To meet the increasing water demand of increasing PopulationTo rise the level of ground waterIn desert & in island this rain water harvesting has providedrelief to the people.IndustrializationIn places where rain fed/ irrigation based crops are cultivatedthrough ground water.Decrease in surface area of Lakes, ponds, tanks etc.
Rain water is the ultimate source of fresh water. Potential of rain to meet water demand istremendous. Rain water harvesting helps to overcome waterscarcity. To conserve ground water the aquifers must berecharged with rain water. Rain water harvesting is the ultimate answer.
Rainwater harvesting is the process of augmenting the natural filtration of rainwater in to the underground formation by some artificial methods. "Consciouscollection and storage of rainwater to cater to demands of water, for drinking, domestic purpose & irrigation is termed as Rainwater Harvesting
How to harvest rainwater? Broadly there are two ways of harvesting rainwater:- (1) Surface runoff harvesting. (2) Roof top rainwater harvesting.
Surface runoff harvesting: In urban area rainwater flows away as surface runoff. This runoff could be caught and used for recharging aquifers by adopting appropriate methods. Roof top rainwater harvestingIt is a system of catching rainwater where it falls. In rooftop harvesting, the roof becomes the catchments, and the rainwater is collected from the roof of thehouse/building. It can either be stored in a tank or diverted to artificial recharge system. This method is less expensive and very effective and if implemented properly helps in augmenting the ground water level of the area.
The basic components of roof top rainwaterharvesting system is given below:-§ Catchment§ Transportation§ First flush§ Filter
The surface that receives rainfall directly is the catchment of rainwater harvesting system. It may beterrace, courtyard, or paved or unpaved open ground. The terrace may be flat RCC/stone roof or sloping roof. Therefore the catchment is the area, which actually contributes rainwater to the harvesting system.
Rainwater from rooftop should be carried throughdown take water pipes or drains to storage/harvestingsystem. Water pipes should be UV resistant (ISI/PVCpipes) of required capacity. Water from sloping roofscould be caught through gutters and down take pipe.At terraces, mouth of the each drain should have wiremesh to restrict floating material.
First flush is a device used to flush off the waterreceived in first shower. The first shower of rains needs to be flushed-off to avoid contaminating storable/rechargeable water by the probablecontaminants of the atmosphere and the catchment roof. It will also help in cleaning of silt and other materialdeposited on roof during dry seasons Provisions of first rain separator should be made at outlet of each drainpipe.
Harvesting since doubts are raised that rainwater maycontaminate groundwater. There is remote possibility of thisfear coming true if proper filter mechanism is not adopted.Secondly all care must be taken to see that underground sewerdrains are not punctured and no leakage is taking place in closevicinity. Filters are used fro treatment of water to effectivelyremove turbidity, colour and microorganisms. After firstflushing of rainfall, water should pass through filters. There aredifferent types of filters in practice, but basic function is topurify water.
Sand Gravel FilterThese are commonly used filters,constructed by brick masonry andfilleted by pebbles, gravel, and sand asshown in the figure. Each layer shouldbe separated by wire mesh.
Charcoal FilterCharcoal filter can be made in-situ or in a drum.Pebbles, gravel, sand and charcoal as shown inthe figure should fill the drum or chamber. Eachlayer should be separated by wire mesh. Thinlayer of charcoal is used to absorb odor if any.
Filtration TankRainwater collected from rooftop of the building is diverted throughdrainpipes to settlement or filtration tank. After settlement filtered wateris diverted to bore wells to recharge deep aquifers. Abandoned borewells can also be used for recharge.Optimum capacity of settlement tank/filtration tank can be designed onthe basis of area of catchment, intensity of rainfall and recharge rate asdiscussed in design parameters. While recharging, entry of floatingmatter and silt should be restricted because it may clog the rechargestructure. "first one or two shower should be flushed out through rainseparator to avoid contamination. This is very important, and all careshould be taken to ensure that this has been done."
1. Level of Water Table: Increase in the level of groundwater is anobvious and visible parameter for success of rainwater harvestingsystems.2. Decrease in the following factors is taken into consideration toassess the groundwater quality before and after rainwaterharvesting:a. Salinityb. Fluoride concentrationc. Nitrate concentrationd. Bacteriological and heavy metal concentration