Erosion of the Hooghly bank at Palta, in North 24-Parganas, is posing a threat to filtered water supply in the city.

About half of the five-km embankment on the eastern bank of the river at Palta is on the verge of being eroded. The river has swallowed more than 12 bighas of land. More land is being eroded every day.

If the embankment is not bolstered, the lifting jetties and underground intake well pump houses, located on the riverfront, will be destroyed by the water. The river will also swamp the Palta waterworks.

Worried over the situation, the mayoral council, in a meeting on Friday, approved a project to protect the embankment near the waterworks.

There are three lifting jetties, about 2,500 ft apart, on the bank of the river. They draw about 300 million gallons of water from the river every day. The water is distributed in Calcutta after treatment.

“My predecessor had spent Rs 30 crore to augment the capacity of the waterworks, but he did not think it important to take steps to protect the set-up from erosion at half the expenditure,” said mayor Bikash Ranjan Bhattacharyya.

According to municipal commissioner Alapan Bandyopadhyay, the preliminary survey revealed that the river has eaten about 500 m into the eastern bank.

Since erosion is a continuous process, more and more land will be eroded in the absence of precautionary measures, he said.

The wall to protect the embankment will cost about Rs 15 crore. The expenditure has been ratified at the mayoral council meeting.

The engineers have planned a full casing concrete piling at double the scour depth along the bank. The piling will go down 24 metres and will be topped with reinforced cement concrete beam, continuous pile cap and a serpentine brick wall.

Chief engineer (water supply) Bibhas Maity said the Palta waterworks, designed and set up by the British engineers 138 years ago, continues to be the mainstay of city’s water supply. Of 5.5 million Calcuttans, more than 3.8 million get filtered water from Palta, about 25 km off Calcutta.

The river water is treated and supplied to the city, 72 hours after it is drawn. Untreated water is kept in the primary sedimentation tank for two days and in the final sedimentation tank for a day before being supplied.

The civic body uses 20 tonnes of alum daily and chlorine cylinders to treat the water.

The Palta waterworks started as a six-million-gallons-daily-capacity treatment plant 138 years ago. It now produces 20 million gallons daily of treated water.