Biofloc for sustainable aquaculture

Debtanu Barman

Aquaculture production depends on feed, dissolved oxygen and maintenance of water quality parameter 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.

There are strong economic incentives for an aquaculture business to be more efficient. High-Density rearing of fish typically requires some waste treatment infrastructure. Bio-floc system uses a counter-intuitive approach, allows or encourages solids and the associated microbial community to accumulate in the water.

As long as there are sufficient mixing and aeration to maintain an active floc in suspension, water quality can be controlled in a better way. Managing biofloc system 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.

Nutritional value of biofloc for aquaculture

Biofloc is a collection (flocs) of algae, bacteria, protozoans, and other kinds of particulate organic matter such as faeces and uneaten feed. 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 as a source of feed ingredients

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 commercial-scale 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.

      Biofloc act as probiotics & its effect

Floc stimulate immune response in fish. It is well established that the immune response of animal 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 give specific nutraceutical and immunogenic effect.

What are the microbes used in probiotics to grow floc

Heterotrophic bacteria (Bacillus subtilis, Bacillus licgenuformis, Bacillus coagulus, with a mix of Nitrobacter and Nitrosomonas) are required for floc growth, then assemblage of algae, green algae, diatoms, zooplankton including rotifer nematodes copepods etc also required for floc growth.  In  Greenwater, biofloc algae dominate and in brown water biofloc bacteria dominates. Floc species show natural growth and succession so the floc quality and watercolour shall be monitored and managed regularly.

How biofloc system works

Biofloc technique is waste treatment method. Biofloc provides two critical services-training 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.

The 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 environment as waste. In biofloc systems, some of this nitrogen is incorporated into bacterial cells that are main components of biofloc. Consumption of this microbial protein, in effect for a second time, contributes to growth.

 Suitable culture species for biofloc fish farming

The biofloc system is benifecial 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, Pangassius, 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

Few types of biofloc systems have been used in commercial 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 pond 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 (Fig-3), where only bacterial processes control water quality.

Importance of mixing and aeration in biofloc system

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 settle out of suspension and may form piles that rapidly consume nearby dissolve 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 providing 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  water- brown 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.

Bacteria begin to grow and develop biofloc, as indicated by a feed consumption in shrimp raceway biofloc systems, assure a solid concentration of 100 to 300mg/L. Imhoff or setting cones are a simple way to index suspended solid concentration.  The cones have marked graduation on the outside that can be used to measure the volume of solid that settles from 1 liter of system water. The interval of time should be standardized and convenient, usually 10 to 20 minutes. Solids also can be measured with a TDS meter. Maintaining a settleable solids concentration of 15 to 30ml/L will provide good functionality in biofloc systems.

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 is constantly depleted by reaction with acid added to water. The activity of nitrifying bacteria is responsible for most losses of alkalinity in intensive 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.

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.

Solids management

In biofloc systems, waste solids are allowed to accumulate and additional solids are encouraged by intensive aeration and carbohydrate additions. Over time, and with sufficient mixing, solids can be accumulated to undesirably high levels (2,000 to 3,000 mg/L). Biofloc systems are typically operated at suspended solids concentrations less than 1,000mg/L and the most often less than500 mg/L.

A suspended solids concentration of 200 to 500mg/L is sufficient for good system functionality and will control ammonia without excessive water respiration. The best increase in suspended solids concentration (250 to 500 mg/L) and the associated rapid increase in water respiration (6mg O₂/L per hour). This requires a five-fold increase in aerator power from 30to 150hp/ha to match the oxygen demand. Most of this increasing energy demand is required to maintain biofloc in suspension.

Dynamics of ammonia

A major goal of water quality management in any aquatic animal production system is maintaining ammonia concentration below toxic levels. In biofloc systems, there are three main processes that control ammonia, algal uptake, bacterial assimilation and nitrification. The transformations and dynamics of ammonia in biofloc system are complex, involving interplay among the algae and bacteria that compete for ammonia. The relative importance of each process depends on many factors, among them the daily feeding rat, suspended solids (biofloc) concentration, ammonia concentration, light intensity, and input carbon-to-nitrogen (C:N) ratio.

Importance to obtain C: N ratio in biofloc

In Biofloc system, a major factor that controls ammonia concentration is the C:N ratio. A feed with 30 to 35 per cent protein concentration has a relatively low C: N ratio, about 9 to 10:1. Increasing the C:N ratio of inputs to 12 to 15:1 favours the heterotrophic pathway for ammonia control. The low C:N ratio of feed can be augmented by adding supplement materials with a high C: N  ratio. Or the inputs C: N ratio can be increased by reducing feed protein content. Ammonia Control through the heterotrophic pathway is often, more stable and reliable than algal uptake or nitrification.

Bacterial assimilation and its action

Many of the early names for biofloc systems included the word “ heterotrophic”, which describes a group of bacteria that, by definition, obtains carbon from organic sources. Despite large inputs of feed to intensive systems, the growth of heterotrophic bacteria in biofloc systems is limited by dissolved organic carbon. To stimulate the production of heterotrophic bacteria,  the C:N ratio of inputs is raised by adding a supplement source of carbohydrate or reducing feed protein level. By this manipulation, heterotrophic bacteria create a demand for nitrogen(as ammonia) because organic carbon and inorganic nitrogen are generally taken up in a fixed ratio that reflects the composition and requirement of bacterial cells. Thus ammonia can be controlled by adding organic carbon to stimulate the growth of heterotrophic bacteria.

Similar to algae, ammonia is “immobilized” while packaged in heterotrophic bacterial cells as protein. Because the growth rate of heterotrophic bacteria is so much greater than that of nitrifying bacteria, ammonia control through immobilization by heterotrophic bacteria occurs rapidly, usually within hours or days if sufficient quality of simple organic carbon(e.g. sugar or starch)is added. The packaging of nitrogen in bacterial cells is temporary because cell turns over rapidly and release nitrogen as ammonia when they decompose. Cells are also consumed by fish or removed as excess solids. As with nitrogen assimilated by algae, microbial protein in flocs containing heterotrophic bacteria can serve as a supplemental source of nutrition for fish and shrimp.

Importance of Nitrification

The two-step oxidation of ammonia to nitrate is called Nitrification. The bacterial process transforms a toxic form of nitrogen (ammonia) to one that is toxic only at high concentrations (nitrate). Over time, nitrate accumulates in low-exchange biofloc systems. In contrast to rapid cycling between dissolved ammonia and algal or bacterial cells, nitrification is responsible for the long-term, the ultimate fate of a large fraction(25 to 50 per cent) of the nitrogen from feed added to intensive.

 Algal uptake

In any biofloc system exposed to sunlight, a dense algal bloom will develop in response to nutrient loading from feeding system. Nutrients released from the decomposition organic matter (including dead algae, faecal solids, and uneaten feed) are rapidly taken up and stored in algae cells. The rate algal uptake in biofloc systems is mainly influenced by underwater light intensity. In biofloc systems with a primary dependence on algal uptake, extended periods of cloudy weather can cause spikes of ammonia concentration. The accumulation of biofloc solids shades out algae and limits ammonia uptake. Daily fluctuation in dissolved oxygen concentration and pH, despite intensive aeration, is another characteristic of biofloc systems where the algal activity is predominant.

Advantages Of biofloc culture system

Bio-floc act as a substitute for fish meal in aquaculture feed.

• The protein level in bio-floc ranges from 15% to 20 %.
• Bio-floc system provides a protective shell to the fish, through a probiotic effect(Depression of Tilapia infection by Streptococcusbacterial 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 bio-floc.
• Enhance growth performance and survival.
• Maintain 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.
• Increased pollution potential from nitrate accumulation.
• Inconsistent and seasonal performance for systems exposed to sunlight.

Conclusion

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 disesase 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 system 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. The 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. He can be reached at email: debtanu080@gmail.com. Mobile: 8334932266.Views expressed are personal.)