Bio-sand water filters
Practical Action
The system has proven effective and has become one of the most widely distributed devices
for household water treatment in developing countries.
The filter consists of a layer of gravel overlain with prepared sand media contained within a
filter body or box.
The difference in a traditional slow sand filter and the newer bio-sand filter is that the drain is
piped back up to between 1 and 8 cm above the sand level allowing a shallow layer of water to
sit on top of the sand, where the biofilm (schmutzdecke) is created. This ensures that the
sand within the system is always covered by water even when no water is added to the system.
Experimental evidence shows that oxygen will still reach the organisms in the sand.
Contaminated
water in
Wooden
cover
Diffusion
plate
Biologically active
bio-film develops
on the surface of
the sand
Sand
Gravel
Concrete body
cast on site
in shuttering
mould
Outlet pipe
and tap for
clean water
Coarse
sand layer
Figure 3: A typical bio-sand filter cross section. Illustration: Practical Action.
Bio-sand filters can be fabricated locally because they use
common materials. The filter consists of a layer of gravel
overlain with prepared sand media contained within a filter body
or box. One of the initial activities is the selection and
preparation of the sand and the gravel that is used in the bio-
sand filter, this activity is important for the effective and
efficient operation of the filter. Poor selection and preparation
of the sand and the gravel can result in the filter performing
badly and resulting in more work rectifying the problem.
The body is often constructed from concrete using a metal
mould. Construction guidelines are available form
http://www.cawst.org and http://www.biosandfilter.org/.
However, other deigns use oil drums or plastic containers and
2
Figure 4: Removal of the concrete
filter from the metal mould. Photo:
Soluciones Prácticas.