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Poison air threat to tank

- Vehicle pollution causing more damage to 105-yr-old reservoir than water corrosion
Tallah tank, a nine-million-gallon structure 105 years old, 10 storeys tall and wider than the average school playground

Tallah tank, the engineering marvel that has been sating the city’s thirst for over a century, has survived the ravages of corrosion from metals in water but its steel body faces a bigger foe: air pollution.

A team of scientists that recently inspected the world’s largest overhead reservoir heaped praise on its architectural integrity but expressed concern about the impact of constant pollution on the structure.

Vehicle pollution in the tank’s immediate surroundings pose a greater threat to the steel body than corrosion from water, they said.

The tank is contiguous to BT Road, a 24-hour corridor for heavy goods vehicles entering and exiting the city and the lifeline between north Calcutta and its suburbs. Since traffic can’t be diverted to spare the tank, the only way to save it is through proper restoration, the scientists recommended.

“We inspected the whole structure and found air pollution to be a major corrosive factor. Corrosion over the past century has been uniform but pollution can expedite it by reacting to steel,” said G.T. Parthiban, principal scientist at the Central Electrochemical Research Institute in Tamil Nadu.

He was part of the team of three research scientists, all experts in corrosion, that spent two days assessing the condition of the tank and discussing plans for its restoration with engineers of the Calcutta Municipal Corporation. The team filed its report to the CMC early this month.

Carbon monoxide in the air, and to some extent nitrogen dioxide, have been identified as the main agents of corrosion. The pH value of water in the tank has been consistently neutral and, therefore, not contributed to corrosion.

“Carbon monoxide being a strong reducing agent, it drags down the iron content, which in turn encourages the growth of metal corrosive microbes like Thiobacillus or Ferrobacillus. Corrosion is thereby enhanced,” Dipak Chakrabarty, former chief scientist at the West Bengal Pollution Control Board, said.

While the lifespan of a steel structure is 50 to 70 years, experts marvel at the structural integrity of the Tallah tank that has stood strong for 105 years (and counting).

Modern-day welding may have made joining pieces of metal with rivets redundant but the experts were surprised to find that the tank, a completely riveted structure, had suffered minimal damage over the years.

“A few preventive measures can extend the life of the tank by another 50 years. It is amazing to see the tank easily complete a century with minimum corrosion,” said Syed Azim, chief scientist and corrosion paint expert at the research institute.

The findings and recommendations of the panel will be appended to an earlier report by the CMC and submitted to the Jawaharlal Nehru National Urban Renewal Mission (JNNURM).

The decision to renovate the nine-million-gallon reservoir was taken after engineers found signs of corrosion in the steel plates of some of the chambers, which if tanked up with fuel would hold enough to power 158 jumbo jets.

A status report jointly prepared by Jadavpur University, Besu, IIT Khargpur and IIT Patna had been submitted to Delhi last September.

While the JNNURM agreed in principle to an overhaul of the heritage structure, it suggested that another “institute of repute” assess the state of the tank before starting the task. The estimated cost of repairs has been pegged at Rs 60 crore.

“Repairs should have been carried out much earlier. We will make sure that Tallah tank undergoes an overhaul,” mayor Sovan Chatterjee said.

“We hope to get the project sanctioned by March,” added a senior engineer in the CMC’s water supply department.

According to a report filed by the state pollution control board, “the predominant source of CO in Calcutta is automobile tailpipe emission”, which is normally at its peak from 8pm to 2am. Also, the northern part of the city is generally more polluted than other parts of the city because of several reasons such as low road space, greater congestion and higher movement of polluted goods vehicles.

Data uploaded on the pollution control board’s website on February 2 states that the CO concentration in north Calcutta’s automatic station located within the Rabindra Bharati University campus was 4.1 micrograms per cubic meter at 11pm, far higher than the 1.5 micrograms recorded by the south Calcutta station at Victoria Memorial.

The national standard is two micrograms per cubic meter.