60 DECEMBER 2012 • WORLD AQUACULTURE • WWW.WAS.ORG important feature because food quality for different aquaculture species is partially dependent on particle size (GaratunTjeldsto et al. 2006, Knights 1983). In the case of cod and Macrobrachium rosenbergii larvae, floc particle size should range from 250 to 1200 µm (de Barros and Valenti 2003). Dissolved Oxygen Concentration Dissolved oxygen (DO) is essential for the metabolic activity of cells within aerobic flocs. Flocs tend to be larger and more compact at higher DO concentrations (Wilen and Balmer 1999), although there is no clear relation between DO concentration and average floc diameter. At low DO levels (0.52.0 mg/L), flocs have poor settling properties, with a sludge volume index (SVI) of 250 mL/g. At higher DO levels (2.0-5.0 mg/L), the SVI is about 100 mL/g. More filamentous bacteria than zoogloeal bacteria occur at DO concentration less than or equal to 1.1 mg/L (Martins et al. 2003). Filamentous bacteria have a greater affinity for oxygen than zoogleal bacteria and thus dominate the floc during periods of low DO concentration (Martins et al. 2003). Bioflocs with a greater SVI can be produced at lower DO levels in biofloc ponds. This gives the aquaculture organisms opportunity to filter flocs from suspension before they fall to the sediment and are lost as food. However, floc with a higher SVI can cause clogging of fish gills. Any organic carbon source added to biofloc ponds causes a decrease in dissolved oxygen levels from aerobic microbial metabolism. To produce biofloc as food, it is advisable to grow the heterotrophic biomass in an external reactor rather than within the culture unit itself. Externally grown flocs can be redirected to the pond as food without inducing stress through depressed DO levels. Organic Carbon Source Organic carbon can be supplied 1) as an additional organic carbon source (e.g., glucose, acetate or glycerol) with highprotein feed, or 2) by reducing the protein content of feed, thereby increasing its organic carbon content (Avnimelech 1999). The quantity of organic matter needed for an intensive pond can be calculated on the basis of the amount of nitrogen excreted by the aquaculture species. The organic carbon source of choice will, to a large degree, determine the composition of flocs produced, mainly the type and amount of energy storage polymers (Hollender et al. 2002, Oehmen et al. 2004). Considerations for carbon source selection include local availability, cost, biodegradability and efficiency of bacteria assimilation. The cost of different organic carbon sources will be the main determining factor in making a choice (Salehizadeh and Van Loosdrecht 2004). With BFT systems, it is best to use organic carbon sources that are considered low-value by-products, such as glycerol, which is a by-product of biodiesel production (Dube et al. 2007). Organic Loading Rate The rate of organic carbon loading is a major technical process factor. Filamentous bacteria have an advantage over non-filamentous bacteria at low substrate concentration because of their greater surface-to-volume ratio. Moreover, filaments can extend from the floc core into the bulk water and are thus exposed to greater substrate concentrations than nonfilamentous bacteria that grow mainly within flocs (Martins et al. 2003). Organic carbon can be added in small amounts, almost continuously, or in larger doses at regular time intervals. The second approach is known as a “feast or famine” regime (Salehizadeh and Van Loosdrecht 2004) and results in transient conditions of substrate limitation. Microbial biomass stores cellular energy reserves, such as poly-β-hydroxybutyrate, under conditions of excess nutrient availability, allowing microorganisms to bridge periods of nutrient shortage. Energy storage products may be of importance to the added value that bioflocs bring to aquaculture. As such, it may not be advisable to apply organic carbon sources continuously if the goal is to produce reserve materials. Temperature The parameters described above can be controlled in aquaculture systems. Temperature influences floc characteristics but is more difficult to control. Temperature is of major importance for microbial metabolism. The influence of temperature on biofloc characteristics is complex. More deflocculation of activated sludge flocs occurs in cold water (4 °C) than cool water (18-20 °C), probably related to decreased microbial activity within flocs (Wilen et al. 2000). At high water temperature (30-35 °C), bulking of sludge (SVI ≥ 500 mL/g) occurs from excessive production of EPS (Krishna and Van Loosdrecht 1999). At intermediate water temperature (2025 °C), flocs are stable and have an an intermediate SVI (200 mL/g). The water temperature in BFT ponds is not a factor that can be easily adjusted without imposing considerable additional operating costs, especially in outdoor ponds. In most cases, prevailing climatic conditions determine the operational temperature. Ecofriendly Approach of BFT in Limited Water Aquaculture Biofloc technology is step towards a more environmentally friendly aquaculture production system. The technology was developed to create economic and environmental benefits by reducing water use, effluent discharges, artificial feed supply 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.
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