Ways to harvest rainwater

The article 'Rainwater harvesting a forgotten solution' (MM, May 1st), has rightly stressed the urgent need for wider practice of Rainwater Harvesting (RWH). The two main factors for the 2003 mandate not having had the desired effect are: 1.The lack of effective enforcement of the mandate, and 2. Lack of information to the public on effective methods of carrying out the harvesting. As a person who has done RWH in over 200 four-storey complexes in Chennai, I offer some guidance on the second aspect.

RWH is extremely efficient and cost-effective for those who happen to have a traditional dug-well in their premises. They just have to connect the various down-take pipes from the terrace directly to the well. The down-take pipes can in many cases be connected to one another at a height along the wall and brought down as one bigger pipe and connected to the well. The horizontal connection can be above ground in places where there is no vehicular movement or concealed just below the ground.

The inlet pipe projecting into the well should have an elbow connected to it so that the rainwater hits the walls of the well and the water runs smoothly down the walls. Otherwise, the momentum of the water falling directly can disturb the mud at the bottom of the well and render the water muddy, particularly in the dry season when the water level tends to be low. Once in two or three years, the well may have to be desilted to remove the deposition of mud at the bottom.

The terrace must be swept clean of mud and leaves once in May and again in September. If there are any overhanging branches on the terrace, these should be pruned. If not pruned, grills have to be fixed at the out-flow points to retain the leaves. This however necessitates the need to periodically check the grills during the rains, as otherwise the leaves will block the grills and cause water stagnation on the terrace. In the case of complexes with more than five stories, neither dust nor leaves will be found on the terrace. If any food is served on the terrace, the floor should be swept and the food droppings collected. If the terrace is merely washed, the food droppings would end up in the well and contaminate it.

If only borewells are available, a safe but not a very efficient method of charging a borewell would be to dig a pit of 3 or 4 or 5 ft dia around the borewell to a depth of 8/10/12 ft (depending on the volume of rainwater to be handled) and stabilise it with RCC rings. When the casing pipe becomes exposed in the pit, replace the exposed section of the pipe with a machine slotted pipe (leaving the bottom and top one foot portions as it is), and cover the slotted section with fine nylon mesh and tie it with polythene cord. Alternately, make horizontal slits on the exposed section using a hacksaw fixed with double blades. This will result in the rainwater entering the casing pipe after filtration and reaching the bottom of the borewell. Although the area for the infiltration of water through the casing pipe during the duration of the rain will be limited, the tank will act as a holding tank for the rainwater which can go into the casing pipe even after the rain has stopped. You can also replace this section with a pipe of bigger diameter provided with slots or slits and covered with nylon mesh. This will increase the area of the openings in the pipe for entry of the rainwater.

A four-inch layer of blue metal at the bottom of the pit will prevent the mud there from being disturbed by rainwater falling with force. This can also be reduced by fixing an elbow to the inlet pipe and extending it close to the wall. Once in a while, the nylon mesh cloth will have to be hosed with water to free it from any fine mud which may clog it.

The well, as also the pit, should be covered with an adequately reinforced concrete cover in several sections so that it can be removed easily by one or two persons for any maintenance. A small service manhole should be provided in the cover for periodic inspection of the well or the pit.

Builders are advised to go in for at least one traditional dug-well in each of their projects as this, generally, will have a good quantity of water and can be sustained by efficient charging of rainwater into it. Electricity consumption for pumping of water will also be much less, compared to deep borewells, because of the shallow depths of these dug-wells.

Persons having only borewells can also think of tapping the shallow water table by going in for a tubewell to draw the water from shallow depths and could provide a pit around it, as detailed above, with the casing pipe having slits or slots and diverting all the terrace water into the pit to charge the tubewell and sustain it. Alternately, they can have a shallow well dug around the borewell and provide the exposed section of the casing pipe with slots or slits as detailed above.

There are suggestions that the terrace water can be first filtered and then put directly into the borewell casing pipe. This is safe only if the filtration is regularly monitored. Otherwise, extraneous matter may get into the borewell and spoil its very functioning.

The rainwater falling in the open spaces around the buildings can also be harvested efficiently and charged to the shallow water table by providing a water trap between the columns of the gate (like the cattle traps of the old days) and connecting it to an absorption pit.

Those needing more information or any clarifications can contact me at isragade@yahoo.com or Dr. Sekar Raghavan, Director, Rain Centre, Mandavalli, at sekar1479@yahoo.co.in.

Indukanth S. Ragade
isragade@yahoo.com