Growing population and the lack of a corresponding infrastructure for waste water treatment are growing concerns. In every country, the generation of domestic sewage escalates, often beyond the capabilities of conventional sewage treatment plants, which include oxidation/waste stabilization pond, activated sludge, trickling filter, aerated lagoons, upflow anaerobic sludge blanket process, etc. At the same time, it is increasingly being recognized that sewage is not just a pollutant, but rather a nutrient resource. Traditional practices of recycling sewage through agriculture, horticulture and aquaculture have been tried in several countries. This article discusses the role of aquaculture-related approaches for processing sewage water.
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Carp polyculture is practiced in most of these waters n Silver carp (Hypophthalmichthys molitrix) n Common carp (Cyprinus carpio var. communis) n Indian major carps – Catla (Catla catla), Rohu (Labeo rohita) and Mrigal (Cirrhinus mrigala) Other medium and minor carps n Bata (Labeo bata) n Reba (Cirrhinus reba) n Mola (Amblypharyngodon mola) are often used as components of fish culture in these waters |
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Problems related to sewage-fed culture systems n Accumulation of silt and high organic matter at pond bottom n Incidence of parasites and fish diseases n Possibilities of pathogens being transferred to humans Solutions n Regulate sewage intake into the ponds n Provide freshwater for dilution n Use of prophylactics n Depuration of fish in freshwater before marketing Several modifications in the system are also being suggested to achieve greater efficiency and sustainability. |
Resource recovery from wastewater is important not only for realizing the
nutrients as food, but also in ensuring that they do not contribute to the
eutrophication of natural waters that are the usual receptors.
Biological treatment of sewage using algal-bacterial associations, macrophytes and fish employ both autotrophic and heterotrophic food chains, rendering the effluent nutrient-deficient and less harmful to the environment into which it is discharged.
Ponds serve as facultative receptacles for sewage treatment, and also provide oxygen input from the photosynthesizing algae and macrophytes. Ponding reduces the bacterial loads by two to three log units and bacteriophage loads by three to four log units even at the loading rates of 100 kg COD (chemical oxygen demand)/ha/day.
An aquaculture-based sewage treatment plant designed in India has incorporated cultivation of duckweeds prior to application of fish ponds and post-fish culture depuration, with the objectives of refinement of sewage-fed fish culture and sewage treatment through aquaculture practices.
The ASTP consists of a set of duckweed ponds, fish ponds and depuration ponds, located at a place 250 m away from the residential area and borewells. Gravitational flow of sewage wherever feasible for sewage intake into the treatment complex will be advantageous.
A model for treating one million liters per day (mld) of sewage, from a population of about 20,000 is described below:
Source: A receiving chamber for sewage feeds the effluent to the ASTP.
Duckweed culture complex: It comprises 18 ponds with brick lining (25 m x 8 m x 1 m), with three series of six ponds in a row. The sewage is retained here for a period of two days, with free passage between the series.
Fish ponds: Two fish ponds (50 m x 20 m x 2 m) receive the treated sewage from the duckweed ponds and retain it for three days.
Depuration ponds: Two depuration ponds (40 m x 20 m x 2 m) with freshwater, also used as marketing ponds, provide for depuration of fish for a week before marketing. As the fish harvest is occasional, these ponds are also used for the culture of grass carp, fed with duckweeds from the system.
Outlet: Sewage outlet drains are provided from the fish and depuration ponds for drainage into natural waters.
Duckweed cultureDuckweeds serve as nutrient pumps, reducing eutrophication effects and providing oxygen through the photosynthesis activity.
The ponds are inoculated with duckweeds to cover roughly one-third of the surface area (400 g/sq m). The approximate growth rates of individual weeds in the sewage-fed culture system are Spirodela 350 g/sq m/day, Wolffia 280 g/sq m/day, Lemna 275 g/sq m/day and Azolla 160 g/sq m/day. The harvested weeds could be used to feed grass carp in the marketing ponds or composted for application in fish ponds and horticulture fields.
The ponds are stocked with Indian and Chinese carps at a density of 10 000 fingerlings/ha (Catla 40%, Rohu 40% and Silver carp 20%). Grass carp, Ctenopharyngodon idella, is stocked in the marketing pond and fed with duckweeds harvested from duckweed ponds. The fish stocks are checked at monthly intervals for their health and growth through sample nettings. By monitoring of dissolved oxygen levels to maintain 3-5 mg/l, the sewage flow is regulated. Fish harvest is carried out 8-12 months after stocking, with mean individual sizes in the range of 600–800 g. About 600-700 kg of fish are harvested from the two fish ponds, working out to a production level of 3-3.5 tons/ha/year and about 400 kg of fish are harvested from the marketing ponds, representing considerable economic returns from the sewage.
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Depending on the area of operation, different fish species could be used. Tilapia (Oreochromis spp.) and Mandarin fish (Siniperca chautsi) are some of the species that are cultured in sewage-fed waters in China and other countries. The ASTP provides for retention of sewage for two days in duckweed ponds and three days in fish ponds. This achieves the desired reduction in nutrient concentrations, BOD, COD and the bacterial populations to meet the standards for discharge into natural waters. The fish produced from the system enables recovery of about 40% of the working costs. |
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This model has been used in several Indian villages for community sanitation and aquaculture, with modifications. Typically, a third of the pond of the size of 0.2–0.4 ha at the inlet end serves as the receptor of sewage from solid wastes from community latrines. This portion is stocked with duckweeds that multiply in the presence of organic matter and effluents that then pass into the adjacent portion of the pond stocked with fish. With a continuous flow, the organic loading is regulated in different seasons. |
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Prepared by:
S. Ayyapan
Sewage is a dark colored, foul-smelling fluid containing organic and
inorganic solids in dissolved and suspended forms. It contains 90 to 99% water,
10-70 mg/l nitrogen, 7-20 mg/l phosphorus, 12-30 mg/l potassium.
Sewage-fed culture
