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<International Source Book On Environmentally Sound Technologies for Wastewater and Stormwater Management> Calcutta system The wastewater-fed fish ponds currently occupy an area of about 2,500 ha although they extended over an area of 8,000 ha up to the late 1950s. They are located in a 12,000 ha waste recycling region for Calcutta city which also includes cultivation of vegetables on wastewater and garbage, and paddy fields irrigated with fish pond effluent (Figure 2.53). The wastewater-fed fish ponds have been developed by farmers over the past 60 years who learned by experience how to regulate the intake of raw sewage into ponds to culture fish.
Calcutta has no wastewater treatment plant and much of the sewage flows untreated into estuarine waters. The main sewers of Calcutta began to function over 100 years ago in 1875. Both sewage and stormwater were discharged through an outfall into a river just outside the city. However the river silted up over the next few decades which lead to the construction, between 1935 and 1940, of sewage and storm water channels, leading to estuarine waters 30 km to the east of the city which receive untreated effluents. About 1930 a landowner discovered he could cultivate fish by letting in wastewater to what was then still a swamp. Soon a large area was converted into wastewater-fed fish ponds. As the sewage or dry-weather flow (DWF) channel usually runs at a higher level than the storm-water flow (SWF) channel, they provide a gravity feed system for the farmers to add sewage to, and drain water from, fish ponds. The area of fish ponds expanded to about 8,000 ha at its peak in the 1950s. As the wastewater-fed fishponds extend from the outfalls of the DWF and SWF channels (both of which now convey untreated wastewater), the wastewater is essentially raw. Drainage and preparation of ponds for fish culture is carried out during the coolest months. Primary fertilization involves filling the ponds with raw sewage and allowing it to undergo natural purification for 20-30 days before fish are stocked. Secondary fertilization consists of introduction of small amounts of wastewater throughout the fish culture cycle to maintain growth of plankton to feed the fish, periodically in small ponds and almost continuously in large ponds. Water may be drained from ponds by gravity to avoid flooding. Major cultured species are Indian major carps, Chinese carps and tilapias. Multiple stocking of fingerlings and multiple or periodic harvesting of fish leads to relatively high yields of 3-8 tonnes/ha/year. There are different types of ownership of wastewater-fed ponds. The majority are under private ownership, often by absentee landlords. There are a few cooperatives and two farms are run as corporations by the West Bengal state government. The area of wastewater-fed ponds has declined over the past 30 years, mainly due to urban expansion. Currently they provide employment for 17,000 poor fishermen and produce 20 tonnes fish daily. Much of the harvest comprises fish < 250 g which are purchased by poor urban consumers. They provide a low-cost, natural wastewater treatment and reuse system for a city that lacks conventional wastewater treatment plants. The area of low lying fish ponds also provides storm-water drainage and a green area or lung for the city. The major threat is from urbanization but industrial pollution causes contamination of fish, particularly heavy metals. Six hundred tanneries discharge 150 kg chromium daily into the wastewater stream that eventually feeds the ponds, although there are plans to relocate the tanneries. Additionally, unregulated discharge of industrial effluents from thousands of small-scale factories threatens the wellbeing of fish and ultimately the public health of urban consumers as the wastewater is not pretreated. Ganga Action Plan Untreated municipal wastewater contributes a major pollution load to the rivers of India. The Ganga Action Plan was introduced by the Government of India with the objective of reducing pollution from discharge of municipal effluents along the entire length of the river Ganges. In the first phase of the initiative, class 1 cities with a population of more than 100,000 inhabitants have been included in the core sector programme which covers treatment and possible reuse of municipal wastewater which were being discharged untreated into the river Ganges. A concept of an integrated wetland system (IWS), for wastewater treatment and resource recovery through aquaculture and agriculture has been formulated based on traditional practice in the East Calcutta Wetlands. Wastewater treatment is linked to improvement of livelihoods of the local community in the rural, peri-urban area by nutrient recovery through aquaculture and agriculture. This has been accepted by the Ganga Project Directorate as a more realistic alternative to conventional mechanical treatment plants under Indian conditions where sufficient land is available. The challenge in replicating the Calcutta East Wetlands experience has been to scale down to an appropriate level i.e., the Calcutta municipal system generates about 750 million litres of wastewater daily and the total resource recovery area covers about 12,000 ha but smaller cities have a design wastewater load of less than 50 million litres per day and much smaller areas of available land for nutrient recovery. An IWS has been developed to date in three municipalities within the Calcutta Metropolitan area at Titagarh-Bandipur, Bally - Kona and Panihati with design loads of 14, 30 and 12 mld, respectively. Resource recovery has been achieved with different levels of success but the three projects provide important lessons for the challenge of transfering traditional, ecologically sound knowledge to an acceptable technological option for wastewater treatment for policy makers, planners and engineers. The first IWS to be constructed was at Bandipur. Wastewater from the Titagarh municipality on the bank of the river Hugly (the name for the river Ganges in West Bengal) is transported several kilometers to the project site. A series of three ponds in series were constructed in a derelict wetland. Up to half load condition (presently it is not more than 20% of the design loading rate of 14 million litres per day) pretreatment takes place in an anaerobic pond with fish culture in two sequential ponds. Under full design load condition the area for fish culture will decline as the first fish pond in series becomes a facultative stabilization pond. Effluent from the fish ponds is used to irrigate about 25 ha of rice fields, with an average production of 3 tonnes/ha without use of chemical fertilizers. About 200 farming households benefit but improved distribution of treated effluent would almost double the number of beneficiaries. Furthermore, the irrigated area will expand in future with increased wastewater flow from the municipality. An important feature of the IWS is the participation of stakeholders: the Calcutta Metropolitan Water and Sanitation Authority, the local village authority, the fish farmer who leases the ponds, and the rice farming households. Agenda - 21 emphasized the need to institutionalize the participation of stakeholders in environmental improvement projects to achieve decentralized decision making and management, in this case empowering a rural community for wastewater treatment and reuse. Another crucial feature is the successful implementation of a revenue generating procedure that should ensure adequate management of the system.
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