Wastewater Treatment through Aquaculture

• Biological treatment involves systematic use of natural activity of the bacteria for biochemical reactions, resulting in oxidation of organic matter into CO2, H2O, N2 and SO4.
• The processes used widely for treatment of domestic sewage are activated sludge and trickling filter methods, oxidation/waste stabilization ponds, aerated lagoons and variations of anaerobic treatment systems;
• The latest one is the Upflow Anaerobic Sludge Blanket (UASB) process. The traditional practices of recycling sewage through agriculture, horticulture and aquaculture, being basically biological processes, have been in vogue in several countries. The sewage-fed fish culture in bheries of Calcutta is world-famous. The emphasis in these practices has been on the recovery of nutrients from the wastewater.
• Taking cue from these practices and deriving from the new databases in different disciplines of wastewater management, aquaculture is being proposed and standardized as a tool for treatment of domestic sewage.

View of duckweed ponds

The wastewater treatment system through aquaculture includes sewage intake system, duckweed culture complex, sewage-fed fish-pond, depuration pond and outlet systems.

• The duckweed culture complex comprises a series of duckweed ponds where aquatic macrophytes like Spirodela, Wolffia and Lemna are grown. The wastewater is taken on gravity or pumped through intake system into duckweed culture system where it is retained for 2 days before allowing to fish-ponds.
• The model developed for treatment of 1 MLD wastewater consists of 18 duckweed ponds of 25 m x 8 m x 1 m, constructed in 3 rows, thus allowing water to pass through a series of three duckweed ponds into fish-ponds by gradient.
• The system includes 2 fish ponds of 50 m x 20 m x 2 m and two marketing ponds/depuration ponds of 40 m x 20 m x 2 m. the system receives primary treated sewage, after removal of solid materials.
• On the other hand, the treatment systems operated at two different locations of Bhubaneswar city for treatment of 8 MLD sewage, however, is designed with certain modifications for handling large volume of wastewater.
• As the optimum intake BOD levels for effective treatment are in the range of 100-150 mg/l, it may be necessary to incorporate an anaerobic unit where organic load and BOD levels are very high.

• The duckweed culture unit helps in removal of heavy metals and other chemical residues that otherwise get into human food chain through cultured fish. These also serve as nutrient pumps, reduce eutrophication effects and provide oxygen from their photosynthesis activity. The wastewater with BOD5 levels of about 100 mg/l can be treated with a total retention time of 5 days, with final BOD5 levels brought down to 15-20 mg/l, meeting the required standards of different parameters for discharge into natural waters.
• Further, taking advantage of high productivity and carrying capacity in sewage-fed system, production level of 3-4 tonnes of carps/ha is achieved in fish culture ponds. The system provides for a biological treatment system with high potential of resource recovery in terms of duckweed and fish. The main limitation of this biological treatment system, however, is the reduction of efficiency of treatment during the winter period and the regions with temperate climatic condition. Comparatively low requirement of land of about 1.0 ha for treatment of 1 MLD sewage and resource earnings partially meeting working costs, makes the system an ideal one for treating wastewater before discharge into natural waters.