Newsletter and Technical Publications
Rainwater Harvesting And Utilisation
An Environmentally Sound Approach for Sustainable
Urban Water Management: An Introductory Guide for Decision-Makers
What Must be Considered
to Design and Maintain Facilities for Rainwater Utilisation?
The effective catchment area and the material used
in constructing the catchment surface influence the collection efficiency and
water quality. Materials commonly used for roof catchment are corrugated aluminium
and galvanized iron, concrete, fibreglass shingles, tiles, slates, etc. Mud
is used primarily in rural areas. Bamboo roofs are least suitable because of
possible health hazards. The materials of catchment surfaces must be non-toxic
and not contain substances which impair water quality. For example, asbestos
roofs should be avoided; also, painting or coating of catchment surfaces should
be avoided if possible. If the use of paint or coating is unavoidable, only
non-toxic paint or coating should be used; lead, chromium, and zinc-based paints/coatings
should be avoided. Similarly, roofs with metallic paint or other coatings are
not recommended as they may impart tastes or colour to the collected water.
Catchment surfaces and collection devices should be cleaned regularly to remove
dust, leaves and bird droppings so as to minimize bacterial contamination and
maintain the quality of collected water. Roofs should also be free from over-hanging
trees since birds and animals in the trees may defecate on the roof.
When land surfaces are used as catchment areas, various techniques are available
to increase runoff capacity, including: i) clearing or altering vegetation cover,
ii) increasing the land slope with artificial ground cover, and iii) reducing
soil permeability by soil compaction. Specially constructed ground surfaces
(concrete, paving stones, or some kind of liner) or paved runways can also be
used to collect and convey rainwater to storage tanks or reservoirs. In the
case of land surface catchments, care is required to avoid damage and contamination
by people and animals. If required, these surfaces should be fenced to prevent
the entry of people and animals. Large cracks in the paved catchment due to
soil movement, earthquakes or exposure to the elements should be repaired immediately.
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.
Conveyance systems are required to transfer the rainwater collected on catchment
surfaces (e.g. rooftops) to the storage tanks. This is usually accomplished
by making connections to one or more down-pipes connected to collection devices
(e.g. rooftop gutters). The pipes used for conveying rainwater, wherever possible,
should be made of plastic, PVC or other inert substance, as the pH of rainwater
can be low (acidic) and may cause corrosion and mobilization of metals in metal
Example of a first flush device installation.
When selecting a conveyance system, consideration should be given to the fact
that when it first starts to rain, dirt and debris from catchment surfaces and
collection devices will be washed into the conveyance systems (e.g. down-pipes).
Relatively clean water will only be available sometime later in the storm. The
first part of each rainfall should be diverted from the storage tank. There are
several possible options for selectively collecting clean water for the storage
tanks. The common method is a sediment trap, which uses a tipping bucket to prevent
the entry of debris from the catchment surface into the tank. Installing a first
flush (or foul flush) device is also useful to divert the initial batch of rainwater
away from the tank.
Gutters and down-pipes need to be periodically inspected and carefully cleaned.
A good time to inspect gutters and down-pipes is while it is raining, so that
leaks can be easily detected. Regular cleaning is necessary to avoid contamination.
Storage tanks for collected rainwater may be located either above or below the
ground. They may be constructed as a part of the building, or may be built as
a separate unit located some distance away from the building. The design considerations
vary according to the type of tank and other factors.
Various types of rainwater storage facilities can be found in practice. Storage
tanks should be constructed of inert material. Reinforced concrete, fibreglass,
polyethylene, and stainless steel are suitable materials. Ferro-cement tanks
and jars made of mortar or earthen materials are commonly used. As an alternative,
interconnected tanks made of pottery or polyethylene may be suitable. The polyethylene
tanks are compact but have a large storage capacity (1,000 to 2,000 litres).
They are easy to clean and have many openings which can be fitted with connecting
pipes. Bamboo reinforced tanks are less successful because the bamboo may become
infested with termites, bacteria and fungus.
Precautions are required to prevent the entry of contaminants into storage
tanks. The main sources of external contamination are pollution from debris,
bird and animal droppings, and insects that enter the tank. Sometimes, human,
animal and other environmental contaminants, which happen to fall into tanks,
can cause contamination. Open containers are not recommended for storing water
for drinking purposes. A solid and secure cover is required to avoid breeding
of mosquitoes, to prevent insects and rodents from entering the tank, and to
keep out sunlight to prevent the growth of algae inside the tank. A coarse inlet
filter is also desirable for excluding coarse debris, dirt, leaves, and other
The storage tank should be checked and cleaned periodically. All tanks need
cleaning and their designs should allow for thorough scrubbing of the inner walls
and floors. A sloped bottom and the provision of a sump and a drain are useful
for collection and discharge of settled grit and sediment. An entrance hole is
required for easy access for cleaning. The use of a chlorine solution is recommended
for cleaning, followed by thorough rinsing. Chlorination of the cisterns or storage
tanks is necessary if the water is to be used for drinking and domestic uses.
Dividing tanks into two sections or dual tanks can facilitate cleaning. Cracks
in the storage tanks can create major problems and should be repaired immediately.
The extraction system (e.g., taps/faucets, pumps) must not contaminate the
stored water. Taps/faucets should be installed at least 10 cm above the base of
the tank as this allows any debris entering the tank to settle on the bottom,
where if it remains undisturbed, will not affect the quality of the water. Rainwater
pipes must be permanently marked in such a way that there is no risk of confusing
them with drinking water pipes. Taps must also be clearly labelled for the user
both in the local language and in clear graphic images. The handle of taps might
be detachable to avoid the misuse by children. Periodic maintenance should also
be carried out on any pumps used to lift water to selected areas in the house
|The following devices are also desirable.
||An overflow pipe leading into either infiltration plants,
drainage pipes with sufficient capacity or the municipal sewage pipe
||An indicator of the amount of water in the storage tank
||A vent for air circulation (often the overflow pipe
Protection against insects, rodents, vermin, etc. may also be required.
|Particular care must be taken to ensure that potable water is not contaminated
by the collected rainwater.
of Required Storage Size
When using rainwater, it is important to recognize that the rainfall is not
constant throughout the year; therefore, planning the storage system with an
adequate capacity is required for the constant use of rainwater even during
dry periods. Knowledge of the rainfall quantity and seasonality, the area of
the catchment surface and volume of the storage tank, and quantity and period
of use required for water supply purposes is critical. For example, in Tokyo,
the average annual rainfall is about 1,400 mm. Assuming that the effective catchment
area of a house is equal to the horizontal line of its roof surface area, and
given that that the roof surface area is 50 m2, the average annual volume of
rainwater falling on the roof may be calculated as 70 m3. However, in practice,
this volume can never be achieved since a portion of the rainwater evaporates
from the roof surface and a portion may be lost to the drainage system, including
the first flush. Furthermore, a portion of collected rainwater volume may be
lost as overflow from the storage container if the storage tank has insufficient
capacity to store the entire collected volume even in a heavy rain. Thus, the
net usable or available amount of rainwater from the roof surface would be approximately
70% to 80% of the gross volume of rainfall. In the above example, the actual
usable amount of rainwater would be about 49 m3 to 56 m3 in a year.