The past and present of bioflocs

In aquaculture, biofloc technology was originally applied to high-density culture of tilapia. In recent years, as a very important means to control water quality, it has been widely used in industrial production of vannamei. For this purpose, please refer to Some literature for everyone. Biofloc Technology, Biofloc Technology, BFT.

What is a biological floc? Simply put, a biological floc is a mass formed by bacteria, microalgae and protozoa attached to the floc in the water body, and the attachment is mainly bacteria. This high-end, atmospheric and high-end name is the creation of the aquatic people. Because this technology has already existed in other fields, but the name is different: it has been used in the sewage treatment industry for more than 100 years, but it is not called "Biofloc", but "Biosludge"; in marine ecology, it is called "Marine" -snow"; called "organic detritus" in the textbooks of traditional aquaculture.

Biological floc culture technology, there are various names, such as carbon and nitrogen balance technology, zero water exchange technology. In fact, it can be considered that the basic principle of this aquaculture technology is carbon and nitrogen balance, the method is biological flocs, and the goal is zero water exchange. It can also be described as follows: biological floc culture technology is based on the principle of stoichiometry, using organic compounds as energy and protein skeleton, using microorganisms (flocs) to convert ammonia nitrogen produced in aquaculture process into biological protein, and realize zero water exchange. Aquaculture technology.

Let's compare traditional pond culture and biofloc culture:

1. Functional organisms: pond culture - algae, biological flocs - bacteria.

2. Nitrogen treatment stoichiometric formula:

16NH4+92C02+92H20+14HCO3+H2PO4->C106H264O110N16P+10602 (traditional pond culture)

NH4+7.08CH2O+HCO3+2.0602->C5H7O2N+6.06H20+3.07CO2 (biofloc culture)

3. No two ponds have the same algal composition. Likewise, no two culture systems can have exactly the same microbial composition of bioflocs.

4. The composition of algae in the same traditional pond will change during the whole culture process. Likewise, the microbial composition of bioflocs in the same culture system is constantly evolving throughout the culture process.

5. Algae are generally not symbiotic, and there is no strict order in which different algae appear to each other, but there are various symbiotic relationships between biological floc microorganisms, and some microorganisms appear in flocs in strict order. The chronological order, for example: ammonifying bacteria - nitrifying bacteria - nitrifying bacteria.

6. It is easy to establish algal ecological diversity in ponds, and it can be completed in a few days; it is not easy for biological flocs to establish microbial ecological diversity, and it takes a long time. Even if the activated sludge is inoculated, it will take 3 to 6 months for a sewage treatment plant to mature in operation and commissioning.

Under the condition of an open (non-closed, non-sterile) environment and naturally entrapped external microorganisms, the main factors that determine the final composition of a microbial ecosystem include: medium composition, potential (dissolved oxygen), salinity and temperature. It can be summed up in one sentence, that is, the conditions determine the end point—the biochemical, physical and other conditions of the system determine the final structural composition of the ecosystem. For example, if a biogas digester is built, no matter what kind of bacteria it starts to pollute or inoculate, it will eventually only form a microbial system that produces biogas.

Biofloc is a very complex ecosystem with relatively perfect biological metabolism. A relatively mature biofloc system is not simply composed of one or several bacteria, but contains hundreds of different functions. and interrelated microbial species.

Factors affecting the microbial population structure of bioflocs are:

(1) Feed

Feed is the largest input in the biofloc culture system, and it is the main body of the "culture medium" that determines the microbial population in the biofloc culture system, and it is one of the main factors affecting the composition of microorganisms. The highest protein content of Penaeus vannamei feeds reported abroad is 68.8%. The protein content of most feeds in my country is generally 40%. There are only four places for feed protein nitrogen to go into a farming system:

①Convert to prawn protein nitrogen; 

②Converted into floc protein nitrogen (including bacteria and algae in free state); 

③ Inorganic nitrogen in residual water; 

④ dissimilation into gaseous nitrogen (leaving the breeding system).

For the conventional biological floc culture system, the protein nitrogen input into the culture system mainly exists in the form of the former two. That is to say, for the conventional biofloc technology system, if the feed protein input into the aquaculture system is not converted into shrimp protein, it will eventually be converted into floc protein and consume carbon source.

Assuming that the protein content of prawns is 16% (wet basis) and the protein content of flocs is 0.81% (wet basis) (the dry matter content of flocs is about 1.8% and the protein content of flocs is about 45%), for feeds with different protein content and protein efficiency , How many bioflocs will theoretically be produced at the same time per 1 dry gram of Penaeus vannamei?

When the protein content of the feed is 38% and the protein efficiency is 35%: 36.68 liters of bioflocs will be produced per 1 kg of shrimp produced;

When the protein content of the feed is 38% and the protein efficiency is 40%: 29.63 liters of bioflocs will be produced per 1 kg of shrimp produced;

When the protein content of the feed is 38% and the protein efficiency is 45%: 24.14 liters of bioflocs will be produced per 1 kg of shrimp produced;

When the protein content of the feed is 40% and the protein efficiency is 35%: 36.68 liters of bioflocs will be produced per 1 kg of shrimp produced;

When the protein content of the feed is 40% and the protein efficiency is 40%: 29.63 liters of bioflocs will be produced per 1 kg of shrimp produced;

When the protein content of the feed is 40% and the protein efficiency is 45%: 24.14 liters of bioflocs will be produced per 1 kg of shrimp produced.

It can be seen that the biological floc production in the biological floc culture system is only related to the protein efficiency, and has nothing to do with the protein content, relative to the shrimp production. In turn, for a given floc biomass, shrimp yield (ie, system loading density) depends on protein efficiency.

The above is just a theoretical analysis. In the actual production process, there are different degrees of nitrogen dissimilation in the biological floc system. For a specific biological floc culture system, it is necessary to make reasonable corrections according to its own floc composition.

(2) Breeding animals

In this system, Penaeus vannamei is the metabolic end product of Penaeus vannamei, feces and urine, which are important components of the "culture medium" of the microbial population in the biofloc system, and one of the main factors affecting the composition of microorganisms.

(3) Carbon source

In the biological floc culture system, the carbon source is the most important input after the feed, and different carbon source types strongly affect the microbial composition of the floc. In theory, any organic carbon that can be used by microorganisms as energy and protein backbone can be used as a carbon source, including all monosaccharides, disaccharides, polysaccharides (such as starch), various organic acids, alcohols, etc., and even cellulose.

From an economic point of view, the carbon source should be the most convenient and cheapest local agricultural and sideline products as the first choice. In addition to the widely used sugar cane molasses and brown sugar, some common agricultural and sideline products that can be used as carbon sources are relatively cheap and abundant, including corn starch, potato starch, tapioca starch, and sweet potato starch. Of course, to use starch as a carbon source, the microorganisms in the flocs must be acclimated in advance.

For ordinary biofloc (nitrogen assimilation) technology, there are two principles for using carbon sources. One is that the carbon source used must be able to synchronize the utilization of carbon sources and ammonia nitrogen by microorganisms; the other is to use The more microbial species these carbon sources have, the better.

Of course, if these agricultural and sideline products can be fermented in advance and then used, the effect will be much better. Many people do not understand the difference between using fermented molasses and using molasses directly. The difference between them is that molasses is just molasses, and the microorganisms that can directly use molasses are relatively simple; while fermented molasses is no longer molasses. After molasses is fermented, it has been converted into tens of thousands of organic compounds, including various organic acids, vitamins, Almost all the enzymes and various bioactive factors required by every cell can promote the growth of various microorganisms. Especially in the early stage of biofloc establishment, the use of fermented molasses is more conducive to the establishment of biofloc biodiversity than the use of molasses. In terms of the frequency of use of carbon sources, it is generally used together with feed for convenience. However, it is not correct to use it once a day or several days, because the secretion of ammonia nitrogen in shrimp is continuous, the growth of microorganisms is also continuous, and the carbon source supplied to the microorganisms must also be continuous. Since molasses has certain physical and chemical properties, such as viscosity and osmotic pressure, large doses in a short period of time will change many physical and chemical properties of aquaculture water, affect gas exchange in water, and even inhibit the growth of some squeamish microorganisms. Therefore, the most ideal form of addition of soluble carbon source is to add a small amount and continuously.

(4) Salinity

Although many microorganisms are basalt and can survive and grow in different salinities, their physiological activities and competitive advantages are different under different salinities; for those microorganisms that are relatively sensitive to salinity, the salinity is different. , microbial species are naturally different.

(5) Composition of trace elements (or ions)

Trace elements are not only nutrients for microorganisms, but also different microorganisms have their own requirements for trace elements, and different composition or lack of trace elements will lead to the formation of different dominant microbial communities. The valence of ions also varies greatly in their ability to act as complexation, bridging, and forming floc structures: monovalent ions (such as sodium, potassium) have no bridging ability, and even block the bacterial surface due to excessive monovalent ions. Negative charges make it impossible to form flocs between bacteria; divalent ions (such as calcium, magnesium) have bridging ability; trivalent ions (such as aluminum, iron) have strong bridging and flocculation ability.

(6) Potential (dissolved oxygen)

Biological flocs have a certain "size". Due to the high density and high oxygen consumption of bacteria, a certain potential gradient will be formed inside and outside the floc "particles": in the case of high dissolved oxygen concentration in the water body and small floc particles , the potential gradient is small, and the proportion of aerobic bacteria in the bacteria forming flocs increases; in the case of low dissolved oxygen concentration in the water body and large floc particles, facultative anaerobic or even anaerobic bacteria in the bacteria forming flocs proportion increased. Dissolved oxygen levels also determine the morphology of a microbial metabolic end product, which in turn affects downstream microbial composition. For example, in yeast, the end product of metabolism is carbon dioxide under hyperoxic conditions, while the end product of metabolism under hypoxic conditions is ethanol. Therefore, under different dissolved oxygen levels, the downstream microorganisms of yeast are different.

(7) Water movement speed

The movement of the water body has a certain "shear force" on the biological flocs. The floc particles are small, oxygen is easy to penetrate, and the oxygen content inside the floc is high, resulting in more aerobic bacteria and less anaerobic bacteria, which affects the respiration type of floc bacteria and the composition of microbial community.

(8)Temperature

Different microorganisms have different temperature adaptation ranges. Temperature not only affects the metabolism, growth and function of microorganisms, but also affects the microbial population.

(9) Other inputs

Such as various water quality conditioners, microbial preparations, etc., especially microbial preparations (microbial active substances) have a "butterfly effect" on the evolution of microbial ecology.

(10) Floc "Age"

Due to the different reproductive cycles of various microorganisms in the flocs, when the rate of renewal (removing or changing water while taking away the flocs) exceeds the reproductive cycle (time of one generation) of certain microorganisms, the proportion of these microorganisms will become less and less. , or even missing.

Shrimp Project Team: Xi Wenqiu

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