Ever since I took up this project of construction of rainwater harvesting system as part of Global Volunteer Days 2009, I had been thinking of putting down my experiences so that others like me can learn and are not as confused as I was! In this post I have detailed construction of the RWHS that we built, note that this is not the only design. You can make more sophisticated systems with more investment.
Well, even an ordinary pit can be called a RWHS, since it stores the rainwater and lets it seep in the ground and increase the moisture level of the surrounding earth. However a major concern with rainwater harvesting is the quality of water that goes underground since if groundwater gets contaminated it can't be purified.
1) What is the source of water you will collect? Rooftop water being directed to the pit through PVC pipes is most common option.
Second option, bit more difficult but cheaper, is to do the landscaping in such a manner that water falling on the ground flows to the pit.
Rooftop water is much more cleaner and the design is less intrusive since pipes can go underground and cause no hindrance to occupants of the building.
In this article I will talk about using PVC pipes for collecting water.
2) What should be the dimension of the pit? We had dug a 6x6x6 pit since it was a small school. This size would be sufficient for a standalone 3BHK house or a multi-story with 3BHK flats. For bigger buildings, you will need to talk to a contractor.
3) Dug the pit, what next? OK, now we need to fill this pit with stones and sand which will act as purifier layers. In our simple design we will have following layers:
a) 40 mm metal: This is the first layer (from bottom) and is half the height of the pit. So in our case it was 3 ft. See in the picture below.
Below: 40 mm metal being dumped.
b) 20 mm metal: Second layer, half the height of first layer. (In our case 1.5 ft.) These are the pebbles used in road construction. See in the picture below.
Below: 20 mm metal being dumped
Don't worry if you don't know about these stones. You will need to find a stone supplier and he will supply these stones.
3) Sand: Third and topmost layer which is about half feet high.
You will notice that we have not filled complete pit, 1 ft is still remaining. Yes we have left this space purposely, so that during the rains, water coming from the pipe doesn't spill over and is collected in this empty space before seeping down.
The pipe coming from roof top has to be directed to this pit, half ft under ground, which means it will be just half ft above the sand layer. In the above pic, see that pipe is below ground level.
The 1 ft of the pit walls which are exposed can be plastered. This will increase the longevity of the walls by guarding against soil collapsing during the rains.
Special case: If there is a handpump or borewell in the vicinity
By making small change to the design, you can not only make your RWHS more effective but also recharge the handpump if it has dried up, due to water table getting depleted.
We had a handpump in the school which we wanted to recharge, so we dug the pit with the handpump in the center of the pit. A handpump has 2 concentric pipes. The internal pipe about 2 inches in diameter is called riser pipe which actually draws in the water from ground. The outer pipe is just plastic casing about 4 inches in diameter which is called bore hole casing. It is used to protect the internal pipe. Both pipes go fairly deep in ground.
After we removed the soil around the handpump, we had six ft of bore casing exposed. In the below photo the white pipe that we see is the casing. (The steel pipes and rods have been put to support the pump.)
We drilled small holes in the bottom 1.5 ft of the casing. This is done so that water being collected in the pit can enter the bore casing and go deeper easily.
The holes have to be arranged in straight vertical lines on all four sides of the pipe, each about 3 inch apart. (see the photo to understand)
A thin wire mesh was tied around the pipe in this area to cover the holes and avoid any mud/soil/tiny pebbles getting in. (see the below photos)
Remaining process of filling the pit remains same.
You will have to cover the pit with an iron grill so that people using this pump do not step on the sand.
Budget: Stones and sand are purchased according to 'load'. For our project we had purchased 2 loads of 40 mm metal, 1 load each of 20 mm metal and sand. 'Load' is the capacity of the truck which dumps these materials.
40 mm metal costs Rs. 1100-1500 per load.
20 mm metal costs Rs. 2500-3000 per load.
sand costs Rs. 1100-1400 per load.
Problem is that even though we don't need that much sand or 20 mm metal, we can't order in fraction (like half load).
PVC pipes are available in a range of prices depending on the thickness, diameter and brand. Thicker the pipe, sturdier and pricier it is. We bought 80 ft (4x20 ft) 4 inch diameter pipes of an ordinary brand, which cost us Rs. 1500. You will have to buy joints (elbows or Ts) according to structure of your building. Add to this shipping charges. All these things cost us nearly Rs. 1900.
Apart from these you will need a bag of cement for plastering which will cost about Rs 400. You will need a plumber for doing pipe fittings, a mason for plastering the pit or redoing any concrete that you break. If you don't dig the pit yourself you will have to hire at least 2 labors also.
There you go! I have mentioned almost everything that you need to know about construction of a good Rainwater Harvesting System. All you need now is a motivated team (I had the greatest team one can have!) and a good plan to go ahead.
(I have tried to be as much discreet about things as possible. If you have any questions or think something can be better explained, let me know through comments.)