How biofloc technology works?
Bioflocs are macroaggregates (flocs) of bacteria, algae, protozoa (also known as zooplankton) and particulate organic matter such as uneaten food and feces. The flocs are held together by a loose matrix of mucus secreted by the bacteria, bound by filamentous microorganisms or held together by electrostatic attraction (Hargreaves 2013). Phytoplankton in the biofloc system could either be introduced into the system through the water that is used during the system start-up or inoculated into the system from a phytoplankton stock. In a green-water biofloc system, phytoplankton can help to control the water quality by uptake of toxic substances like ammonia-nitrogen. Being autotrophic, phytoplankton can also perform photosynthesis in the presence of sunlight, thereby enriching the system with oxygen produced.
In a biofloc system, locally available cheap carbon sources such as wheat flour are added into the system to manipulate the C/N ratio in order to stimulate heterotrophic bacteria growth as well as control inorganic nitrogen concentration in the system through assimilating of ammonia into bacteria as single-cell microbial protein. The microbial protein (biofloc) will then be eaten by the fish, thereby recycling protein that are excreted from the fish as only 20-25% of fed protein is retained in the fishes raised in intensive system, with the remainder being excreted into the system as ammonia and organic nitrogen in feces and feed residues.
Constant intensive turbulent mixing is also essential in a BFT system in order to keep the solids suspended in the water column at all times. Without mixing, bioflocs can settle out of suspension and form dense piles that rapidly consume nearby dissolved oxygen, creating an anaerobic zone. These zones can lead to the release of chemical compounds such as hydrogen sulfide, methane and ammonia that are toxic to shrimps and fish. In some practice, sludge banks are resuspended periodically by moving and repositioning aerators, creating a turbulent condition.