Recommended for
Roof Area between 1000 to 1500 sq. meters.
Annual rainfall up to 800 mm. With a daily limit up to 30 mm.
Drilling of new single Injection borewell with assured yield > 2820 lits/hour indicating presence of productive shallow/ deep aquifer/s.
Methodology
Drill a new borewell of at least 6” diameter within the premises at geohydrologically appropriate location. This borewell should yield at least 2820 lits/hour, then only it will accept good quantity of the filtered rooftop water and recharge the aquifer. Usually, this borewell is not used for pumping purposes. Hence, it is treated as a Recharge Shaft.
Preferably use perforated (slit type) M.S. casing pipe of at least 6 meters depth for the borewell during drilling. After drilling grout only, the lower 2 m part of the casing using cement slurry. Wrap silicon mesh of < 70 micron in upper 4 m part of the casing. Cover the borewell tightly with the metal cap.
Channelize the rainwater collected on the roof of the building/s into one or two vertical drainpipes of at least 4” in Diameter. Bring this water through a single horizontal drainpipe of at least 4” diameter, laid down over or below the land surface. Join this drainpipe to the inlet pipe. If required, construct a small chamber of 0.5 m x 0.5 m x 0.5 m dimension for installation of a two-way valve.
Install a two-way ball bearing valve to the drainpipe, where it connects the inlet pipe of the filter media. Close this valve during the first spell of rain and allow the rooftop water to go into the drain chamber. Also close this valve during the high intensity rainfall spells.
Using RCC slab, construct a settling tank of 3 m x 3 m x 3 m (L x W x D) size to collect the roof top rainwater. In continuation with this, construct a second small chamber (inspection) of 1.5 m x 1 m x 1 m size to lay down a horizontal pipe to carry the settled water from the settling chamber to the filter bed. In continuation with this inspection tank, construct a filtration tank of 2 m x 3 m x 3.5 m (L x W x D) size using RCC slab, to collect and filter the roof top rainwater. Please refer to the drawing for further detail.
This complete tank assembly is to be constructed below the ground. Hence it is necessary to cover the tank with an RCC slab. Provide one manhole to each chamber within the slab as shown in the drawing for cleaning and emergency purposes.
In the filtration chamber create a filter bed composed of Sand, Gravel, Activated Charcoal, & Pebbles. Lay down the material very carefully so that the casing and silicon mesh, wrapped over the borewell, will not get damaged. This filter material is readily available.
Initially lay a 700 mm layer of pebbles at the bottom of the filter. Cover it with silicon mesh of < 70 micron from the top. Over this silicon mesh place a 700 mm thick uniform size layer of gravel (size 3 to 5 mm). This is also separated by a silicon mesh of the same size. Add a sand layer of 400 mm thickness over the mesh followed by an activated charcoal layer of 150 mm thickness. Again, lay down 450 mm of sand bed over the activated charcoal. All these layers are necessarily separated by silicon mesh.
For carrying the filtered & clean water from the filtration chamber, lay down a 2” dia PVC pipe at the bottom of the inspection chamber. Put a two-way valve in the middle for safety. This valve is also to be connected to the drain chamber. This valve is to be opened when the borewell starts overflowing.
In the settling chamber provide two overflow pipes to drain out the excess water collected in it. This is required for high intensity rainfall spells. These holes will help to maintain the appropriate hydrostatic pressure within the settling chamber and only the stagnant water will pass into the filtration chamber.
Process
The rooftop channelised water will come through the inlet pipe into the settling chamber. This water is allowed to fall over a small slab (8” long and 2” thick) to avoid the erosion of the tank walls. This slab will also help in reducing the velocity of the inflow water. Slowly the settling tank will get filled by the roof top rainwater. Soon after the water level in the chamber reaches the level of outlet pipe it will start flowing into the filtration chamber.
As soon as the water column increases into the settling chamber, slowly the water becomes stagnant. This helps in converting the turbulent flow into laminar flow. It also allows to settle the silt load present within the roof top rainwater.
When the water enters the filtration chamber, it will start spreading over the sand layer and because of high porosity it will go down naturally. During this movement heavier clay and silt particles present in the water will start depositing into the pore spaces present in the sand and gravel layers. After entering the pebble layer, the water will settle down and remaining dust, clay and silt particles will get deposited at the bottom of the filter.
The activated charcoal helps in adsorption of the chemical and bacterial contaminants present in the water. Hence, it is the most important and non-negotiable constituent of the artificial filter bed.
The clean water will slowly fill the pebble layer. Then it will start flowing through the PVC outlet pipe joined with the borewell. The velocity of this water will be very less. Hence the aquifer/s present within the borewell will accept continuously.
Precautions
During the first rainfall spell, close the inlet pipe two-way valve and drain the first rooftop runoff water. This will help in avoiding the heavy contaminant (including bacterial) load 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.
When inflow of water increases in the settling chamber and outflow from the filtration chamber decreases, excess water collected there will automatically get drained out through two overflow pipes given in the settling chamber.
If the borewell starts overflowing, then immediately close the outflow valve present in the inspection chamber and drain the excess water into the drain chamber. At the time of cleaning the filter keep the outflow valve closed. This will help protect the borewell from siltation, contamination, or damage.
Maintenance
Every year, it is necessary to clean the roof of the building, settling chamber, and filter chamber prior to the monsoon.
Immediately after the monsoon is over, take out the stagnant water in the settling chamber. Also take out the silt settled at the bottom of the tank. For cleaning purposes use the manhole given in the settling and filtration chambers.
Every year during March-April take out the upper layer of sand and wash it thoroughly. After due drying, top up the sand layer with this washed sand.
Once in two years take out the complete filter bed i.e. sand, activated charcoal, gravel and pebble layers. Clean, wash, dry them properly, and then place them again in a filtration chamber. Replace the activated charcoal after 3 to 4 years.
The society/management of the building should train their personnel for the annual maintenance of the filter and make appropriate provision of funds.
Every year check the water quality parameters (chemical and bacteriological) of the borewell water at least during pre- and post-monsoon seasons.