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3.7 Rainwater Harvesting for Multiple Purpose Use

Technical Description

This technology employs one of the oldest methods of rainwater harvesting in Bangladesh; namely, the use of the roof of a house to collect rainwater, and, by means of a gutter, convey the collected water to a pot for immediate use or to a storage place for later use. This system of rainwater harvesting is associated with various kinds of roofing materials including cement, corrugated iron (C.I. sheeting), thatched straw, etc. These roofing materials span the gamut of materials found within a community: cement roofed houses tend to be owned by the richer persons in a community, corrugated iron sheet roofed houses by the middle income persons, and the thatched roof houses by the poorer segments of the community. Gutters, too, are made from different materials, including corrugated iron sheets, palm wood, betel nut wood, bamboo, and banana plant leaves, etc. Storage pots are also of different types, ranging from earthen jars, locally called motkas which are of three different sizes, small, medium and large; to oil drums; to cemented tanks with steel fabricated covers. For immediate use, water is usually collected in buckets or wide-mouthed pots locally called gamlas. Sometimes, medium or large underground or above ground cement water tanks are also constructed where large quantity of water is to be stored. Rainwater from the gutters may also be stored in ditches around the house. Large earthen jars or cement storage tanks with steel plate covers are generally used by the wealthier section of the community and are semi-permanently placed inside or immediately outside of the house. In all cases, the harvested rainwater is used to meet household demand for such needs as bathing animals, washing dirty clothes, and other nonpotable purposes.

Figure 14. Rainwater Harvesting from a Corrugated Iron Roof

In the case of cement roofed houses, water storage tanks are generally constructed immediately below the roof with an opening at one side. Rainwater is directed to the storage tank by erecting a 3" to 4" brick-cement boundary. The slope is adjusted in such a way so that rainwater automatically runs into the storage tank through the opening. The opening can be closed with a steel plate cover. The boundary also has an opening which is used to bypass the storage tank and convey excess rainwater through a PVC pipe to the ground. The water from the storage tank is provided to the lower floors of the house through rubber piping, and abstracted through a plastic tap.

In the case of corrugated iron roofed houses, the corrugated iron sheets are usually inclined at angles between 30 and 40 degrees. Water collected in gutters placed at the lower edge of the corrugated iron sheeting is conveyed by gravity into motkas as shown in Figure 14. Water is supplied to the user through opening in the top of the motka which is usually kept covered.

Figure 15. Palm Wood Guttering Used to Capture Water

Figure 16. Storage Tanks

In the case of straw roofed houses, the thatch is also generally inclined at an angle of between 30 and 40 degrees. Water running off the thatch is collected in palm wood or bamboo gutters and conveyed into motkas or buckets at the convenience of the household as shown in Figures 14 and 15. In each case, the first flush of rainwater is allowed to flow out as the storage container as it is generally very dirty. The subsequent water is stored in the motkas or other storage containers.

Extent of Use

In places where there are chronic freshwater shortages, like Gourikhali, Kumkhali, Dacope, Ramnagar, Kaulashganj, and Shyamnagarin in the southwest of Bangladesh, rainwater obtained using roof catchments is used throughout the year for household purposes (see also the previous discussion of rainwater harvesting for domestic water supply use). Elsewhere in the southwest of Bangladesh, rainwater is used to supplement available water through January and February. In other parts of the country, rainwater is usually used for meeting immediate household needs such as bathing, feeding cows, washing clothes and dishes, etc. The stored water retains its colour and taste for about two months after the monsoon. .

Figure 17. Storage Jars for Household and Barnyard Use.

A significant number of people living in saline areas without access to tubewells at reasonable depths or other suitable surface water sources use rainwater for drinking purposes as well as other household purposes like cooking, washing and bathing. When people do not use rainwater, it appears to be the lack of suitable storage pots or lack of knowledge of the technology for harvesting rainwater. Such persons often make use of other water sources such as ponds, which are usually contaminated. Rooftop rainwater harvesting has also been observed in hilly areas of Mymensingh District, where most middle class families were observed to use rainwater for washing and other nonpotable domestic purposes.

Operation and Maintenance

Maintenance of rainwater harvesting systems is simple, consisting of the cleaning of catchment areas, conveyance systems, and storage devices.

Level of Involvement

This technology is implemented at the household level.

Effectiveness of the Technology

Unless carefully constructed and managed, the quality of the water stored in the motkas gradually deteriorates as a result of infestation by insects and pests, settling of dust, breeding of mosquito larvae, etc. However, if the stored rainwater is filtered and the lid of the storage container tightly closed after each use, its quality can be maintained up to two months. If storage is prolonged, the stored water may acquire odours and its colour may change. Previously, local Koi, Singh or Magur fish ( Anabas testudinews, Heteropreutes sp., Clarias batrachus) were used to eat the mosquito larvae and other insects. However, these fishes also discharge their own excreta into the water which degrades the quality of water. As people have become more conscious of these facts, the use of fish for maintaining water quality is fast decreasing. If these fishes are grown at all, they are harvested for consumption. Currently, people sometimes use alum or other locally made flocculent aids like burnt shell to clarify the water. Water purifying tablets are very infrequently used.

Generally, the availability of a means to purify the stored water is a severe constraint to maintaining rainwater quality; usually well-to-do families practice sound preservation procedure, while to disadvantaged cannot afford to do so as they do not have motkas of an appropriate size and may lack the cash necessary to buy flocculent aids. In extreme cases, the poor use contaminated rainwater stored in inappropriate containers or use dirty pond water.

Suitability

This technology is suitable for use in areas with adequate rainfall.

Costs

The capital cost of this technology is associated mainly with the purchase and installation of the gutters and storage tanks. The cost of the gutters varies depending on the type, ranging from $1.00 to $1.50 for bamboo, $4.00 to $5.00 for palm wood, and $10.00 to $15.00 for corrugated iron sheet.

Advantages

Rainwater harvesting is a relatively inexpensive and popular technology.

Disadvantages

Disadvantages of harvesting rainwater include contamination by pollutants from the roof mixing with the water, and degradation of the stored water due to contamination by toads, mosquitos, cockroaches, etc.

Cultural Acceptability

There are no known problems associated with the use of this technology.

Further Development of the Technology

HEED-Bangladesh, an NGO located in Dacope, stores water in a fibre glass tank pumped by centrifugal pump from a nearby rainwater pond. Water tanks could also used in conjunction with such a pumping system to store rainwater on the rooftop.

Information Sources

Mohammed Aslam, Saleh Ahmed Chowdhury, Alamgeer Faridul Hoque, and S.R. Sanwar, Intermediate Technology Group, House 32, Road 13A, Dhanmondi, Dhaka, Bangladesh, Tel. 880 2 811 934, Fax: 880 2 813 134, E-mail: itdg@itbangla.bdmail.net.

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