Bio Floc Technology (BFT) is a new approach toward management of ponds, in most cases intensive tilapia or shrimp ponds.
Water scarcity, the demand for bio-security and economy, all leads to minimizing water exchange, even down to zero water exchange. Under these conditions, a major problem is the accumulation of ammonia and nitrite, both toxic to shrimp and fish. One way to solve this problem in low water exchange systems, is to recycle the water through a bio-filter system. Recirculating Aquaculture Systems (RAS) are known to work well, yet they have two major problems: First they are expensive in both investment and maintenance and secondly, they recycle water but do not recycle feed residues. Feed is becoming more and more expensive and its recycling is essential.
An alternative approach, the Bio Floc Technology (BFT) is based upon the activity of the microbial community within the pond. Water treatment is done within the pond, with no need for a separate water treatment component. Very dense microbial community develops when water exchange is limited. Typically, we find 10-1000 million microbial cells (107-109) in 1 cm3 of pond water. If we add carbonaceous material (molasses, starch, tapioca and others) to adjust the C/N ratio in feeds to 15-20, the microbes take up the ammonium from the water and create microbial protein. By the adjustment of the C/N ratio, the nitrogen problem can be easily and consistently solved.
An important feature of BFT is the ability to recycle proteins. In conventional aquaculture, only about 20-25% of feed protein is retained by fish or shrimp. The rest is excreted to the water, mostly as toxic ammonium. With BFT the ammonium is converted to microbial protein (through the addition of carbohydrates) that can be used as a protein source. The micro-organisms in the water tend to aggregate and form bio-flocs that can be filtered and harvested by tilapia or shrimp. It was found that 20% of protein eaten by shrimp or fish growing in BFT systems came from bio flocs harvesting. The amount of feed protein utilized by shrimp in BFT systems is double than that in traditional ponds, since the protein is practically used twice: Once when pellets are eaten by fish and then when the bio flocs are harvested. The doubled feed efficiency is a very important factor, especially now, when feed costs are rising.
Both protein recycling and water quality control are achieved through the addition of carbonaceous feed and adjustment of the C/N ratio.
Protein is an expensive feed component. In addition, it is, at least partially, made of fish meal, a component that is scarce and its harvest in the ocean leads to environmental damage. Thus, the fact that protein utilization rises from 15-25% in conventional ponds to 45% in BFT is very important.
The utilization of microbial flocs as a source of feed protein leads to a lower expenditure on feed. Avnimelech reported that feed cost for tilapia production was lowered from $0.84/kg fish in conventional ponds to $0.58 in BFT. McIntosh reported that feed cost using the lowered protein diet in Belize Aquaculture was about 50% as compared to conventional shrimp farming.
An important advantage of biofloc systems is their contribution in limiting development of shrimp and fish diseases. These processes were lately discussed in a special workshop dealing with the effects of biofloc and related systems on shrimp and fish diseases.
BFT systems are environmentally friendly, mostly due to the fact that there is almost no release of nutrient rich drainage water to the environment.
How to do it?
Construction of ponds
Aeration and mixing is an essential feature determining the planning of the pond.
Most ponds have radial or elliptic water flow modes, concentrating the settled organic waste in the center. Ponds can be round (or close to it) or rectangular. The ponds should have a slope toward the hydraulic center, to create a sludge trap. An alternative design is a closed raceway, made by a partition (not necessarily a tight and rigid partition) in a rectangular pond. A low lying sludge trap should be made in the exit corner of the pond. Sludge traps should be equipped with a wide diameter drainage pipe
If possible, ponds should be lined. In soil ponds without lining, the intensive water flow can erode pond banks and add significant amounts of fine soil particles to the bottom sludge. In addition, the large amounts of organic residues sinking to the pond bottom will mix with soft mud and create a deep sludge layer that is hard to control. All this will generate anaerobic condition in the pond bottom failing fish (or shrimp) production. Lining can be made using plastic sheets (HDPE), soil – cement mixture or compacted Laterites.
Intensive BFT fish ponds carry a biomass of up to 30 kg/m3 and daily feed of up to 600g/m3. These ponds generate very high TSS and sludge that has to be drained daily. Pond size is limited by the ability to thoroughly well mix ALL points in the pond and effectively drain sludge. Typical size of tilapia ponds with a biomass of 20-30 kg/m2 is 50 m2 to about 1000m2 . Shrimp BFT ponds carry a lower biomass (up to about 2 kg/m3) lower feed ration and suspended solids. Typical size of such ponds is around 1 ha.
Aeration and mixing
The dense fish biomass and consequently high feeding rations both lead to high oxygen consumption. Reliable means to ensure proper aeration and maintenance of adequate oxygen concentration is essential, including full proof backup systems for cases of failure (If some thing can get wrong, it will). In addition, the pond should be thoroughly mixed, to prevent the presence of stagnant corners where sludge accumulates and anaerobic conditions develop and prevail.
Aeration and mixing are both provided by a variety of aerators, mostly paddle wheel aerators. Combination with aspirators and with air-lifts is recommended.
Paddle wheel aerators are effective means to add oxygen to the water, however dissipate most of the energy toward the surface layers of the water and only a fraction to mix and aerate bottom layers of the pond. There is a need for effective aerators that combine aeration and mixing of the whole water column. To obtain radial or elliptic water flow aerators are placed in parallel to the dykes, yet, should not be placed close to the dyke. A distance of about 20 meters from the dyke is recommended (25-30% of pond width). Aspirators or air-lifts should be located in the center of the pond or in sites where sludge accumulates, to get resuspension of organic particles. Resuspension of organic particles enable aerobic metabolism and recycling of the organic residues and prevents anaerobic sludge accumulation.
Required aeration capacity is approximated to about 1 kw for 500 - 1000 kg shrimp or fish, though higher aeration efficiencies are obtained if growing conditions are optimal. Drainage of excessive sludge also raises aeration efficiency.
Water exchange in BFT ponds should be limited, both due to economics, environmental concerns and as a pre-requisite for the development of significant microbial community. Water exchange rates for intensive fish ponds (ca 10-50 kg fish/m3) should be up to 5-10% per day, whereas shrimp ponds carrying up to 2 kg/m3 can be managed with practically no water exchange, except for sludge drainage.
Feeding, C/N control
The microbial processes in the pond are controlled through the proper adjustment of feed application and feed composition, and control of the applied C/N ratio. Carbonaceous substrates are metabolized, about 50% of which are converted to microbial proteins accompanied by the immobilization of inorganic nitrogen, especially that of ammonium. It has to be noticed that some of the ammonium is removed by nitrification. A combination of nitrification and nitrogen assimilation by bacteria is more than welcome.
Carbonaceous substrates can be added in response to excessive inorganic nitrogen levels, at a rate of about 20 g carbonaceous substrate to remove 1 g nitrogen, as an integral part of feed application. To prevent excessive nitrogen build up C/N ratio of feed materials addition should be 15-20, though less carbon addition is needed if you got significant nitrification.
Many carbonaceous substrates can be used, such as wheat flour, corn or potato starch, cassava meal, molasses and others. Price considerations are valid, since there is no clear preference to any substrate. The substrate has to be finely ground or water dispersible so as to stay long in the water column. The carbonaceous substrate can be integrated with the normal feed pellets (giving the desired C/N ratio) or applied separately, as powder or molasses spread onto the water or as separate low protein pellets applied in addition to conventional feeds in separate application times.
It is quite easy to monitor total ammonia nitrogen (TAN) and other inorganic N species and to adjust the carbon addition so as to avoid excessive nitrogen or carbon application levels.
Control of sludge
Accumulation of sludge and the generation of anaerobic pockets is a common cause for failure of BFT ponds. The anaerobic reactions lead to the release of toxic compounds, affecting both fish and microbial reactions. The accumulation of sludge should be monitored, preferably by getting into the pond. An increase of nitrite concentration is often an indication of the existence of anaerobic pockets.
Preventing sludge accumulation or correcting un-desired accumulation can be done by adjustment of aerators location and types.
Closing the cycle of sludge accumulation depends on the potential uses or disposal of the sludge. Most probably there will be more detailed environmental regulation of that matter. A few possible uses of the sludge, such as fertilization and amelioration of agricultural soils (sludge of salty ponds may be a problem unless washed out of salinity), feeding of fish in adjacent extensive ponds or use in mixtures
Bio Floc Technology (BFT) is a new approach toward management of ponds, in most cases intensive tilapia or shrimp ponds.
Water scarcity, the demand for bio-security and economy, all leads to minimizing water exchange, even down to zero water exchange. Under these conditions, a major problem is the accumulation of ammonia and nitrite, both toxic to shrimp and fish. One way to solve this problem in low water exchange systems, is to recycle the water through a bio-filter system. Recirculating Aquaculture Systems (RAS) are known to work well, yet they have two major problems: First they are expensive in both investment and maintenance and secondly, they recycle water but do not recycle feed residues. Feed is becoming more and more expensive and its recycling is essential.
An alternative approach, the Bio Floc Technology (BFT) is based upon the activity of the microbial community within the pond. Water treatment is done within the pond, with no need for a separate water treatment component. Very dense microbial community develops when water exchange is limited. Typically, we find 10-1000 million microbial cells (107-109) in 1 cm3 of pond water. If we add carbonaceous material (molasses, starch, tapioca and others) to adjust the C/N ratio in feeds to 15-20, the microbes take up the ammonium from the water and create microbial protein. By the adjustment of the C/N ratio, the nitrogen problem can be easily and consistently solved.
An important feature of BFT is the ability to recycle proteins. In conventional aquaculture, only about 20-25% of feed protein is retained by fish or shrimp. The rest is excreted to the water, mostly as toxic ammonium. With BFT the ammonium is converted to microbial protein (through the addition of carbohydrates) that can be used as a protein source. The micro-organisms in the water tend to aggregate and form bio-flocs that can be filtered and harvested by tilapia or shrimp. It was found that 20% of protein eaten by shrimp or fish growing in BFT systems came from bio flocs harvesting. The amount of feed protein utilized by shrimp in BFT systems is double than that in traditional ponds, since the protein is practically used twice: Once when pellets are eaten by fish and then when the bio flocs are harvested. The doubled feed efficiency is a very important factor, especially now, when feed costs are rising.
Both protein recycling and water quality control are achieved through the addition of carbonaceous feed and adjustment of the C/N ratio.
Protein is an expensive feed component. In addition, it is, at least partially, made of fish meal, a component that is scarce and its harvest in the ocean leads to environmental damage. Thus, the fact that protein utilization rises from 15-25% in conventional ponds to 45% in BFT is very important.
The utilization of microbial flocs as a source of feed protein leads to a lower expenditure on feed. Avnimelech reported that feed cost for tilapia production was lowered from $0.84/kg fish in conventional ponds to $0.58 in BFT. McIntosh reported that feed cost using the lowered protein diet in Belize Aquaculture was about 50% as compared to conventional shrimp farming.
An important advantage of biofloc systems is their contribution in limiting development of shrimp and fish diseases. These processes were lately discussed in a special workshop dealing with the effects of biofloc and related systems on shrimp and fish diseases.
BFT systems are environmentally friendly, mostly due to the fact that there is almost no release of nutrient rich drainage water to the environment.
How to do it?
Construction of ponds
Aeration and mixing is an essential feature determining the planning of the pond.
Most ponds have radial or elliptic water flow modes, concentrating the settled organic waste in the center. Ponds can be round (or close to it) or rectangular. The ponds should have a slope toward the hydraulic center, to create a sludge trap. An alternative design is a closed raceway, made by a partition (not necessarily a tight and rigid partition) in a rectangular pond. A low lying sludge trap should be made in the exit corner of the pond. Sludge traps should be equipped with a wide diameter drainage pipe
If possible, ponds should be lined. In soil ponds without lining, the intensive water flow can erode pond banks and add significant amounts of fine soil particles to the bottom sludge. In addition, the large amounts of organic residues sinking to the pond bottom will mix with soft mud and create a deep sludge layer that is hard to control. All this will generate anaerobic condition in the pond bottom failing fish (or shrimp) production. Lining can be made using plastic sheets (HDPE), soil – cement mixture or compacted Laterites.
Intensive BFT fish ponds carry a biomass of up to 30 kg/m3 and daily feed of up to 600g/m3. These ponds generate very high TSS and sludge that has to be drained daily. Pond size is limited by the ability to thoroughly well mix ALL points in the pond and effectively drain sludge. Typical size of tilapia ponds with a biomass of 20-30 kg/m2 is 50 m2 to about 1000m2 . Shrimp BFT ponds carry a lower biomass (up to about 2 kg/m3) lower feed ration and suspended solids. Typical size of such ponds is around 1 ha.
Aeration and mixing
The dense fish biomass and consequently high feeding rations both lead to high oxygen consumption. Reliable means to ensure proper aeration and maintenance of adequate oxygen concentration is essential, including full proof backup systems for cases of failure (If some thing can get wrong, it will). In addition, the pond should be thoroughly mixed, to prevent the presence of stagnant corners where sludge accumulates and anaerobic conditions develop and prevail.
Aeration and mixing are both provided by a variety of aerators, mostly paddle wheel aerators. Combination with aspirators and with air-lifts is recommended.
Paddle wheel aerators are effective means to add oxygen to the water, however dissipate most of the energy toward the surface layers of the water and only a fraction to mix and aerate bottom layers of the pond. There is a need for effective aerators that combine aeration and mixing of the whole water column. To obtain radial or elliptic water flow aerators are placed in parallel to the dykes, yet, should not be placed close to the dyke. A distance of about 20 meters from the dyke is recommended (25-30% of pond width). Aspirators or air-lifts should be located in the center of the pond or in sites where sludge accumulates, to get resuspension of organic particles. Resuspension of organic particles enable aerobic metabolism and recycling of the organic residues and prevents anaerobic sludge accumulation.
Required aeration capacity is approximated to about 1 kw for 500 - 1000 kg shrimp or fish, though higher aeration efficiencies are obtained if growing conditions are optimal. Drainage of excessive sludge also raises aeration efficiency.
Water exchange in BFT ponds should be limited, both due to economics, environmental concerns and as a pre-requisite for the development of significant microbial community. Water exchange rates for intensive fish ponds (ca 10-50 kg fish/m3) should be up to 5-10% per day, whereas shrimp ponds carrying up to 2 kg/m3 can be managed with practically no water exchange, except for sludge drainage.
Feeding, C/N control
The microbial processes in the pond are controlled through the proper adjustment of feed application and feed composition, and control of the applied C/N ratio. Carbonaceous substrates are metabolized, about 50% of which are converted to microbial proteins accompanied by the immobilization of inorganic nitrogen, especially that of ammonium. It has to be noticed that some of the ammonium is removed by nitrification. A combination of nitrification and nitrogen assimilation by bacteria is more than welcome.
Carbonaceous substrates can be added in response to excessive inorganic nitrogen levels, at a rate of about 20 g carbonaceous substrate to remove 1 g nitrogen, as an integral part of feed application. To prevent excessive nitrogen build up C/N ratio of feed materials addition should be 15-20, though less carbon addition is needed if you got significant nitrification.
Many carbonaceous substrates can be used, such as wheat flour, corn or potato starch, cassava meal, molasses and others. Price considerations are valid, since there is no clear preference to any substrate. The substrate has to be finely ground or water dispersible so as to stay long in the water column. The carbonaceous substrate can be integrated with the normal feed pellets (giving the desired C/N ratio) or applied separately, as powder or molasses spread onto the water or as separate low protein pellets applied in addition to conventional feeds in separate application times.
It is quite easy to monitor total ammonia nitrogen (TAN) and other inorganic N species and to adjust the carbon addition so as to avoid excessive nitrogen or carbon application levels.
Control of sludge
Accumulation of sludge and the generation of anaerobic pockets is a common cause for failure of BFT ponds. The anaerobic reactions lead to the release of toxic compounds, affecting both fish and microbial reactions. The accumulation of sludge should be monitored, preferably by getting into the pond. An increase of nitrite concentration is often an indication of the existence of anaerobic pockets.
Preventing sludge accumulation or correcting un-desired accumulation can be done by adjustment of aerators location and types.
Closing the cycle of sludge accumulation depends on the potential uses or disposal of the sludge. Most probably there will be more detailed environmental regulation of that matter. A few possible uses of the sludge, such as fertilization and amelioration of agricultural soils (sludge of salty ponds may be a problem unless washed out of salinity), feeding of fish in adjacent extensive ponds or use in mixtures
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