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3.3 Rooftop Rainwater Harvesting for Domestic Water Supply
Rainwater may be collected from any kind of roof. Tiled or metal roofs
are easiest to use, and asbestos sheet roofs, especially when damaged,
should not be used as asbestos fibres may be released into the harvested
water. This technology has been used in Assam State (northeast India),
where a more traditional reliable drinking water source has not been
identified for a number of villages. In the State of Assam, rainwater
harvesting is accomplished primarily through household rain catchment
structures which are best suited for use in the villages in hilly areas,
where people live in scattered huts or in small settlements. The
technology also has been adopted in the neighbouring State of Meghalaya,
where polythene sheet covering is used as a rooftop catchment on thatched
roofs. Storage of rainwater collected from rooftop catchments is typically
informal. Buckets, basins, oil drums, etc. are commonly placed under the
eaves in order to store water to supplement normal water supplies. Such
water is rarely used for drinking purposes.
Household rooftop rainwater collection systems consist of the following
Guttering: Guttering collects the rainwater runoff from the roof
and conveys the water to the downpipe. Gutters may be constructed of plain
galvanised iron sheets or of local materials such as wood, bamboo, etc.
All gutters should have a mild slope to avoid the formation of stagnant
pools of water. Gutters with a semicircular cross-section of 60 mm radius
are sufficiently large to carry away most of the intense monsoonal
Down pipe: A vertical down pipe of 100 mm to 150 mm diameter is
required to convey the harvested rainwater to the storage tank. An inlet
screen (#20 wire mesh) to prevent entry of dry leaves and other debris
into the down pipe should be fitted.
Foul Flush Diversion: The first flush of water from the roof is
likely to contain dust, dropping and debris which has collected on the
roof. This contaminated water should be diverted from the storage tank to
avoid polluting the stored rainwater. Such a diversion can be achieved
manually by including a ninety degree elbow on the down pipe so that the
pipe can be turned away from the storage tank to divert the flow for the
first 5 to 10 minutes of a storm. Alternatively, separate storage for the
initial flow of rainwater may be provided in the form of a pipe with
sufficient volume to contain the foul flush. Once this volume is exceeded,
additional rainfall will flow into the storage tank. The contaminated
water may be discharged after each heavy rain by removing a plug. Figures
6 and 7 illustrate these alternatives.
Filter: A filtering system may be placed between the down pipe,
after the foul flush system, and the storage tank. Filters can be
constructed using locally available materials such as sand, gravel, or
charcoal, etc. placed within a container to a depth of 1.2 m. The media
and the cross-sectional area of the filter should be chosen to provide a
rate of filtration adequate to pass 5 to 7 m of water per hour.
Storage Tank: The size of the storage tank in a particular area
should be matched to the volume of water expected to be harvested based
upon the area of the roof. The volume of the tank should also be related
to the quantity of water required by its users, and be appropriate in
terms of cost, resources required and construction methods.
Figure 6. Foul Flush System for Rooftop Rainwater
Some elements which should be considered in designing a storage tank
include the following:
- An accessway with an area of about 0.25 m2 (0.5 m x 0.5 m) to allow
periodic cleaning of the tank.
- A double pot chlorinator of 5 l capacity to provide continuous
- A vent pipe and overflow pipe (fitted with screens) of 100 to 150 mm
diameter to minimize the build up of gases and to allow excess water to
exit the storage tank.
- An outlet pipe of 100 to 150 mm diameter located at the bottom of the
tank to allow the tank to be drained for cleaning (separate from the
service tap which should be located above the bottom of the tank).
- A water level indicator, in the form of graduated transparent plastic
pipe for above ground tanks or float system for underground tanks, to
assist the owner to gauge water use from the system.
Figure 7. Alternate Methods of Diverting Contaminated
Storage tanks may be constructed above ground and fitted with a self
closing tap provided near the base of the tank, or underground and fitted
with a hand pump, depending on the height of the house and other site
specific conditions. Underground tanks should be constructed with the top
30 cm of the tank above ground level to minimize debris from the
surrounding land surface being washed into the tank.
Extent of Use
This technology is widespread in the State of Assam, and is best suited
for use in heavy rainfall areas.
Operation and Maintenance
The technology is simple to install and operate, and requires minimal
Maintenance consists of:
- regular cleaning of the catchment surface (i.e., the rooftop) and
storage tank to avoid physical and bacteriological contamination of the
- periodic inspection of the catchment surface for leaks, especially
when thatched roofs are used;
- regular cleaning of the filters to maintain good water quality and
acceptable rate of filtration.
Level of Involvement
Use of this technology is primarily at the community level, with an
emphasis on individual household level involvement for maintaining the
technology. Assistance in designing, sizing and constructing the
technology may be provided by governmental bodies such as extension
services or the village panchayat in the rural areas.
The technology is highly cost-effective since it uses locally available
materials for constructing the system. Capital costs are limited to the
cost of gutters, down pipes, filters and storage tanks. However, all these
can be constructed using low cost materials thereby reducing the overall
cost of the project. The recurring costs for maintenance of the system
include regular cleaning and leak prevention which can be easily
undertaken by the members of the household.
Effectiveness of the Technology
If the storage tank is of a suitable size, this technology can meet the
minimum standard for the supply of drinking water at a rate of 22 l per
capita per day. The harvested water is of good quality, with low turbidity
and no objectionable tastes, odours and colours. The water is soft and may
be slightly acidic. Bacteriologically, rainwater is generally very good
and free from organic matter, but it may be contaminated by material that
has accumulated on the rooftops. Wherever water quality tests have been
carried out, some bacteriological contamination of water from roofs has
been found. However, it is to be appreciated that a greater chance of
contamination comes in the storage tanks in which the water is kept for
long periods prior to consumption. As noted, storage tanks should be
provided with close fitting covers, screens, and self-closing abstraction
points to minimize the chance of the stored water being contaminated by
materials entering the tanks after the rainwater is stored.
The principle advantages of rooftop rainwater harvesting are: - The
capital low cost of rooftop rainwater harvesting, which is much less
expensive than conventional water supply technologies. - The ease of
construction, operation and maintenance of rooftop rainwater harvesting
technologies. - The minimal operating costs, which include reduced
chemical and energy costs compared to conventional water supply schemes.
Disadvantages of the rooftop rainwater harvesting technology as:
- The potential for the water to be polluted by birds droppings, dust,
etc., that accumulates on the rooftop requiring a regular programme of
maintenance of the roof surface and filter.
- The reliance on rainfall.
Further Development of the Technology
Rooftop rainwater harvesting is generally considered to be a fully
Paul, A.B. 1989. Rain and Dew as Sources of Water Supply in Assam - Some
Aspects, Journal of Indian Water Works Association, Jan-March, pp