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Tallah tank on BT Road is the world’s largest overhead water reservoir. Picture by Sanjoy Ghosh

A nine-million-gallon tank that’s 104 years old, 10 storeys tall, wider than the average school playground and strong enough to survive a century — and a World War II bombing scare — with just 14 leaks. Gulp!

But it’s finally time for an overhaul at Tallah tank, the BT Road behemoth without which much of old Calcutta would be left high and dry.

The decision to renovate the world’s largest overhead reservoir was taken after engineers found signs of corrosion in the steel plates of some of the chambers, which if tanked up with aviation fuel would hold enough to power 158 jumbo jets.

A tank so large would need a gargantuan effort just to identify all the problem areas, let alone renovate the structure, according to municipal commissioner Khalil Ahmed.

So the Calcutta Municipal Corporation has decided to commission a status report by RITES, a subsidiary of the railways, and Jadavpur University before deciding how to go about the task.

“We would need to dry up the cells that store water to replace the steel plates with new ones. We intend to also ascertain the condition of the iron pillars that support the structure and, if required, strengthen them,” a CMC engineer said.

Jadavpur University professor Gokul Mondol said a preliminary inspection showed that the rivets used in the plates had become weak. “A thorough inspection of the structure is required before we can arrive at a conclusion on the overall condition of this great overhead tank.”

Based on the status report, the civic body will draw up a project estimate and put the contract out to tender.

“The cost of overhauling won’t be less than Rs 25 crore,” mayor Sovan Chatterjee said.

That is 500 times more than the Rs 5 lakh spent on building the Tallah tank in 1909!

“We are looking at a global tender considering the requirement of special-grade steel and workmanship to properly renovate this marvel of British engineering,” Bibhas Maity, director-general of water supply in the CMC, said.

Engineers said the proposed overhaul would start by the end of this year or early 2014. Nobody knows yet whether the overhead tank would have to be taken off regular water-supply duty for some time, but CMC officials said alternative arrangements would be made.

Tallah tank had last undergone “minor repairs” in 2009, when it turned 100. “That was part of the centenary makeover,” an official said.

Engineers find it “amazing” that in all these years of uninterrupted service, the tank has sprung a mere 14 leaks. “For a tank of this size, this is unbelievable. The leaks that reappear are quickly repaired and we haven’t yet had an incident that has ever disrupted water supply to the areas we serve,” said director-general Maity.

The water stored in Tallah is enough to bathe and sate the old city’s thirst for a day and Salt Lake’s for two days.

Clayton, Son & Co of Leeds had been the contractor for the tank commissioned in August 1907 and completed two years later. The material used in building the structure included special anti-corrosive plates shipped to Calcutta from the UK. Fabrication was done at the site.

Sir Edward Baker, Lt Governor of undivided Bengal under the British, had inaugurated the tank on November 18, 1909, in the presence of A. Earle Esq, an officer of the erstwhile Indian Civil Service who was then the chairman of the corporation.

The corporation’s chief engineer W.B. MacCade and assistant engineer A. Pierce were present, too.

So famous was the Tallah tank that it had been part of “enemy plans” not only during World War II but also the two subsequent wars with Pakistan. On one occasion, the authorities had planted artificial grass cover on the roof of the tank to make it look like a playground to the pilots of enemy aircraft.