Project based Learning
RAIN WATER HARVESTING SYSTEM
Rajashri Dinkar Dahibhat
Priyanka Sampat Datir
Prof. Rajashri Bam
RAIN WATER HARVESTING
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 check dams.
NEED OF RAIN WATER
This is perhaps one of the most frequently asked question, as to why one should harvest
There are many reasons but following are some of the
To arrest ground water decline and augment ground
To beneficiate water quality in aquifers.
To conserve surface water runoff during monsoon.
To reduce soil erosion
To inculcate a culture of water conservation
Selected surface Area
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
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
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
Choosing a tank size that is appropriate in terms of costs, resources and
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 collection.
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.
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.
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
Design of groundwater recharge
In places where the withdrawal of water is more than the rate of recharge
an imbalance in the groundwater reserves is created. Recharging of aquifers
are undertaken with the following objectives:
To maintain or augment natural groundwater as an economic resource
To conserve excess surface water underground
To combat progressive depletion of groundwater levels
To combat unfavorable salt balance and saline water intrusion
= 4.25 cu. m. (4,250 litres)
In designing a recharge trench, the length of the trench is an important
factor. Once the required capacity is calculated, length can be calculated by
considering a fixed depth and width.
Design of an aquifer recharge system
The proper design will include the following considerations:
Selection of site:
Recharge structures should be planned out after conducting proper hydro-
geological investigations. Based on the analysis of this data (already
existing or those collected during investigation) it should be possible to:
1) Define the sub-surface geology.
2)Determine the presence or absence of impermeable layers or lenses
that can impede percolation
3) Define depths to water table and groundwater flow directions
4) Establish the maximum rate of recharge that could be achieved at
Design of recharge structures and
For designing the optimum capacity of the tank,
the following parameters need to be
1.) Size of the catchment
2.) Intensity of rainfall
3.) Rate of recharge, which depends on the
geology of the site
The capacity of the tank
The capacity of the tank should be enough to retain the runoff occurring
from conditions of peak rainfall intensity. The rate of recharge in
comparison to runoff is a critical factor. However, since accurate recharge
rates are not available without detailed geo-hydrological studies, the rates
have to be assumed. The capacity of recharge tank is designed to retain
runoff from at least 15 minutes rainfall of peak intensity. (For Delhi, peak
hourly rainfall is 90 mm (based on 25 year frequency) and 15 minutes peak
rainfall is 22.5 mm/hr, say, 25 mm, according to CGWB norms).
For an area of 100 sq. m.,
volume of desilting tank required in Delhi = 100 x 0.025 x 0.85
= 2.125 cu. m. (2,125 litres)
Design of a recharge trench
The methodology of design of a recharge trench is similar to that for a
settlement tank. The difference is that the water-holding capacity of a
recharge trench is less than its gross volume because it is filled with porous
material. A factor of loose density of the media (void ratio) has to be
applied to the equation. The void ratio of the filler material varies with the
kind of material used, but for commonly used materials like brickbats,
pebbles and gravel, a void ratio of 0.5 may be assumed.
Using the same method as used for designing a settlement tank:
Assuming a void ratio of 0.5, the required capacity of a recharge tank
= (100 x 0.025 x 0.85)/0.5
Ways of harvesting rainwater
Ways of harvesting rainwater
Roof top rain water harvesting system
Surface runoff rain water harvesting system
Rainwater harvesting in Singapore
The average annual rainfall of Singapore is
2400 milimeters. Inspite of 50% of the
land area being used as a water
catchment, almost 40-50 percent of its
water requirement are imported.
Research and development
Research and development work had been done for
1) Utilization of the roofs of high-rise buildings,
2) Use of run-off from airports for non-potable uses,
3) Integrated systems using the combined run-off from
industrial complexes, aquaculture farms and educational
Singapore has a rising demand for water and is on the lookout
for alternative sources and innovative methods of harvesting
Rooftop water collection systems:
1. Changi Airport:
Changi Airport system collects and treats rainwater, which
accounts for 28 to 33% of its total water used, resulting in
savings of approximately S$ 390,000 per annum. The potential
for using these rooftops as catchments are high. The system
developed have been result of intensive research. A simple
computer programmed was developed and monogram
prepared relating the roof area, tank size and roof water
1 .Changi Airport:
2. High Rise Buildings:
In this system implemented in a 15 storey building, the
collected roof water was diverted to two rainwater tanks and
the water was used only for flushing. The water quality was
acceptable in terms of color, turbidity and bacteriological
content though the total solids and chloride levels were
marginally higher. A simple dual mode system was
incorporated in the collection tanks which were placed on the
roof of the building. An economic appraisal established that
there was an effective saving of 13.7% of the water. The cost
of the rainwater was s$0.395 (us$0.25) per cubic meter (cum)
as against the cost of potable water which was s$0.535 (us$
3. Urban Residential Area:
In an urban residential area of about 742 hectares
having a total of 49,000 flats, rainwater harvesting
was very effective. Using further modified computer
programmed, it was possible to compute the volume
of potable water to be pumped when there was no
stored rainwater and also determine the frequency of
4. Capturing Urban Runoff:
By 1986, the growing need for water led to the establishment
of the lower Seletar-Bedok water scheme where almost nine
per cent of the total land area was used. The most important
feature of this scheme is that almost one-quarter of this
catchment is in urban area having high rise buildings and
industries and surface run-offs were subject to a wide verities
of contaminants. Hence the control of the water pollution and
relevant technologies were the main priorities of the scheme.
The lower involved the damming of the Sungei Seletar
estuary, which has a catchment area of 3200 ha, and forming
of lower Seletar reservoir. The reservoirs are interconnected
and raw water from Bedok reservoir is treated to potable levels
before distribution. The rest of the catchment of 2,625 ha was
primarily urban and both the runoffs are directed to Bedok
Rainwater harvesting technologies are simple to install and
Local people can be easily trained to implement such
technologies, and construction materials are also readily
It is convenient in the sense that it provides water at the
point of consumption, and family members have full
control of their own systems, which greatly reduces
operation and maintenance problems.
Running costs, also, are almost negligible.
Water collected from roof catchments usually is of
acceptable quality for domestic purposes.
It is collected using existing structures not specially
constructed for the purpose.
It has few negative environmental impacts compared to
other water supply project technologies.
Although regional or other local factors can modify the
local climatic conditions, rainwater can be a continuous
source of water supply for both the rural and poor.
Depending upon household capacity and needs, both the
water collection and storage capacity may be increased as
needed within the available catchment area.
Disadvantages of rainwater harvesting technologies are mainly
due to the limited supply and uncertainty of rainfall.
Adoption of this technology requires a *bottom up* approach
rather than the more usual *top down* approach employed in
other water resources development projects.
This may make rainwater harvesting less attractive to some
governmental agencies tasked with providing water supplies in
developing countries, but the mobilization of local government
and NGO resources can serve the same basic role in the
development of rainwater-based schemes as water resources
development agencies in the larger, more traditional public
water supply schemes.
Tips to ensure quality of harvested rain
For high quality performance. Following aspects should be taken care
Just before the arrival of monsoon, the rooftop/catchmet area has to be
The roof outlet on the terrace should be covered with a mesh to prevent
entry of leafs or other solid waste into the system.
The filter materials have to be either replaced or washed properly before
The diversion valve has to be opened for the first 5 to 10 minutes of rain to
dispose off the polluted first flush.
All polluted water should be taken away from the recharge structures.
The depth of bores (of recharge structures) shall be finalised depending on
the actual site condition
Do's and Don'ts
Harvested rainwater is used for direct usage or for recharging aquifers so
following precautionary measures should be taken while harvesting
Roof or terraces uses for harvesting should be clean, free from dust, algal
Roof should not be painted since most paints contain toxic substances and
may peel off.
Do not store chemicals, rusting iron, manure or detergent on the roof.
Nesting of birds on the roof should be prevented.
Terraces should not be used for toilets either by human beings or by pets.
Provide gratings at mouth of each drainpipe on terraces to trap leaves
debris and floating materials.
Do's and Don'ts
Provision of first rain separator should be made to flush off first rains.
Do not use polluted water to recharge ground water.
Ground water should only be recharged by rainwater.
Before recharging, suitable arrangements of filtering should be provided.
Filter media should be cleaned before every monsoon season.
During rainy season, the whole system (roof catchment's, pipes, screens,
first flush, filters, tanks) should be checked before and after each rain and
preferably cleaned after every dry period exceeding a month.
At the end of the dry season and just before the first shower of rain is
anticipated, the storage tank should be scrubbed and flushed off all
sediments and debris.