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Review article
Nitrogen removal techniques in aquaculture
for a sustainable production
Roselien Crab
a,b
, Yoram Avnimelech
c
, Tom Defoirdt
a,b
,
Peter Bossier
b
, Willy Verstraete
a,
⁎
a
Laboratory of Microbial Ecology and Technology (LabMET), Ghent University, Coupure Links 653, 9000 Gent, Belgium
b
Laboratory of Aquaculture and Artemia Reference Center, Ghent University, Rozier 44, 9000 Gent, Belgium
c
Faculty of Agricultural Engineering, Technion, Israel Institute of Technology, Haifa 32000, Israel
Received 16 February 2007; received in revised form 2 May 2007; accepted 5 May 2007
Abstract
As the aquaculture industry intensively develops, its environmental impact increases. Discharges from aquaculture deteriorate
the receiving environment and the need for fishmeal and fish oil for fish feed production increases. Rotating biological contactors,
trickling filters, bead filters and fluidized sand biofilters are conventionally used in intensive aquaculture systems to remove
nitrogen from culture water. Besides these conventional water treatment systems, there are other possible modi operandi to recycle
aquaculture water and simultaneously produce fish feed. These double-purpose techniques are the periphyton treatment technique,
which is applicable to extensive systems, and the proteinaceous bio-flocs technology, which can be used in extensive as well as in
intensive systems. In addition to maintenance of good water quality, both techniques provide an inexpensive feed source and a
higher efficiency of nutrient conversion of feed. The bio-flocs technology has the advantage over the other techniques that it is
relatively inexpensive; this makes it an economically viable approach for sustainable aquaculture.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Water quality; Sustainability; Biofilters; Periphyton; Bio-flocs technology; C/N ratio
Contents
1. Overview of problem................................................... 2
2. N removal outside the culture unit ............................................ 3
2.1. Earthen treatment ponds or reservoirs....................................... 3
2.2. Biofiltration .................................................... 4
3. N removal within the culture unit ............................................ 7
3.1. The periphyton treatment technique........................................ 7
3.2. Bio-flocs technology ............................................... 8
Aquaculture 270 (2007) 1 –14
www.elsevier.com/locate/aqua-online
⁎Corresponding author. Tel.: +32 9 264 59 76; fax: +32 9 264 62 48.
E-mail address: Willy.Verstraete@UGent.be (W. Verstraete).
URL: http://labmet.ugent.be (W. Verstraete).
0044-8486/$ - see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.aquaculture.2007.05.006
4. Conclusions and future perspectives .......................................... 11
Acknowledgements ...................................................... 11
References .......................................................... 11
1. Overview of problem
Aquaculture is a rapidly growing food producing
sector. The sector has grown at an average rate of 8.9%
per year since 1970, compared to only 1.2% for capture
fisheries and 2.8% for terrestrial farmed meat-production
systems over the same period (FAO, 2004). In contrast to
aquaculture, capture fisheries landings as a whole is
stagnant. Although catch rates for some species did not
decline during the 1990s, most ocean fisheries stocks are
now recognized as fully or over fished. The worldwide
decline of ocean fisheries stocks and the further expansion
of the human population are an incentive for the further
growth of aquaculture. Despite the growth of the sector,
aquaculture production still needs to increase 5-fold in the
next 2 decades in order to satisfy the minimum protein
requirement for human nutrition (FAO, 2004).
The intensive development of the aquaculture
industry has been accompanied by an increase in
environmental impacts. The production process gener-
ates substantial amounts of polluted effluent, containing
uneaten feed and feces (Read and Fernandes, 2003).
Discharges from aquaculture into the aquatic environ-
ment contain nutrients, various organic and inorganic
compounds such as ammonium, phosphorus, dissolved
organic carbon and organic matter (Piedrahita, 2003;
Sugiura et al., 2006). The high levels of nutrients cause
environmental deterioration of the receiving water
bodies. In addition, the drained water may increase the
occurrence of pathogenic microorganisms and introduce
invading pathogen species (Thompson et al., 2002).
To produce 1 kg live weight fish one needs 1–3kgdry
weight feed (assuming a food conversion ratio about 1–3)
(Naylor et al., 2000). About 36% of the feed isexcreted as
a form of organic waste (Brune et al., 2003). Around 75%
of the feed N and P are unutilized and remain as waste in
the water (Piedrahita, 2003; Gutierrez-Wing and Malone,
2006). An intensive aquaculture system, which contains
3 ton tilapia, can be compared on a biomass basis to a
human community with 50 inhabitants (Helfman et al.,
1997). This intensive aquaculture system can also be
compared on grounds of waste generation to a community
of around 240 inhabitants (Aziz and Tebbutt, 1980;
Flemish government, 2005). It can thus be concluded that
live fish biomass generates approximately 5 times more
waste than live human biomass. The reason is that the
scope of digestion in fish is limited; a relatively large
fraction of feed remains undigested and is excreted
(Amirkolaie, 2005). The feeding habit of fish is reflected
in the digestive anatomy. The gut length of fish is short
and the ratio of gut length to body length is small
(Hertrampf and Piedad-Pascual, 2000). For instance, the
intestine of carp is 2.0–2.5 times longer than the body,
while that of cattle and sheep is respectively 20 and 30
times longer. The human intestine is about 3 to 4 times
longer than the body. Consequently, in fish, the chyme
stays in the gut only for a short time. For this reason, fish
feed must have a high digestibility. Typically, fish body
contains 65 to 75% protein (Hertrampf and Piedad-
Pascual, 2000). In addition, fish use proteins for energy
production to a large extent, unlike terrestrial animals that
use mostly carbohydrates and lipids (Hepher, 1988). Fish
protein requirement, therefore, is about two to three times
higher than that of mammals. Ammonium is one of the
end products of protein metabolism (Wals h a n d Wright ,
1995). All these factors contribute to the high nitrogen
residues in aquaculture water (Fig. 1). In water, NH
3
(ammonia) and NH
4
+
(ammonium) are in equilibrium
depending on the pH and the temperature (Timmons et al.,
2002). The sum of the two forms is called total
ammonium nitrogen (TAN). Although both NH
3
and
NH
4
+
may be toxic to fish, unionized ammonia isthe more
toxic form attributable to the fact that it is uncharged and
lipid soluble and consequently traverses biological
membranes more readily than the charged and hydrated
NH
4
+
ions (Körner et al., 2001). Ammonia-N is toxic to
commercially cultured fish at concentrations above
1.5 mg N/l. In most cases, the acceptable level of union-
ized ammonia in aquaculture systems is only 0.025 mg
N/l (Neori et al., 2004; Chen et al., 2006). However, the
toxicity threshold depends strongly on the species, size,
fine solids, refractory organics, surface-active com-
pounds, metals, and nitrate (Colt, 2006).
In addition to the generation of large amounts of waste,
the use of fishmeal and fish oil as prime constituents of
feed is another non-sustainable practice in aquaculture.
Approximately one-third of the global fishmeal produc-
tion is converted to aquaculture feeds (Delgado et al.,
2003). The proportion of fishmeal supplies used for fish
production increased from 10% in 1988 to 17% in 1994
2R. Crab et al. / Aquaculture 270 (2007) 1–14
and 33% in 1997 (Naylor et al., 2000). Hence, aquaculture
is a possible panacea, but also a promoter of the collapse
of fisheries stocks worldwide. The ratio of wild fish:fed
farmed fish (both live weight base) is about 1.41:1 for
tilapia and 5.16:1 for marine finfish, (Naylor et al., 2000).
Purchase of commercially prepared feed for fish culture
comprises 50% or more in the production costs; this is
primarily due to the cost of the protein component
(Bender et al., 2004). On average some 25% of the
nutrient input of these feed sources is converted into
harvestable products (Avnimelech and Lacher, 1979;
Boyd, 1985; Muthuwani and Lin, 1996; Avnimelech and
Ritvo, 2003). To make further sustainable increase of
aquaculture production possible, the search for inexpen-
sive protein sources and a higher efficiency of nutrient
conversion of feed is needed.
2. N removal outside the culture unit
The most common water purification treatments in
aquaculture systems can be subdivided in different types
of water treatments: 1) earthen treatment ponds or
reservoirs, and 2) a combination of solids removal and
nitrification tanks as also used in domestic wastewater
treatment plants. It should be noted that the real nitrogen
removal processes are those that involve the release of
fixed nitrogen back to the atmosphere (van Rijn et al.,
2006). However, these are not discussed here.
2.1. Earthen treatment ponds or reservoirs
This treatment procedure consists of the direct linkage
of, and water recirculation between the intensive produc-
tion ponds and treatment ponds. The effluent water of the
production pond is retained in a basin for several hours to
days to allow natural physical, chemical, and biological
processes to improve its quality for reuse (Diab et al., 1992;
Hargreaves, 2006). Important practical parameters in this
system are the hydraulic retention times of the intensive
fish culture unit and the treatment pond, homogeneous
mixing of the treatment pond, and the periodic aeration of
the pond sediment by drainage. The use of treatment ponds
encounters problems due to algal collapse and anaerobiosis
of the sediment (van Rijn, 1996). The main disadvantage is
the unstable purification resulting from unpredictable
fluctuations of phytoplankton biomass and speciation in
Fig. 1. Nitrogen cycle in aquaculture ponds with a long hydraulic residence time. The N-input considered is formulated feed. A part of the feed
remains unconsumed in the system (Franco-Nava et al., 2004b). The consumed feed is partially converted into fish biomass and partially excreted as
ammonium or egested as feces (Jiménez-Montealegre et al., 2002). The uneaten feed and feces contribute to the organic mater load of the system. The
microbial decomposition of organic matter in the system leads to increased levels of TAN and nitrite, both harmful to fish even at low concentrations
(Meade, 1985; Jiménez-Montealegre et al., 2002; Torres-Beristain et al., 2006). The TAN present in the system may be transformed into nitrite, nitrate
and gaseous nitrogen. The formation of nitrogen gas is considered negligible in aquaculture ponds (El Samra and Olàh, 1979). The bacteria present in
the water and sediment carry out these nitrogen transformations by nitrification and denitrification. Both TAN and nitrate can be assimilated by the
phytoplankton, present in the water column. The phytoplankton can be consumed by the cultured organism (Turker et al., 2003). The consumption of
phytoplankton by fish is minimal in this network. In stagnant water ponds TAN tends to accumulate within the system due to insufficient nitrification
activity (Grommen et al., 2002).
3R. Crab et al. / Aquaculture 270 (2007) 1–14
the treatment pond (Hargreaves, 2006). An important
advantage is that the microalgae grown in the treatment
pond can be used to produce a second crop, such as bivalve
seed or Artemia, which can be sold to generate income
(Wang, 2003).
A possible system configuration comprises of a fish
farm with nutrient assimilation by molluscs and seaweed
(Fig. 2). Here, nutrients released in the culture system can
be converted into plant or other biomass, which can easily
be removed and may often be a valuable by-product. The
nutrient-assimilating photoautotrophic plants can be used
to turn nutrient-rich effluents into profitable resources
(Neori et al., 2004). Biofiltration by plants generates in the
culture system a mini-ecosystem, in which, if properly
balanced, plant autotrophy counters fish (or shrimp) and
microbial heterotrophy, not only regarding nutrients but
also with respect to oxygen, pH and CO
2
(Neori et al.,
2004). As a result, plant biofiltration diminishes the net
environmental impact of aquaculture production systems.
Today's integrated intensive aquaculture approaches,
developed from traditional extensive polyculture, inte-
grate the culture of fish or shrimp with vegetables,
microalgae, shellfish and/or seaweed (Neori et al., 2004).
By dividing the production process into stages, we can
increase the constancy of the biomass in the system and
improve the utilization efficiency of the physical facility
(Wan g , 2 003).
2.2. Biofiltration
The treatment methods that are applied to treat
aquaculture wastewater are broadly classifiable into phy-
sical, chemical and biological processes. Physical unit
operations apply physical forces to remove contaminants.
Solid removal is accomplished by sedimentation (settle-
able solids) or mechanical filtration (suspended and fine
solids) (van Rijn, 1996). Two commonly used types of
mechanical filtration in aquaculture include screen fil-
tration and expendable granular media filtration (Twa r-
owska et al., 1997; Franco-Nava et al., 2004a). For fine
solids removal, foam fractionation –a process also
referred to as air stripping or protein skimming –is often
employed (Timmons, 1984; Hussenot, 2003). Chemical
unit processes used for aquaculture wastewater treatment
are customarily used in conjunction with physical unit
operations and biological processes. The inherent disad-
vantage of most chemical unit processes is that they are
additive processes; the chemicals tend to stay for a major
part in the water. This is a significant factor if the waste-
water is to be reused. The main chemical unit process used
in aquaculture is disinfection by means of ozonation
(Summerfelt, 2003). Disinfection by UV irradiation is
considered as a credible alternative to chemical disinfec-
tion, because of the absence of toxic by-products which
are usually generated and identified during chemical
disinfection (Hassen et al., 2000). These techniques avoid
the addition of chemical substances that are hazardous to
the cultured organism. Biological processes are the most
important ones with respect to aquaculture wastewater
treatment and the major biological process is nitrification.
Nitrification is carried out in a variety of systems, which
can be grouped into 2 general types: emerged (rotating
biological contactors, trickling filters) and submerged
(e.g. fluidized bed filters, bead filters) fixed film filters
(van Rijn, 1996; Ling and Chen, 2005; Malone and
Pfeiffer, 2006). Biological filters are used for freshwater
Fig. 2. Integrated farming: nitrogen and phosphorus budget (after Kautsky, 2004).
4R. Crab et al. / Aquaculture 270 (2007) 1–14
and marine operations (Hovanec and DeLong, 1996;
Gutierrez-Wing and Malone, 2006; Malone and Pfeiffer,
2006). This paper reviews recirculating systems on
biofiltration technologies for freshwater systems.
Nitrification in the bacterial film of the biofilter is
affected by a variety of parameters such as substrate and
dissolved oxygen concentrations, organic matter, temper-
ature, pH, alkalinity, salinity and turbulence level (Satoh
et al., 2000; Chen et al., 2006). Nitrifying bacteria are
sensitive organisms and are extremely susceptible to a
wide variety of inhibitors such as high concentrations of
ammonia and nitrous acid, low dissolved oxygen levels
(b1 mg/l) and pH outside the optimal range (7.5–8.6)
(Masser et al., 1999; Villaverde et al., 2000; Ling and
Chen, 2005). Nitrification, and especially the second step
(NO
2
→NO
3
), is very sensitive to even traces of sulphides
(Joye and Hollibaugh, 1995). Sulphides are present in
sediments and in sludges accumulated in intensive
aquaculture systems. For higher C/N ratios, the hetero-
trophic bacteria out-compete nitrifiers for available
oxygen and space in the biofilters (Michaud et al.,
2006). Hence, nitrification necessitates a low C/N ratio.
Fig. 3 illustrates the N cycle in aquaculture systems
equipped with an external biofilter.
Rotating biological contactors have been used in the
treatment of domestic wastewater for decades and are now
widely used as nitrifying filters in aquaculture applica-
tions. Rotating biological contactor technology is based
on the rotation of a submerged substrate, which is made of
high-density polystyrene or polyvinyl chloride, attached
to a shaft (Tawfik et al., 2004; Park et al., 2005; Brazil,
2006). Nitrifying bacteria grow on the media and because
of the rotation they alternately contacting nitrogen rich
water and air. As the rotating biological contactor rotates,
it exchanges carbon dioxide, generated by the bacteria,
with oxygen from the air. In general, rotating biological
contactor systems are divided into a series of independent
stages or compartments (Lavens and Sorgeloos, 1984;
Brazil, 2006). Compartmentalization creates a plug-flow
pattern, increasing overall removal efficiency. It also pro-
motes a variety of conditions where different organisms
can flourish to varying degrees. As the water flows
through the compartments, each subsequent stage
receives influent with a lower organic content than the
previous stage; the system thus enhances organic removal
(UN, 2003; Watten and Sibrell, 2006). Complimentary,
the rotating biological contactor has low head require-
ments to move water through the vessel. This advantage
implies passive aeration and carbon dioxide removal, and
low chance of clogging (Brazil, 2006).
Miller and Libey (1985) demonstrated that a rotating
biological contactor provided better TAN areal removal
rates, in the range of 0.19–0.79 g TAN/m
2
day, than a
packed tower or fluidized bed reactor (0.24 g TAN/m
2
Fig. 3. Nitrogen cycle in aquaculture systems equipped with an external biofilter. The nitrogen cycle is similar to that of in water ponds with a long
hydraulic residence time, but now the water rich in TAN is sent to an external biofilter. In this biofilter, nitrification is enhanced and through nitrite,
nitrate is formed out of TAN. Nitrate is less toxic to fish than is TAN or nitrite (Meade, 1985; Lyssenko and Wheaton, 2006). Although such a system
avoids TAN to accumulate, nitrate build up may take place. The biofilter creates space and optimal conditions for nitrifying bacteria to grow.
5R. Crab et al. / Aquaculture 270 (2007) 1–14
day), when treating the same fish culture water at
comparable hydraulic loadings. Brazil (2006) described
the performance and operation of a rotating biological
contactor in a tilapia recirculating aquaculture system.
The system obtained an average TAN areal removal rate
of about 0.42 g/m
2
day. Increasing influent dissolved
organic carbon levels decreased ammonia removal ef-
ficiency. However, there was no detectable relationship
between the feed loading rate and ammonia oxidation
performance. In addition to organic loading, mass and
hydraulic loading, rotational speed and staging affected
the ammonia oxidation performance.
Trickling filters consist of a fixed medium bed
through which aquaculture wastewater flows down-
wards over a thin aerobic biofilm (Eding et al., 2006).
As it trickles down, the water is continuously oxygen-
ated, while the carbon dioxide is degassed and removed
by the ventilated air. Trickling medium has a specific
surface area ranging from 100 to 1000 m
2
/m
3
. Finturf
artificial grass (284 m
2
/m
3
), Kaldnes rings (500 m
2
/m
3
),
Norton rings (220 m
2
/m
3
) and Leca or light weight clay
aggregate (500–1000 m
2
/m
3
) are some of the most fre-
quently used media (Greiner and Timmons, 1998;
Lekang and Kleppe, 2000; Timmons et al., 2006a).
The organic material present in the wastewater is adsorbed
on the biological slime layer and degraded by aerobic
microorganisms.
Kamstra et al. (1998) reported TAN areal removal
rates between 0.24 and 0.55 g TAN/m
2
day for a com-
mercial-scale trickling filter. For three different applied
filter medium types in commercial farms and a range of
hydraulic surface loading conditions, the highest ob-
served TAN areal removal rate for a trickling filter was
1.1 g TAN/m
2
day, with an average TAN areal removal
rate of about 0.16 g TAN/m
2
day (Schnel et al., 2002;
Eding et al., 2006). Lyssenko and Wheaton (2006) re-
ported TAN areal removal rates of 0.64 g TAN/m
2
day.
In the same study they found similar TAN areal removal
rates for a submerged expandable upflow sand filter.
Downflow microbead filters are combinations of
trickling filters and granular type biological filters
(Timmons et al., 2006a). The use of floating media in
downflow configurations has the advantage of being
capable of using smaller media and the associated higher
specific surface areas. As the recirculating water passes
through the packed bed, suspended solids are captured
and biofiltration processes are active (Malone and
Beecher, 2000). The configuration offers the added ad-
vantage of using high hydraulic loadings without the need
for sophisticated mechanical structures in the reactor to
retain the media within the reactor vessel (Greiner and
Timmons, 1998). The medium consists of polystyrene
beads that are 1–3 mm in diameter and have a porosity of
36–40% (Timmons et al., 2006a). Depending on these
features the specific surface area ranges from 1150 to
3936 m
2
/m
3
(Greiner and Timmons, 1998; Malone and
Beecher, 2000; Timmons et al., 2006a).
Greiner and Timmons (1998) observed TAN areal
removal rates of about 0.45–0.60 g/m
2
day. A study
using a commercial microbead filter system reported
an average TAN areal removal rate of 0.30 g/m
2
day
(Timmons et al., 2006a).
Fluidized sand biofilters have been widely adopted in
recirculating systems that must reliably maintain ex-
cellent water quality (Summerfelt, 2006). Filter sand has
a high specific surface area, i.e. 4000–20000 m
2
/m
3
and
has a moderate cost (Summerfelt, 2006). A disadvantage
of the FBS is that they do not aerate, as do trickling
filters (Summerfelt, 2006). Therefore, additional aera-
tion is needed. These filters also must operate within a
narrow water flow range in order to maintain proper bed
expansion (Summerfelt, 2006).
Miller and Libey (1985) demonstrated that the TAN
removal rate of a fluidized bed reactor was around
0.24 g N/m
2
day. Timmons and Summerfelt (1998)
found similar rates in their research.
Table 1 gives an overview of the average TAN areal
removal rate and the cost per kg of fish produced per
Table 1
General overview of the average TAN areal removal rate for frequently used biofilters in aquaculture systems
Biofilter type Average TAN areal removal rate Cost
a
References
(g TAN/m
2
day) (Euro/kg yr)
Rotating biological contactor 0.19–0.79 1.143 Miller and Libey, 1985; Brazil, 2006
Trickling filter 0.24–0.64 1.036 Kamstra et al., 1998; Schnel et al., 2002; Eding et al.,
2006; Lyssenko and Wheaton, 2006
Bead filter 0.30–0.60 0.503 Greiner and Timmons, 1998; Timmons et al., 2006a
Fluidized sand biofilter 0.24 0.198 Miller and Libey, 1985; Timmons and Summerfelt, 1998
Also the costs for various biofilter choices based upon their capitalization cost to support a 454 ton per year tilapia farm are summarized.
a
Data from Timmons et al. (2006b).
6R. Crab et al. / Aquaculture 270 (2007) 1–14
year for each biofilter type. Rotating biological con-
tactors have the highest TAN areal removal rate,
followed by bead biofilters and trickling filters, and
fluidized sand biofilters. Although rotating biological
contactors show good performance concerning TAN
removal rate, they are together with trickling filters more
expensive then the other biofilter types discussed.
Fluidized sand biofilters and bead biofilters are the
least expensive options for water treatment when the
cost per kg of fish produced per year is considered.
3. N removal within the culture unit
The three nitrogen conversion pathways naturally
present for the removal of ammonia–nitrogen in
aquaculture systems are photoautotrophic removal by
algae, autotrophic bacterial conversion of ammonia–
nitrogen to nitrate–nitrogen, and heterotrophic bacterial
conversion of ammonia–nitrogen directly to microbial
biomass (Ebeling et al., 2006).
Developing and controlling dense heterotrophic mi-
crobial flocs in the water column or attached micro-
organisms called periphyton can accelerate the biological
removal of organic and inorganic wastes in ponds
(Avnimelech, 2005; Azim et al., 2003a,c). These
processes are integral parts of the culture unit (Har-
greaves, 2006). An important advantage is that microbial
bio-flocs and periphyton can be consumed and used as a
source of feed by the cultivated organisms (Burford et al.,
2003, 2004; Hari et al., 2004; Azim and Wahab, 2005;
Keshavanath and Gangadhar, 2005). As explained in the
following paragraphs, both approaches are possible
solutions for water quality problems, and can decrease
the use of fish oil and fishmeal utilization in aquaculture.
3.1. The periphyton treatment technique
The periphyton community consists of attached
aquatic biota on submerged matrices. It harbours algae,
bacteria, fungi, protozoa, zooplankton and other inverte-
brates (Azim et al., 2005). As with phytoplankton,
periphyton can be found in almost every type of water
body from small ponds to large oceans and in trophic
conditions that range from the most oligotrophic to the
most eutrophic (Azim and Asaeda, 2005). Given adequate
light, up to about 0.5 m depth in the water, high rates of
photosynthesis and autotrophic production can be
achieved (Craggs et al., 1996; Vermaat, 2005). Values
for periphyton productivity are typically in the range of
1–3gC/m
2
substrate day or 2–6gdrymatter/m
2
day
(Azim et al., 2005). Periphyton entraps organic detritus,
removes nutrients from the water column and helps
Fig. 4. Nitrogen cycle in extensive aquaculture ponds with substrates for periphyton growth. The nitrogen cycle is similar to that of in water ponds
with a long hydraulic residence time, but now TAN concentration does not build up in the water column, neither is the nitrate concentration. The
periphyton community takes up both TAN and nitrate and edible biomass is formed. The cultured fish can graze on the periphyton community and
hence nitrogen, originating from wasted feed and excretion by fish, is redirected towards the cultured organism and therefore, this technique enhances
the overall efficiency of nutrient conversion of feed.
7R. Crab et al. / Aquaculture 270 (2007) 1–14
control the dissolved oxygen concentration and the pH of
the surrounding water (Azim et al., 2002; Dodds, 2003;
Bender et al., 2004).
Supplying substrates improves the nitrogen-related
processes developing in the water column and the nitro-
gen flow is mainly linked to autotrophic and heterotrophic
activity that takes place in the periphyton (Fig. 4)
(Milstein, 2005). The beneficial influence of periphyton
on the water quality in different aquaculture systems has
been investigated, as well as the impact of grazing by fish
on periphyton communities (Huchette et al., 2000; Azim
et al., 2001, 2002, 2003a,b,c, 2004). Not all fish are able to
graze on periphyton; morphological and physiological
adaptations to periphyton grazing are required (Azim
et al., 2005). Although direct experimental evidence is
scarce, the aquaculture fish species that can effectively
utilize the periphyton assemblage are probably more
numerous than those that are exclusively phytoplankti-
vorous (van Dam and Verdegem, 2005). Besides spe-
cialist (macro)herbivores, more general detritus and
benthos feeders can also thrive on periphyton (van Dam
et al., 2002).
Periphyton has an average C/N ratio of 10 (Azim and
Asaeda, 2005). Its assimilation capacity is around 0.2 g N/
m
2
day. From this it is clear that one needs a large surface,
which allows periphyton growth, to treat intensive
aquaculture wastewater without compromising the water
quality. Besides N removal, biomass is formed. The yield
is around 4 g dry matter/m
2
day and the protein content of
periphyton is around 25% of the dry matter (Azim et al.,
2002, 2005). This corresponds to a particular feed
quantity that can diminish the overall feed cost.
Besides the large area needed, the problem within this
system is that the process is completely dependent on the
availability of sunlight (Azim and Asaeda, 2005). On
cloudy days or on days with insufficient sunlight, the
maximum nitrogen uptake rate will not be reached.
Another problem is the laborious task to harvest the
periphyton. One can conclude that application of the
periphyton treatment technique in the intensive aquacul-
ture sector is not feasible. Nevertheless, the technique of
using this natural feed may be significant, particularly in
smaller, extensive-level aquaculture systems in develop-
ing countries. The addition of the ‘periphyton loop’in
aquaculture ponds can be accomplished by adding static
substrates to the pond (Azim et al., 2005), such as poles
horizontally planted in the ponds. Substrates used are
bamboo, hizol and kanchi (Azim et al., 2002, 2003c).
Since periphyton can be easily cultured in modified fish-
ponds and demands little management, the benefits may
be substantial.
3.2. Bio-flocs technology
Suspended growth in ponds consists of phytoplankton,
bacteria, aggregates of living and dead particulate organic
Fig. 5. Amorphous aggregate, which consists of phytoplankton, bacteria, aggregates of living and dead particulate organic matter, and grazers (The
flocs were examined with light microscopy, and digital images were captured with a 1-CCD camera).
8R. Crab et al. / Aquaculture 270 (2007) 1–14
matter, and grazers of the bacteria (Fig. 5)(Hargreaves,
2006). If carbon and nitrogen are well balanced in the
solution, ammonium in addition to organic nitrogenous
waste will be converted into bacterial biomass (Schneider
et al., 2005). By adding carbohydrates to the pond,
bacterial growth is stimulated and nitrogen uptake
through the production of microbial proteins takes place
(Avn i m e lech, 1999). This promoted nitrogen uptake by
bacterial growth decreases the ammonium concentration
more rapidly than nitrification (Hargreaves, 2006). Immo-
bilization of ammonium by heterotrophic bacteria occurs
much more rapidly because the growth rate and microbial
biomass yield per unit substrate of heterotrophs are a
factor 10 higher than that of nitrifying bacteria (Har-
greaves, 2006). The microbial biomass yield per unit
substrate of heterotrophic bacteria is about 0.5 g biomass
C/g substrate C used (Eding et al., 2006).
In natural environments, microorganisms tend to form
amorphous aggregates. The settlingvelocity of these flocs
appears not to relate to the square of the size, as expected
from Stokes' law (Logan and Hunt, 1987, 1988). If an
aggregate is highly porous, fluid streamlines will pene-
trate the aggregate resulting in advective flow through it.
This will improve the supply of nutrients to the cells
present in the aggregate and will decrease the settling
velocity of the flocs in the pond.
Using the relative uptake factor γ, defined as growth
rate of aggregated cells/growth rate of free cells, one can
make a comparison of the substrate uptake by aggregated
versus dispersed cells. Fig. 6 depicts the relative uptake
predictions for microbial cells in permeable flocs (Logan
and Hunt, 1988). The power input to the fluid originates
from the aeration of the ponds. Different aeration tech-
niques are available, such as diffuser aeration, mechanical
aeration and packed column aeration. For turbulent fluids,
the mean shear rate Gis determined from the power input
to the fluid per unit volume of the fluid. In intensive
aquaculture systems the average power input to the fluid is
around 1–10 W/m
3
=10
1
–10
2
cm
2
/s
3
or 10bGb100 s
−1
(Boyd, 1998; McGraw et al., 2001; Schuur, 2003). At
these moderate mixing rates, cells growing in permeable
aggregates can profit from advective flow and grow better
than single dispersed cells (γN1). One can calculate that
the relative growth rate of aggregated cells in this energy
regime is greater than the growth rate of free cells (Logan
and Hunt, 1987, 1988). When more intense aeration is
applied, the advantage of growing in flocs disappears and
cells growing solely show higher growth.
We can conclude that the biological flocs can be
considered as a kind of fast growing microbial mixed
culture in which the ‘waste’-nitrogen is recycled to
young cells, which subsequently are grazed by the fish
(Fig. 7). Uptake of the bio-flocs by fish depends most
probably on the fish species and feeding traits, fish size,
floc size and floc density (Avni m e lech, 20 0 7). With
respect to feeding, this technique operates at “neutral
cost”, because it upgrades starch to protein. Moreover,
one does not need to invest in an external water treatment
system. This method is applicable to extensive as well as
intensive aquaculture systems. In addition, the heterotro-
phic microbial biomass is suspected to have a controlling
effect on pathogenic bacteria (Michaud et al., 2006).
Preliminary results at our laboratories have shown the
presence of poly-β-hydroxybutyrate in bio-flocs. PHB-
accumulating bacteria may abate pathogenic bacteria in
aquaculture (Defoirdt et al., 2007; Halet et al., 2007).
Fig. 6. Relative growth prediction for microbial cells in permeable flocs (after Logan and Hunt, 1988).
9R. Crab et al. / Aquaculture 270 (2007) 1–14
In what follows, some examples are discussed con-
cerning performance of the bio-flocs technology in prac-
tice in freshwater systems.
Avnimelech (1999) pointed out the use of the C/N ratio
as a control element in aquaculture systems. Nitrogen
control was induced by adding carbohydrates to the water,
and through the subsequent uptake of nitrogen by
heterotrophic bacteria. This resulted in the synthesis
of microbial proteins that can be eaten by the cultured fish
species. Experiments with sediment suspension amended
with about 10 mg N/L ammonium and glucose at a
concentration 20 times higher than that of the TAN
showed that almost all the added ammonium disappeared
over a period of about 2 h. Avnimelech et al. (1994) found
that protein utilization by fish in intensive bio-floc
systems is almost twice as high as the protein utilization
in conventionally fed intensive aquaculture ponds, due to
a recycling of the excreted nitrogen into utilizable
microbial protein. Protein recovery by tilapia rose from
23% in the control to 43% in the floc treatment. It was
concluded from this study that the price of feed for fish
production using sorghum supplemented granules (pellets
containing only 20% protein and sorghum as a carbona-
ceous substrate) is just about 50% of the conventional cost
when 30% protein pellets were used.
The bio-flocs technology is also applicable to saline
systems, as discussed below.
Hari et al. (2004) facilitated the development of
heterotrophic bacteria and the related in situ protein
synthesis by increasing the C/N ratio of the feed and by
further increasing the C/N ratio through carbohydrate
addition to the ponds. The added carbohydrate facilitat-
ed increased heterotrophic growth thereby augmenting
shrimp production. The levels of inorganic nitrogen
species in the water column were lower due to uptake by
heterotrophic bacteria, making farming more sustain-
able. The TAN levels in the water column in the study
were 0.01 mg/L, which is low compared to levels
reported in other studies (0.5–3.0 mg/L) (Hopkins et al.,
1993). Consumption of microbial flocs increased
nitrogen retention from added feed by 13% (Hari
et al., 2004).
Burford et al. (2004) promoted the growth of the
natural microbiota in ponds by routine addition of grain
feed (18–22% protein) and molasses as carbon sources.
Fishmeal-based feeds and ammonia from shrimp
excretion were used as nitrogen sources. The study
supports the theory that natural biota can provide a
nitrogen source for shrimp, and that flocculated particles
are likely to be a significant proportion of this.
Hari et al. (2006) reported that carbohydrate addition
in combination with a decreased dietary protein level
improved the sustainability of shrimp farming in exten-
sive shrimp culture systems through 1) increased nitro-
gen retention in harvested shrimp biomass, 2) reduced
demand for feed protein, 3) reduced concentrations of
potentially toxic TAN and NO
2
–N in the system, and 4)
reduced water based nitrogen discharge to the
Fig. 7. Nitrogen cycle in bio-floc ponds. The nitrogen cycle is similar to that of in water ponds with periphyton. In contrast to theperiphyton system, this
system is also applicable to intensive systems. The added carbon source, together with the waste nitrogen, is converted into microbial bio-flocs, which in
turn can be eaten by the cultured organism. This technique provides aninexpensive protein source with a higher efficiency of nutrient conversion of feed.
10 R. Crab et al. / Aquaculture 270 (2007) 1–14
environment. If carbohydrate was added to the water
column to enhance heterotrophic bacterial protein
production, the protein level in the diet could be
reduced from 40% to 25%, without compromising
shrimp production.
4. Conclusions and future perspectives
Possible effluent treatment technologies in aquaculture
are diverse. The challenge to the designers of aquaculture
systems is to develop systems that maximize production
capacity per cost unit of capital invested. To do so, com-
ponents used in recirculating systems need to be designed
and developed to reduce the cost of the unit while main-
taining reliability. The bio-flocs technology, the periphyton
treatment technique, integrated treatment ponds, fluidized
sand biofilters, bead filters, trickling filters and the rotating
biological contactors can be considered as good effluent
treatment technologies. The bio-flocs technology provides
a sustainable method to maintain water quality in
aquaculture systems and moreover concurrently fish feed
is produced. Since the purchase of commercially prepared
feed in fish culture has a share of 50% or more in the
production costs, an effluent treatment technique that
maintains water quality and simultaneously produces in
situ fish feed has a large asset over other techniques.
Additional research in this field concerning management
of the floc production, the floc dynamics in intensive
aquaculture systems, the nutritional value of flocs and the
health effects of flocs is needed, more specifically the
effect on growth and survival of the cultured organisms.
Also microbiological aspects need further investigation,
particularly the microbiological characterization of the
flocs, possible manipulation of the microbial community
and presence of pre/probiotic organisms in the microbial
community of the flocs are challenging fields of interest.
Acknowledgements
This work was supported by the “Instituut voor de
aanmoediging van Innovatie door Wetenschap en
Technologie in Vlaanderen”(IWT grant no. 51256). A
special thank to the reviewers (ir. Han Vervaeren and ir.
Peter Deschryver) for critically reading the manuscript.
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