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Rain
Collection
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Rainwater harvesting is a
technology used for collecting and storing rainwater from rooftops,
the land surface or rock catchments using simple techniques such as
jars and pots as well as more complex techniques such as underground
cisterns. Commonly used systems are constructed of three principal
components; namely, the catchments area, the collection device, and
the conveyance system.
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Rooftop runoff is the
easiest rainwater to collect. It is collected via gutters and
down spouts to rain barrels and/or larger cisterns either above or
below ground. As the rooftop is the main catchment area,
the amount and quality of rainwater collected depends on the area and
type of roofing material. Reasonably pure rainwater can be collected
from roofs constructed with metal roofs, aluminum or cement sheets,
tiles and slate.
- Storage tanks for collecting rainwater
harvested using guttering may be either above or below the ground.
Precautions required in the use of storage tanks include provision of
an adequate enclosure to minimize contamination from human, animal or
other environmental contaminants, and a tight cover to prevent algal
growth and the breeding of mosquitoes. Open containers are not
recommended for collecting water for drinking purposes. Various types
of rainwater storage facilities can be found in practice. Among them
are cylindrical Ferro cement tanks and mortar jars. The Ferro cement
tank consists of a lightly reinforced concrete base on which is
erected a circular vertical cylinder with a 10 mm steel base. This
cylinder is further wrapped in two layers of light wire mesh to form
the frame of the tank. Mortar jars are large jar shaped vessels
constructed from wire reinforced mortar. The storage capacity needed
should be calculated to take into consideration the length of any dry
spells, the amount of rainfall, and the per capita water consumption
rate. In most of the Asian countries, the winter months are dry,
sometimes for weeks on end, and the annual average rainfall can occur
within just a few days. In such circumstances, the storage capacity
should be large enough to cover the demands of two to three weeks. For
example, a three person household should have a minimum capacity of 3
(Persons) x 90 (l) x 20 (days) = 5 400 l.
- Conveyance systems are required to
transfer the rainwater collected on the rooftops to the storage tanks.
This is usually accomplished by making connections to one or more
down-pipes connected to the rooftop gutters. When selecting a
conveyance system, consideration should be given to the fact that,
when it first starts to rain, dirt and debris from the rooftop and
gutters will be washed into the down-pipe. Thus, the relatively clean
water will only be available some time later in the storm. There are
several possible choices to selectively collect clean water for the
storage tanks. The most common is the down-pipe flap. With this flap
it is possible to direct the first flush of water flow through the
down-pipe, while later rainfall is diverted into a storage tank. When
it starts to rain, the flap is left in the closed position, directing
water to the down-pipe, and, later, opened when relatively clean water
can be collected. A great disadvantage of using this type of
conveyance control system is the necessity to observe the runoff
quality and manually operate the flap. An alternative approach would
be to automate the opening of the flap.
- In order to safely fill a rainwater
storage tank, it is necessary to make sure that excess water can
overflow, and that blockages in the pipes or dirt in the water do not
cause damage or contamination of the water supply. The design of the
funnel system, with the drain-pipe being larger than the rainwater
tank feed-pipe, helps to ensure that the water supply is protected by
allowing excess water to bypass the storage tank. A modification of
this design is shown in Figure 5, which illustrates a simple
overflow/bypass system. In this system, it also is possible to fill
the tank from a municipal drinking water source, so that even during a
prolonged drought the tank can be kept full. Care should be taken,
however, to ensure that rainwater does not enter the drinking water
distribution system.
- Maintenance is generally limited to the
annual cleaning of the tank and regular inspection of the gutters and
down-pipes. Maintenance typically consists of the removal of dirt,
leaves and other accumulated materials. Such cleaning should take
place annually before the start of the major rainfall season. However,
cracks in the storage tanks can create major problems and should be
repaired immediately. In the case of ground and rock catchments,
additional care is required to avoid damage and contamination by
people and animals, and proper fencing is required.
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