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Newsletter and Technical Publications
<Proceedings of the International Symposium on Efficient Water Use in Urban
Areas
- Innovative Ways of Finding Water for Cities ->
Session 2: Harvesting and Utilisation of Rainwater
ECONOMIC AND WATER QUALITY ASPECTS OF RAINWATER CATCHMENT
SYSTEMS
Adhityan Appan
(President, International Rainwater Catchment Systems Association)
School of Civil & Structural Engineering, Division of
Environment & Water Resources
Nanyang Technological University, Nanyang Avenue,
Singapore
INTRODUCTION
Basic perceptions in engineering have been used for the collection, storage
and use of water from time immemorial. With the advent of technological
developments, harnessing techniques have advanced considerably. The twin factors
that realise such development are economic viability and the sustenance of an
acceptable quality of water. On the one hand, the funds expended for any water
project should ensure that the final product viz., water, should be costed such
that it's use is within the means of the potential user while on the other, this
very same product should have a quality level that should be of an acceptable
order. The main objectives of this paper are to define and explain the design,
construction, maintenance aspects of water resources development, with emphasis
on rainwater catchment systems, in terms of economic viability and water
quality. Special emphasis will be made on experience accrued, in terms of
design, construction and maintenance in developing countries like Thailand,
Indonesia and the Philippines where there has been considerable development in
rainwater catchment systems. Systems developed in Singapore, in an urban
environment, will also be mentioned in terms of methodology and economic
aspects.
ECONOMIC & ASSOCIATED FACTORS
The major driving force in providing water, be it for domestic, industrial,
commercial, irrigation or other purposes is its inherent cost. Conventional
freshwater sources are limited to 2% of the available water resources of the
world and the development of the sources, be they from rivers, impoundments,
lakes or groundwater, largely dictate the viability of a project. The rest of
the brackish and salt water can be abundant sources, but there has not been a
sufficient break-through in treatment methodologies so as to make such sources
economically viable. The main components in typical systems are the design of
storage volumes and all associated factors that go towards the collection,
treatment and distribution of treated water. The whole system will have to be
costed and due attention also be paid to the utilization of systems involving a
number of sources.
- Design of the system: The conventional design of any water resource
system concentrates on the storage capacity of a reservoir or, in the case of a
smaller rainwater catchment system, a cistern. The design of such storage
systems has been formalized as long ago as 1883(Rippl) when the cumulative
inflows were graphically plotted against the time to obtain a reliable yield or
reliable supply corresponding to a specific reservoir volume. Subsequently, many
other designs have come into existence. Typical examples are simulation analyses
where the changes in storage are of a finite reservoir are calculated using a
mass storage equation (McMohan et al, 1972), minimum flow approaches wherein the
lowest flows for various durations are plotted against the duration forming
drought curve (Waitt, 1945), extension of the mass curve and minimum flow
approaches to provide the probability of storage failure by developing a series
of drought curves (Alexander, 1962) and a series of probability matrix methods
(Moran, 1959, Gould, 1961).
In a more recently developed input/output system
(Appan, 1982), discretised rainfall time intervals as low as 15 minutes have
been used in combination with corresponding demands to arrive at storage volumes
that will ensure minimum waste. These systems tend to depict the real situation
more accurately as they cater more for cistern systems where the storage volumes
are relatively small.
In most cases, the major costs in water supply systems are associated with
the storage systems.
- Types of rainwater catchment systems: The costing of such systems
will largely depend on the magnitude of the project. Collection systems can vary
from simple types within a household to bigger systems where a large catchment
area contributes to an impounding reservoir from which water is either
gravitated or pumped to water treatment plants. The categorization depends on
widely varying factors like the sizes and nature of the catchment areas, whether
the systems are in developed or developing countries etc., Some of the systems
are described below.
- Simple roofwater collection system in developing countries: In
this system, the main components are the size of the cistern, the piping that
leads to the cistern and the appurtenances within the cistern. The materials and
the degree of sophistication of the whole system will largely depend on the
availability of funds. On the one hand there are highly expensive tanks utilised
in affluent locations in Hawaii whereas in others there is evidence of simple
and much cheaper systems in which the cisterns have been made with ferro-cement
(Vadhanavikkit, 1983), bamboo reinforcement (Thiensiripipat, 1983) etc, In all
these systems, every single element that contributes towards the collection
system has to be designed and costed. In some cases, the water is subjected to
some filtration processes whereas in others they have been nominally disinfected
with some bleaching powder (Krishna, 1991).
- Marginally larger systems in educational institutions, airports etc:
When the systems are larger, but do not take the scale of a major project, the
overall system can become a bit more complicated. Typical cases are, the
collection of rainwater from the roofs and grounds in institutions, their
storage in underground reservoirs, some minor treatment and then use for
non-potable uses (Appan, 1998). An even larger scale of such activity exists in
the Changi Airport in Singapore (Appan et al, 1995) wherein the rainfall from
the runways and the surrounding green areas is diverted to two impounding
reservoirs. One of the reservoirs is designed to balance the flows during the
coincident high runoffs and incoming tides, and the other reservoir is used to
collect the runoff. The waters are used primarily for non-potable functions such
fire-fighting drills and toilet flushing. Such collected and treated water
accounted for 28 to 33% of the total water used and savings per annum amounted
to S$ 390,000.
- Roofwater collection systems in high-rise buildings in urbanized
areas: Almost 86% of Singapore’s population lives in high-rise buildings
(HDB, 1994). A light roofing is placed on the roofs to act as catchment and the
collected roofwater can be kept in separate cisterns on the roofs for
non-potable uses. Using such systems, studies have been carried out much earlier
(Appan 1982, Appan et al, 1987). In the most recent study (Appan et al, 1997)
involving an urban residential area of about 742 ha a model was developed to
determine the storage volume of the cistern, taking into consideration the
non-potable water demand and the actual rainfall at 15 minute intervals. This
approach resulted in an effective saving of 4% of the water used, the volume of
which did not have to be pumped from the ground floor. As a result of savings in
terms of energy costs, deferred capital and water saving, the cost of collected
roof water amounted to S$0.96 against the then cost of S$1.17 per cubic meter.
- Collection of stormwater in urbanized catchments: In the
Sungei Seletar - Bedok Water Scheme (Appan 1977), 45% of the catchment of 5825
ha lies in urban areas. Urban storm water is collected in 8 stormwater ponds and
directed to the Bedok reservoir which has a yield of 14x 106 m3/d. The urban
area has high-rise buildings and the surface runoff is subject to a wide variety
of contaminants. Hence control of water pollution is of cardinal importance. Six
government departments met very frequently and chalked out programs for strict
pollution control. The quality of the raw water, which has runoff from a better
catchment, is of a high order (see Table 4). The overall cost is considerable,
as all aspects of keeping catchments clean have to be considered.
- Conventional large water abstraction systems: The very large
water resource projects involving the construction of dams and impounding
reservoirs, with or without hydro-power, fall outside the purview of the smaller
schemes.
- Some self-supporting simple systems in Asia: It is known that even in
the second millinium there existed cisterns (Ozis, 1982). With the advent of
larger water supply systems in the earlier part of this century, large capital
outlay came from governments who not only undertook to supply water but also
subsidized such systems (Prempridi, 1982). But, over a period of time, as
capital - intensive systems could not be financed, there has arisen, in many
developing countries, the need to supply drinking water to the people at very
low costs. This situation has lead to considerable effort and energy being put
in countries like Thailand, Indonesia and Philippines to build and maintain
simple systems. These systems adopted a "total" concept (Appan and Lee, 1987)
wherein the economic, social and cultural aspects of the location were taken
into consideration with emphasis placed on the utilization of local available
unskilled labour and indigenous building materials. Most important of all, an
appropriate financial model was developed thus ensuring that the potential user
could pay for the system installed in his household. Some of the successful
schemes are as follows:
- Thailand: With foreign capital not forthcoming for major water
schemes and the continuous demand for water escalating, in 1982 the emphasis
shifted to providing potable water for rural areas through shallow and deep
wells and rain harvesting programs. Funds were allocated for planned communal
systems to be executed by village councils, foreign agencies, government or
private organizations. A program that was very successful was that by the
Population and Community Development Association (PDA), a non-profit
organization.
The operations carried have been well-defined (Hayssen, 1983) and primarily
involved the study of rainfall statistics to identify the feasible areas for the
implementation of roofwater collection systems. The areas needing water most,
low repayments and a simple system of collection were the main features of the
systems. The modus operandi proved to be quite successful and villagers who were
trained b the PDA carried out construction work for the whole village, the
necessary hard-core labour being provided by the villagers
This scheme, with some foreign aid, proved to be so encouraging that an
elaborate program was carried out in 1983 wherein indigenous material like
bamboo reinforced concrete (brc) was used to cut costs by half. Interest was
generated in materials and studies were carried out plastic, steel, bricks and
inter-locking mortar, Ferro-cement etc were tried out. The costs at that time
varied from US$1.29 to US$0.14 per cubic metre of volume of tank.
- Indonesia: Even as early as 1979, the criteria used to execute
such rainwater catchment systems was to select building material that was
locally available, the design and construction should be within the
understanding and technical capability of the villagers. Most important of all
was the fact that the introduction of such collection systems should not
conflict with the way of existing life. All these factors had to be met with
besides which the cost had to be within that set by the government budget
(Doelhamid, 1982).
By a systematic process of experimentation and application, brc and
ferro-cement were introduced and a methodology was evolved for the introduction
, and propagation of such simple systems (Aristanti, 1983). The general approach
was very much akin to the Thai model and ensured that all level of villagers got
involved. A non-governmental organization, known as Dian Desa, harnessed the
special characteristic of Indonesian villagers who placed great importance on
the joy and value of working together. It was ensured that skills were developed
by community participation, and there was appropriate transfer of such skills
including appropriate maintenance. In all cases, local lifestyles, tradition and
local opinions were taken into consideration and it was always ensured that
project schedules did not conflict with local time constraints.
The major feature of this scheme was the management of necessary financial
backing . There was very limited external funding besides which the economic
status of the users was very low. The most deserving cases of the "poorest of
the poor" were identified and two she goats were lent to the family. When these
bear (normally) four young ones, two of these were returned to the owner and the
other two belonged to the borrower. The borrower than looked after the two young
ones and when they had grown up used them as payment for the cisterns that had
been built earlier in their premises (Aristanti, 1983).
- The Philippines: Propagation of roofwater collection systems
had the advantage in that a suitable study was done with respect to the Thai and
Indonesian models before embarking on the project. Pilot project areas were
selected and a total approach (Appan & Lee, 1987) was adopted to ensure that all
the lessons learned were fully utilised to improve the quality of water being
harnessed. Cost-wise ferro-cement was considered the most appropriate building
material. Monitoring of initially built 30 tanks was carried out to ensure
bacterial purity of the water and health education was imparted to ensure that
collection methods, systems etc, were well looked after.
Again, since the villagers were extremely poor and had obtained the basic
material for construction, they were convened to get their suggestions on the
most appropriate payment method. They were ultimately very much in favour in
engaging in hog-raising where the project authority provided the piglets and
basic training on how to care for them. The proposal was very similar to the
"two she-goat system" in Indonesia.
In all these countries, the extra effort has been put to make potable water
to be attainable by the poorest of the poor at a cost that can be borne by them.
Most important of all, there are number of NGO's in these countries, who have
embarked on rigorous systems of maintenance so as to ensure that the existing
rainwater collection systems that can not only be financed by the villagers but
can also be sustained and continue to give water of an acceptable quality level.
- Economic benefits by integrated systems: In most countries there
exist some form of conventional large schemes for the supply of water. However,
with the uncontrolled population explosions, particularly in developing
countries, and the concentration of most of humanity in emerging megacities
(ADB, 1993), water demands are far exceeding the rate of development of
projects. However, with limited funding and the inability of the impoverished
users to pay for the water, there is a great demand to try to conserve the use
of available water, to use urban catchments due to the extension of cities to
the hinterland (Appan, 1998a) and to establish simple systems that can meet the
needs of the urban poor. One such system is the development of individual
roofwater collection systems which can be integrated with existing systems
besides which it will have a considerable impact on both the rising demands and
savings in deferred capital costs (National Water Council, 1980). When such
relatively cheap rainwater catchment systems are established, they can replace
the proposed new conventional projects. Consequently, these projects can be
postponed which means capital borrowing can be delayed leading to considerable
saving for the water authority.
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