What is Rain water harvesting – types, advantages, structures

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Rain water harvesting is an age-old technique to conserve water. Our ancestors harvested rain just as naturally as they tilled the ground to grow crops. Today, it’s the need of the hour to revive this centuries-old system. It simply means catching and holding rain where it falls and using it judiciously. You can store it in tanks or you can use it to recharge ground water.

WHAT IS RAIN WATER HARVESTING ?

Rain Water Harvesting is the collection and storage of rain water that runs off from catchment areas like roofs, pavements roads, parks, open grounds, etc. This rain water can be collected and stored as surface storage in tanks or can be recharged into the ground water.

Rain water falling on rooftops and other areas and where sufficient space is not available for surface storage is guided into sub-soil water through various techniques.

Structures generally used for this purpose are :
1. Recharge pit.
2. Recharge trenches.
3. Recharge shafts.
4. Trench with a recharge bore wells.
5. Recharge through existing dug wells.
6. Recharge through abandoned tube wells, abandoned hand pumps.
7. Percolation tanks etc.

Jab Aakah Se Dharti Par Pani Barse Hum Boond Boond Ke Liye Kyun Tarse ?

rain water harvesting structure
rain water harvesting structure

 

NEED OF THE HOUR RAIN WATER HARVESTING

With an ever-growing population, finite resources of the world arc decreasing. There is a continuous increase in demand of potable water supply for NCT, Delhi, and other parts of the country. Water is no longer an abundant resource and we have exploited ground water to meet the demand for water. This has resulted in an alarming fall in the ground water levels. Therefore, it is essential to arrest this situation by adopting Rain Water Harvesting because we need to give back nature what we take from it.


Rainwater Harvesting – All Important Facts

HOW MUCH CAN WE ACTUALLY SAVE?
A concrete, tiled and paved area of 100 sq.m. size yields about 55 thousand liters of rain water annually, in this case, we can save up to 40 thousand litres. The water saved is about four times the annual drinking requirement of five member family.

TWO TYPES OF RWH RECHARGE CHAMBERS

  1. Rectangular recharge chamber for rwh.

Remember. The above calculations give the ‘effective capacity i.e., the space inside the pit that will actually hold water. So the depth x width x length of the pit should be equal to the Figs. 48.5 and 48.6. The depths should be calculated from the inlet pipe down to the top of the filter media.

Rectangular recharge chamber for rain water harvesting
Rectangular recharge chamber for rain water harvesting

HOW TO MAKE AND MAINTAIN RAIN WATER HARVESTING SYSTEM?

Your Rain Water Harvesting system should be designed in such a way that :
1. Even if it rains with high intensity continuously for 1 hour, your system should be able to store and recharge the run off that flows into it.
2. It is safe, easy to make, and easy to maintain.
3. It complies with the local municipal authority guidelines so that you are able to avail of the RWH rebate, if any.

Based on this, some recommendations for implementation, operation, and maintenance of RWH system are :
1. Calculate the water holding capacity of the pit (in cubic meters or kiloliters that you will build by using this simple formula) Roof up area’ ( in sq.m) x 0.8″ ( run off coefficient for roof top/concrete area) x 0.025″** ( average maximum rainfall intensity in meters per hour) = roof top area x 0.02
2. Instead of filter media as mentioned in Fig 48.7, residents may also use multiple layers of jute mats in recharge chambers/modular filters in rain water pipes from rooftops. The objective is simply to arrest the silt in the rain runoff generated from the catchments before its percolation into the natural soil strata.

3. An over flow pipe in recharge structures should be provided leading out/falling into municipal storm water drains/open areas. Under no circumstances should they be connected to the sewer.
4. Recharge structures with requisite structural use soundness must be implemented and adhered to all the construction of structural norms. The structure should not pose any danger to people and buildings. The thickness of RCC cover slabs and reinforcement shall dependent on structural loads,
5. Fitter media consisting of brick aggregates ( 40/50/63. mm size) charge and activated carbon/coarse sand (1.5-2.0 mm) geotextile membrane ought to be provided. For membrane even thick blankets can be used.

BENEFITS OF RAIN WATER HARVESTING

Recharge of ground water through rain water is a long continuous process. It is an investment for our next generation.
Besides recharge of ground water, rain water harvesting benefits us in many ways like.
1. Improving the quality of ground water.
2. Reducing soil erosion as surface run off is reduced.
3. Choking of storm water drains & flooding of roads during monsoon is minimized,
4. Saving of energy. One meter rise in ground water level saves about 0.4 kWh of electricity.

HOW TO HARVEST RAIN WATER?

Rain water falling on the roof is collected and injected into ground water reservoirs through the following structures.
1. A dry/unused dug well can be used as a recharge structure.
2. The recharge water is guided through a pipe to the
bottom of well or below the water level to avoid Scouring of bottom and entrapment of air bubbles in the aquifer.
3. Before using the dug well as recharge structure, its bottom should be cleaned and all the fine deposits should be removed.
4. Recharge water should be silt-free.
5. It should be cleaned regularly.
6. It is suitable for large buildings having a roof area of more than 1000 sq.m.
7. Periodic chlorination should be done for controlling bacteriological contamination.

rain water harvesting recharge through hand pump
rain water harvesting recharge through hand pump

AN ABANDONED/RUNNING HAND PUMP
1. An abandoned/running hand pump can be used for recharge.
2. The structure is suitable for the small building having the roof area of up to 150 sq.m.
3. Water is diverted from the roof top to the handpump through a pipe of 50 to 100 mm diameter.
4. For running hand pumps a closing valve is fitted in the conveyance system near the hand pump to avoid entry of air in the suction pipe.
5. Recharge water should be silt-free.
6. During the recharging period, the water extracted from the hand pump should be utilized after proper chlorination.

RECHARGE PIT
1. Recharge pits are constructed for recharging the shallow aquifer.
2. These are constructed generally 1 to 2 m wide and 2 to 3 m deep.
3. The roof top rain water is channelised to the well and recharge under gravity flow conditions.
4. After excavation, the pits are refilled with pebbles and shoulders.
5. Cleaning of the pit should be done periodically.
6. It is suitable for small buildings having a roof top area of up to 100 sq.m.
7. Recharge pit may be of any shape i.e., circular, square, or rectangular.
8. If the pit is of a trapezoidal shape, the side slope should be steep enough to avoid silt deposition.

RECHARGE TRENCH
1. It is constructed when permeable strata of adequate thickness is a shallow depth.
2. It is a trench of shallow depth filled with pebbles and boulders.
3. These are constructed across the land slope.
4. The trench maybe 0.5 to 1 m wide, 1 to 1.5 m deep, and 10 to 20 m long depending upon the availability of land and roof top area.
5. It is suitable for buildings having a roof area of 200
6. Cleaning of the trench should be done periodically.

GRAVITY HEAD RECHARGE WELL
1. Bore wells/tube wells can be used as recharge structures.
2. This technique is suitable where.
(a) Land availability is limited.
(b) When the aquifer is deep and overlained by impermeable strata clay.
3. The roof top rain water is channelised to the well and recharge under gravity flow conditions.
4. Recharge water should be silt-free.
5. The well can also be used for pumping.
6. Most suitable for the areas where ground water levels are deep.
7. The number of recharging structures can be determined in limited areas around the buildings depending upon roof top area and aquifer characteristics.

RECHARGE SHAFT
1. A recharge shaft is dug manually or drilled by the reverse/direct rotary method.
2. Diameter of recharge shaft varies from 0.5 to 3 m depending upon the availability of water to be recharged.
3. It is constructed where the shallow aquifer is located below the clayey surface.
4. Recharge shaft is backfilled with boulders, gravel, and coarse sand.
5. It should end in more permeable strata (sand).
6. Depth or recharge shaft varies from 10-15 m below ground level.
7. Recharge shaft should be constructed 10 to 15 m away from buildings for the safety of the building.
8. It should be cleaned regularly by scraping the top layer of sand of refilling it periodically.

WHAT PRECAUTIONS ARE NEEDED FOR RAIN WATER HARVESTING STRUCTURES?

It is essential that the rainwater harvesting systems and its components once constructed are maintained in a proper way for their effective utilization.
Some of the precautions that should be taken are :
1. Implement the rain water harvesting scheme in a proper way, adopt the system/method which is most efficient and economical as per site conditions. The solution to any water harvesting problem is site-specific.

2. Install rain water harvesting structure at the construction stage itself as it is more economical and easy than installing in an old building or house.
3. In general, harvest rain water in areas where the ground water level is at a depth of more than 5.00 m below ground level.
4. Entry of highly turbid water into recharge well should be prevented.
5. Keep the roof and catchments area clean so as to harvest clean rain water.
6. Itis preferable to have a silt chamber before the recharge structure to trap the silt.
7. Provide mesh filters at the mouth of drain pipes to exclude the debris.
8. Domestic waste water sewage water and contaminated water should not be allowed for recharging.
9. For harvesting rain water of storm water drains, it is essential that they carry only the rainwater and there is no inner mixing of waste water.
10. Do not store any chemicals, oils, paints, lubricant or fertilizers on the catchment areas.
11. Periodical maintenance for recharge structures to be carried out.

DO’S AND DON’TS
DO’S
1. It has to be ensured that no wastewater enters recharge structures.
2. Depths or recharge structures should be 1.0 m -4.0 m.
3. Recharge systems should be located at a safe distance away from the buildings/foundations.
4. Run off coefficient for bituminous road/paved areas and open/green areas without steep slopes should be 0.6 and 0.1 respectively.
5. Only non-polluted rain water from the catchment area should be directed to recharge structures.
6. All catchment areas should be cleaned before the onset of the monsoon to avoid any contamination.
7. Recharge structures are to be cleaned after every 2 rainfalls during the rainy season.
8. Post-monsoon cleaning and maintenance of recharge chambers shall be carried out.

DONTS
1. Roof should not be painted since most paints contain. toxic substances and may peel off.

2. Chemicals, rusting iron, manure, or detergent should not be stored on the roof.
3. Terraces should not be used for toilets either by humans begins or by pets.
4. Polluted water should not be used to recharge ground water.
5. Rain water should not be harvested whcre post monsoon ground water levels are less than 5 m.

USES OF RAIN WATER HARVESTING

1. Tamil Nadu. Tamil Nadu was the first state to make rain water harvesting compulsory for every building to avoid ground water depletion. The scheme has been implemented in all rural areas of Tamil Nadu. Posters all over Tamil Nadu including rural areas create awareness about harvesting rain water. Since its implementation, Chennai had a 50% rise in water level in five years and the water quality significantly improved.
2. Karnataka. In Bangalore, adoption of rain water harvesting is mandatory for every owner or the occupier of a building having a site area measuring 6 ft ( 18.3 m) x 40 ft ( 12.2 m) and above and for newly constructed building measuring 30 ft ( 9.1 m) x 40 ft (12.2 m) and above dimensions. Bangalore water supply and sewerage board has initiated and constructed “Rain Water Harvesting Theme Park” in the name of Sir. M. Visvesvarya in 1.2 acres (4900 m2) of land situated of Jaya Nagar, Bangalore. In this park, 26 different types of rain water harvesting models are demonstrated along with the water conservation tips.
3. Rajasthan. Rain water harvesting has traditionally been practiced by the people of the Thar Desert. Many ancient water harvesting system in Rajasthan have now been revived. Water harvesting systems are widely used in other areas of Rajasthan, as well, for example the chauka system from the Jaipur district.
4. Maharashtra. At present, in Pune, rain water harvesting is compulsory for any new housing society to be registered.

5, In Mumbai. Maharashtra, rain water, harvesting is being considered as a good solution to solve the water crisis. The Mumbai city council is planning to make rainwater harvesting mandatory for large societies.

ADVANTAGES OF RAIN WATER HARVESTING

1. Provides self-sufficiency to the water supply.
2. Reduces the cost for pumping of ground water.
3. Provides high-quality water, soft and low in minerals.
4. Improves the quality of ground water through dilution when recharged to ground water.
5. Reduces soil erosion in urban areas.
6. Roof top rain water harvesting is less expensive.
7. Rain water harvesting systems are simple which can be adopted by individuals.
8. Roof top rain water harvesting systems are easy to construct, operate and maintain.
9. In hilly terrains, rain water harvesting is preferred.
10. In saline or coastal areas, rain water provides good quality water and when recharged to ground water, it reduces salinity and also helps in maintaining a balance between the fresh-saline water interface.
|11. In islands, due to the limited extent of fresh water aquifers, rain water harvesting is the most preferred source of water for domestic use.
12. In the desert, where rain fall is low, rain water harvesting has been providing relief to people.

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