Nearly 2.5 lakh tonnes of silt have been picked up from a 7km stretch of Tolly’s Nullah since December 2022 when the Calcutta Municipal Corporation (CMC) started dredging the canal, officials of the civic body said on Monday.
However, the canal is still a happy waste dumping ground for people living along the waterway and suffers because of the direct discharge of untreated sewage.
Pollution in Tolly’s Nullah means pollution of the Hooghly since the canal drains into the river.
There are plans to stop the discharge of liquid waste. The National Mission for Clean Ganga has agreed to fund a Rs 653.72-crore project to build three sewage treatment plants and redirect the sewage lines now discharging untreated sewage into Tolly’s Nullah to the sewage treatment plants and a drainage pumping station.
River scientists and engineers believe that if the discharge of waste is stopped, the condition of Tolly’s Nullah will see a marked improvement.
At present, Tolly’s Nullah has turned into a sewage canal with stench emanating from the waters and mosquitoes breeding on multiple stretches, where the flow of water has almost stopped.
“Many people still consider Tolly’s Nullah as a garbage vat. They keep throwing waste into it. The state government has given us Rs 32 crore to dredge the 15.5-km stretch of Tolly’s Nullah between Doi Ghat and Sonarpur. We need to improve the quality of water, too,” mayor Firhad Hakim, who inspected the progress of the dredging work on Monday, said.
“There are a lot of encroachments along the banks. Today, I found new places have been encroached, places that were without any encroachments earlier.”
The dredging started in December last year and is likely to be completed by mid-2024, said a CMC engineer. “We have so far picked up about 2.5 lakh tonnes of silt from Tolly’s Nullah. On an average, about 3 metres of silt is being removed from under the water,” said a CMC engineer.
Civic officials and engineers said the dredging, which is being done after at least a decade, will not clean the canal, for which the direct discharge of liquid waste and the throwing of solid waste into the water has to stop.
Metro found large pipes as well as drainage pipes from individual houses discharging liquid waste into Tolly’s Nullah during Monday’s inspection. Officials said these were untreated sewage that made the canal’s water extremely polluted.
An analysis of water samples taken from Tolly’s Nullah in February 2021 revealed high levels of pollutants. The analysis, by the state pollution control board, showed that the BOD (biochemical oxygen demand), a parameter of wastewater and sewage pollution, at Bansdroni was 40mg per litre during low tide. The limit is 3mg.
The amount of dissolved oxygen along the entire stretch of the canal was nil. Dissolved oxygen is required for aquatic animals to thrive in any water body.
A senior engineer of the CMC said the civic body will start a project under the National Mission for Clean Ganga to stop the discharge of liquid waste into the canal.
“There are two things that will be done under the National Mission for Clean Ganga project. All the drainage lines that discharge sewage into
Tolly’s Nullah, between Doighat and Chetla, will be redirected to the sewer lines of Calcutta. From there, the sewage will go to the Ballygunge drainage pumping station through underground sewer lines and discharged into the Bidyadhari river, via the East Calcutta Wetlands,” said the engineer.
“For the rest of Tolly’s Nullah, three new sewage treatment plants will be built. The lines that discharge sewage into Tolly’s Nullah directly will be diverted to any of these treatment plants. The plants will reduce the pollutants in the sewage and make it fit for discharge into the Nullah,” said the engineer.
The CMC has floated a tender for the Rs 653-crore project. The company that wins the contracts will get 30 months to complete the tasks.