A study of East Kolkata Wetlands
The East Kolkata Wetlands is one of the largest human made wetland which caters to the ecosystem service of managing sewage water of the city. It receives daily input of 350 million gallons and covers 125 sq kilometers. The wetlands are also used to treat Kolkata's sewage, and the nutrients contained in the wastewater sustain fish farms and agriculture.
The name East Calcutta Wetlands was coined by Dhrubajyoti Ghosh, who reached this incredible but neglected part of the city when, while working as an engineer for the Government of West Bengal's Water & Sanitation Department, he was finding answer to understand exactly happens to the cities sewage problem.
Depth of bheris (ponds) are shallow 50 to 150 cm or 3-4 feet and ponds as large as 40 ha. They yield on average 7 tons/ha/yr, 2-4 times that of fresh water ponds. Series of ponds required for ideal waste water fishery – egg ponds, nursery ponds, rearing ponds, stocking ponds and finally harvesting ponds.
Pond preparation steps – pond preparation done in colder months when growth of carps is slow.
1) Draining of water from pond by pumping
2) Sun drying the pond bed
3) Sludge remove and ploughing of pond bed
4) Prepare 1 ft excavated channel as border around bheri, preparation and repair of dykes. Surfaces of dykes can be used for growing horticulture crops.
5) Waste water allowed into the pond
6) Application and mixing of lime – ensure alkalinity of introduced water for coliform removal
7) Embankment using water hyacinth after few days – saves dykes from wave, provides shelter to fishes, removes heavy metals from water, supplies oxygen through photosynthesis
8) Bamboo sluice helps prevent entry of unwanted fish and escape of cultured fish
Primary fertilization -
1) Sewage drawn from canal through bamboo sluice and allowed to stabilize for 15-20 days. Gate prevents entry of large solid suspended matter
2) Photosynthesis activity in presence of large amount of sunlight. Phytoplankton uses organic matter and themselves are fish feed.
3) Once the water turns completely green (due to phytoplankton), stocking of fish is initiated.
Fish stocking – starts in middle of feb or march
1) Before stocking, some are kept in egg pond to test pond conditions
2) Fish stocking takes place at different times depending on intensity and suitability of operation
Secondary fertilization –
1) Introduction of waste water from time to time is carried out throughout the growth cycle
2) Sewage (1-10%) is taken into ponds throughout the culture period
3) In bigger ponds, water level is maintained through constant inflow and outflow
4) Care is taken to maintain DO above threshold level for fish sustenance
5) Requirement of sewage is determined by observing water colour, transparency, temperature and depth
6) Nutrient rich nature helps avoid supplementary feed. During rainy season due to dilution of water, they are fed with supplementary feed.
1) Growth cycle ends with harvesting during August and continues to December
2) Rotational cropping carried out to maintain steady supply for market
3) After completion of one cycle, restocking at the rate of 1 kg fingerlings/ 5 kg harvested fish is carried out
4) Another harvest phase follows after 15 days of restocking
5) Drag nets are generally applied for harvesting using encircling technique. For fishes like tilapa and common carp, hand picking is preferred.
Feed used- mustard oilcake & Mahua Khoal and ready fish meal
Replication of Bheri –
Depth of ponds – 2.5 to 3 feet. Size- 1.15 to1.32 ha.
Waste water drain close to ponds (approx. 3 feet far). Surrounding lands of the ponds cultivated with vegetables, bananas.
For aquaculture, maximum permissible limits – pH :8.5, alkalinity: 200mg/l, hardness: 300mg/l. DO levels should be atleast 5 to 6 mg/l
Salient points for successful implementation of the model:
1) Requirement of shallow ponds/pond of depth 3-4 feet for successful penetration of sunlight to the bottom
2) Place needs to receive lot of sunlight for the growth of plankton from the nutrients in the sewage
3) Pond/ponds need to have drainage pipes close by for receiving the sewage. There should be a continuous supply of sewage of high quality of nutrients to minimise the use of secondary feed.
The study was carried out by M.S Sharath, an intern at ATREE.