Urban Sewage Canal sediment in Kolkata Metropolis (India) is a potent producer of greenhouse gases

The large network of sewage canals criss-crossing Kolkata Metropolis carry considerable sewage load of about 4.5 million people. In an incubation study, substantial carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) production were observed in sediment samples collected from the canal bed at five sites, located at city outskirt, urban slum area, industrial area, residential area and agricultural area. The maximum weekly total production of CO2 and CH4 were 201.6, 137.4 mg kg-1 sediment, respectively, while N2O production was 37.1 mu g kg-1 sediment. This translated into maximum annual production of 28609.5, 19498.6 and 5.3 t y- 1 CO2, CH4 and N2O, respectively, from the sediment pool lying within 0.30 m depth in entire canal network. The CO2-equivalent (100-year time horizon) production in the sediment of canal network was estimated to be 0.56 Tg y- 1. The estimated productions underline the chances of substantial emissions of CO2, CH4 and N2O from such large urban sewage canal networks. Sewage canals as GHG source remains under-represented in greenhouse gas (GHG) inventories due to lack of understanding and data on GHG production in canal sediments and their potential subsequent emissions. This study highlights that sewage canals deserve serious consideration in the preparation of GHG budgets in regions hosting large network of sewage canals.