WWW.WAS.ORG • WORLD AQUACULTURE • MARCH 2018 37 (CONTINUED ON PAGE 38) volume biofilter, equal to 50 percent of the total volume of all fish tanks was designed to ensure sufficient organic matter oxidation and nitrification. This was considered to be a cheaper way to remove organic matter without extra energy consumption. The biofilter consists of four connected tanks with different functions. The first tank is filled with a high density of cheap brush material to capture and oxidize most of the organic matter generated in the system. The second and third tanks are filled with filter materials with high surface area for nitrification. The fourth tank is filled with brush materials and provided with strong aeration from the bottom and is operated as a trickling filter to remove dissolved gases. A low-lift, high-flow-rate centrifugal pump provides a recirculation rate of one complete system volume per hour. To maintain dissolved oxygen concentration near saturation, pure oxygen is injected into an oxygenation tank next to the biofilter. UV light sterilization is applied between the outlet of the biofilter and fish tanks to control pathogens. Ozonation is an optional compartment that can be combined with the protein skimmer in the case of emergent disease control. To adapt to the live fish market, a RAS unit consists of 8-10 small fish tanks with a surface area of 20-40 m2 each (Fig. 7). Such small tanks facilitate grading, harvest, and sale of small batches of live fish daily (Fig. 8). The whole room is designed as a closed space. An arched roof and steel supports are sprayed with rate between the Bohai Sea and the Yellow Sea. Nevertheless, the high demand for live fish and rich and cheap geothermal energy resources in this region are favorable for marine aquaculture development in Bohai Bay. Marine fish consumption in China is different from that in other countries. Most fish are sold to restaurants live at a market size of 500-800 g, and served at the table as a whole fish, rather than as a fillet. Thus, live fish are usually stocked in aquaria of restaurants for several days (Fig. 3), which is needed to minimize stress during long-distance transportation. A Simple Recirculating Aquaculture System Appropriate for the Bohai Bay Situation In 2000, the first farm equipped with a recirculating aquaculture system (RAS) was set up in Tianjin, a main city on the Bohai Bay coast (Figs. 4 and 5). After several successful technical modifications and innovations, the production capacity has increased from 11,000 m2 in 2000 to 45,000 m2 in 2016. The RAS is designed to save water treatment costs and stabilize water quality, which should result in lower investment in equipment, electrical power, and maintenance, but allow high-density culture. In RAS, water from the culture tank is treated through a series of physical, chemical and biological process — such as screening, foam fractionation, disinfection, and biofiltration — before returning to culture tanks (Fig. 6). To reduce the investment, a drum filter was replaced by a flow-through, curved-screen, welded-wedge filter, and a protein skimmer was used to remove small particles and dissolved organics that are not removed by the curved-screen filter. To compensate for the low efficiency of the curved screen, a largeFIGURE 4. Haifa fish farm, the first company to use RAS in China. FIGURE 5. Overview of a RAS fish farm in the Bohai Bay area. FIGURE 6. Diagram of the recirculating aquaculture system: fish tank (1), curved screen filter (2), pump (3), protein skimmer (4), biofilter (5), degassing tank (6), oxygenation tank (7), UV light (8), optional ozone generator (9), and liquid oxygen storage tank (10).
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