Catchment area/Roof top harvesting: It is the surface which receives rainfall directly; can be a paved/unpaved area. The roofing material texture influences the flow, evaporation, leakage and inefficiencies in the collection process of rain water. It recovers good quality water.
Surface run off: It is the water that flows away from the catmint area after falling on its surface in the form of rain. Efficiency of rain water collection for paved areas is 10% and for unpaved areas is 60%. The quality of water depends on the nature of the run off area and the impurities present in the soil.
Underground water recharging
Storage tank: Rain water once harvested can be stored in a tank. The size of tank for small collection should be minimum of 500 cubic meter capacity with RCC structure. For large collection, earthen lagoon with precast cement slabs laid over 250 micron thick pre-membrane lining can be used. The storage tank should be above the ground level and depth can be up to a maximum of 3.5 m.
Rain water harvesting for Industrial purpose: Rain water can meet a major portion of fresh water demand for boiler and cooling tower applications. Rain water harvesting system includes: a) conduits from the catchment surface to the storage area b) storage tanks where collected rain water is stored c) pre-treatment of rain water.
Conduits are the pipelines/drains that carry rain water from catchment area to the storage tanks. The material used is PVC and GI. It is essential to provide an intermediate collection tank of a lesser capacity to collect the rain water and pump it to the storage pond.
Pre treatment: The quality of rain water depends on harvesting methods like roof top harvesting, surface run-off and ground water harvesting.
Ground water: It will normally contain the leacheates of the soil. Depending on the quality of subsoil, it requires pre-treatment like softening/demineralization.
Rain water for recharging ground water aquifers: Rainwater may be let into the ground water aquifers with or without pre-treatment through open wells, bore wells, recharge trenches and recharge pits.
Rain water for individual house plot: If water is properly collected and recharged to the ground within the plot through a recharge pit, there is a great possibility of increasing groundwater table.
The pit may be excavated to a depth of 3m with an area of 12sq.mt. The size of the pit mainly depends on the total size of the plot and maximum rainfall received in that area. Of the 3m depth, 1m from the bottom may be filled with sand and gravel so that it will act as a filter and a good permeable material. Top of the pit should be covered with RCC slab with a provision of man hole which can be removed for clearing accumulated silt and clay in the over a period of time. A meshed entrance is provided for recharging the rainfall water. Walls of the pit should be lined in order to protect from possible leakages from the sewer lines or from the storm runoff of the roads. Periodical removal of the clay and silt accumulated in the bottom of the pit is highly essential to ensure good infiltration. High quality water can be recharged if we avoid recharging the first 2 to 3 minutes run off which may contain dust and other organic matter.
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