Rainwater harvesting

RAINWATER HARVESTING

Rain water harvesting (RWH) is a technique of collection and storage of rainwater into natural reservoirs or tanks, or the infiltration of surface water into subsurface aquifers. One method of rainwater harvesting is rooftop harvesting. With rooftop harvesting tiles, metal sheets or plastics can be used to intercept the flow of rainwater and provide a household with high-quality drinking water and year-round storage. Other uses include water for gardens, livestock, irrigation etc.

The reasons for using rainwater harvesting systems are:

• To improve water supply, food production, and ultimately food security.

• To benefit water insecure households or individuals in rural areas.

• To greatly contribute to income generation owing to food security.

 

1. COMPONENTS OF RAINWATER HARVESTING SYSTEM

The common components of a rainwater harvesting system are detailed below:

1. Catchments

The surface which directly receives the rainfall and provides water to the system is called catchment area. It can be a paved area like a terrace or courtyard of a building, or an unpaved area like a lawn or open ground. A roof made of reinforced cement concrete (RCC), galvanized iron or corrugated sheets can also be used for water harvesting.

2. Coarse Mesh

It prevents the passage of debris when provided in the roof.

3. Gutter

Gutters are channels which surround the edge of a sloping roof that collects and transports rainwater to the storage tank. Gutters can be semi-circular or rectangular and mostly made locally from plain galvanized iron sheet. Gutters need to be supported so that they do not sag or fall off when loaded with water. The way in which gutters are fixed mainly depends on the construction of the house. Mostly iron or timber brackets are fixed to the walls to support gutters.

4. Conduits

Conduits are pipelines or drains that carry rainwater from the catchment or roof top area to the desilting chamber of harvesting system as shown in Fig. 1. Commonly available conduits are made up of material like polyvinyl chloride (PVC) or galvanized iron (GI).

5. First-flushing

A first flush device is a valve which ensures flushing out of first spell of rain away from the storage tank that carries a relatively larger amount of pollutants from the air and catchment surface.

6. Filters

The filter is used to remove suspended pollutants from rainwater collected from roof top water. Sand filter is generally made up of three layers consisting of coarse sand, gravel and boulders as shown in Fig. 1. The types of filter generally used for commercial purpose are Sand filter, Slow sand filter, Charcoal water filter and Horizontal roughing filter.

7. Storage facility

Storage tanks are used to store the harvested water. There are various options available for the construction of these tanks with respect to the shape, size, material of construction and the position of tank. The tank shape may be cylindrical, square or rectangular.

Material of construction: Reinforced cement concrete (RCC), masonry, Ferro cement etc. Position of tank: Depending on land space availability these tanks could be constructed above ground, partly underground or fully underground. Some maintenance measures like disinfection and cleaning are required to ensure the quality of water stored in the container.

If harvested water is used to recharge the underground aquifer/reservoir, then some of the structures mentioned below are used.

8. Recharge structures

Rainwater harvested can also be used for charging the groundwater aquifers through suitable recharge structures like dug wells, bore wells, recharge trenches and recharge pits.

Certain recharge structures promote the percolation of water through soil strata at shallower depth (e.g., recharge trenches, permeable pavements). Others conduct water to greater depths from where it joins the groundwater (e.g. recharge wells). A typical dugwell recharge structure is shown in Fig. 1.

At many locations, existing structures like wells, pits and tanks can be modified as recharge structures, eliminating the need to construct any fresh structures.