RAINWATER HARVESTING FAQS

1. What is Roof Top Harvesting?
To collect and store the rainwater which falls on the terrace of the buildings/houses.The water collected from the terrace is of good quality and it can be stored in tanks/sumps either for direct use or can be diverted to the existing bore well/open well for ground water recharge and storage..

2. What are the precautions ot be taken for roof top harvesting?
The terrace of the building should be maintained clean. A grill/mesh has to be fixed at the entrance of the rain water pipe in the terrace to arrest large particles such as papers, leaves, etc. A filter chamber has to be provided to filter small/minute dust particles before diverting the rain water into the storage tank or open well/bore well.

3. How to harvest rain water in the open spaces?
Rain water collected in the open spaces, being relatively dirty in nature, cannot be used for direct recharge of the well and therefore used for ground water recharge, using appropriate recharge methods/structures. In the absence of open well, the roof-top water may also be harvested along with open space water.

4. What are the precautions to be taken while harvesting rain water from open space around the building?
A dwarf wall of (7.5 cm height) has to be constructed at the entrance/gate to avoid run off into the street/road. If man holes are present (sewerage/waste water line) the height of which has to be raised a little to avoid draining of rain water through the manholes.

5. What are the RWH methods used for ground water recharge?
There are various methods available for rain water recharge into the ground which depend on the nature of sub-surface formation, extent of the area from where rain water is collected. Some of the simple and cost effective RWH structure methods are:

  • Percolation Pit
  • Percolation Pit with Bore
  • Recharge Trench
  • Recharge Trench with Bore
  • Recharge well (shallow/small)
  • Recharge well (deep/large)

6. What are the methods suitable for sandy sub-soil area?
As the sandy soil facilitates easy percolation of rain water, shallow recharge structures such as percolation pits, recharge trenches and shallow/small recharge wells are enough for sandy sub-soil areas.

7. What are the methods suitable for areas with clay sub-soil and hard rock areas?
Since clay sub-soil formation is impervious in nature and is having poor permeability, deep recharge structures such as percolation pit with bore, recharge trench with bore and deep/large recharge wells are needed for deep percolation into the underlying sandy layer.
For hard rock areas it is advisable to construct recharge wells the size of which depend on the extent of the area/building.

8. Whether existing structures could be used for RWH?
Yes. Existing structures such as defunct bore wells, unused/dried up open wells, unused sump, etc. can be very well used for RWH instead of constructing recharge structures to reduce the total cost.

9. Whether RWH helps to get immediate benefits?
Yes. In case roof top rainwater harvesting where the water is collected in storage tanks/sumps after filtering, the water is available for use the moment it rains.
In case of ground water recharge where the quality of ground water is poor or moderate considerable improvement in quality would be observed from three to five years, if continuous recharge of rain water is effected into an open well. However, slight improvement can be seen within weeks of rain if RWH structures have been installed.
As far as improvement in ground water table is concerned, the improvement can be seen even during one rain fall season if large number of people have done RWH in a locality.
In short, rain water is relatively pure form of water and when it is added to the relatively poor quality of ground water, the quality of that water will improve due to dilution. More the water harvested, better will be the result.

10. Is it necessary to construct all types of recharge structures in buildings?
Not necessary. In areas with alluvial sand, recharge structures would not be required unless the open spaces are covered with cement pavements. In other areas depending upon the area one or two recharge structures are enough to meet the requirement of an average sized house. Preference must be given to roof-top harvesting using existing open well. When roof-top harvesting using sumps and existing sumps and open well is practiced, it would take care of 60% to 75% of the rain water recharge in an ordinary/normal house. In such cases, one or two simple structures would suffice to harvest the rain water from the remaining open spaces around the building.

11. How much cost would it be require to install RWH?
While existing open well is used for RWH an amount of Rs.1,250/- is required for constructing the filter chamber, and connecting pipeline. The approximate cost for other structures are:

1. Percolation pit ( one no.) Rs. 650/-
2. Percolation pit with Bore (one no.) Rs. 1,200/-
3. Recharge Trench Rs. 650/- per metre
4. Recharge Trench with Bore Rs. 900/- per metre
5. Recharge well (shallow/small) (one no.) Rs. 4,100/-
6. Recharge well (deep/large)(one no.) Rs. 7,500/-

12. Whether RWH is made mandatory?
RWH is made mandatory by CMWSS Board, CMDA and Corporation of Chennai. Building plans would not be sanctioned by CMDA & Corporation of Chennai unless RWH proposals are included in the plan. Similarly, new water and sewerage connections would not be sanctioned by CMWSS Board if the same is not complied with.

13. Whether any firms/contractors approved by the Government to undertake RWH?
CMWSS Board has prepared a list of contractors to carry out various types of RWH structures as per the model/design and cost approved by the Bard.

14. To whom public can approach to know more about RWH?
The residents of Chennai and sub-urban areas may contact the following address for technical advise free of cost or for any other details regarding RWH:

Senior Hydrogeologist,
RWH Cell/CMWSSB
No.1, Pumping Station Road,
Chintadripet, Chennai-2.
Telephone: 8535697 (direct) / 8535257/8535976 Extn. 295 &
Website: chennaimetrowater.tn.nic.in

15. Is there any subsidy/incentive provide by Government agencies to instal RWH structures?
No, not in monitor terms. However biggest incentive is to become partially independent as for as water needs are concerned. That is a much bigger incentive.

16. Whether roof top water is suitable for drinking and cooking?
Though the rain water which falls on roof is pure but, still when it falls on roof and on the way to sump, some dirt, dust particles etc. are carried away with it. Therefore, it is advisable to filter this water and boil it before using for cooking and drinking.

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