The water most of Calcutta drinks comes from a 108-year-old overhead tank that is one-and-a-half times the size of the football pitch at Salt Lake stadium. Tallah tank, the steel behemoth that is hard to miss as you enter BT Road, is being renovated for the first time since it was built in 1909 at the cost of Rs 11 lakh.
Deepankar Ganguly and Pradip Sanyal of Metro climbed 110ft up the tank — around 10 storeys tall — and then 16ft below into its cavernous belly to find out the secrets it holds.
What we saw
The tank we have all seen from a distance is actually not one but four of its kind. One of these four chambers has been emptied out for the first round of repairs. After draining out the water, workmen in Wellingtons have been using shovels to scrape the silt that has accumulated on the floor and walls over the years.
The steel plates on the roof, each measuring 4.5ft x 2ft, have been removed at two places and makeshift stairs built to go into the tanks. It doesn’t feel like a tank inside, more like a steel fortress without any ventilation. The saving grace, or relief, is that lights have been installed. On the flip side, illumination has made the place hotter still.
The lack of ventilation is killing, and shirts are soaked in sweat in no time.
What will be repaired
A technical assessment by the department of construction engineering at Jadavpur University (JU) showed:
1) Deviation between actual fabrication and drawing
2) Leaks from the bottom plate in some areas
3) Loose rivets
4) Corrosion
5) Displacement of the timber sleepers between the bed of the tank and the steel structure on which it rests. These are meant to reduce corrosion and provide a cushion
6) Growth of vegetation on the timber sleepers
What is being done now
A tank built with a budget of Rs 11 lakh is undergoing a Rs 76-crore renovation. Bridge and Roof Co. (India) Ltd, a central government undertaking, has been entrusted with the job.
Once the silt is removed, the rusty rivets and plates will be replaced. The structure on which the tank rests will be reinforced, leaks plugged and all vegetation removed from the timber sleepers.
In the early 1970s, some of the concrete slabs on the roof of the tank had been replaced and that is about the only repair that the tank needed in over a century.
How a 108-year-old mechanism still works
Chief engineer William Bernard MacCabe had created a mechanism to fill the entire four-chamber tank and empty it through a single 5ft diameter pipe. But if necessary, each chamber can be isolated. “This century-old mechanism devised by MacCabe has worked perfectly,” said Bibhas Maity, director general of water supply in the Calcutta Municipal Corporation.
Why Calcutta won’t be affected during the renovation
Since one chamber has been decommissioned for repairs as of now, Tallah is supplying 6.75 million gallons of water less every day. Calcutta’s per capita supply is 32.9 gallons a day, which means a daily shortfall of 6.75 million gallons. In terms of population, 1.84 lakh-odd households would have been affected had the civic body not made alternative arrangements.
To compensate for the shortfall, extra pumps have been installed at Palta waterworks as well as Tallah and several old pumps replaced to ensure an extra supply of seven million gallons.
“I hope people will not feel the shortage in supply as we have made arrangements. The renovation of Tallah tank was long overdue,” mayor Sovan Chatterjee told Metro.
What is the project timeline
Bridge and Roof Co. has 28 months from July 2017 to finish the job. Engineers hope to wrap up the project by December 2019.
