Wetlands just outside India’s Kolkata city have for generations provided tonnes of food daily and thousands of jobs as they filter sewage through fish ponds.

But rapid urbanisation is threatening the ecosystem.

Conservationists warn that pollution and strong-arm land grabs are putting a lifeline for the megacity’s 14 million residents at risk.

“We are destroying the environment,” said Tapan Kumar Mondal, who has spent his life farming fish in the ingenious system of canals and ponds stretching across about 125 square kilometres (48 square miles).

“The population … has increased, there is a pressure on nature, they are ruining it,” the 71-year-old added.

Listed as a wetland of global importance under the United Nations Ramsar Convention, the waters offer natural climate control by cooling sweltering temperatures – and act as valuable flood defences for low-lying Kolkata.

But Dhruba Das Gupta, from the environmental group SCOPE, said short-sighted building development was encroaching on the wetlands.

“The wetlands are shrinking,” said the researcher, who is trying to finance a study of what is left of the waters.

Every day, 910 million litres of nutrient-rich sewage flow into the wetland, feeding a network of about 250 hyacinth-covered ponds.

“Sunlight and the sewage create a massive plankton boom,” said K Balamurugan, chief environment officer for West Bengal state, explaining that the microorganisms in the shallow fish ponds feed rapidly growing carp and tilapia.

Once the fish have had their fill, the water run-off irrigates surrounding rice paddies and the remaining organic waste fertilises vegetable fields.

“The sewage of the city is being naturally treated by the wetlands,” Balamurugan said, giving them the nickname the “kidneys of Kolkata”.

The community-developed system was created by “the world’s foremost connoisseurs of wastewater wise use and conservation,” according to its UN Ramsar listing, which also warns it is under “intense encroachment stress of urban expansion”.

The wetlands system processes about 60 percent of Kolkata’s sewage free of charge, saving the city more than $64m a year, according to a 2017 University of Calcutta study.