Biofloc Systems for Sustainable Production of Economically Important Aquatic Species: a review
Drawbacks, Limitations, and Management Aspects of BFT Systems
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| 7. Drawbacks, Limitations, and Management Aspects of BFT Systems
Despite their use in commercial applications since the mid-1990s, BFT systems are still facing serious drawbacks and operational difficulties. For example, the dependency of out- door systems on prevailing weather conditions often results in fluctuations in water quality due to changes in microalgae bloom. Therefore, for sustainable aquaculture production, certain factors such as site location, light intensity, and season of the year should be taken into consideration when setting up outdoor BFT systems. Furthermore, the concentration of total suspended solids (TSS) should be carefully monitored. The desirable concentration of TSS for aquaculture ranges from 500 to 1000 mg L −1 , above which turbidity, visibility, and FCR are increased, thus leading to poor growth performance and poor productivity [ 98 ]. According to Avnimelech et al. [ 36 ], monitoring the following water quality parame- ters is vital: • NO 2 . Nitrite is highly toxic to fish if present in levels above 1 mg L −1 . This means the presence of anaerobic regions, which lead to the accumulation of sludge. This will therefore necessitate the changing of aerators to increase levels of dissolved oxygen required by aerobic microbes to convert nitrite to nitrates.
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• TAN. Total ammonia nitrogen below 0.5 mg L −1 indicates that the system is working properly. An increase in TAN above this level warrants the addition of carbon into the system. • DO. Dissolved oxygen should not fall below 5 mg L −1 . Below this level, more aerators should be added to the system to provide more oxygen. • Floc volume (FV) should be in the range of 5 to 50 mL L −1 and this can be monitored using Imhoff cones. When FV concentrations are above 50 mL L −1 , sludge should be removed, and if below 5 mL L −1 , carbohydrates should be added. The slow establishment of nitrifying bacteria within the BFT system is also one of the drawbacks of this technology. It takes more than one month for the initial bioflocs to develop, which might affect aquatic life at sensitive stages of their life cycle. Luo et al. [ 102
] have recently shown that a strategy of a one-time carbohydrate addition at a C/N ratio maintained at 20:1 has a good nitrification performance. However, the authors did not show how fast the nitrifying bacteria were established; thus, more research is needed. De Morais et al. [ 103
] have also shown that increasing the aeration at a rate of 33.75 L/min in Litopenaeus vannamei (Boone, 1931) biofloc culture speeded up the establishment of nitrifying bacteria, whereas a low aeration rate slowed down the process. Jiménez-Ordaz et al. [
104 ] have recently demonstrated that the addition of Schizochytrium sp., L. fermentum (TD19), and two diatoms (Grammatophora sp. and Navicula sp.) can induce the formation of bioflocs in a hyper-intensive culture of P. vannamei reared in a BFT system. Therefore, more research is needed to optimize the conditions necessary for the fast establishment of slow-growing nitrifying bacteria in BFT systems. Another serious limitation of BFT is high energy costs. Aerators and pumps require energy for their normal operation and any incident of power failure can cause huge economic losses. Likewise, high energy costs (hydroelectricity) make BFT systems less feasible to small-scale farmers and those from developing countries. Hence, there is an urgent need to search for cheaper and environmentally cleaner sources of energy that would sustainably permit intensive cultivation and result in maximum profits. Another important concern of the BFT system is the development of off-flavors (geosmin and 2- methylisoborneol) that lower the quality and market cost of BFT-produced fish and shrimps. These off-flavors develop as a result of high turbidity, filamentous cyanobacteria, and Actinomycetes. Transferring fish and shrimps to clean running water before harvest would alleviate this problem but this is costly and unsustainable. However, Schrader et al. [ 105
] suggested that lowering the feeding application rates could decrease the production of off-flavors, particularly 2-methylisoborneol (MIB) in channel catfish (Ictalurus punctatus). Another strategy is to introduce certain microorganisms, such as those from the Bacillaceae family, into the BFT system designs as bioreactors. These have been reported to play a vital role in the degradation of geosmin and MIB [ 106 ,
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