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Rain Water Harvesting Technologies in Sri Lanka
SARID, January 2004
By Vinod Moonesinghe
(Writer is the
editor of the Environment Foundation Ltd., Sri Lanka & a SARID project
coordinator)
Water is a resource
under considerable pressure from population growth and global warming,
so alternative, low-cost sources take on an added importance. Among these
sources rainwater harvesting stands out, especially in Third World countries
with high rainfall.
In Sri Lanka, notwithstanding
the saying of King Parakramabahu the Great that not a drop of water should
flow unused to the sea, the uninterrupted supply of water for domestic
purposes has long been taken for granted. The water-supply infrastructure
has not kept pace with needs and rainwater harvesting has not been promoted
as in India, where urban authorities have amended their by-laws to facilitate
it.
In 1995, the World Bank
assisted Community Water Supply and Sanitation Project (CWSSP) conducted
a study of the feasibility of rainwater harvesting for domestic use, incorporating
rainwater harvesting into official water supply schemes in Sri Lanka.
This led to the development of some low-cost technologies that helped
the spread of rainwater harvesting, which had been retarded somewhat due
to the high cost of storage and other facilities (1). Of note was the
‘Pumpkin Tank’, a low-cost (given as SL Rs 5,000 or Sterling
£ 77) pumpkin-shaped ferrocement tank for use above ground level
(2), (Fig.1) and a 5m3 underground brick built tank, based loosely on
the design of the Chinese below ground biogas tank - the Sri Lankan engineer
who designed the system had studied for some years in China, an example
of cross fertilisation of appropriate technologies (3), (Fig. 2)
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Fig. 1: Pumpkin Tank
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Fig. 2: Underground Tank
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.So
far, there are just 14,000 rain water harvesting systems functioning in
the country. Of this total, Badulla, Hambantota and Matara districts have
39%, 25% and 8% respectively, while the next six, Moneragala, Putlam,
Kegalle, A’pura, Kalutara, Kurunegala have 22% between them. The
remaining 15 districts (including the urban agglomeration around Colombo)
account for only a tiny proportion. So there is obviously a lot of scope
for the expansion of rain water harvesting (4).
Lanka Rain
Water Harvesting Forum
In March 1996, a small
group of people, from a range of institutions which had been involved
in rain water harvesting, established the Lanka Rain Water Harvesting
Forum (LRWHF) as a single institution for propagating it in this country.
Among its objectives are the identification of existing rain water harvesting
practices in Sri Lanka, the further development of techniques for collection
and the initiation of further study to make recommendations for maintaining
good practices.
The LRWHF carries out
research and development into low cost storage techniques and the suitability
for rainwater harvesting of different roofing materials. While monitoring
the durability of present practices, it surveys the water user pattern
and water security and tests the quality of collected water. It has established
an information network on a district basis - linking it with the users,
agencies, NGO’s and individuals - and a pool of skilled masons who
are trained in planning, design and constructions of rainwater harvesting
systems (5).
Starting in August
1998, the LRWHF participated in a three-year research programme, funded
by the European Commission, called the Domestic Roofwater Harvesting in
the Humid Tropics project, carried out by an international consortium
known as the Roofwater Harvesting Research Group. The aim of the project
was ‘to generate reliable information about domestic roofwater harvesting
for water policy planners, supply professionals and, ultimately, householders’
(6).
Several technologies
were identified by this research. One was a 3-cubic-metre (106 cu ft)
tank for use at ground level: a rectangular 1.5 x 1.5 x 1.5m (5’
x 5’ x 5’) brick structure with a concrete base, cement rendered
inside and out, costing about SL Rs 5,000 (stg £75.00). The cover
is a fine nylon mesh which acts as a filter for debris. A PVC downpipe
delivers water - collected on roof by conventional factory-made poly-vinyl-chloride
(PVC) guttering - into the tank through the cover. Water is extracted
through a small removable section in the nylon mesh, using a small bucket.
The tank has an overflow and washout and is easy to clean because the
walls are at chest height (7).
Competition
The LRWHF also tries
to promote the application of rain water for domestic purposes through
information, communication and awareness-raising.
In June 2003, the LRWHF advertised a competition to design rain water
harvesting systems and components for urban households. There were two
main categories, Medium Income housing and Low Income housing, plus a
sub-category for components (water filters, devices for flushing out the
first fall of rain). The designs were either to be incorporated into new
house designs or else to be added onto existing houses.
Of the prize winning
designs, several stand out (8). The first prize in the Middle Income category
was won by A.N. Abeywickrama of Matara. His design had some unique features.
The feed from roof was through a mesh filter into a cylindrical tank (brick
with 8 pie-slice-shaped covers). The outlet was fed through a PVC pipe
with holes in it and a sand filter and pumped into the main storage tank.
An ingenious ball-float device for removing first rain completed the system.
Danushka Bandara, a
school student from Anuradhapura, won the second prize with an original
design, with the tank as part of the roof itself and requiring no pumping.
The tank was covered by a 1/16” (1.6 mm) corrosion-resistant mesh.
At the mid-level of the tank lay a charcoal / rubble / sand / cotton layered
filter, sandwiched between another 1/16” (1.6 mm) corrosion-resistant
mesh (on top) and a 1/32” (0.8mm) corrosion-resistant mesh below.
There were two joint
second prizes instead of a first prize in the Low Income category:
GJ Bandara of Polonnaruwa.
Based on a large black plastic tank, with a cylindrical aluminium inlet
filter (filled with rubble) lowered into the access hole at the top. A
plastic or tin bucket or can, filled with rubble, was hung on the outlet
tap, to serve as a filter.
KPSP Perera of Kotte.
The main innovation was a storage tank based on a thick tube of 3 ft (0.9
m) wide polythene (used for packaging and concrete work), slit along one
edge and sunk into a 2 ft (0.6 m) deep, 2ft (0.6 m) wide trench. The upper
cover of the tank was another thick polythene. Extraction was by hand-pump.
The system had been operational for three months at the time the design
was submitted (Figs. 3 and 4).
Polythene Tank
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Fig. 3: Uncovered
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Fig. 4: Covered
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The
Third Prize was won by RAAR Ranasinghe from Veyangoda whose system was
based on 500 litre volume Ferro cement tank. It had a U-trap built into
the inlet from the gutter to trap dirt and remove it, via a pipe, to the
first-flush device. The inlet filter was an 18” (450 mm) tall cylinder
with a 4” (100 mm) layer of sand and a 6” (150 mm) layer of
rubble and coconut charcoal. The outlet filter was a PVC strainer on the
outlet pipe, raised inside the tank to prevent ingress of sediment, leading
to a bib-tap and washout valve.
The prize for components
was won by Dulip Gunawardena of the National Water Supply & Drainage
Board, the components being an outlet filter and a depth gauge. The former
consisted of a floating device (shaped like a lotus) with a flexible hose
attached. The floating device had v-shaped notches through which the surface
water could flow, any floating debris being trapped between the teeth.
The depth-gauge consisted of a floater made of 3 PET bottles fixed together,
with a plastic pipe extending upwards with depth markings on it.
Although the number of entries was relatively low, the respose to this
competition show that there is considerable talent innovative talent in
Sri Lanka which could be utilised in further developing low-cost domestic
rain water harvesting. What is required is a greater effort to further
this technology. The state should provide incentives (not necessarily
monetary – as for instance the Indian Municipal by-law amendments)
in order to properly harness this potential in a meaningful manner.
References
1. Tanuja Ariyananda,
Rainwater harvesting as a water supply option in rural Sri Lanka, East
Asia 2000 Regional Rainwater Utilisation Symposium, International Rainwater
Catchment Systems Association.
http://www.ircsa.org/Meeting%20Reports/East%20Asia%202000%20Regional
%20Rainwater%20Utilization%20Symposium/Rainwater%20harvesting%20as
%20a%20water%20supply%20option%20in%20rural%20Sri%20Lanka-tanuja.pdf
2. University of Warwick
Development Technology Unit, Domestic Roofwater Harvesting Research Programme,
Case Study 1: the Sri Lanka Pumpkin Tank. http://www.eng.warwick.ac.uk/DTU/pubs/rn/rwh/cs01/cs1.html
3. University of Warwick
Development Technology Unit, Domestic Roofwater Harvesting Research Programme,
Case Study 2: Underground brick dome tank, Sri Lanka.
http://www.eng.warwick.ac.uk/DTU/pubs/rn/rwh/cs02/cs2.html
4. Lanka Rain Water
Harvesting Forum, Distribution of Household RWH Systems in Sri Lanka.
http://www.webasia.com/rainwaterharvesting/statusRWHinSL.htm
5. Lanka Rainwater
Harvesting Forum.
http://www.rainwaterharvesting.com
6. University of Warwick,
Development Technology Unit, Domestic Roofwater Harvesting Research Programme,
Case Study 3: 3.0m3brick built storage tank, Sri Lanka.
http://www.eng.warwick.ac.uk/DTU/pubs/rn/rwh/cs03/cs3.html
8. Information supplied
by Tanuja Ariyarnanda of LRWHF.
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