Thursday, July 29, 2021
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Rainwater harvesting in housing societies

Rainwater harvesting assumes greater importance in a City like Mumbai. It is indeed a pathetic position with so much rain water is going to drains. We are at a loss how to plan for the future as the city is expanding beyond our expectations and the need for storage of rainwater is of greater value when there is the absence of rain during one particular monsoon season. No one is worried to Nationalise the rivers in India or save rainwater for future use. Thus we feel that

1.The lack of effective enforcement of the mandate

2. Lack of information to the public on effective methods of carrying out rainwater harvesting.

Rain Water Harvesting (RWH) is extremely efficient and cost effective for those who happen to have a traditional dug well in the 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 level 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 water falling directly can disturb the mud at the bottom of the well and render water muddy, particularly during 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 October. If there are any overhanging branches on the terrace these should be pruned. If not pruned, grills have to be fixed at the outflow points to retain the leaves. This, however, necessitates the need to periodical checking the grills during rains, as otherwise the leaves will block the grills and cause clogging. In 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 wept and the food droppings collected. If the terrace is merely washed, the food dropping would end up in the well and contaminate water.

If only bore wells are available, a safe but not very efficient method of charging a borewell would be to dig a pit of 3 or 4 or 5 ft diameter around the bore well to a depth of 10 feet say, keeping the volume of rainwater to be handled and then 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. Alternatively, 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 bore well. 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 bigger in 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 small service manhole should be provided in the cover for periodic inspection of the well or 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 bore well because of the shallow depths of these dug-wells. Persons having only bore wells can also think of tapping the shallow water table by going for a tube well to draw the water from shallow depths and could provide a pit around it, as detailed above, with the casing having slits or slots and diverting all the terrace water into the pit to charge the tube well and sustain it.

Alternately, they can have a shallow well dug around the bore well and provide the exposed section of the casing pipe with slots or slits as detained above. There are suggestions that the terraced water can be first filtered and then put directly into the bore well casing pipe. This is safe only if the filtration is regularly monitored. Otherwise, the extraneous matter may get into the borewell and spoil its functioning. The rainwater falling in the open spaces around the building can also be harvested efficiently and charged to the shallow water table by providing water trap between columns of the gate and connecting it an absorption pit. Making rainwater useful for domestic purpose is the right step in the right direction instead of just wasting rainwater.

(The views expressed by the author in the article are his/her own.)
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