Rainwater harvesting
Practical Action
tank inlet, and then replaced again once the initial first flush has been diverted. This method
has obvious drawbacks because there has to be a person present who will remember to move
the pipe.
Other systems use tipping gutters to achieve the same purpose. The most common system
(as shown in Figure 7a) uses a bucket which accepts the first flush and the weight of this
water off-balances a tipping gutter which then diverts the water back into the tank.
The bucket then empties slowly through a small-bore pipe and automatically resets. The
process will repeat itself from time to time if the rain continues to fall, which can be a
problem where water is really at a premium. In this case a tap can be fitted to the bucket
and will be operated manually. The quantity of water that is flushed is dependent on the
force required to lift the guttering. This can be adjusted to suit the needs of the user.
Figure 7b: Floating ball first flush system
Another system relies on a floating ball
that forms a seal once sufficient water has
been diverted (see Figure 7b). The seal is
made as the ball rises into the apex of an
inverted cone. The ball seals the top of
the ‘waste’ water chamber and the diverted
water is slowly released, as with the bucket
system above, through a small bore pipe.
Again, the alternative is to use a tap. In
some systems (notably one factory
manufactured system from Australia) the
top receiving chamber is designed such
that a vortex is formed and any particles in
the water are drawn down into the base of
the vortex while only clean water passes
into the storage tank. The ‘waste’ water
can be used for irrigating garden plants or
other suitable application. The debris has
to be removed from the lower chamber
occasionally.
Although the more sophisticated methods provide a much more elegant means of rejecting
the first flush water, practitioners often recommend that very simple, easily maintained
systems be used, as these are more likely to be repaired if failure occurs.
Filtration systems and settling tanks
Again, there are a wide variety of systems available
for treating water before, during, and after storage.
The level of sophistication also varies from
extremely high-tech to very rudimentary. A German
company, WISY, have developed an ingenious
filter which fits into a vertical downpipe and acts
as both filter and first-flush system. The filter
(Figure 8) cleverly takes in water through a very
fine (~0.20mm) mesh while allowing silt and
debris to continue down the pipe. The efficiency of
the filter is over 90%. This filter is commonly used
in European systems.
The simple trash rack has been used in some
systems but this type of filter has a number of
associated problems: firstly it only removes large
debris; and secondly the rack can become clogged
easily and requires regular cleaning.
Figure 8 : the WISY filter (downpipe and
6 high-capacity below ground versions)
Source: WISY Catalogue