Calcutta: The meandering Hooghly is gobbling up one of its banks in Palta, North 24-Parganas, endangering a facility that treats half the volume of water Calcutta consumes.

The Calcutta Municipal Corporation (CMC) has floated a tender to strengthen the river bank adjoining the water treatment plant. The tender was published in September and potential bidders have till December 5 to respond.

"A mild-steel sheet will be erected along 1.8km of the river bank on which the treatment plant is located. This will protect the bank from erosion and save the plant," an engineer of the civic body said.

"The erosion may not create any immediate problem but will pose a serious risk to the plant in the near future. If not arrested in time, the erosion will gain in intensity and slowly eat up the plant's area."

The water treatment plant, 27km from Esplanade, was built between 1868 and 1870.

The plant has three jetties jutting into the river, which are used by workers responsible for the upkeep of the pipes that draw water from the Hooghly for treatment.

"If the erosion is unchecked, the jetties and the pipes may collapse one day," the engineer said.

The British had selected the spot for setting up the plant because the riverbank facing it was the outer curve of a meander.

"Because of being located on the outer curve, availability of water has never been a problem for the plant," an engineer said. "Adequate availability of water is necessary for the plant to function to its optimum capacity."

Old records show that while the depth of the river along the bank facing the plant was 14-17m, it was just 3-4m along the opposite bank.

However, the location of the bank - the outer curve of the meander - has also ensured that it is susceptible to erosion.

"The flow of high volume of water is eating into the bank. Thanks to its position - the outer face of the curve - the water is hitting the bank with full force," the engineer said. "While the bank adjoining the plant is facing erosion, the opposite bank is witnessing siltation."

The water treated at the Palta plant is sent to the Tallah reservoir in north Calcutta, 23km away, through a dedicated line. From Tallah, the water flows into the CMC distribution network.

The civic body has four other water treatment plants - in Garden Reach, Dhapa, Jorabagan and Watgunge.

The five together treat 435 million gallons of water daily. The Palta plant alone treats 210 million gallons.

Work is under way to increase the capacity of the Palta plant by 20 million gallons a day. The Rs 112-crore project has to be finished in 30 months.