Biofloc systems were developed to improve environmental control over production. In places where water is scarce or land is expensive, more intensive forms of aquaculture must be practiced for cost-effective production. There are strong economic incentives for an aqua¬culture business to be more efficient with production inputs, especially the most costly (feed) and most limiting (water or land). High-density rearing of fish typically requires some waste treatment infrastructure. At its core, biofloc is a waste treatment system.
Biofloc systems were also developed to prevent the introduction of disease to a farm from incoming water. In the past, standard operation of shrimp ponds included water exchange (typically 10 percent per day) as a method to control water quality. In estuarine areas with many shrimp farms practicing water exchange, disease would spread among farms. Reducing water exchange is an obvious strategy for improving farm biosecurity. Shrimp farming began moving toward more closed and intensive production where waste treatment is more internalized.
Biofloc systems use a counter-intuitive approach— allow or encourage solids and the associated microbial community to accumulate in water. As long as there is sufficient mixing and aeration to maintain an active floc in suspension, water quality can be controlled. Managing biofloc systems is not as straightforward as that, however, and some degree of technical sophistication is required for the system to be fully functional and most productive.
Composition and nutritional value of bioflocs:
Bioflocs are aggregates (flocs) of algae, bacteria, proto¬zoans, and other kinds of particulate organic matter such as feces and uneaten feed. Each floc is held together in a loose matrix of mucus that is secreted by bacteria, bound by filamentous microorganisms, or held by electrostatic attraction (Fig. 1). The biofloc community also includes animals that are grazers of flocs, such as some zooplank¬ton and nematodes. Large bioflocs can be seen with the naked eye, but most are microscopic. Flocs in a typical green water biofloc system are rather large, around 50 to 200 microns, and will settle easily in calm water. ood but rather variable. The dry-weight protein con-tent of biofloc ranges from 25 to 50 percent, with most estimates between 30 and 45 percent. Fat content ranges from 0.5 to 15 percent, with most estimates between 1 and 5 percent. There are conflicting reports about the adequacy of bioflocs to provide the often limiting amino acids methionine and lysine. Bioflocs are good sources of vitamins and minerals, especially phosphorus. Bioflocs may also have probiotic effects.
Dried bioflocs have been proposed as an ingredient to replace fishmeal or soybean meal in aquafeeds. The nutritional quality of dried bioflocs is good, and trials with shrimp fed diets containing up to 30 percent dried bio¬flocs show promise. Nonetheless, it is unlikely that dried bioflocs could replace animal or plant protein sources used in commercial-scale aquafeed manufacturing because only limited quantities are available. Furthermore, the cost-effectiveness of producing and drying biofloc solids at a commercial scale is questionable.
What biofloc systems do?
Bioflocs provide two critical services—treating wastes from feeding and providing nutrition from floc consumption. Biofloc systems can operate with low water exchange rates (0.5 to 1 percent per day). This long water residence time allows the development of a dense and active biofloc community to enhance the treatment of waste organic matter and nutrients. In biofloc systems, using water exchange to manage water quality is minimized and internal waste treatment processes are emphasized and encouraged.
Research with shrimp indicates that culture water contains growth-enhancing factors, such as microbial and animal proteins, that boost production. Flocs are a supple¬mental food resource that can be grazed by shrimp or tilapia between feedings of pelleted diets.
A potential benefit of biofloc systems is the capacity to recycle waste nutrients through microbial protein into fish or shrimp. About 20 to 30 percent of the nitrogen in added feed is assimilated by fish, implying that 70 to 80 percent of nitrogen added as feed is released to the culture environ¬ment as waste. In biofloc systems, some of this nitrogen is incorporated into bacterial cells that are a main compo¬nent of biofloc. Consumption of this microbial protein, in effect for a second time, contributes to growth.
Research with shrimp and tilapia suggests that for every unit of growth derived from feed, an additional 0.25 to 0.50 units of growth are derived from microbial pro¬tein in biofloc systems. In other words, 20 to 30 percent of shrimp or tilapia growth is derived from the consumption and digestion of microbial protein. This benefit is reflected in improved feed conversion, one of the best predictors of system profitability and business sustainability. However, the value of flocs in nutrition is limited at the highest levels of production intensity because the contribution of feed to growth of cultured animals is overwhelming.
Suitable culture species:
A basic factor in designing a biofloc system is the species to be cultured. Biofloc systems work best with species that are able to derive some nutritional benefit from the direct consumption of floc. Biofloc systems are also most suitable for species that can tolerate high solids concentration in water and are generally tolerant of poor water quality. Species such as shrimp and tilapia have physiological adaptations that allow them to consume biofloc and digest microbial protein, thereby taking advantage of biofloc as a food resource. Nearly all biofloc systems are used to grow shrimp, tilapia, or carps. Channel catfish and hybrid striped bass are examples of fish that are not good candidates for biofloc systems because they do not tolerate water with very high solids concentrations and do not have adaptations to filter solids from water.
Basic types of biofloc systems:
Few types of biofloc systems have been used in com¬mercial aquaculture or evaluated in research. The two basic types are those that are exposed to natural light and those that are not. Biofloc systems exposed to natural light include outdoor, lined ponds or tanks for the culture of shrimp or tilapia and lined raceways for shrimp culture in green¬houses. A complex mixture of algal and bacterial processes control water quality in such “greenwater” biofloc systems. Most biofloc systems in commercial use are greenwater.
However, some biofloc systems (raceways and tanks) have been installed in closed buildings with no exposure to natural light. These systems are operated as “brown- water” biofloc systems, where only bacterial processes control water quality.