Kolkata sewer repairs

Year:2011

Duration:4 years

Cost:£32m

Country: Kolkata, India

What did this project achieve?

Refurbish a network of 130-year-old brick sewers under the city of Kolkata

Kolkata’s sewer system was built in 1875 by the British rulers of continental India. Kolkata (then called Calcutta) was the country’s administrative capital.

Designed by engineer William Clark, Victorian Kolkata’s sewer network was rivalled only by those of London and Hamburg in Germany.

By the late 20th century more than 100 years of silt had reduced the system’s effectiveness.

Sanitation and hygiene problems were made worse by flooding during the annual monsoon season. The Kolkata Municipal Corporation (KMC) drew up plans to refurbish around 26km of brick sewers in the oldest section of the system.

Centre of the project was the overhaul of 3 trunk sewers about 12km long. These sewers were the backbone of an ageing system under a heavily congested modern city.

Difference the project has made

Apart from improvements to sanitation and drainage the refurbished system has helped control the flooding that hits the city during monsoon season.

Kolkata receives an average annual rainfall of more than 1,800mm – mostly during the monsoon. The city is often called Venice during the heavy rains as boats ferry stranded people through the streets.

Although there is still flooding the upgraded system means water levels drop faster, people can get around more easily and there’s less disruption to business.

How the work was done

Laying more sewers under Kolkata wasn’t an option – the city has 25,000 residents per square kilometre and only 6% of the land is roads. Access would have been nearly impossible. The only alternative was to restore and refurbish what was already there.

KMC engineers decided to line tunnels with glass reinforced polymer (GRP). GRP is much smoother than the original brick. This means less friction and faster flowing water – increasing the network’s capacity.

Before the sewers could be lined with GRP, they had to be cleared. Decades of sewage deposits meant the resulting silt couldn’t always be shifted with high pressure water jets or suction machines. Workers had to use jackhammers or chisels instead.

After cleaning and before lining with GRP tunnels were repaired and grouted to restore their classic egg shape. An egg shaped tunnel helps maintain a good flow of water in dry weather.

“​‌

Last monsoon, you had 430mm of rainfall in just four days. But were the streets flooded for long? No. This is our success story.

KMC engineer speaking in 2014

Fascinating facts

More than 50,000m3 of silt was removed from the brick sewers during the first phase of the project. The waste was disposed of in secure sanitary landfills.

All work had to be carried out from 11pm - 5am as Kolkata had no standby system to divert the flow of sewage. With businesses closed and people asleep there was a much lighter flow of waste into the sewers.

The monsoon season disrupted work for around 3 - 4 months every year. The increased flow of water from storm drains linked to the sewer network made conditions more dangerous for project workers.

People who made it happen

  • Project commissioners: Kolkata Municipal Corporation (KMC)
  • Project team leader: Dr. Nilangshu Bhusan Basu, principal chief engineer of KMC’s planning and development department

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