Design
of storage tanks
The volume of the storage tank can be determined by the following
factors:
- Number of persons in the household: The greater the number
of persons, the greater the storage capacity required to
achieve the same efficiency of fewer people under the same
roof area.
- Per capita water requirement: This varies from household
to household based on habits and also from season to season.
Consumption rate has an impact on the storage systems design
as well as the duration to which stored rainwater can last.
- Average annual rainfall
- Period of water scarcity: Apart from the total rainfall,
the pattern of rainfall -whether evenly distributed through
the year or concentrated in certain periods will determine
the storage requirement. The more distributed the pattern,
the lesser the size.
- Type and size of the catchment:Type of roofing material
determines the selection of the runoff coefficient for designs.
Size could be assessed by measuring the area covered by
the catchment i.e., the length and horizontal width. Larger
the catchment, larger the size of the required cistern (tank).
Dry
season demand versus supply approach
In this approach there are three options for determining the
volume of storage:
- Matching the capacity of the tank to the area of the
roof
- Matching the capacity of the tank to the quantity of
water required by its users
- Choosing a tank size that is appropriate in terms of costs,
resources and construction methods.
In practice the costs, resources and the construction methods
tend to limit the tanks to smaller capacities than would otherwise
be justified by roof areas or likely needs of consumers. For
this reason elaborate calculations aimed at matching tank
capacity to roof area is usually unnecessary. However a simplified
calculation based on the following factors can give a rough
idea of the potential for rainwater colection.
Illustration
Suppose the system has to be designed for meeting drinking
water requirement of a five-member family living in a building
with a rooftop area of 100 sq. m. The average annual rainfall
in the region is 600 mm (average annual rainfall in Delhi is
611 mm). Daily drinking water requirement per person (drinking
and cooking) is 10 litres.
Design
procedure:
Following details are available:
Area of the catchment (A) = 100 sq. m.
Average annual rainfall (R) = 611 mm (0.61 m)
Runoff coefficient (C) = 0.85 1. Calculate the maximum
amount of rainfall that can be harvested from the rooftop:
Annual water harvesting potential = 100 x 0.6 x 0.85
=
51 cu. m. (51,000 litres)
2. Determine the tank capacity: This is based on the dry
period, i.e., the period between the two consecutive rainy
seasons. For example, with a monsoon extending over four
months, the dry season is of 245 days.
3. Calculate drinking water requirement for the family
for the dry season
= 245
x 5 x 10
= 12,250
litres
As a safety factor, the tank should be built 20 per
cent larger than required, i.e., 14,700 litres. This
tank can meet the basic drinking water requirement of
a 5-member family for the dry period. A typical size
of a rectangular tank constructed in the basement will
be about 4.0 m x 4.0 m x 1.0 m
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Salient features of this approach:
- Simplest approach to system design but is relevant only
in areas where distinct dry seasons exist
- Provides a rough estimate of storage volume requirements
- This method does not take into account variations between
different years, such as the occurrence of drought years.
It also entirely ignores rainfall input and the capacity
of the catchment to deliver the runoff necessary to fill
the storage tank.
- This technique can be used in the absence of any rainfall
data and is easily understandable to the layperson.These
points are especially relevant when designing systems in
the remote areas of developing countries where obtaining
reliable rainfall data can be difficult.
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