Rainwater harvesting apparatus on the roof of the Salt Lake office of The Association of Engineers, India. Picture by Sanat Kumar Sinha

The groundwater level in and around the city is fast receding even as the demand for water, met by groundwater and river water, outstrips supply by 14 million gallons per day (MGD). The gap can be closed by harvesting rainwater, according to The Association of Engineers, India (AEI).

To demonstrate the process, including collecting of rainwater from rooftops, the association held a two-day training session in April. The Calcutta Municipal Corporation (CMC) urban planners as well as engineers working with realtors attended the training. Such training sessions will be held once every three months.

“A person needs 135l of water daily. Rainwater that accumulates on roofs can meet a household’s needs for more than 200 days a year,” explained Chittaranjan Haldar, the AEI secretary.

“To store rainwater, one has to install pipes to bring it to the ground level, where it would be filtered and stored in a reservoir, which can be built on the ground or below it,” added Haldar.

Madhusudan Bhattacharjee, the former head of the Jadavpur University mechanical engineering department and the president of AEI, said: “We can harvest 48 MGD of water in Calcutta, which is more than three times the supply gap.”

He added: “A rainwater harvesting set-up will soon be installed in the Jadavpur University civil engineering building. Water will flow down from the eighth floor terrace to the fourth floor, where it will be filtered.”

The AEI will ask the CMC to provide a 50 per cent rebate on expenditure for setting up harvesting infrastructure and fee for sanctioning the plans of the houses that do so. The association will also recommend property tax rebate for those who set up harvesting equipment.

The approximate cost of installing (rooftop) rainwater harvesting equipment in an under-construction two-storeyed building with a 100-sq m terrace is Rs 16,000. For ready houses, rainwater harvesting equipment will cost about Rs 20,000 more.

“During construction of a house, a reservoir has to be built for soaking the bricks. It can later double as the reservoir for rainwater harvesting,” said Bhattacharjee.