Household Water Treatment and Safe Storage Factsheet: Source ...

Household Water Treatment for Fluoride Removal
Factsheet: Clay
Potential Treatment Capacity
Very Effective For: Somewhat Effective For: Not Effective For:
Fluoride
Turbidity
Bacteria
Protozoa
Helminths
Viruses
Chemicals
What Is Clay?
Clay is a very fine textured earthy material. It is
composed mainly of very small particles of
hydrous aluminium silicates, other minerals
and may include other materials. It is used for
making pottery (ceramics), brick and tile. Both
clay powder and fired (baked) clay are capable
of removing fluoride and other contaminants
from water. The ability of clay to clarify turbid
water is well known and it is believed to have
been used in households in ancient Egypt
(WHO, 2006).
Clay can be used in powder form in a bucket
system, or freshly fired clay/brick chips can be
used in column filters. The use of clay powder
in column filters is possible, but it is
troublesome because of difficulties in packing
the columns and controlling the flow.
How Does It Remove Contamination?
Clay is a good flocculent and absorbent for
removing fluoride, because of its relatively high
density (the particles are heavy).So once the
fluoride attaches to the clay particles, it settles
out well.
The best clay for fluoride removal has high
levels of iron oxide and aluminium (e.g.
bauxites, goethite/ hematite). The removal
process is an ion exchange between fluoride
and iron or aluminium.
Operation
Domestic clay column filters are normally
packed using fired (burnt) clay chips. The fired
clay chips can be found as waste from the
manufacturing of brick, pottery or tile.
The Clay Column Defluoridator (pictured) is an
example of a burnt clay filter used in Sri Lanka.
It is a layered column of
freshly fired brick chips,
pebbles and crushed
coconut shells. Water is
passed through the unit
upwards (from the bottom
to the top). The filters can
be made out PVC pipe or
cement. In the columns,
brick chip sizes are
generally between 15 and
20 mm.
The firing/burning of the
clay is important because it
activates the aluminium
oxide which reacts with the
fluoride. Once the clay is
fired it is also easier to
break into clay chips.
Column filter used in
Sri Lanka
(Credit: WHO, 2006)
In the bucket system, clay powder is added at
large dosages to water, stirred and left to settle
for several hours. The clean water is scooped
or decanted off the top. The sludge in the
bottom of the bucket must be disposed of
appropriately away from water sources. This
method cannot be used for source water with
high concentrations of fluoride (above 3 mg/L,
WHO 2006).
Clay pottery can also be used if the water is
allowed to drip through the clay. Since water is
often stored in clay pots in many cultures this
method may be quite feasible in communities
where the aluminum oxide concentration in the
soil (and therefore in the clay pots) is high. The
storage time in the pots varies depending on
the aluminum oxide level in the clay.