World Aquaculture Magazine - June 2021

WWW.WA S .ORG • WORLD AQUACULTURE • JUNE 2021 51 Imhoff cone. After ten days, a notable increase in the floc volume of more than 30 mL/L can be observed in raceways. Harvesting of floc can be initiated once the floc volume is >50 mL/L. Water from raceways is pumped to a harvesting chamber fitted with nylon mesh of different mesh size. Generally, 50- µ mmesh is recommended for harvesting biofloc from raceway ponds. On average, 5-8 kg of wet weight floc can be harvested per day from a 17-t capacity raceway. In general, one third of the wet weight floc can be obtained by drying. Shade drying of biofloc is easiest. However drying wet floc in an oven at 40 C for 48 h is also recommended. The cost of producing dry biofloc is approximately Rs 27/kg (US$ 0.37/kg) based on the source of water used for biofloc production. Photobioreactor The photobioreactor system (Figs. 4-6) consists of 2-m long acrylic tubes arranged in four rows and five columns for a total tube length of 20 m that provides a capacity of 100 L/unit. The bioreactor extends to a length of 1.4 m and height of 1.5 m. The total space for the photobioreactor is 6.1 × 9.1 m. A 1-hp pump motor is connected between the bioreactor and the acrylic pipe unit and provides a flow rate of 40-70 L/min. The pump is capable of operating the bioreactor up to a maximum height of 20 m in the acrylic tubes. Moreover, it runs in the anti-gravity flow to allow culture growth at the optimum flow speed range. For illumination, LED lights were fixed to pass through the acrylic tubes for the fast growth of the biofloc culture in the photobioreactor. An agitator provides suspension of the culture medium and avoids settlement of denser particles. A 1-hp Tuple motor is installed with an enclosed gear box in the agitator which can run up to the maximum of 250 rpm. Three solenoid valves (CO 2 , O 2 and water chilling) are installed in the photobioreactor unit. These valves can maintain the pressure from 0.3-10 bar. Solenoid valves are connected to the PLC control system (Fig. 5) so that they can function to maintain the culture automatically. The water-cooling solenoid valve maintains optimum temperature and CO 2 and O 2 valves maintaining the optimum oxygen and pH in the biofloc culture. A rotometer functions to adjust the pressure of atmospheric air inside the bioreactor and can be managed with the operation of a control valve. Ten plug-ins are connected accordingly for various operating functions of the photobioreactor. Data can be archived in the PLC system and can be utilized further when required. PLC 8.5 software has been used to acquire and record data from transmitters. The photobioreactor can utilize both solar and the artificial light sources. Supply of light to the photobioreactor can be maintained continuously by integrating artificial solar light devices. About 8000 lumens is provided to the bioreactor to optimize growth of the culture. FIGURE 5. Photobioreactor with PLC system (white box). FIGURE 6. Photobioreactor tube array. ( C O N T I N U E D O N P A G E 5 2 )