WWW.WAS.ORG • WORLD AQUACULTURE • DECEMBER 2012 59 Biofloc Technology Biofloc technology combines the removal of nutrients from water with the production of microbial biota, which can be used by some fish and shrimp as food. Bioflocs are macroaggregates of diatoms, macroalgae, fecal pellets, exoskeletons and the remains of dead organisms, bacteria, and invertebrates. Biofloc technology systems in aquaculture are based on activated-sludge wastewater treatment systems. Biofloc technology is based on intensive growth of heterotrophic bacteria, which consume organic carbon and immobilize inorganic nitrogen, depending on the C/N ratio. Removal of nitrogen from culture water by means of biofloc technology (BFT) is regulated by the balanced addition of carbon. This transformation is achieved by adding different types of organic carbon and results in the production of microbial protein that can be used as fish food. One striking feature of biofloc systems is very low retention of nutrients by microbes. About 7 percent of the nitrogen and 6 percent of the phosphorus in feed are retained by conversion to microbial biomass (Schneider et al. 2005). However, when carbon and nitrogen are balanced in the water and microbial assimilation of the ammonia is engineered efficiently, complete retention can be obtained. Avnimelech (1999) demonstrated that 10 mg NH4N/L could be almost completely removed within 5 hours after the addition of glucose at a C/N ratio of 10, without the accumulation of nitrite and nitrate. Bioflocs have the potential to be used as fish food with good nutritional quality. Biofloc technology can stimulate microbial development in BFT ponds that will lead to improved nutrient efficiency and reduced need for dietary fishmeal and fish oil. Biofloc technology can improve the efficiency in the use of nutrients from feed. Dried biofloc can be used as a protein source in aquafeeds. This article describes the factors that regulate the nutritional quality of bioflocs. Biofloc Constituents and Characteristics Microbial flocs consist of a heterogeneous mixture of microorganisms, particles, colloids, organic polymers, cations and dead cells (Jorand et al. 1995). Living microbial cells are only 2-20 percent of the organic fraction of sludge flocs. The total organic content of flocs may be 60-70 percent (Wilen et al. 2003). Flocs can reach more than 1000 µm in diameter. Typical flocs are irregular in shape, have a broad distribution of particle sizes, are fine and easily compressible, with more than 99 percent porosity, and are permeable to fluids (Chu and Lee 2004). The density of floc material is low, just slightly greater than that of water. As a consequence, flocs sink rather slowly (1-3 m/h, Sears et al. 2006). Good-settling flocs are not necessarily lost by sedimentation because aeration devices keep them in suspension. Extracellular polymeric substances (EPS) form a matrix that encapsulate microbial calls and play a major role in binding floc components. The presence of EPS in activated sludge wastewater systems can be substantial, up to 80 percent of the total mass (Hantula and Bamford 1991, Liu and Fang 2003). EPS typically consist of polysaccharides, protein, humic compounds, nucleic acids and lipids (Zita and Hermansson 1994). They are produced as slime or capsule layers under various nutritional conditions, particularly nutrient limitation (Steiner et al. 1976). Critical Control Factors The nutritional value and morphological characteristics of bioflocs are dependent on a number of operational parameters. Inasmuch as floc is an aggregate of several components, prevailing conditions in culture units strongly affect biofloc composition and characteristics. Mixing Intensity In biofloc systems, one of the main functions of paddlewheel aerators is to keep bioflocs in suspension. In aquaculture systems, a wide range of aeration devices are used, ranging from 0.1 to 10 (up to 100) W/m3 (Boyd 1998). The correct number and position of paddlewheel aerators used in ponds are critical. The steady-state floc particle size mainly depends on the number and mixing intensity of aeration devices. The steady-state floc particle size distribution reflects an equilibrium between the rate of aggregation and the rate of breakage and dispersion as affected by turbulent mixing (Chaignon et al. 2002, Spicer and Pratsinis 1996). At greater mixing intensities, shear rates will be high and the average floc size will decrease because of increased floc breakage. For BFT systems, the steady-state floc size is an Factors Controlling Biofloc Characteristics Vikas Phulia1*, Babita Mandal, Aritra Bera, Soibam Khogen Singh, Rakesh Das and Ankur Jamwal (CONTINUED ON PAGE 58) Biofloc technology combines the removal of nutrients from water with the production of microbial biota, which can be used by some fish and shrimp as food. Bioflocs are macroaggregates of diatoms, macroalgae, fecal pellets, exoskeletons and the remains of dead organisms, bacteria, and invertebrates. Biofloc technology systems in aquaculture are based on activated-sludge wastewater treatment systems.
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