This post is for everyone in India who wants to implement a rain water harvest in their homes but don't know how to do go about the same.
First the definition: Rain water harvesting means of storing rainwater for useful purposes - like drinking, washing, gardening etc. instead of just letting all the rain water run off into the storm drain (in other words the road next to your house).
Motivating factors: If you are asking why I and you should invest in rain water harvest, basically it is for us to do a bit for the nature and reduce our own water expenses. According to some theories, the next world war is going to be fought over water. Water is becoming more and more scarce, your ground water level is constantly droping and what better way than using some of the water which falls right on your house to offset some of your domestic demands? There are other advantages too. You will prevent rainwater runoffs causing road erosions (I mean what is left of as a road - I bet your city doesn't have a good storm drain), your water bills will reduce, your health could improve - the water you harvested is probably more clean than the one supplied by the corporation/municipality(at least you are in-charge of quality of the water). While using ground water (say from borewell), there is a risk of "Radon" exposure. Radon is a radioactive gas found mostly in deep wells. The amount of radon depends on various factors - rock makeup, depth, region etc. However, it is a common notion that radon exposure increases with older wells (as the water level goes down). I'm no geologist. But my guess, if you implement rain water harvesting/re-charge of borewells, it is going to minimize your exposure to radon (in case you have a borewell).
If you have a little more money and are little enterprising, you could let excess rain water which you cannot use, to recharge your ground water as well - by diverting the water into the borewell which you dug at the time of constructing the house and kept on sucking water for construction as well as present consumption.
According to some statistics, in Bangalore, in a 60 X 40 site if you harvest rain water without wasting a single drop in the entire year, the water so harvested is sufficient to take care of 100 families with 4 members in each for an entire year. With that statistic(okay take it with a pinch of salt), if you own a house (of whatever dimension) and do not harvest rain water, and continue to cry about your water woes, first blame yourselves then the city corporation.
Ok, enough preaching, now some practical ways of implementing rain water harvesting.
The next sections will discuss a few designs, techniques to implement rain water harvest at homes/apartments with an option to recharge ground water as well.
1. Simple with Syntex (Plastic) tanks.
The simplest technique involves diverting your rain water pipe(s) to a single location, running the water through a filter (sponge, wads of clothes) and collecting the water in a drum/storage. You could use two metal grids at the top of the barrel and wedge the filter (sponge/cloth) between the two. You could also cover the filter with a cover if you desire so.
Excess water beyond your storage capacity would get wasted and run off like it did before you implemented this system. However, this is better than doing nothing. It might at least provide you with enough water to wash clothes, your car or pet during rainy season.
2. Large Storage with dirty water outlet
This idea is a little more complex and a more engineered (needs plumber's assistance). In Idea #1, it assumes the barrel can be easily portable and cleaned. For example, the first rains could bring in lots of dirt and you may not want to use it. In case you have a large storage(like a sump), discarding the water you collected from first rain with silt and dirt from your unclean terrace may be a pain. In this design, install a run off valve at a level lower than the intake of the sump. During summer, before the first rains come, keep the dirty water run off valve open. After the first few rains, close the valve and start collecting and using the rain water.
Again a filter at the top of sump intake is assumed - this could be a layer of sponges wedged between two metal grid/grill. The dirt accumulated on top of the filter will automatically run off through the excess water runoff pipe - if they are of floating type. Once in a while you can access the filter and remove any sediments (like soil, pebbles) caught in the filter - or even replace it say once in a year. You could also put a mesh/filter at the top of the terrace where the water gets into the piping.
Make sure there is free flow of water in this system. If there is blockage at the excess runoff, water may accumulate in the pipeline and start collecting over your terrace as well.. Not a bad idea if your terrace is 100% leak proof - it will serve as a reservoir!!. However, in most cases, given the quality of construction you could afford and also due to ethics of your contractor, this may not happen. You will mostly develop few damp spots in your terrace if you are not careful in ensuring free flow of water.
Also, make sure your sump is well insulated from external elements (like the street drain overflowing) and creating a feed into your sump during flood conditions. A valve on the excess water run-off is not a bad idea in such cases to prevent 'feedback'.
3. Storage/Rain water recharge
As a variant of Option #2 above, move the excess water runoff pipe below the filter and connect it to a borewell shaft or open well (if you have one). As simple as that ! In case you had a borewell which did not give you the desired yield, try recharging it - make something out of the money that got sunk.
4. Green design (for new constructions)
- At the time of working with your architect, let him know that you are going to do rain water harvesting (to take care of natural incline of the terrain).
- While reviewing the plumbing lines, ask your architect where the run off of sewage is vs rain water run off (corresponding ducts). Ideally, they should be on different sides of the house (for easy maintenance, prevention of contamination etc.).
- Construct a sump for rain water collection (optional).
- Construct a sink pit 10 feet deep, 4 feet wide(dia). If you have a borewell in your house, the sink pit can be around the borewell. Remove the casing, sorround the bore duct piping with nylon mesh. Lay a layer (2 feet) of large gravel, 1 feet of medium size gravel, 1 feet of char coal (to absorb odor), 2 feet of sand leaving 4 feet of clearance for rain water recharging. The depths, layers could be different depending on the catchment area and other practical limits. At the top of the sink pit, leave a excess discharge outlet to the storm drain, where normally rain water would flow without rain water harvesting.
From my personal experience, rain water harvesting works. When my new house was constructed, a borewell was dug and hit source of water at 169 feet depth. I insisted with my architect to implement a rain water harvesting plan. After impelementing a sink pit around the bore well, spending around 25,000 rupees extra, I was wondering if it was really worth the trouble. After couple of years, during a maintenance work with the borewell submersible pump, I discovered the water depth had improved. Instead of reaching water at 169 feet, it was now available at 50 feet depth.
