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1. Dear Ajith, Mr. Ajith is to

Dear Ajith,

Mr. Ajith is to be commended for striving to design an inexpensive user-friendly filter for rainwater harvesters. Since he has asked for views of others on his innovation, the following comments are offered:

For   assessing the   practicability of any rainwater filter, the following criteria need to be considered, as the target volume of water to be collected is quite high:

  • The collection area
  • The suspended solid content  of the feed water
  • The volume of water that it can filter before it needs maintenance such as removal of mud from the clogged filter
  • The rate of filtration (i.e. litres per minute)
  • User-friendliness i.e.  They should need minimum human intervention; time taken for the media replacement and skill needed for the replacement should be limited.
  • Cost of replacement of the filter media
  • Capital cost

In the filter designed by Mr. Ajith, items, 5, 6 and 7 are satisfied but in the absence of more precise information from him on the other criteria, my preliminary assessment is it may be suitable for a small terrace area in a place where the rainfall is gently spread and not heavy and inn short spurts.
However, based on my field experience, I would state that the filter area is very limited and the pores in the sponge will get clogged very quickly. If it is filtering large quantities (given that it is draining terrace rainwater into a well) without getting clogged, this could simply be because the terrace is more or less free of any mud and so the rainwater is practically free of any fine mud. Moreover, in apartment complexes, the volume of rainwater will be many times larger than that in Mr. Ajith’s house and there will be multiple terrace-pipes coming down and so installing filters at each down take point and maintaining them will not be convenient.
In my two –decade experience in RWH, I have not come across any filter yet that fulfils the criteria cited above. My solution for the problem of filtration in over 150 residential apt complexes in Chennai was to recommend sweeping the terrace clean of mud and leaves before the monsoons break. The problem of leaves is solved in most cases by just pruning the over-hanging branches of the trees that have grown higher than the terrace. Also, if the terrace water is to be drained into a shallow dug well or a sump, even if some mud comes along with the water into the sump or well, it will be only sterile mud and even this will settle at the bottom (just as the mud from the piped water supply settles at the bottom).It may also be pointed out that there is already a lot of mud at the bottom of the shallow well! Again, when this water is pumped into the over-head tank, there will be again settlement of mud at the bottom. Those who use this water for cooking can simply filter it through a piece of cloth, if necessary. We put an elbow  at the terminal point of the pipe inside the well so that the water does not fall down with momentum and create turbulence in the standing water column of the well ( particularly when the water level is low). The elbow directs the water on to the wall of the well which neutralizes the momentum and the water drains down smoothly along the wall of the well.
In the case of sloped roofs, it may not be feasible to sweep the roof and here, the first good rain of the season will wash away all the mud and leaves and this can be drained and only the water collected thereafter can be collected and stored.


Dr. Indukanth S. Ragade
Ex-Vice-Chairman of Alacrity Foundations Pvt. Ltd.


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