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Biofloc Technology – A Practical Guide Book

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19
World AquAculture
Reprinted from World Aquaculture 34(4):19-21. ©2003 World Aquaculture Society
Control of microbial activity
in aquaculture systems: active
suspension ponds
Yoram avnimelech1
The control of aquaculture systems
through the manipulation of microbial
activity became an important and com-
monly discussed technology arising from
the development of intensive aquaculture.
Aquaculture depended, traditionally, on
algal activity. Growing sh, including
crustaceans or other cultured aquatic
animals in water bodies depends, in tra-
ditional aquaculture, on algae generating
the base for the food chain or food web,
to feed the sh. Basic properties related to
algae control are listed in Table 1. Algal
productivity in non-fed ponds depends on
the level of inorganic nutrients, mostly
phosphorus (P) and nitrogen (N). Thus,
one of the rst stages of intensication
was to add inorganic fertilizers to the
pond. Following the elimination of nutri-
ents las a limiting factor, as is common in
well fertilized or fed ponds, productivity
becomes limited by solar radiation reach-
ing the algae. With optimal solar radiation
Table 1. Comparison of algae and bacterial controlled systems.
Property Algae Control Bacteria Control
Energy source Solar radiation Primarily organic matter
Occurrence Ponds with low organic matter Dominance in ponds with high supply and
concentration. Algae density increases concentration of organic substrate, normally
with the availability of nutrients up to limited to intensive ponds with zero to low
limitation of light. water exchange.
Sensitivity to Light is essential (activity is decreased Does not need light. Adapts to a variety of
environmental variables on cloudy days). Crashes are common. conditions. Crashes are exceptional.
Effect on oxygen Oxygen is produced during the day and Oxygen is consumed.
consumed at night.
Relevant activities Primary production: produces organic Degradation of organic matter. Nitrication.
matter and oxygen. Ammonia uptake. Production of microbial protein.
Inorganic nitrogen control Uptake driven by primary production Uptake of nitrogen affected by the C:N ratio
Maximum Capacity ca. 0.7 g NH+/m2/ in organic matter. Practically unlimited
day. capacity.
Potential capacity Normally, daily primary production Limited by substrate concentration
does not exceed 4 g O2/m2. and rate constant of degradation.
and algal activity, primary productivity
(PP) can reach a maximal daily level of 8-
10 g carbon (C) per m2 (Boyd and Tucker
1998). However, normal high values are
in the range of about 4 g C/m2 day (Wetzel
1975). The assimilated organic carbon
is the base of the food web. Only part
of it may be consumed directly by sh,
inasmuch as there is no control over the
species of algae that grow in the pond.
Algae supply other services to the
pond. A product associated with carbon
assimilation is the production of oxygen:6
CO2 + 6 H2O -- C6H12O6 + 6O2 ( 1 )
Primary production of 4 g C/m2 day
adds 10.7 g/m2 of oxygen; yet, only a
fraction of that is a net addition to the
pond. In the long run, all the organic
carbon is respired and all of the oxygen
that is produced is then consumed. The
benet to the pond stems from the fact that
part of the organic matter settles as dead
algae or other particulate organic matter,
and its microbial degradation takes place
only later. This is the reason for oxygen
problems in old ponds, where the sedi-
ment oxygen consumption becomes an
important fraction of the pond oxygen
balance. This is also the reason for the
recommended drying of the pond bottom
to oxidize the organic matter by exposing
it to the air. An additional reason leading
to inefcient use of the produced oxygen
by algae is that oxygen is produced mostly
in the top water layer, leading to oxygen
supersaturation in that layer and to its
release to the atmosphere.
One of the problems associated with
the intensication of pond aquaculture
is the accumulation of ammonia in the
water. This accumulation places the sh
at risk from elevated levels of the toxic
ammonia (NH3) and nitrite (NO2
-) spe-
cies. Algae help to alleviate this problem
because they take up nitrogen from the
20 December 2003
depends on an ample supply of oxygen, though
their sensitivity toward low oxygen is far be-
low that of sh. The number of bacteria in
zero exchange intensive ponds was found to
be in the order of 107-108 cells/ml (McIntosh
2000). The level of organic matter in ponds
tends to reach a steady state as a result of
a balance between the addition of organic
matter and its microbial degradation.
Continual aeration is conducive to the
development of nitrifying bacteria that
oxidize the excreted ammonia, rst to
nitrite and then to nitrate (NO3
-). Nitrate
is not toxic to sh at the levels found
in ponds and it serves as an oxidizing agent, preventing the
development of anaerobic conditions which can lead to several
fermentation processes and to the build up of toxic reduced
compounds. Nitrifying bacterial development is rather slow. It
may take 2-3 weeks from the day the pond is lled until the full
oxidation of ammonia to nitrate, with a possible short period of
nitrite accumulation when the rst stage of ammonia oxidation
has developed while the second is still evolving (Avnimelech
et al. 1986).
Though intensive nitrication is a very efcient mechanism
to reduce the accumulation of ammonia and nitrite, microbial
control enables a better and more efcient methods of reducing
toxic nitrogen metabolites. Bacterial cells are made primarily of
proteins. The carbon:nitrogen ratio of most microbes is about 4:5.
When bacteria are fed organic substrates that contain mostly carbon
(starch, molasses, cassava meal), they must take up nitrogen from
the water to produce the protein needed for cell growth and multi-
plication. Thus, the addition of carbonaceous materials leads to the
conversion of inorganic deleterious nitrogen to microbial proteins.
Roughly, 20-25 g of carbonaceous material are needed
to convert 1 g of ammonia nitrogen into
microbial protein (Avnimelech 1999).
This process is relatively fast and it is
possible to reduce an elevated level of
ammonia to any desired level within a
period of 1-3 days. One feature of this
mechanism is the possibility of using
the microbial protein as a source of
protein for sh.
The availability of microbial pro-
tein is not trivial nor is harvesting. Individual bacteria
are too small to be harvested physically by sh. In addition, the
digestibility of the bacterial protein must be determined. In a
series of tank and commercial size pond experiments it was found
that tilapia ingest and digest the microbial protein (Avnimelech
et al. 1989). Similar results were found with shrimp (McIntosh
2000) and bass (Milstein et al. 2001). The harvesting of the bac-
teria were possible because in dense microbial suspensions the
organisms tend to form ocs that were visible, having a size of a
few tenths of a millimeter. These ocs can be directly harvested
or ltered. MacKintosh (2001) reported that in the Belize zero
exchange shrimp ponds occulation was enhanced through the
re-use of water from ponds that had a good occulation, seem-
ingly by selection of bacteria with high occulation potential.
water as a part of their metabolism. As shown in
equation (1), the basic activity of the algae leads
to the production of glucose. However, a
basic component of algal cells is a
protein that is synthesized from
glucose using nitrogen that is
adsorbed from the water. Ni-
trogen constitutes about 1/6
of the carbon in algal cells
(Wetzel 1975), thus the am-
monia uptake potential of a pond
with PP of 4 g C/m2 day, is 0.66 g of
ammonia nitrogen per day. A pond
carrying 10 tons of sh/ha and fed
with 40 percent protein feed at a
rate of 2 percent bodyweight, gets
1.24 g N/m2 day, and roughly 75
percent of it, or 0.93 g N/m2, is released into the water. Thus, the
algae that can serve as a good buffer against ammonia toxicity
cannot function when pond intensication rises. Moreover, algal
control suffers from inherent instability. One factor is the strict
demand for light. A series of cloudy days, very common in the
tropics, leads to severe light limitations and to drastic reduction
of primary productivity. Oxygen concentration can be drastically
reduced and ammonia concentration increased following a series
of cloudy days.
To raise yields and to get better control of oxygen in ponds,
sh farmers introduce aerators into the pond system. Aerators
provide oxygen and, at the same time, mix the water. Intensive
aeration provides optimal conditions for aerobic bacteria and is
a feature typically found in bio-techno-
logical microbial-based industries. An
additional factor leading to the micro-
bial dominance in intensive ponds is
the limitation on water exchange.
Water exchange is one of the few controls
in algae dominated systems. When the water contains a
surplus of algae, or a surplus of organic materials, oxygen
consumption exceeds supply. One means of overcoming
this is to exchange the organic rich water with cleaner water.
However, this practice is limited because of environmental
concerns and regulations, the risk of disease infestation result-
ing from the un-controlled introduction of water and, in part,
the expense of pumping. When water exchange is limited, as in
the case of zero exchange or minimal exchange regimes, organic
matter builds up in the water. Organic matter is the substrate
needed for the development of a heterotrophic microbial com-
munity, microbes that get their energy by metabolizing organic
molecules. Intensication, aeration, mixing and limited water
exchange lead to the development of microbial dominance in
the pond. In intensive sh culture systems using heterotrophic
microbial control, active suspension ponds (ASP) were studied
and implemented in the last two decades (Avnimelech et al
1992, Avnimelech et al. 1994, McIntosh 2000, Chamberlain et
al 2001).
Typical features of microbial dominant systems are presented
in Table 1. The size of the microbial population depends on the
supply of organic matter and the stability of the aerobic community
21
World AquAculture
References
Avnimelech Y. 1999. Carbon/nitrogen ratio as a control element in
aquaculture systems. Aquaculture 176:227-235
Avnimelech, Y., B. Weber, B. Hepher, A. Milstein and M. Zorn. 1986.
Studies in circulated sh ponds: Organic matter recycling and
nitrogen transformation. Aquaculture and Fisheries Management
17: 231-242.
Avnimelech, Y., S. Mokady and G.L. Schroeder. 1989. Circulated ponds
as efcient bioreactors for single-cell protein production. Bamidgeh
41:58-66.
Avnimelech, Y., S. Diab and M. Kochva. 1992. Control and utilization
of inorganic nitrogen in intensive sh culture ponds. Aquaculture
and Fisheries Management 23:421-430.
Avnimelech Y., M. Kochva and S. Diab. 1994. Development of controlled
intensive aquaculture systems with a limited water exchange and
adjusted carbon to nitrogen ratio. Bamidgeh 46:119-131
Avnimelech, Y.. and G. Ritvo. 2001. Aeration, Mixing and Sludge
Control in Shrimp Ponds. Global Aquaculture Advocate 4:51-53.
Boyd, C.E. and C.S. Tucker. 1998. Pond aquaculture water quality
ma nag eme nt. Kl uwe r Aca d emi c Pub lic ati ons . Dord rec ht,
Netherlands.
Chamberlain, G., Y. Avnimelech, R.P. McIntosh and M. Velasco. 2001.
Advantages of Aerated Microbial Reuse Systems with Balanced
C:N I: Nutrient transformation and water quality benets. Global
Aquaculture Alliance Advocate 4: 53-56.
McIntosh R.P. 2000. Changing paradigms in shrimp farming: V.
establishment of heterotrophic bacterial communities. Global
Aquaculture Alliance Advocate 3:52-54.
McIntosh R.P. 2001. Changing paradigms in shrimp farming: V.
establishment of heterotrophic bacterial communities. Global
Aquaculture Alliance Advocate 4:53-58.
Milstein, A., Y. Avnimelech, M. Zoran and D. Joseph. 2001. Growth
performance of hybrid bass and hybrid tilapia in conventional and
active suspension ponds. Bamidgeh 53:147-157.
Wetzel, R. G. 1975. Limnology. W.B. Saunders Co, Philadelphia,
Pennsylvania USA.
Table 2. Effect of added carbon substrate (wheat
our) on yield, feed conversion ratio, protein
conversion ratio (protein in feed/protein gain
in sh) and feed cost in intensive tilapia
ponds. Averages based on three replicates
in commercial scale ponds. (Adapted from
Avnimelech et al. 1994.)
Protein level in feed 30 percent 20
percent
Experiment 1 (51 days)
Feed C:N ratio 11.1 16.6
Daily gain (% body weight) 1.59 2.0
Food conversion ratio 2.62 2.17
Protein conversion ratio 4.38 2.42
Feed cost (US$/kg sh) 0.85 0.58
Experiment 2 (30 days)
Feed C:N ratio 16.6 11.1
Daily gain (% body weight) 1.63 2.22
Food conversion ratio 2.62 2.02
Protein conversion ratio 4.35 2.18
Fish or shrimp utilize
only about 25 per-
cent of the protein
in the feed. High
protein diets are
needed to com-
pe nsa te for the
low utilization
rate. This may be
a dominant factor
in feed expense,
inasmuch as pro-
tein is the most expensive fraction
of the feed. In addition there are global
uctuations in the quantities of animal protein
sources available that can be problematic. The utilization of protein
in active suspension ponds is doubled. The sh eat and digest the
feed-originated protein and use it again by ingesting and digesting
the microbial protein, actually a recycling of the non-utilized feed
protein. A demonstration of this effect, obtained in a eld trial, is
presented in Table 2. Very similar results were obtained in com-
mercial shrimp ponds (McIntosh 2000). The higher utilization of
feed leads to signicantly lower feed costs (Table 2).
Preliminary results indicate that microbial dominance may
help induce disease resistance in sh. Several observations as
well as a controlled experiment (Avnimelech and Ritvo 2001)
indicated that a dense heterotrophic population can act as an
antagonist against pathogens. This effect, a probiotic control
mechanism, deserves further study.
Conclusions
Microbial controlled ponds, (ASP), provide a stable control
over processes in the pond. The microbial control is stable, does
not depend on light intensity and is not sensitive to population
crashes. Microbial control leads to efcient degradation of waste
materials, efcient nitrication and, through the manipulation of
the C:N ratio, facilitates the control and recycling of nitrogen
and doubles the protein utilization.
The basic demands to establish and maintain microbial
dominance are:
a. The pond gas must be well aerated and mixed.
b. Concentration of organic substrates has to be high to support
the heterotrophic population. This is achieved in intensive
systems with zero or restricted water exchange. An addition
of organic substrates in the beginning of the culture cycle
may help promote the development of the microbial com-
munity.
c. Microbial controlled ponds rely on the development of natural
microbial populations. Presently, there is no microbial inocu-
lum proven to be effective in promoting the development of
microbial controlled ponds.
d. The efciency of microbial controlled ponds must be evalu-
ated for the given organism to be cultured.
Notes
1Dept of Agricultural Engineering, Technion, Israel Inst. of Technology,
Haifa, 32000, Israel agyoram@tx.technion.ac.il
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... Compared to the traditional system, the higher BOD concentrations in the BFT treatments were due to bio oc development; bio ocs have oxygen demand in the water(Kuhn 2012). However, BOD values estimated in the present study were in the acceptable ranges for sh cultivation, as suggested in previous studies(Das 1997;Deka 2015).Floc volume is considered as the fundamental marker of bio oc formation(Avnimelech 2012). The oc developed in the BFT treatments likely in uenced alkalinity and hardness in the BFT system with sh excretions concentrating residues and the metabolic activity of the microorganisms in the water(Lee et al. 2013). ...
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