Newsletter and Technical Publications
<Sourcebook of Alternative Technologies for
in Small Island Developing States>
PART B - ALTERNATIVE TECHNOLOGIES
1. TECHNOLOGIES GENERALLY APPLICABLE TO ISLAND STATES
1.1 Freshwater Augmentation Technologies
1.1.1 Rainwater Harvesting
A rainwater harvesting system consists of a rainwater catchment surface,
conveyance system, and water storage tank(s). Figure 8 shows a schematic
of a typical rainwater catchment system.
Rainwater harvesting systems can serve households or communities of
various sizes. Household systems generally catch rain from the rooftops of
homes and store it in tanks adjacent to the homes. Water is drawn from the
tanks by means of taps at the base of the tanks. In some cases rainwater
may be reticulated within a house using a pump/pressure system.
Alternatively the tank may be partly buried and a handpump used to
withdraw water. In cases where the majority of homes have thatched roofs,
community systems are popular. The roofs of large community buildings,
such as churches and schools, are used as catchment surfaces and the water
is stored in large tanks adjacent to these buildings (Figure 8).
Alternatively, if no suitable catchment surface is available, a separate
catchment surface is built adjacent to, or directly over, the water
storage tank. Residents of the community walk to these tanks, draw water
from a tap at the base of the tank, and transport it back to their homes
for drinking or cooking. In some cases, individual homes with thatched
roofs have also built separate catchment surfaces serving household
Materials commonly used in the construction of the roofs are corrugated
aluminium and galvanized iron, concrete, asphalt or fibreglass shingles,
tiles with a neoprene-based coating, and mud, which is used primarily in
rural areas. Roofs are generally sloped to avoid ponding and roof coatings
are required to be non-toxic. The effective roof area and the material
used in constructing the roof influence the collection efficiency and
water quality. The growing number of permanent-material roofs, as opposed
to straw, grass, or palm leaves, is encouraging rainwater harvesting in
many countries. Because natural roofing materials attract rodents and
insects, and often yield contaminated and coloured water, most people find
them objectionable for use as a collecting surface. In such cases,
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. These surfaces should be fenced
to prevent the entry of people and animals.
Conveyance systems usually consist of gutters and drain pipes that
deliver rainwater from the catchment area into the storage tanks. The
conveyance systems should be of inert material to avoid adverse affects on
water quality. Ground catchments would normally use pipes and/or open
channels to convey rainwater to the storage tanks/reservoirs.
The rainwater ultimately is stored in a storage tank, which should also
be constructed of inert material. Reinforced concrete, ferrocement,
fibreglass, polyethylene, or stainless steel have been found to be
suitable. Storage tanks may be constructed as part of a building or may be
a separate unit some distance away. In Bermuda and the United States
Virgin Islands, where most buildings are designed with rooftop catchments,
storage tanks are usually constructed under the building. These tanks are
built in two sections to facilitate cleaning. On a large number of islands
in the Pacific Ocean and the South China Sea, ferrocement tanks are
becoming increasingly popular.
Figure 8. Schematic of a rainwater catchment system.
Important factors to incorporate into the design of a storage tank
include adequate capacity; division of tank into two sections or dual
tanks to facilitate cleaning (when possible); provision of a sloped bottom
and provision for collection of settled grit and sediment; overflow
protection; inclusion of a manhole for easy access for cleaning; provision
of a vent for air circulation (often the overflow pipe); and, protection
against insects and rodents. A water-level indicator is an optional
refinement of this technology.
The quantity of water available from a rainwater harvesting system
depends on the size of the catchment surface, the percentage catchment
surface area that is guttered, the efficiency of the gutters in
transporting the water, and the size of the storage tank. If a catchment
surface is too small, it may not provide sufficient water to fill the
tank. Furthermore, the rainfall pattern and user-demand are also factors
that must be taken into account.
Extent of Use
Rainwater collection systems are extensively used by most SIDS,
especially those low-lying islands where rainwater catchments constitute
the major part of the water supply for the inhabitants. This supply will
often be supplemented by groundwater. In St. Lucia, the type of tank
varies from 200 l, used oil drums, to varying sizes of polyethylene
plastic tanks (usually 1 300 to 2 300 l), to underground concrete tanks
with capacities of 100 m3 to 150 m3. In the Turks
and Caicos Islands, there are a number of Government-built, public
rainwater catchment systems. Government regulations make it mandatory that
all developers construct a water tank large enough to store 400 l/m2
of roof area.
Rooftop and purpose-built catchments also are common place in the
Bahamas. One settlement (Whale Cay) has a piped distribution system based
on rooftop-collected water. On New Providence, most of the older houses
collect rooftop rainwater and store it in tanks averaging 70 000 l
capacity. Industrial use of rooftop-collected rainwater is also practised,
and a preliminary assessment of rainwater harvesting from the impervious
surfaces of Nassau International Airport has been carried out. In
multi-storied apartment buildings and other areas serving large
concentrations of people (such as hotels and restaurants), water
augmentation using rainwater from rooftop storage tanks has been
The Islas de la Bahia, off the coast of Honduras, meet a substantial
proportion of their potable-water needs from rooftop catchments. Rainwater
catchment systems are practically the sole water supply source for a small
group of islands north of Venezuela; these arid islands experience only
500 mm to 700 mm of rainfall per year, and have largely saline groundwater
reserves which cannot be used for potable purposes.
Rainwater harvesting is widely used in Male, as well as on the other
islands of the Maldives. Rainwater is harvested and stored in both public
and private tanks. In Male, the public rainwater storage tank is 8 500 m3,
which provides an average water supply of 0.14 m3/person. The
private rainwater storage capacity is estimated to be approximately 10 000
m3, or 0.16 m3/person
In the Federated States of Micronesia, in response to a 1982 cholera
epidemic on Chuuk, the State government constructed 14 m3
ferrocement tanks for all of the households on several islands. On Majuro,
Marshall Islands, a rainfall catchment system was installed at the
airstrip to serve about 14 000 people. Adjacent reservoirs, into which the
rainwater is pumped from the catchments, have capacities of about 57 000 m3
of raw water and 7 600 m3 of treated water. On the high
islands, rainwater harvesting is often used to supplement drinking and
Operation and Maintenance
Rainwater harvesting systems require minimal attention with respect to
their operation. Contamination of water as a result of contact with
certain materials can be avoided by using appropriate materials during
construction, or selecting tanks made from acceptable materials. The major
concern is to prevent the entry of contaminants into the tank while it is
being replenished during a rainstorm. Bacterial contamination can be
minimized by keeping the rooftop surfaces and drains clean. The main
causes of bacterial pollution are from debris, bird and animal droppings,
and insects that enter the tank. The following maintenance guidelines
should be considered in the operation of rainwater harvesting systems:
- Use an installed first flush (or foul flush) device, which directs the
initial batch of rainwater away from the tank to avoid the entry of debris
from the catchment area into the tank.
- Check and clean the storage tank periodically. All tanks need
cleaning, and their designs should allow for the thorough scrubbing of the
inner walls and floors using a chlorine solution (followed by thorough
rinsing). This sometimes can be difficult to accomplish without emptying
- Cover and ventilate the tank to avoid mosquito breeding, prevent
insects and rodents from entering the tank, and minimise the growth of
- Chlorinate the storage tanks as necessary if the stored water becomes
contaminated using slow-release chlorine tablets. (Most times the
rainwater is used without treatment.)
- Maintain gutters and downpipes. A good time to inspect gutters and
downpipes is while it is raining, so that leaks can easily be detected.
Regular cleaning is necessary to avoid contamination.
Community systems require community involvement and organisation for
effective maintenance, while household systems require a correspondingly
smaller scale involvement by residents. In some cases, where the water is
pumped, periodic, preventive maintenance is required on the small pumps
that lift water to selected areas of a house or building, or provide
public supply from underground storage tanks; more-often-than-not,
however, only repairs after breakdowns are done. Additional requirements
for ground catchments include fencing the paved catchment to keep out
trespassing animals (primarily small livestock such as goats, cows,
donkeys and pigs) that can affect water quality, cleaning the paved
catchment of leaves and other debris, and repairing large cracks in the
paved catchment that result from soil movements, earthquakes, and/or
exposure to the elements
Problems commonly encountered in maintaining the system at an efficient
operating level are the lack of availability of chemicals required for
appropriate treatment and the lack of adequate funding.
Level of Involvement
Household rainwater catchment systems can be constructed by, and
maintained at, the community or individual level, with a minimum of
training by skilled technicians. Construction and maintenance of ground
catchment systems (i.e., airstrip, roads, and specially constructed
surfaces) require skilled engineers and operators.
The cost of this technology varies considerably depending on location,
type of materials used, and level of implementation. The components that
need to be costed include roofing materials; gutters; conveyance pipes;
and, storage tanks.
Storage tanks costs (1995 prices) average $125 for a 1 500 l plastic
tank in St Lucia. In the Federated States of Micronesia, storage tank
construction costs range from $126 for a 1.3 m3 ferrocement tank with
corrugated metal cover to $327 for an 11.4 m3 ferrocement tank with
concrete cover. Intermediate-sized tanks range in cost from $148 for a 1.9
m3 ferrocement tank with corrugated metal cover to $300 for a 5.3 m3
ferrocement tank with concrete cover. In the Maldives, a 10 m3 ferrocement
tank costs $1 500 to construct, while a 2.5 m3 HDPE tanks costs $ 500. In
The Philippines, the cost per m3 capacity of locally-constructed rainwater
tanks is $48 for ferrocement tanks, $38 for inter-locking block tanks, and
$70 for reinforced concrete tanks.
Effectiveness of the Technology
In most cases, a rainwater catchment system cannot meet demand during
extended dry periods. Hence, it is often be necessary to have an
alternative source to supplement the rainwater supply. Notwithstanding,
rainwater may be the only available source of water in some locations.
This technology has good potential for use in the very small islands,
and, in many cases, could be the only option for a low-tech, economical
alternative to meeting freshwater demand. It also has good potential for
community-based system management. This technology is best suited to
islands that have evenly-distributed rainfall throughout the year.
Rainwater harvesting systems provide water at the point where it is
needed, and the systems may be owner operated and managed. The systems
also provide a water supply buffer for use in times of emergency or
breakdown of the public water supply systems, particularly during natural
disasters. The systems also provide a good supplement to other water
sources, and relieves pressures on other water sources. Construction,
operation, and maintenance are not labour intensive, and the physical and
chemical properties of the rainwater may be superior to those of
groundwater or desalinated water that may have been subjected to
Rainwater harvesting systems are completely dependent upon the frequency
and amount of rainfall. There will be shortages during dry spells or
prolonged droughts, which can be exacerbated by low storage capacities. If
greater storage capacities are provided, the additional construction and
operation costs may be too expensive for some households. Hence, drought
management may be required. Also the water may become contaminated if the
storage tanks are not adequately covered, and uncovered or poorly covered
storage tanks can be unsafe for small children. Contamination can also
occur from dirty catchment areas. Water treatment is infrequent in many
countries due to lack of adequate resources, and lack of treatment may
lead to health risks. In the case of community systems, people have to
walk significant distances for water if a distribution system is not in
place. All systems require maintenance to minimise wastage through broken
gutters, drainpipes, leaking storage tanks or outlet taps.
Rainwater-catchment systems encourage users to conserve water, as the
responsibility of the operation and maintenance rests with the individual
Further Development of the Technology
Rainwater harvesting is a very well-developed technology, with only
minor improvements being considered necessary. There are however a large
number of factors which could contribute to the optimisation of this
technology. In order to optimise the rainwater catchment systems, there
are various analyses techniques that can be used that require daily
rainfall data. Where rainfall data are not available, the establishment
and operation of a rain gauge should be encouraged. If possible,
evaporation information should also be collected. In addition, the
transfer of data and designs between planners and designers in the region
should be encouraged.
In the design-phase, detailed investigations should be conducted to
determine the actual demand for rainwater. This can be done through
questionnaires and surveys. It should be noted that it may be impossible
to fully meet demand, especially during dry periods. Simple procedures for
control the use of rainwater supplies during droughts need to be
developed. A procedure where water rationing begins when the water level
in the tank drops below a certain point is necessary to avoid emptying the
tank during extended dry periods. Investigations should also be made into
the most appropriate measures to be used for the collection of rainwater
(i.e., into the type of materials used, the sizes of buildings, etc).
Collection from flat roofs is discouraged as algae and dirt can collect on
these surfaces and contaminate the runoff. Sometimes unpainted roofs are
preferred as some paints contain toxic elements. In all cases, the concept
of first flush (foul flush), where the initial batch of rainwater is
directed away from the tank, should be supported and simple,
easy-to-operate flushing devices provided. Identification of the various
types of first-flush devices currently in use throughout the region should
be undertaken, and assessments of appropriate types carried out to improve
them. Likewise, more emphasis should be placed on designing cost-effective
guttering. Guttering needs to be adequate and simple to collect rainwater
from a sufficient area relative to the size of the tank to ensure optimal
filling of the storage tank (often not enough roof area is guttered).
Gutters need to be properly designed and fixed (structural and hydraulic
problems are often found).
There are many different tank designs, and a number of different
materials are used in tank construction, such as ferrocement, HDPE (black
polyethylene), galvanised iron, and fibreglass. Some standardisation of
materials, at least at the village level, may be desirable from a
maintenance and replacement point of view. Also, the volumes of the
storage tanks need to be based on rainfall analyses and predicted water
usage, although other factors such as cost, ease of construction, and
cultural considerations may limit the size. It may be also appropriate to
standardise the volume of the tank, at least at the village level. Greater
involvement of the public health department in the monitoring of water
quality should be promoted.
Ensuring adequate operation and maintenance of the rainwater systems may
be a problem. Continuous and repetitive public information campaigns and
training is required. Some methods for increasing the impact of the public
information and training programmes are to use influential people (e.g.,
pastors, doctors, teachers, and health educators) in the programming; to
establish village-based water committees (also useful for promoting safe
and adequate sanitation); and, to include the training as part of the
educational curriculum in schools. User fees may be appropriate for
community rainwater systems, as the revenue could cover the cost of
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