HEALTH AND SANITATION
49
<< Continued from page 47
1. Sedimentation
Sedimentation is recommended as simple treatment of
water prior to application of other purification treatments
such as filtration and disinfection methods. It is a physical
treatment process used to reduce the turbidity of the
water. Small particulate suspended matters (sand, silt and
clay) and some biological contaminants are removed from
water under the influence of gravity. The longer the water
is allowed to sediment, the more the suspended solids and
pathogens will settle to the bottom of the container. The
addition of special chemicals or some natural coagulants,
such as indigenous plants, can accelerate sedimentation.
Four common chemicals used are aluminium sulphate
(alum), polyaluminium chloride (PAC or liquid alum), alum
potash and iron salts (ferric sulphate or ferric chloride).
Some indigenous plants are also traditionally used in
some countries, depending on the local availability,
to help with sedimentation. In countries like Malawi,
Sudan, Egypt, and Malaysia, the application of Moringa
oleifera seeds extract in water coagulation and softening
has received a lot of attention. Another natural plant
coagulant known to clarify turbid surface water is
Strychnos potatorum. This type of coagulant is reported
to be used in countries like Southern and central parts
of India, Sri Lanka, and Burma. Much of the suspended
material can be removed by simply allowing the water
to stand and settle for some time. This retention time
(from one hour up to two days, the longer the better) is
required to settle particles to the bottom. Storing water
for at least one day will also promote the natural die-off
of some bacteria.
Simple sedimentation is often effective in reducing water
turbidity, but it is not consistently effective in reducing
microbial contamination. However, most viruses, bacteria,
and fine clay particles are too small to be settled out
by simple gravity se dimentation. However, attachment
of these smaller particles (bacteria and viruses) to
suspended particles would result in the formation of flocs
that can then settle to the bottom of the tank due to their
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1
Post-storage treatment of the water is critical for both
health of the users and maintenance of the system. The
level of treatment will depend on the intended use of
the water. Water used for irrigation does not require the
same level of treatment as water used for potable indoor
purposes. To maximise effectiveness, a multi-barrier
approach where more than one method of treatment is
used is recommended.
approaches comprise: sedimentation, filtration methods,
and disinfection methods (to be covered in the next
article of the series). Those methods are discussed
further in the following sections of this document.
2
increased mass. The addition of coagulants reduces the
time required to settle out suspended solids and is very
effective in removing fine particles.
2. Filtration methods
Filtration is commonly used to reduce turbidity and remove
pathogens. It is a physical process that involves passing
water through filter media. There are several types of
filters; some are designed to grow a biological layer that
kills or inactivates pathogens and improves the removal
efficiency. Various types of filters are used by households
around the world, including bio-sand filters, ceramic pot
filters, ceramic candle filters, and membrane filters.
• Sand filters
Studies have reported that bio-sand filters (BSF) are
capable of removing 81–100% bacteria and 99.98–
100% protozoa from harvested rainwater. Reports
indicate that treatment of harvested rainwater with BSF
can reduce bacteria, viruses, and protozoa by up to
4-log reduction. Further reports show that turbidity can
be reduced by 84% with BSF. Other studies have also
reported on the removal of microorganisms and turbidity
by iron oxide coated sand filters. Results showed that
the coated filter medium was able to remove 99% of
coliforms and 96% lead. Biosand filters, however, have
been reported to have a limited virus removal efficiency.
1.
2.
A common filter
installed to a storage
tank eliminates
organic materials
entering the system.
Organic materials are
the most common
elements to negotiate
in pre-storage
harvesting treatment.
Continued on page 51 >>
August 2018 Volume 24 I Number 6