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Newsletter and Technical Publications

<Sourcebook of Alternative Technologies for Freshwater Augumentation
in Some Countries in Asia>

3.3 Rooftop Rainwater Harvesting for Domestic Water Supply

Technical Description

Rainwater may be collected from any kind of roof. Tiled or metal roofs are easiest to use, and asbestos sheet roofs, especially when damaged, should not be used as asbestos fibres may be released into the harvested water. This technology has been used in Assam State (northeast India), where a more traditional reliable drinking water source has not been identified for a number of villages. In the State of Assam, rainwater harvesting is accomplished primarily through household rain catchment structures which are best suited for use in the villages in hilly areas, where people live in scattered huts or in small settlements. The technology also has been adopted in the neighbouring State of Meghalaya, where polythene sheet covering is used as a rooftop catchment on thatched roofs. Storage of rainwater collected from rooftop catchments is typically informal. Buckets, basins, oil drums, etc. are commonly placed under the eaves in order to store water to supplement normal water supplies. Such water is rarely used for drinking purposes.

Household rooftop rainwater collection systems consist of the following elements:

Guttering: Guttering collects the rainwater runoff from the roof and conveys the water to the downpipe. Gutters may be constructed of plain galvanised iron sheets or of local materials such as wood, bamboo, etc. All gutters should have a mild slope to avoid the formation of stagnant pools of water. Gutters with a semicircular cross-section of 60 mm radius are sufficiently large to carry away most of the intense monsoonal rainfall.

Down pipe: A vertical down pipe of 100 mm to 150 mm diameter is required to convey the harvested rainwater to the storage tank. An inlet screen (#20 wire mesh) to prevent entry of dry leaves and other debris into the down pipe should be fitted.

Foul Flush Diversion: The first flush of water from the roof is likely to contain dust, dropping and debris which has collected on the roof. This contaminated water should be diverted from the storage tank to avoid polluting the stored rainwater. Such a diversion can be achieved manually by including a ninety degree elbow on the down pipe so that the pipe can be turned away from the storage tank to divert the flow for the first 5 to 10 minutes of a storm. Alternatively, separate storage for the initial flow of rainwater may be provided in the form of a pipe with sufficient volume to contain the foul flush. Once this volume is exceeded, additional rainfall will flow into the storage tank. The contaminated water may be discharged after each heavy rain by removing a plug. Figures 6 and 7 illustrate these alternatives.

Filter: A filtering system may be placed between the down pipe, after the foul flush system, and the storage tank. Filters can be constructed using locally available materials such as sand, gravel, or charcoal, etc. placed within a container to a depth of 1.2 m. The media and the cross-sectional area of the filter should be chosen to provide a rate of filtration adequate to pass 5 to 7 m of water per hour.

Storage Tank: The size of the storage tank in a particular area should be matched to the volume of water expected to be harvested based upon the area of the roof. The volume of the tank should also be related to the quantity of water required by its users, and be appropriate in terms of cost, resources required and construction methods.

Figure 6

Figure 6. Foul Flush System for Rooftop Rainwater Harvesting

Some elements which should be considered in designing a storage tank include the following:

- An accessway with an area of about 0.25 m2 (0.5 m x 0.5 m) to allow periodic cleaning of the tank.

- A double pot chlorinator of 5 l capacity to provide continuous disinfection.

- A vent pipe and overflow pipe (fitted with screens) of 100 to 150 mm diameter to minimize the build up of gases and to allow excess water to exit the storage tank.

- An outlet pipe of 100 to 150 mm diameter located at the bottom of the tank to allow the tank to be drained for cleaning (separate from the service tap which should be located above the bottom of the tank).

- A water level indicator, in the form of graduated transparent plastic pipe for above ground tanks or float system for underground tanks, to assist the owner to gauge water use from the system.

Figure 7

Figure 7. Alternate Methods of Diverting Contaminated Runoff

Storage tanks may be constructed above ground and fitted with a self closing tap provided near the base of the tank, or underground and fitted with a hand pump, depending on the height of the house and other site specific conditions. Underground tanks should be constructed with the top 30 cm of the tank above ground level to minimize debris from the surrounding land surface being washed into the tank.

Extent of Use

This technology is widespread in the State of Assam, and is best suited for use in heavy rainfall areas.

Operation and Maintenance

The technology is simple to install and operate, and requires minimal maintenance.

Maintenance consists of:

- regular cleaning of the catchment surface (i.e., the rooftop) and storage tank to avoid physical and bacteriological contamination of the rain water;

- periodic inspection of the catchment surface for leaks, especially when thatched roofs are used;

- regular cleaning of the filters to maintain good water quality and acceptable rate of filtration.

Level of Involvement

Use of this technology is primarily at the community level, with an emphasis on individual household level involvement for maintaining the technology. Assistance in designing, sizing and constructing the technology may be provided by governmental bodies such as extension services or the village panchayat in the rural areas.

Costs

The technology is highly cost-effective since it uses locally available materials for constructing the system. Capital costs are limited to the cost of gutters, down pipes, filters and storage tanks. However, all these can be constructed using low cost materials thereby reducing the overall cost of the project. The recurring costs for maintenance of the system include regular cleaning and leak prevention which can be easily undertaken by the members of the household.

Effectiveness of the Technology

If the storage tank is of a suitable size, this technology can meet the minimum standard for the supply of drinking water at a rate of 22 l per capita per day. The harvested water is of good quality, with low turbidity and no objectionable tastes, odours and colours. The water is soft and may be slightly acidic. Bacteriologically, rainwater is generally very good and free from organic matter, but it may be contaminated by material that has accumulated on the rooftops. Wherever water quality tests have been carried out, some bacteriological contamination of water from roofs has been found. However, it is to be appreciated that a greater chance of contamination comes in the storage tanks in which the water is kept for long periods prior to consumption. As noted, storage tanks should be provided with close fitting covers, screens, and self-closing abstraction points to minimize the chance of the stored water being contaminated by materials entering the tanks after the rainwater is stored.

Advantages

The principle advantages of rooftop rainwater harvesting are: - The capital low cost of rooftop rainwater harvesting, which is much less expensive than conventional water supply technologies. - The ease of construction, operation and maintenance of rooftop rainwater harvesting technologies. - The minimal operating costs, which include reduced chemical and energy costs compared to conventional water supply schemes.

Disadvantages

Disadvantages of the rooftop rainwater harvesting technology as:

- The potential for the water to be polluted by birds droppings, dust, etc., that accumulates on the rooftop requiring a regular programme of maintenance of the roof surface and filter.

- The reliance on rainfall.

Further Development of the Technology

Rooftop rainwater harvesting is generally considered to be a fully developed technology.

Information Sources

Paul, A.B. 1989. Rain and Dew as Sources of Water Supply in Assam - Some Aspects, Journal of Indian Water Works Association, Jan-March, pp 59-64.

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