Recommended for
Regular rainfall - more than 500 mm.
Insufficient space on the surface.
Methodology
Channelize the rainwater collected on the roof of the building/s into one or two vertical drainpipes of at least 4” diameter PVC pipes. Bring this water through a single horizontal drainpipe of same diameter, laid down over a slab of the building or on the land surface depending upon the position of the storage tank. If slab support is not available, then lay down iron beams.
Install a two-way ball bearing valve to the drainpipe. Attach a first flush pipe to the valve to drain out the first rainfall water into the drainage. Close this valve during the first spell of rain and allow the runoff water to go into the drain chamber. Also close this valve during the high intensity rainfall spells.
Using a reducer to the valve, carry the rooftop rainwater through a 3” diameter PVC down pipe into the storage tank.
Estimate the roof water availability using this relation = Annual Rainfall (in mm) x Roof area (in sq. m) x Runoff Coefficient.
Runoff Coefficients
| Type of Roof | Coefficient |
|---|---|
| Concrete | 0.7 |
| Tiled | 0.75 |
| GI Sheet | 0.9 |
Depending upon the water availability and need decide the size of the water storage tank. e.g. For 100 sq. m tiled roof with annual average rainfall of 800 mm ~ 60000 liters of water will be available.
The storage tank is constructed/ installed partially below the ground.
The storage tank is usually covered from all sides. Three openings are kept in it. One is for the lid on the top, second is for allowing the filtered water to enter it from the top side and third is for outlet at the bottom. Fix a valve to this outlet pipe. This is necessary to control the use of stored water.
Though the roof top rainwater appears to be clean it is necessary to use the online filters comprised of coarse sand, activated charcoal, coconut fibre, pebbles, and gravels. Such filters are readily available in the local market.
Immediately after the valve install the online filter either on the storage tank or on the ground.
Fix a small pipe & elbow to the filter to carry the filtered water into the storage tank.
Process
The rooftop channelised water will come through the online filter into the storage tank.
As soon as the storage tank gets filled, completely cover the lid tightly.
For domestic purposes the stored water can be directly used without any treatment. Just open the outlet valve and use the stored water.
For drinking water usage, the stored water is collected into a small tank of 1000 to 2000 lits. Chlorination will have to be carried online using govt approved capsules.
Precautions
During the first rainfall spell, close the two-way valve and drain the rainwater through the drainpipe. This will help in avoiding the heavy contaminant load from entering the chambers. During dry spells (dry days > 10) within the monsoon adopt the same procedure.
During high intensity rainfall spells (>25 mm/hour) open the inlet pipe two-way valve, so that maximum roof top water will drain directly into the drain chamber.
Do not allow the sunlight and dust to enter the storage tank.
Before using the stored water for drinking purposes, it is necessary to get it tested for the chemical and bacteriological contaminations. If the results are within the permissible limits as per IS standard, then only use it after due chlorination.
Maintenance
Every year before the monsoon, clean the roof area properly.
After complete usage of stored water, clean and dry the storage tank in bright sunlight. For next year’s usage first rinse the tank by using the first inflow of rainwater.