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Rain Water Harvesting / RWH

Regulations in Maharashtra :

On 14th Feb. 2002, Water Supply & Sanitation Department of Government of Maharashtra has issued a G.R approving Rain Water Harvesting as a means of improving water supplies. The G.R. details various techniques of Rain Water Harvesting, their costing & availability of funds. In furtherance Municipal Corporation of Greater Mumbai has issued a Circular insisting provisions for of Rain Water Harvesting while developing plots having area more than 1000 Sq.Mts, as I.O.D. conditions from 1st Oct. 2002. The design of Rain Water Harvesting system shall be prepared by approved consultant in the field & shall be to the satisfaction of the Corporation. Further all Centrally Air conditioned building shall have their own waste water treatment plant & treated waste shall be used for cooling purposes.

Need for Rain water harvesting :

Nature replenishes the ground water resources annually through rainfall; by way of infiltration though soil layers. Due to urbanization, the soil surface exposed to natural recharge gets reduced. Therefore, natural recharge is diminishing, resulting in drying of wells. Ground water source has the benefit of availability where water is needed and during emergencies and scarcity period, the public at large or NGOs should take measure to improve the ground water recharge by rain water harvesting to maintain the reliable and sustainable ground water resource for supplementary domestic and industrial needs by ground water balance use.

Rain water Harvesting Aims and Objectives & Scope :

Rainwater harvesting may be defined as process of augmenting the natural infiltration of rainwater or surface run off into the ground by some artificial methods. The methods suggested are recharge through pits, trenches, bore wells shafts by directly diverting run off water into existing or disused wells or conserving the rain water by artificial storing and using the same for human use. The choice and effectiveness of any particular method is governed by local hydrological and soil conditions and ultimate use of water.

Methods of Rainwater Harvesting in Urban Cities.

Broadly the rain water can be harvested by two methods
● Store the rainwater in containers above or above grounds or below grounds;
● Recharge into soil for withdrawal later by ground water recharging basis.

Rainwater can be stored in tanks
Rainwater can be recharged into the ground.

Any rain water harvesting will have four elements :

● Catchment area;
● Conduits;
● Settlement Tank
● Recharge facility or storage facility.

Catchments area :

The catchment is the area or surface, which receives rainfall directly. It can be any surface such as paved area like a terrace or courtyard of building or an unpaved area, like Lawn or open ground. Temporary structures like sloping sheds can also act as catchment. Run-off factor determines the quantity of water which will be available from the catchment run-off factors for wooded or grassy land is very less say 10% rest is absorbed by percolation, whereas run-off factor for paved or terraced area is 70 to 80% as most of the rainwater is available for recharge or storage except for water lost due to evaporation.

Conduits:

Conduits or the pipes carry rainwater from the catchment or roof tops to harvesting/storage system. The conduits may be of any materials like PVC, asbestos or galvanized or locally available metal like earthen pipes or bamboos which have been used in North-Eastern States of India.

Type of Catchment Coefficients	Coefficients
Roof Catchments
- Tiles	0.8- 0.9
- Corrugated metal sheets	0.7- 0.9
Ground surface coverings
- Concrete	0.6- 0.8
- Brick pavement	0.5- 0.6
Untreated ground catchments
- Soil on slopes less than 10 per cent	0.0 - 0.3
- Rocky natural catchments	0.2 - 0.5
Untreated ground catchments
- Soil on slopes less than 10 per cent	1.0 - 0.3
- Rocky natural catchments	0.2 - 0.5

Based on the above factors the water harvesting potential of a site could be estimated using the formula given below.

Water harvesting potential = Rainfall (mm) x Area of catchment x Runoff coefficient

Different Methods Of R.W.H.

Absorption Pit Method

Apercolation / absorption pit is a hand bore made in the soil with the help of an augur and filled up with pebbles and river sand on top. The depth of these pits will be anywhere between 4 and 8 meters depending on the nature of the soil. If the soil is clayey, the pit has to be dug to a depth till a reasonably sandy stratum is reached. The diameter of these pits will be 25 cm (10 inches). A square / circular collection chamber with silt arrester is provided at the top

Constructed in the open space at required intervals.
Size " 1m x 1m x 1.5m (depth)
Filled with broken bricks / pebbles
Suitable for sandy sub - soil area
One unit for 300 sq.ft area (approx.)

Percolation pit with bore method

A borehole to be drilled at the bottom of the percolation pit. Bore hole size : 150 - 300 mm dia with 10 -15 ft depth (approx.) Filled with broken bricks. Suitable for clay area.

Note :
Above structures are meant for area with small catchment like individual houses. 2. RCC slab cover is optional. 3. Top (1') portion may be filled with sand.

Percolation Absorption Well

These wells are constructed using cement rings readily available in the market. The diameter of these rings range from 2 ft to 6 ft. The depth to which these wells are dug depends on the nature of the soil and the diameter depends on the number of roof top pipes that are likely to be connected to each one of these wells. These wells are left unfilled and are covered with RCC slabs of suitable thickness to facilitate free pedestrian and vehicular movement on the ground.
Rainwater from the terrace is diverted to the existing open well using PVC pipes through a filter chamber.
The minimum size of the filter chamber is 2" x 2" x 2" filled with broken bricks in the bottom and sand on the top. The chamber may be covered with RCC slab.

Artificial Recharge of ground water

Augmentation of ground water resources becomes necessary, when, in a given area or basin the annual development of ground water exceeds the annual replenishment. When the natural recharge is slow and not upto the expected/required level, we have to go in for artificial recharge of ground water. Various methods of artificial recharge can be broadly classified in to two viz. Surface techenique and sub surface technique.

Artificial Recharge through Subsurface Dyke

The following structures will come under the surface technique group.
Contour bunds
Percolation tanks
Irrigation tanks
Individual well recharge
The sub surface techniques include
Sub surface dykes
Recharge tube wells
Recharging trenches
Injection wells

Site Selection for artificial recharge
Various Techniques in the selection of site for AR
Assessment of sub surface potential for ground water recharge
Favourable recharging zones in Chennai city

Rainwater harvesting - Borewell Method

Roof top rain water may also be diverted to a borewell
Settlement / filter tank of required size has to be provided.
Overflow water may be diverted to a percolation pit nearby.
The rate of recharge through borewell is less effective than open wells.
Defunct borewells may also be used.

Maintenance Tips for Rainwater harvesting structures

Always keep the surroundings of the rank clean and hygienic
Remove Algae from the roof tiles and asbestos sheets before the monsoon
Drain the tank completely and clean from inside thoroughly before the monsoon
Clean the water channels (gutters) often during rainy season and definitely before the first monsoon rain
Avoid first 15 or 20 minutes of rainfall depending on the intensity of rain. Use the first flush arrangement to drain off this first rain water.
Change the filter media every rainy season
Cover all inlet and outlet pipes with closely knit nylon net or fine cloth or cap during non-rainy season to avoid entry of insects, worms and mosquitoes.
Withdraw water from the system at the rate of 5 litres /head/day. This will ensure availability of water throughout the water scarcity period.
Leakage of cracks in the ferro cement storage tank shall be immediately attended to by cement plastering. This will avoid major repairs due to the propagation of cracks.
Heavy loads should not be applied on the lid, particularly many people should not stand on the lid.
Water should not be allowed to stagnate in the collection pit.
The tap should have lock system so that pilferage or waste of water is avoided.
The filter materials shall be washed thoroughly before replacing in the filter bucket
The coconut coir in the filter unit definitely needs regular replacement in rainy season, because, in wet conditions it rots and spoils water quality
In coastal areas, the tank may be painted outside by corrosion resistant paint once in 3 years and in other areas lime (Calcium Carbonate) based white wash may be applied not only for beauty but also for cleanliness

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