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Newsletter and Technical Publications PART B - TECHNOLOGY PROFILES 1. Technologies for Supplementing Freshwater Supply 1.1. Rainwater Harvesting: Microcatchment Systems Arid and semi-arid regions typically suffer from severe water shortages. This general phenomenon is even more acute in West Asia, due to its high rate of population growth and urbanization, which has resulted in serious gaps between the available water supply and the demands for the water. Most of the large cities in the western Asia region are either in semi-arid zones (e.g., Damascus, Amman, Sanaa) or in hyper-arid zones (e.g., Dubai, Abu Dhabi, Kuwait, Muscat, Aden). These cities require a large, and continuously increasing, supply of water. Rural communities in semi-arid regions that lack adequate groundwater sources also require viable methods and techniques to secure potable, domestic and irrigation water, and to supplement their limited water resources, which are often threatened by drought or increasing scarcity. To help address these freshwater demands, microcatchment rainwater harvesting techniques refer to the technologies that ensure the collection and storage of rainwater within the area of its direct use. 1.1.1. Rainwater Harvesting from Residential Rooftops Successful applications of this technology have been observed in Amman (Jordan), Edlib and Quneitra (Syria), West Bank highlands (Palestine) and Lebanon, where annual rainfall varies between 300-500 mm. The technology was also historically used in drier climates in Yemen (Aden) and Syria (Rasafe). In the semi-arid zones of West Asia, about 65-80% of the annual rainfall occurs during an approximately four-month period, with the remaining months having little or no rainfall. Accordingly, rainwater harvesting from residential rooftops represents a viable alternative under certain natural and demographic conditions to help satisfy domestic water demands during the dry season and throughout the year. Technology Description As with most of the water harvesting technologies, this technology consists of three components, including (1) collection, (2) conveyance and (3) storage systems. The residential rooftop constitutes the rainwater collection system. The rooftop surface should (1) be smooth, (2) not absorb large amounts of dust, (3) of uniform slope, and (4) not covered with harmful paint especially those made of bitumen. If the roof is painted, the paint should not contain significant amounts of any heavy metals (lead, mercury). The direction of the prevailing winds should be considered in determining the location of the rooftop. Application of chemical fertilizers via aircraft should be avoided in agricultural areas. Routine cleaning of the rooftop also is required to maintain an acceptable quality of the collected rainwater. The conveyance system usually consists of gutters to receive the rainwater flow from the rooftop, and convey it to vertical pipes leading to the storage tank. The slope of the gutters and pipes should be adequate to ensure the immediate drainage of the rainwater. The pipes should be constructed of non-toxic material, such as aluminum, plastic or fiberglass. They should also be equipped with screens, to prevent debris, sand and other dirt from entering the cistern. The storage tank (cistern) should be constructed of concrete, galvanized zinc, plastic, fiberglass, rubber or stainless steel. It can be of differing shapes, depending on its storage capacity and on the specific site conditions, and can be constructed either above or below the ground surface. It is usually equipped with a device to indicate the existing storage inside the cistern. A cistern (Figure 1) typically consists of (1) storage tank, (2) sediment trap (to receive the rainwater and retain sediment and other debris before entering the storage tank), and (3) extraction opening (manhole) at the top of the cistern, compatible with convention extraction methods (e.g., hand pump). Protection measures should be taken to prevent all water pollutant sources from entering the cistern, especially for underground cisterns, which should be situated 20-30 cm above the ground level. Cisterns also should not be built near sewer systems, to avoid any possible contamination from this source. Some design considerations for protecting the quality of the collected and store water supply are described herein. Debris Separation and Water Filtration Manual, semi-automatic or automatic mechanisms can be used to separate the settled and deposited material that accumulates on the rooftop during the dry season. The first flush of rainwater is directed away from the cistern. Special manual valves to divert the flow at the feeding pipe to the cistern can be provided. Special drainage of the dirt settled at the bottom of the cistern alsocan be incorporated. A small sand filter also can be attached to the cistern to secure better water quality in it. Cover and Ventilation A cover and a screen are placed on the storage tank to prevent pollutants and living organisms entering the water, and to prevent mosquito breeding and algal growth resulting from exposure of the water to the sun. The installation of screens on all pipe inlets, outlets, manholes and drainage pipes is required. The cover design should include a release and ventilation opening covered with a screen. Drainage ditch It is essential to design the cistern to prevent the release of sediments with the water. This can be done with drainage water openings in the floor of the cistern. Water Treatment The water settlement process constitutes a part of the cistern system. The infiltration mechanism can be a sand filter attached to the cistern structure, or a separate filter device in which the water drawn from the cistern is treated and chlorinated.
Figure 1. Use of House Roof for Filling the Cistern. Disinfection methods include chlorination of the clear water container, through adding doses of chlorine, iodine or sodium carbonate, or using separate charcoal filters. If no disinfection method exists, boiling the water is essential if it is to be used for drinking purposes.
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