Kolkata: British-era sewerage system turns corner with tech, precise restoration

Hindustan Times

4 days ago

Amalendu Biswas, a retired government employee from central Kolkata, was often forced to miss office after a downpour as the lane in front of his house would be inundated almost waist-deep. He said Muktaram Babu Street, Manicktala, Thanthania, MG Road, and Chittaranjan Avenue were flooded even after a light rain. The sewerage system was built between 1860 and 1875. (Sourced)
'The scene was almost the same in parts of Kolkata. Photos of boats at Bodyguard Lines in Alipore [housing police personnel] after a heavy downpour were published in every newspaper,' said Biswas.
Kolkata's over 150-year underground sewerage system has since turned a corner, overcoming silt and neglect thanks to a mix of technology and targeted restoration.There was hardly any water logging in July, even as Kolkata received the second-highest rainfall for the month since 2010. The mechanisation has helped in faster and more efficient desilting, which was previously done manually. The work, which started in 2007-08 was scaled up in the last 10 years.
The India Meteorological Department's regional meteorological centre (RMC) recorded around 669 mm of rain this July, against a normal of 371 mm. RWC head HR Biswas said that there were at least three days of intense rain in July when Kolkata received over 80 mm of rainfall within hours.
Large parts of Kolkata would earlier get inundated after moderate (15 mm–64 mm) and heavy rainfall (64 mm–115 mm). Some pockets were waterlogged even after a light rain (up to 15 mm).
A police constable said there was hardly any water logging in the Bodyguard Lines despite the heavy spells of rain in July. 'Earlier, police personnel would need rubber boats to reach the office after a heavy downpour. There is now maximum ankle-deep water after a heavy downpour, but even that recedes fast.'
British officer William Clark designed the system between 1860 and 1875, the first such mechanism in Asia and the world's third, rivalling London and Hamburg (Germany), to discharge rainwater up to 6 mm per hour.
Kolkata Municipal Corporation (KMC)'s former principal chief engineer, Nilangshu Bhusan Basu, said that siltation reduced the sewer system's hydraulic capacity. 'The century-old brickwork was in poor shape and on the verge of collapse in many stretches,' Basu said.
He added that the pumping stations were too old to handle the pressure. 'The run-off reaching the sewers was increasing because of the destruction of wetlands, which once acted as a sponge. Even the city's outfall canal system was heavily silted.'
KMC member-mayor-in-council Tarak Singh said that the civic body refurbished the system by desilting clogged sewers using new machinery. He added that a complete overhaul was out of question as it would take years and thousands of crores. 'We did a targeted intervention in vulnerable and water logging-prone areas.'
Officials said digging up the entire system and laying new pipes was infeasible in a city with a density of over 24,300 people per square km and a road space of around seven per cent. The sewers were heavily clogged with blockages of 70%-90%. There was hardly any desilting.
KMC director general Santanu Kumar Ghosh said they repaired the system. 'The tunnels were lined with glass-reinforced polymer (GRP). It was smoother than the brick walls and ensured a faster flow of water. The fast-flowing water would help the sewer be desilted for a longer time.'
The sewers were unclogged before the repair. Singh said around 36,500 MT of silt was removed in 2014-15. 'In 2024-25, it rose to 2,28,600 MT. We removed around 184400 MT of silt in 2020-21, when the [Covid] pandemic was at its peak. In the last decade, the KMC removed around 1.77 million MT of silt to unclog the system,' said Singh.
At least seven types of machines, including gully pit emptier machines, jet cum suction machines, and blow-vac machines, were used to remove the silt. Singh said the number of gully-pit emptier machines has increased to 40 from 15 in 2004-05, jet-cum-suction from seven to 63, and blow vac from one to eight. 'The mechanisation helped in faster and efficient desilting.'
A KMC engineer said stretches needing repair and reinforcement are dried up by pumping out the water. 'The cleaning team then goes down after taking safety precautions. Machines are used to extract the silt. Once the stretch is unclogged, the tunnels are repaired and grouted to restore their classic egg shape.'
The engineer said that an egg-shaped tunnel helps maintain a good flow of water in dry weather. 'Then the GRP liners are inserted to make the tunnel stronger.' He said the pumping stations were upgraded, and their number increased.
Jadavpur University School of Water Resource Engineering professor Pankaj Kumar Roy called unclogging a mammoth task. 'In some stretches, the silt had become hard like rock and had to be cut. Unclogging the sewers and reinforcing them with GRP gave the sewers a fresh lease of life.'
Over ₹5,000 crore was spent over the last 10 years on the project from KMC's revenue, central and state government funds, and an Asian Development Bank loan. Kolkata has around 2,800–3,000 km of underground sewerage and drainage system, with the new tunnels reinforced with concrete.
A former KMC engineer said the sewers should be declared a heritage, calling it an engineering marvel, with the British-era brickwork in most stretches still holding strong.