Ecologically subsidised city
Let us take an ecologist's look at Kolkata. It has the river Hooghly to its west from where it draws water. Another river, the Kulti Gong, 28 kilometres to Kolkata's east, drains the city's wastewater into the Bay of Bengal. No other mega-urban ecosystem can boast of such assistance from a river jacket. Besides, the city is endowed with a copious supply of groundwater: dig a hole and get any amount of water.
If all this is not enough, Kolkata has a huge wetland to its east, the East Calcutta Wetlands. This is a unique ecosystem that uses wastewater in fisheries and agriculture and in the process purifies it to a standard that compares very well with what comes out from many urban secondary level treatment plants. Even by conservative estimates, this has saved the city authorities from spending Rs 400 crores on capital expenditure and incurring a recurring maintenance cost on water treatment plants