there is not enough
food or oxygen for them to survive.

Created by

CAWST is a Canadian non-profit organization focused on the principle that clean water
changes lives. Safe water and basic sanitation are fundamentals necessary to empower the
world’s poorest people and break the cycle of poverty. CAWST believes that the place to start
is to teach people the skills they need to have safe water in their homes. CAWST transfers
knowledge and skills to organizations and individuals in developing countries through
education, training and consulting services. This ever expanding network can motivate
individual households to take action to meet their own water and sanitation needs.

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