Rainwater Harvesting for Reconstruction
Practical Action
Filtration Systems and First-Flush
It is often necessary to include filtration and ‘first-flush’ components in a RWH system, where
sporadic rainfall washes debris that has collected on a roof into the guttering system. If this
debris is allowed into the storage facility, it may contaminate the water in there.
Filtration may consist of a simple
sand/charcoal/stone bed, which will filter
efficiently but slowly due to the percolation
rate through the sand. A simpler method may
use a cloth or mesh to catch larger debris but
prevent overflow.
More complex systems have been developed
by manufacturers and are in common use in
developed countries; an example is WISY, a
German manufacturer that has created a
downpipe filter that is approximately 90%
efficient. The website can be found here. A
similar solution is provided by Guttermate in
the UK, which is demonstrated in Figure 5.
Many of these solutions are too expensive to
be considered in the PCR context, and
simple techniques as described may have to
be used.
Figure 5: ‘Guttermate’ downpipe filter, Source: www.guttermate.co.uk
A first flush system can be incorporated to
ensure that large amounts of debris do not
block a filter or enter a storage tank after an
initial rainfall following a dry period. An
example is shown below, whereby a floating
ball in a pre-storage tank will rise to the
surface and eventually seal, diverting water
directly into the storage facility.
It may be more practical in a PCR context to
choose a more simple option, as it will be
easier to maintain and duplicate for large
displaced populations. Post-storage
treatment can provide additional filtration to
the stored water and ensure it is of a safe
level for consumption.
Figure 6: Ball First-Flush concept
Illustration: Ahmed, S. (from Practical Action technical
brief ‘Rainwater Harvesting’ (2008)
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