Bioflocs are conglomerates of microbes, algae, protozoa and other organisms, together with detritus, dead organic particles. The basic principle of the biofloc technology (BFT) is the retention of waste and its conversion to biofloc as a natural food within the system.
There are strong economic incentives for an aquaculture business to be more efficient. High-density rearing of fish typically requires some waste treatment infrastructure. Biofloc systems use a counterintuitive approach, allowing or encouraging solids and the associated microbial community to accumulate in the water. Aquaculture production depends on feed, dissolved oxygen and maintenance of water quality parameters in an optimal range. Basically, where the microbial community growth is promoted and utilized for removal of waste accumulated in water in the form of uneaten feed, faecal matter with excreted ammonia and derived total nitrogen. As long as there is sufficient mixing and aeration to maintain an active floc in suspension, water quality can be controlled in a better way. Managing biofloc systems is not straight forward as that, however, and some degree of technical sophistication is required for the system to be fully functional and the most productive.
Each floc is held together in a loose matrix of mucus that is secreted by bacteria, bound by the filamentous microorganism, or held by electrostatic attraction. The biofloc community also includes animals that are grazers of floc, such as some zooplankton and nematodes. Large biofloc can be seen with the naked eye, but most are microscopic. The nutritional quality of biofloc to the cultured animal is good but rather variable. The dry-weight protein content of biofloc ranges from 25 to 50 per cent, with most estimates between 30 and 45 percent. Fat content ranges from 0.5 to 1.5 per cent, with most estimates between 1 and 5 percent. There are conflicting reports about the adequacy of biofloc to provide the often limiting amino acids methionine and lysine. Biofloc is a good source of vitamins and minerals, especially phosphorus. The core advantage in biofloc systems is whatever the waste around from feed or faecal matter from fish which accounts for about 70% of the total energy given in feed, goes to the system as the waste. Thus at intensive culture operation maintaining water quality becomes a challenge. So reusing the lost nutrients through floc development becomes a wise way for increasing feed efficiency and maintaining water quality.
Dried biofloc has been proposed as an ingredient to replace fishmeal or soybean meal in aquafeeds. The nutritional quality of dried biofloc is good, and trials with shrimp feed diets containing up to 30 percent dried biofloc show promise. Nonetheless, it is unlikely that the dried biofloc could replace the animal or plant protein sources used in commercialscale aqua feed manufacturing because only limited quantities are available. Furthermore, the cost-effectiveness of producing and drying biofloc solids at commercial scale has to be standardized.
As probiotics and its effect
Floc stimulate immune response in fish. It is well established that the immune response of animals reared in the biofloc based systems is better than the other control culture. A possible reason is the presence of bacteria and other microbial assemblages that give specific nutraceutical and immunogenic effects.
How biofloc system works
Biofloc technique is a waste treatment method. Biofloc provides two critical service straining waste from feeding providing nutrition to the fish 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 exchanges to manage water quality is minimized and internal waste treatment processes are emphasized and encouraged.
Suitable culture species
The biofloc system is beneficial for the species that are able to derive some nutritional benefit from the direct consumption of floc. Biofloc systems are also most suitable for the species that can tolerate high solids concentration in water and are generally tolerant of poor water quality such as Tilapia, Pangasius, Magur, Singhi and Common carp etc. Some of them like Tilapia, Common carp, and shrimps have physiological adaptations that allow them to consume biofloc and digest microbial protein, thereby taking advantage of biofloc as a food resource.
Basic types of biofloc systems
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 for shrimp & fish culture in greenhouses. A complex mixture of algal and bacterial processes control water quality in such “Greenwater” biofloc systems. However, some biofloc systems (raceways and tanks) have been installed in a closed building with no exposure to natural light. These systems are operated as “brown-water” biofloc systems, where only bacterial processes control water quality.
Advantages of biofloc culture system
• Biofloc acts as a substitute for fish meal in aquaculture feed.
• The protein level in biofloc ranges from 15% to 20 %.
• Biofloc system provides a protective shell to the fish, through a probiotic effect (Depression of Tilapia infection by Streptococcus bacterial problem).
• Reduced need for water exchange.
• Higher stocking densities of fishes.
• Biosecurity can be maintained.
• Better feed utilization & reduced FCR.
• Better nutrition by continuous consumption of biofloc.
• Enhance growth performance and survival.
• Maintains favourable water quality and enhanced production.
• Reduction in feed cost.
• Reduction in toxic metabolites.
• Reduction in stress.
• Reduction in the pathogen.
• Production (Carrying capacity): 5% to 10% better than the normal system.
• FCR low – between 0.6 to 1.0.
• Production cost is lower by around 15% -20 %.
• Disadvantages of biofloc system
• Reduced response time because water respiration rates are elevated.
• Start-up period required.
• Alkalinity supplementation required.
• Increasedpollutionpotential from nitrate accumulation.
• Inconsistent and seasonal performance for systems exposed to sunlight.
Importance of mixing and aeration
Intensive turbulent mixing is an essential requirement of biofloc systems. Solids must be suspended in the water column at all times or the system will not function. Without mixing, biofloc settles out of suspension and may form piles that rapidly consume nearby dissolved oxygen. These anaerobic zones can lead to the release of hydrogen sulphide, methane and ammonia that are highly toxic to shrimp and fish. Solids can be removed by periodic flushing or by pumping sludge from the pond centre. In intensive, green water raceways for shrimp, water respiration rates range from 2 to 2.5mg O2/L per hour, although it can be as high as 6 mg O2/L per hour. It is absolutely essential to provide sufficient aeration or oxygenation to meet this high oxygen demand and to maintain concentration at safe levels. These high respiration rates also indicate that the response time in the event of a system failure is very short, often less than 1 hour. Thus monitoring, alarms, and emergency power systems are required elements of biofloc systems.
Effect of green waterbrown water biofloc transition
Floc develops gradually in this system and first, the system will abruptly transition from green water, an algal system to brown water, bacterial system. As daily feeding rate increases from 100 to 200kg/ha (10 to 20g/m2), the water will appear green with the dense algae bloom. Algal uptake is the main mechanism for ammonia control. The aerator power required at this feeding rate is about 25 to 30hp/ha. At a daily feeding rate of 300 kg/ha, there is an abrupt shift when the lack of light at very high algal density hinders photosynthesis.
Importance of liming for alkalinity management
Alkalinity is the capacity of water to buffer or resist changes in pH in response to additions of acids or base. Water in biofloc systems should be maintained with a sample reserve of alkalinity because it biofloc systems. Over time, acid produced by nitrification wears down the reserve of alkalinity in the water. Once alkalinity is depleted, pH can drop steeply, inhibiting bacterial function, including that of the important nitrifying bacteria.is constantly depleted by reaction with acid added to water. The activity of nitrifying bacteria is responsible for most losses of alkalinity in intensivebiofloc systems. Over time, acid produced by nitrification wears down the reserve of alkalinity in the water. Once alkalinity is depleted, pH can drop steeply, inhibiting bacterial function, including that of the important nitrifying bacteria. Denitrification and sludge treatment Alkalinity can be recovered in denitrification units. Nitrate accumulates in most intensive biofloc systems because of ongoing nitrification. If unchecked, nitrate concentration reflects the cumulative feed loading to the system. Nitrate accumulation can be tempered by partial dilution through water exchange.
Biofloc is an eco-friendly fish farming technology. Biofloc technology offers benefits in improving aquaculture production that could contribute to the achievement of sustainable development goals. This technology can be an innovative strategy for disease control and prevention such as antibiotics, antifungal, probiotics & prebiotics application and also could result in higher productivity with less impact on the environment.
Furthermore, biofloc systems may be developed and performed in integration with other food production. This technology has the obvious advantage of minimizing water requirement and organic matter and it turns improving farm biosecurity by exclusion of pathogens, augmentation of natural food and improvement of FCR, providing a stress-free environment. Biofloc technology is still in its infant stage. A lot more research is needed to optimize the system e.g. in relation to nutrient recycling, MAMP production and immunological effects. Biofloc technologies have the potential to revolutionise the aquaculture system.
(The author is the Founder & CEO, Aqua Doctor Solutions, Kolkata. Views expressed are personal.)