Monday, December 13, 2010

Rain water harvesting - design

My primary motive is to give some ideas to others who might be considering RWH. It also has value in the similarities I see to our software projects where we invariably work with different constraints and choices we make during design having repercussions later down the line post deployment.
Background:The house is a 30+ year old construction which we purchased in 2001 and got it renovated. At that point of time we had not thought of RWH. Last year I started to seriously consider doing RWH within the constraints posed by the house without too many major changes.
First constraint: I could not do groundwater recharge as there is no existing well / borewell nor space to now dig one.
Second constraint: The simplest option available was to connect all the down-pipes (that bring down the rain water from the roof) together, add a pop-up filter (there is one readily available for Rs 2000 designed by a scientist at IISc Shivakumar) and then feed the water into the sump. The issue here was that we very rarely (almost never ) use the sump as the BWSSB supply directly fills the overhead tank (house is a single floor construction). Feeding the rain water into the sump meant that we would now be forced to use the pump and consume more electricity. It was a tradeoff between saving water and consuming electricity. I decided against this option. Looking back I feel now that it would have been lot simpler and possibly a cheaper option too but then I would not have had this opportunity to tell an interesting tale. I guess same goes for the design options and decisions we taken in our projects too. Hindsight is 20/20 !
So I had to figure out a RWH store and use solution which is based on gravity flow ….

The existing architecture itself provided opportunities to do so.
The car portico roof was about 2 feet below the main house roof. This gave me an idea to set up a tank here into which rain water from the main roof could flow. Now given the height difference between the two roofs this had to be a rectangular tank to maximize the storage volume. The first option thought of was to buy an outdoor loft tank. The maximum capacity available was 1000 litres. Couple of factors which made the decision against the PVC tank. One the roof space available was much more and two there was a danger of coconuts, fronds from the neighbors yard falling and breaking the tank. (A sidenote: The neighbor cut down all the coconut trees recently and replaced them with potted ornamental plants instead!) So I planned for a brick construction which utilized the space available to the maximum and provided a capacity of about 2000 litres. So far so good…
The main roof was sloped from the middle (on an east west axis) towards north and south. Given the position of the portico in the south east corner and the drain pipes being on the north and south walls of the house it would be a complex process to connect all of them to the tank on the portico (ref diagram 1). As a result only the three pipes on the south side were changed to connect to the tank. This however meant only half the water collecting on roof was being collected.
I kept thinking of the other half and finally hit upon another solution. The toilet inside the house at the north west corner had a large attic which was virtually empty. This again provided an opportunity to set up a storage to which the drain pipes on the north side of the house could be connected. In this case I went for a PVC loft tank. The space on the attic was sufficient to put up a 1000 litre tank. However as I later envisioned the process of getting the tank into the house and putting it up I realized that the narrow passage and the toilet door posed a constraint which could not be overcome. There was simply no space to move the tank in even though there was space on the attic. So deployment issues forced a change of design. Atleast I was lucky that this struck me before I had the tank brought home! As I result I had to settle for a 750 litre loft tank. A side note is that though originally I opted for Sintex but the delays the dealer had in procurement (loft tanks are not used so widely and it had to come all the way from Gujarat) made me to switch to Kaveri which was immediately available. One more constraint was that I did not want to drill big holes into the outer wall (being an old construction did not want to take chances) and had to figure out a way to get the rain water pipe in through the ventilator. Further since the tank was inside the house and overflow pipe had also to be arranged which would take the excess water outside. All the pipes running to the loft tank would not make a pretty sight but considering the utility aspect I decided to forego on the aesthetics (which anyways is not high on my priority list even otherwise!) So the design with a custom tank for the front on the portico, loft tank on the attic inside the toilet was ready and the plans for how the drain pipes would link upto them was in place. Now it was time for implementation which is another story in itself…!


sapio said...

Hi Jaga,

Thank you for this information. I am also looking at RWH option and even we do not have space for a well. However we do use our sump and that is where i plan to store the rain water. So if I go for pop up filter, what would be my estimated cost? and do you have contacts of people who can install this? Thanks in advance

Jaga said...

The last I checked the popup filter was around Rs2000 (this is from Shivakumar of KSCST, IISc). I do not have any specific contacts but I guess these days there are plenty of people doing it ever since BWSSB started issuing notices.

Krishna said...

Thank you for your wonderful informative website. I am need of the filter but it is around 4500 with the contractors of kscst. We are in need of 2 filters which comes to 9000. So, therefore can you please help me getting them where you had bought.