3.6 Rainwater harvesting for community water supply

Technical description

Rainwater is collected from rooftops of buildings using corrugated galvanized iron (CGI) sheets as the roofing material, a half-cut HDPE pipe gutter, and HDPE down-pipe to collect rainwater in ferrocement storage tanks. Some tanks have separate tapstands.

Figure 13. A Community-based Rainwater Harvesting System.

This technology was introduced into Nepal for the first time in 1988. As a pilot project, a 20m³ ferrocement tank was built to collect water from the roof of a middle school in the Village of Daungha. The system helped to fulfil the drinking water demand of about 300 students and teachers. The success of this community-based system, shown in Figure 13, encouraged the villagers to build more such systems, including an additional storage tank for the middle school. Two further 20m³ storage tanks were constructed to harvest rainwater from the Village Committee Office building and Primary School building in 1989.

Extent of Use

This technology is an example of a community-based rainwater harvesting system in Nepal. The example is from the Village of Daungha, in the Gulmi District in West Nepal. The interest of villagers and their active participation have helped to promote the development of such rainwater harvesting systems in Nepal, and there are currently 11 ferrocement tanks for rainwater storage in the region.

Operation and Maintenance

The following maintenance work is required in the operation of this technology:

- Regular cleaning of the rooftops and gutters: Rainwater is generally considered to be free of contamination, and, hence, it requires no treatment. Nevertheless, the roofs and gutters should be cleaned regularly to remove particulates and accumulated materials. Rainwater of the first few hours of the beginning of rainy season should not be collected in the tank, but should be used for flushing the roofs and gutters. Asbestos cement sheets and metal sheet roofing coated with lead-based paints should be avoided as they may be dangerous to health.

- Frequent cleaning of the storage tanks: The storage tank is the most expensive part of any rain water catchment system and determining the most appropriate capacity for any given locality and catchment area will critically affect both its cost and the amount of water available for supply. To ensure the longevity of the tank and quality of the water supply, regular cleaning of the tank should be carried out.

- Inspection of gutters and feeder pipes and valve chambers to detect and repair leaks: Rooftops are commonly used as catchments, even though ground catchments can provide a larger catchment areas, yield a greater volume of water to be collected, and are cheaper to construct. However, the use of ground catchments competes with agriculture for available land. Hence, in Nepal, both flat roofs, with tiles or plastered concrete leading to a floor drain, and sloped roofs are used. Sloped roofs are preferred to flat roofs because sloped roofs are accessible only for cleaning and repair purposes, and the harvested rainwater has less chance of being contaminated. Regular inspection of the drainage systems and conveyance systems minimizes water loss.

However, because this technology can be constructed and maintained locally, the Nepalese projects have been handed over to the users committees which bear the overall responsibilities for operation and maintenance of the systems. Such works as may be beyond the capacities and means of the users committees are carried out by the District Water Supply Offices.

Level of Involvement

Implementation of this technology has involved both the local communities and the government.

Costs

Based upon the experience in Nepal, the total cost of a 20 m³ water supply project, supplying 1 224 I/day, is about $24 620, or about $121 per person. Although the operation and maintenance costs of the system are negligible, the capital cost is too high for individual households and a rural community to invest independently in such a system. Thus, the cost of these community-based systems was divided between the government and community in the following ratio:

The cost of a 20 m³ ferrocement tank is about $2 000.

Effectiveness of the Technology

A total of 183 m³ of rainwater is collected annually in the storage tanks served by 160 m² of rooftop catchment area in the Daungha Village. Assuming that the water is used for drinking purposes only, it is estimated that about 73 m³ of water could meet the needs of the 34 residents for a year. With this system, water collected during the monsoon helps meet demand during the dry season. Of the remaining volume of water harvested, the per capita allocation of six litres per capita per day is considered to be too low to completely meet the demand for water for personal hygiene. However, the estimated 20 litres per capita per day supply for domestic use could be met from a household level 22 m³ rainfall harvesting system. Such a system would require a rooftop area of 49 m² roof area and would provide water to an average household of seven members.

Suitability

This technology is suitable for use in areas with adequate rainfall and in villages having a cluster of roofs or large buildings.

Advantages

The main advantage of having a community-based rainwater harvesting system is the time savings accrued in fetching water. This frees up time which may be utilized for other economic activities. Women, who traditionally gather water, gain more time for child care, social activities and income generating activities.

Disadvantages

The disadvantage of this technology is its high initial cost and per capita cost in small settlements.

Further Development of the Technology

While the technology may be considered to be fully developed, in order to determine the potential rainwater supply for a given catchment, reliable rainfall data (mean annual rainfall and its distribution) are required for a period of at least 20 years. Improved rainfall distribution analysis methods could enhance the utilization of this technology.

Information Sources

Contacts

I. Sainju, Civil Engineer, Department of Civil Aviation, Maintenance Branch, Tribhuvan International Airport, Kathmandu.

R. B. Tamang, Department of Civil Aviation, Repeater Station, Phulchoki, Lalitpur, Nepal.

Bibliography

Department of Water Supply and Sewerage, District Water Supply Office, Gulmi 1993. Daungha Rainwater Collection Water Supply Project, 1993, His Majesty's Government of Nepal.

Department of Water Supply and Sewerage, District Water Supply Office, Tanahu 1991. Management of Water Supply Project, 1991, His Majesty's Government of Nepal.

Gould, J.E. 1991. Rainwater Catchment Systems/or Household Water Supply, Environmental Sanitation Reviews No. 32, ENSIC, Asian Institute of Technology.

Ministry of Housing and Physical Planning and U.S. Peace Corps/Nepal 1989. Rainwater Catchment Tank Construction Technical Manual, His Majesty's Government of Nepal.