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3. FRESHWATER AUGMENTATION
3.1 General Rainwater Harvesting Technologies (2)
Extent of Use
The history of rainwater harvesting in Asia can be traced back to about
the 9th or 10th Century and the small-scale collection of rainwater from
roofs and simple brush dam constructions in the rural areas of South and
South-east Asia. Rainwater collection from the eaves of roofs or via
simple gutters into traditional jars and pots has been traced back almost
2 000 years in Thailand (Prempridi and Chatuthasry, 1982). Rainwater
harvesting has long been used in the Loess Plateau regions of China. More
recently, however, about 40 000 well storage tanks, in a variety of
different forms, were constructed between 1970 and 1974 using a technology
which stores rainwater and stormwater runoff in ponds of various sizes
(see case studies in Part C, Chapter 5). A thin layer of red clay is
generally laid on the bottom of the ponds to minimize seepage losses.
Trees, planted at the edges of the ponds, help to minimize evaporative
losses from the ponds (UNEP, 1982).
Rainwater Harvesting Project in The Philippines
In The Philippines, rainwater harvesting was initiated in 1989 with
the assistance of the IDRC, Canada. About 500 rainwater storage tanks
were constructed in the Capiz Province during this project. The
capacities of the tanks varied from 2 to 10 m3, and the tanks were
made of wire framed ferrocement. The construction of the tanks
involved building a frame of steel reinforcing bars (rebar) and wire
mesh on a sturdy reinforced concrete foundation. The tanks were then
plastered both inside and outside simultaneously, which reduced their
susceptibility to corrosion when compared with metal storage tanks.
The Philippine rainwater harvesting system was implemented as a part
of the income generating activities in the Capiz Province. Initially,
loans were provided to fund the capital cost of the tanks and related
agricultural operations. Under this arrangement, the project
participant took a loan of $200, repayable over a three year period,
and covering the cost not only of the tank but also for one or more
income generating activities such as purchase and rearing of pigs
costing around $25 each. Mature pigs can sell for up to $90 each,
which provided an income generating opportunity that could provide
sufficient income to repay the loan. This innovative mechanism for
financing rural water supplies helped to avoid the type of subsidies
provided by many water resources development projects in the past.
(Source: Gould, 1992)
Operation and Maintenance
Maintenance is generally limited to the annual cleaning of the tank and
regular inspection of the gutters and down-pipes. 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. However, cracks in the storage tanks can create major
problems and should be repaired immediately. In the case of ground and
rock catchments, additional care is required to avoid damage and
contamination by people and animals, and proper fencing is required.
Level of Involvement
Various levels of governmental and community involvement in the
development of rainwater harvesting technologies in different parts of
Asia were noted. In Thailand and The Philippines, both governmental and
household-based initiatives played key roles in expanding the use of this
technology, especially in water scarce areas such as northeast Thailand.
The capital cost of rainwater harvesting systems is highly dependent on
the type of catchment, conveyance and storage tank materials used.
However, the cost of harvested rainwater in Asia, which varies from $0.17
to $0.37 per cubic metre of water storage (Table 10), is relatively low
compared to many countries in Africa (Lee and Vissher, 1990).
Compared to deep and shallow tubewells, rainwater collection systems are
more cost effective, especially if the initial investment does not include
the cost of roofing materials. The initial per unit cost of rainwater
storage tanks (jars) in Northeast Thailand is estimated to be about $1/l,
and each tank can last for more than ten years. The reported operation and
maintenance costs are negligible.
TABLE 10. Costs of Rainwater Catchment Tanks in Asia
(Lee and Vissher, 1990)
||Annual Equivalent Cost $/m3
|Reinforced Cement Jar
| Concrete Ring
|Wire Framed Ferrocement
| Wire FramedFerrocement
Effectiveness of the Technology
The feasibility of rainwater harvesting in a particular locality is
highly dependent upon the amount and intensity of rainfall. Other
variables, such as catchment area and type of catchment surface, usually
can be adjusted according to household needs. As rainfall is usually
unevenly distributed throughout the year, rainwater collection methods can
serve as only supplementary sources of household water. The viability of
rainwater harvesting systems is also a function of: the quantity and
quality of water available from other sources; household size and per
capita water requirements; and budget available. The decision maker has to
balance the total cost of the project against the available budget,
including the economic benefit of conserving water supplied from other
sources. Likewise, the cost of physical and environmental degradation
associated with the development of available alternative sources should
also be calculated and added to the economic analysis.
Assuming that rainwater harvesting has been determined to be feasible,
two kinds of techniques--statistical and graphical methods--have been
developed to aid in determining the size of the storage tanks. These
methods are applicable for rooftop catchment systems only, and detail
guidelines for design of these storage tanks can be found in Gould (1991)
and Pacey and Cullis (1986, 1989).
Accounts of serious illness linked to rainwater supplies are few,
suggesting that rainwater harvesting technologies are effective sources of
water supply for many household purposes. It would appear that the
potential for slight contamination of roof runoff from occasional bird
droppings does not represent a major health risk; nevertheless, placing
taps at least 10 cm above the base of the rainwater storage tanks allows
any debris entering the tank to settle on the bottom, where it will not
affect the quality of the stored water, provided it remains undisturbed.
Ideally, storage tanks should cleaned annually, and sieves should fitted
to the gutters and down-pipes to further minimize particulate
contamination. A coarse sieve should be fitted in the gutter where the
down-pipe is located. Such sieves are available made of plastic coated
steel-wire or plastic, and may be wedged on top and/or inside gutter and
near the down-pipe. It is also possible to fit a fine sieve within the
down-pipe itself, but this must be removable for cleaning. A fine filter
should also be fitted over the outlet of the down-pipe as the coarser
sieves situated higher in the system may pass small particulates such as
leaf fragments, etc. A simple and very inexpensive method is to use a
small, fabric sack, which may be secured over the feed-pipe where it
enters the storage tank.
If rainwater is used to supply household appliances such as the washing
machine, even the tiniest particles of dirt may cause damage to the
machine and the washing. To minimize the occurrence of such damage, it is
advisable to install a fine filter of a type which is used in drinking
water systems in the supply line upstream of the appliances. For use in
wash basins or bath tubs, it is advisable to sterilise the water using a
chlorine dosage pump.
The augmentation of municipal water supplies with harvested rainwater is
suited to both urban and rural areas. The construction of cement jars or
provision of gutters does not require very highly skilled manpower.
Rainwater harvesting technologies are simple to install and operate.
Local people can be easily trained to implement such technologies, and
construction materials are also readily available. Rainwater harvesting 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. As it is collected using
existing structures not specially constructed for the purpose, rainwater
harvesting 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.
Water Quality Considerations and Local People's Preferences
Rain water harvesting systems, especially those sourced from rooftop
catchments, can provide clean water for drinking purposes. The quality
of the water, however, is largely dependent on the type of roofing
materials used and the frequency of cleaning of the surface. A study
carried out by Wirojanagud et al. (1989, as cited by Gould, 1992) on
189 rainwater tanks and jars in Thailand showed that only 2 of the 89
tanks sampled, and none of the 97 rainwater jars sampled, contained
pathogens. Based on the results of bacterial analyses, 40% of the 189
tanks and jars sampled met the WHO drinking water standards. All of
the tanks and jars sampled met the WHO standards for heavy metals,
including the standards for cadmium, chromium, lead, copper and iron.
In northeast Thailand, where the groundwater, the only readily
available source of water, is highly saline, the local people are
aware of the water quality benefits to be had by using rainwater.
Before the Thai government launched the rainwater harvesting program
in 1986, the local people made use of rainwater harvested from
thatched roofs as well as groundwater obtained from shallow tubewells.
During a recent field visit to the area, the local people stated that
they were afraid of drinking water from deep tubewells, even though
the groundwater abstracted from the deep tubewells was reported to be
less saline and suitable for drinking water purposes. When asked, the
local people mentioned that they preferred shallow tubewell water
because it had a nicer taste than the water from deep tubewells;
however, they preferred water from the thatched roofs because of sweet
taste. After the Thai government launched the rainwater harvesting
program in 1986, many villagers in this region of Thailand replaced
the thatched roofs with zinc sheets to increase the volume of
rainwater harvested. Every house now has 6 000 l capacity jars for
rainwater collection, and the jar manufacturing industry has been
commercialized in the area. The demand for jars remains greater than
the ability of the manufacturing firm's capacity to supply.
Rainwater harvesting is an accepted freshwater augmentation technology
in Asia. While the bacteriological quality of rainwater collected from
ground catchments is poor, that from properly maintained rooftop catchment
systems, equipped with storage tanks having good covers and taps, is
generally suitable for drinking, and frequently meets WHO drinking water
standards. Notwithstanding, such water generally is of higher quality than
most traditional, and many of improved, water sources found in the
developing world. Contrary to popular beliefs, rather than becoming stale
with extended storage, rainwater quality often improves as bacteria and
pathogens gradually die off (Wirojanagud et al., 1989). Rooftop catchment,
rainwater storage tanks can provide good quality water, clean enough for
drinking, as long as the rooftop is clean, impervious, and made from
non-toxic materials (lead paints and asbestos roofing materials should be
avoided), and located away from over-hanging trees since birds and animals
in the trees may defecate on the roof.
Further Development of the Technology
Rainwater harvesting appears to be one of the most promising
alternatives for supplying freshwater in the face of increasing water
scarcity and escalating demand. The pressures on rural water supplies,
greater environmental impacts associated with new projects, and increased
opposition from NGOs to the development of new surface water sources, as
well as deteriorating water quality in surface reservoirs already
constructed, constrain the ability of communities to meet the demand for
freshwater from traditional sources, and present an opportunity for
augmentation of water supplies using this technology.
Gould, J.E. 1992. Rainwater Catchment Systems for Household Water
Supply, Environmental Sanitation Reviews, No. 32, ENSIC, Asian
Institute of Technology, Bangkok.
Gould, J.E. and H.J. McPherson 1987. Bacteriological Quality of
Rainwater in Roof and Groundwater Catchment Systems in Botswana, Water
Nissen-Petersen, E. (1982). Rain Catchment and Water Supply in Rural
Africa: A Manual. Hodder and Stoughton, Ltd., London.
Pacey, A. and A. Cullis 1989. Rainwater Harvesting: The Collection
of Rainfall and Runoff in Rural Areas, WBC Print Ltd., London.
Schiller, E.J. and B. G. Latham 1987. A Comparison of Commonly Used
Hydrologic Design Methods for Rainwater Collectors, Water Resources
UNEP [United Nations Environment Programme] 1982. Rain and Storm
water Harvesting in Rural Areas, Tycooly International Publishing
Wall, B.H. and R.L. McCown 1989. Designing Roof Catchment Water Supply
Systems Using Water Budgeting Methods, Water Resources Development,