Water Treatment During Reconstruction
Practical Action
Multi-Stage Filtration
Slow-sand filters are only effective if the turbidity of the water is sufficiently low. One way to
ensure that this is the case is to include a multi-stage filtering process in the construction; the
initial stage is known as a ‘Roughing Filter’, due to the large size of the gravel and crushed
stones.
The diagram below explains the basic layout of a multi-stage filtration plant. More details are
available from the International Water and Sanitation Centre, and can be found at
http://www.irc.nl/page/31601.
Filtration Stages
Terminal
Disinfection
Raw
water
source
Dynamic
Gravel Filter
Coarse
Gravel Filter
Slow Sand
Filter
Safety
Barrier
Figure 5: Layout for a multi-stage filtration system (Illustration by A. Elias based
on Galvis, G. IRC 2006)
Solar Distillation
Distillation can remove virtually all salt, nitrates, and heavy metal such as arsenic from water as
well as pathogens and other biological contaminants from water. Solar distillation is a low cost
but low volume approach to treating water. Essentially water is evaporated by the sun and
condenses on a cooler surface from where it is collected. A common design of a solar still is
shown below; a glass or plastic plate is fixed on top of the still to increase the temperature in the
still, and the bottom of the still is lined with a black material such as bituminous paint, butyl
rubber, epoxy enamel, fibreglass painted black or aluminium painted black, to act as a heat
absorber.
It is also important that the whole still is well-insulated to improve efficiency. The sides and base
of the still are typically brick or concrete. Moulding of stills from fibreglass was tried in
Botswana. This was more expensive than a brick still and more difficult to insulate sufficiently,
but has the advantage of being transportable. This technology may also be suitable for certain
transitional stages. More information is available in the Practical Action technical brief Solar
Distillation.
Figure 6: Schematic of a single-basin still. Illustration: Otto Ruskulis for Practical Action.
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