World Aquaculture - September 2023

WWW.WAS.ORG • WORLD AQUACULTURE • SEPTEMBER 2023 31 phytoplankton production so as to increase water use efficiency and (3) reducing water pollution impacts by avoiding direct discharges into surface waters (Mandal et al. 2018). The cumulative effect of these services can yield protein-rich food from aquaculture to help sustain growing populations. Making Sewage Usable for Aquaculture Characteristics of sewage water. Composition and concentration of domestic sewage vary greatly, with time and place. Used after domestic needs, all the liquid waste materials which are mixed with used water and pass through sewerages include organic compounds, detergents, mineral elements, hormones and scanty amount of pharmaceuticals. Generally, raw sewage comprises about 99 percent water and 1 percent solid materials, of which 70 percent are organic and 30 percent inorganic, apart from pathogenic constituents. Further, organic solids are typically 65 percent protein, 25 percent carbohydrate and 10 percent fat, whereas inorganic solids include silt and minerals. However, the gradation of sewage involves three categories, such as (1) strong, (2) medium, and (3) weak based on biological oxygen demand (BOD) levels. Strong sewage with BOD around 400 ppm is treated and converted to weak sewage with BOD < 30 ppm, which is suitable for use in aquaculture. Potential of sewage. Universally, sewage was previously considered as a valuable organic fertilizer. One L of sewage might generate an average amount of 0.05g of nutrients, primarily N and P. Inflow of 1.0 L of sewage has been calculated to yield an average amount of 0.31 g biomass in the form of fish reared in sewage-fed water bodies (Mandal et al. 2015). Potentiality of sewage has been quantified in terms of fertilizer inputs in culture systems. A rough estimate of 470 kg N ha-1 and 63 kg P ha-1 were available as fertilizer in the form of ammonia nitrogen (NH4-N) and phosphate (P2O5) when 10.5×103 m3 ha-1 of sewage effluent was applied in sewage-fed ponds over 8.5 months, resulting in an average yield of 3.5t of fish (Mandal et al. 2015). Fish production varied between 3.0 and 6.0 t/ha/ yr, creating employment for 2-2.5 persons/ha/yr directly and 1-1.5 persons indirectly as ancillary workers. This amounted to a total of 15,500 laborers’ engagement for the EKW. This sewage-fed aquaculture system operates without supplementary feed or aeration (Ghosh 2018). Treatment of sewage. A sewage treatment plant (STP) was not available in Kolkata city, instead only a Sewage Pumping Station (SPS). Domestic sewage, which was run through closed drains inside the city, was stored temporarily in the SPS before discharge into open drains. The BOD level of raw domestic sewage was around 400 ppm, treated as strong sewage which traveled over a range from 10 to 90 km from the SPS to the nearest and farthest sewage-fed bheris, respectively. While travelling long distances through the open shallow drains (0.3m depth) under bright sunlight, strong sewage is partially treated, becoming mediumto-weak sewage. The BOD level of sewage under such conventional treatment varied between 50-100 ppm before its entry into the bheris. The periphery of all bheris was covered with water hyacinth (Eichhornia crassipes) extending a distance of 1 m from the dykes, and retained with bamboo fencing (Figure 2). Water hyacinth served to accumulate excess amounts of nutrients so as to leave the water body with optimal conditions for phytoplankton production. FIGURE 3. An illustrative diagram of sewage-fed aquaculture practice in EKW. FIGURE 2. Water hyacinth restricted with bamboo fencing in the periphery of water bodies. (CONTINUED ON PAGE 32)