World Aquaculture Magazine - June 2021

50 JUNE 2021 • WORLD AQUACULTURE • WWW.WA S .ORG Raceways are larger than photobioreactors in treating the same organic load. The cost per unit volume of raceways is lower because raceway structure is built with an excavation along with the rudimentary protection against erosion, so that the total cost is less. An advantage of the larger volume is that occasional toxic loads may be diluted and hence their effect will be reduced. Similarly, sudden organic and hydraulic overloads can be accommodated more easily. The relative disadvantage of raceways is that, in the absence of a final settler, the effluent in principle has the same composition as the mixed effluent so that biodegradable material and suspended solids will be discharged. As a consequence, the effluent quality of raceways is poor in terms of BOD, COD and TSS concentration. For the ex-situ production of biofloc, two raceways were designed and fabricated using galvanized iron (GI) pipes (Fig. 1). A HDPE liner of 500 g/m 2 (gsm) was installed over frames. A shaded raceway that is 45 × 9 × 1.5 ft (13.7 × 2.7 × 0.46 m) of 17 t capacity was used for production of biofloc meal to serve as an ingredient in shrimp or fish feed. Ten side frame panels were constructed using ¾-inch and 1½-inch GI pipe. The raceway fabrication arch work was constructed with ½-inch GI pipe and the distance between arches was 4.5 m. The total raceway capacity was 30 t and welding works for the arch set up covers the entire raceway (Fig. 2). The roofing arch of the raceway tank is made and covered by UV-protected HDPE liners to prevent sunlight intrusion into the raceway. A central partition or baffle was provided to ensure circular flow of water (Fig. 3). The raceway is shallow (25-40 cm depth) because optical absorption and self-shading by the algal and bacterial cells limits light penetration. Usually, a relatively low cell density is achieved using a raceway pond system (<1 g dry weight/L). The raceway ponds are equipped with two 1-hp paddlewheel mixers for water circulation and suspension of biofloc. Water flow can be adjusted based on biofloc production and harvest. A detachable harvest chamber was designed to harvest biofloc produced in raceways. The inlet of the harvest chamber is connected to an outlet of the raceway and collection of concentrated biofloc is facilitated by a submersible pump. Excess water drained from the harvest chamber is returned to raceways for further fertilization. The system is characterized by highly turbulent flow, a thin layer of suspension (less than 1 cm), and high ratio of exposed surface area to total volume, and thus can achieve dramatically higher volumetric yield (up to 40 g/L) than open ponds. The overall yield obtained from this system is around 20-25 g/m 2 d. Protocol for Biofloc Production in Raceways Used water from fish/shrimp ponds after proper screening of toxic compounds and waste solids is pumped into raceways. Inorganic fertilizers are added to enrich culture water to produce biofloc. An optimal C:N ratio of 20:1 is recommended for large- scale biofloc production. Regular addition of carbon sources for the maintenance of C:N ratio has to be determined by regular monitoring of TAN concentration. Floc volume in the raceway ponds can be assessed using an FIGURE 2. End view of a biofloc production raceway. FIGURE 3. Internal view of a biofloc production raceway. FIGURE 4. An isometric view of the photobioreactor.