Journal of Water Resource and Protection, 2013, 5, 5-9
http://dx.doi.org/10.4236/jwarp.2013.57A002 Published Online July 2013 (http://www.scirp.org/journal/jwarp)
Recirculating Systems for Pollution Prevention in
Aquaculture Facilities
Juan Ramírez-Godínez1, R. Icela Beltrán-Hernández1, Claudia Coronel-Olivares1,
Elizabeth Contreras-López1, Maribel Quezada-Cruz2, Gabriela Vázquez-Rodríguez1*
1Autonomous University of Hidalgo State, Department of Chemistry, Mineral de la Reforma, Mexico
2Technological University of Tecamac, Tecamac, Mexico
Received April 22, 2013; revised May 24, 2013; accepted June 30, 2013
Copyright © 2013 Juan Ramírez-Godínez et al. This is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
ABSTRACT
As all other forms of livestock production, fish farming has numerous environmental impacts. Water pollution is one of
the most significant outcomes, since aquaculture effluents contain non-ingested food and fish dregs that affect the re-
ceiving water bodies when discharged without any treatment. Conventional pollutants (suspended solids, dissolved or-
ganic matter and nutrients), as well as pesticides, heavy metals and emerging pollutants (as antibiotics and hormones),
are commonly found in these effluents. Recirculating aquaculture systems (RAS, systems that integrate the treatment
and the reuse of water in the process) are an invaluable alternative for preventing water pollution by diminishing both
the volume and the eutrophication potential of the effluents. Based on our review of the extant literature in the field, we
conclude that activated carbon-based biofilters are a favorable technology to achieve a level of water quality that is
compatible with environmentally-sound aquaculture practices.
Keywords: Fresh Water Production; Biofilter; Nitrogen Removal; Biological Activated Carbon
1. Introduction
Fish is an exceptional source of good-quality proteins,
lipids and a wide variety of essential nutrients. Produc-
tion of farmed fish, or aquaculture, is probably the fastest
growing food sector worldwide, as now it accounts for
nearly 40% of the world fish production [1]. Aquaculture
production has expanded 12-fold in the last three decades
(1980-2010) [1], and the reliance on farmed fish will
certainly increase alongside with world population [2].
Until two decades ago, when extensive technologies
prevailed, aquaculture was considered an environmen-
tally-sound activity. Several traditional techniques even
functioned as efficient water treatment systems, thereby
contributing to the abatement of pollution [3]. But re-
cently, with the adoption of more intensive production
systems, the sustainability of aquaculture has been ques-
tioned. Environmental concerns arise from both the in-
creased use of resources (as land, water, feed and energy)
and the concomitant waterborne and airborne emissions
of the farms. The risks inherent to aquaculture [4,5] can
be summarized as in the following list.
 Habitat alteration or destruction
 Generation of organic-rich sediments
 Excessive freshwater consumption
 Modification of water temperature and flow rate pro-
files
 Water pollution
 Modification of the biotic index
 Transmission of infections from farmed organisms to
wild stock
 Emergence and spread of antibiotic resistance
 Genetic risk of escaped culture animals
 Introduction of exotic species
 Diminution of wild fish stock for farming carnivorous
species
 Multi-use conflicts for resources
However, for some authors, even larger-scope impacts
of aquaculture should be taken into account, such as
greenhouse gases originating from energy consumption
and their contribution to global warming, ocean acidifica-
tion and ozone layer depletion [6].
Here we examine the effects of aquaculture on the
quality of receiving water bodies, with emphasis on the
impacts of freshwater fish production in ponds. This re-
view examines the water quality requirements of the in-
*Corresponding author.
C
opyright © 2013 SciRes. JWARP

J. RAMÍREZ-GODÍNEZ ET AL.
6
dustry and summarizes the extant literature concerning
the pollutants expected to be presented in the effluents
from intensive aquaculture facilities. We present also the
recycling aquaculture systems (RAS) as an efficient al-
ternative for pollution prevention in these facilities.
2. Farmed Fish Production and Water
Quality
An aquatic farm (Figure 1) has water quality levels to
maintain, which are very dependent on the species culti-
vated. The main requirements concern dissolved oxygen,
pH, ammonia and nitrites [3]. In salmonid culture, dis-
solved oxygen levels are not allowed to be less than 5
mg/L for more than a few hours. Although carp and tila-
pia in farms can tolerate lower concentrations (ranging
from 3 to 4 mg/L), the optimum levels of dissolved oxy-
gen are higher, and so the desirable range is usually above
5 mg/L. For pH values, the desirable range for fish pro-
duction is 6.5 - 9.0 [3].
Toxicity of ammonia is generally attributed to the con-
centration of the unionized ammonia molecule (NH3), due
to its ability to move across cell membranes [7]. Median
lethal concentrations (LC50) over a 96-hour period of ex-
posure to unionized ammonia have been established for
rainbow trout (0.32 mg/L), bluegill (0.4 - 1.3 mg/L) and
channel catfish (1.5 - 3.1 mg/L) [8]. Since chronic expo-
sure to low concentrations of ammonia may reduce
growth and also increase the susceptibility to diseases,
some authors consider the maximum tolerable concentra-
tion to be 0.1 mg/L, although the preferred level is lower
(the EPA standard for rainbow trout is 0.02 mg/L) [8].
The content of unionized ammonia is determined by the
concentration of total ammonia nitrogen (TAN), pH and
temperature. In this way, at a TAN of 5 mg/L and pH of
9.0, typical fish would be dead in hours, while with pH
less than 6.0, ammonia would have negligible impacts at
the same TAN concentration [7]. Concerning nitrites, the
Figure 1. Farmed carp production.
suggested maximum level for prolonged exposure in hard
freshwater is 0.1 mg/L [3]. The main mechanism of nitrite
toxicity relies on the transformation of hemoglobin to
meta-hemoglobin, which lacks the capacity to bind oxy-
gen irreversibly [9].
3. Pollution Caused by Freshwater
Aquaculture Effluents
Modern aquaculture depends upon the supply of nutrient
inputs. However, for some species, a large fraction of the
food ration can remain uneaten (e.g., European eels and
tilapias spill around 1% - 10% and 10% - 30% of the ra-
tion, respectively [4]). Thus, on the one hand, the rates of
supply and assimilation of nutrient inputs are decisive
factors of the farm outputs, in particular for intensive op-
erations in open aquaculture systems [10]; on the other
hand, overfeeding should be avoided due to its large im-
pact on water quality.
Aquacultural wastes include all materials used in the
process which are not removed from the system during
harvesting [11]. These wastes are mainly associated to
uneaten feed or excreta, chemicals and therapeutants
added to the ponds, and can be discharged either in the
sediments or in the farm effluents. Sediments are usually
collected intermittently or at the end of the production
cycle and consist of inorganic and organic particulate
material. By contrast, effluents are commonly discharged
on a continuous basis over the production cycle and con-
tain both dissolved and particulate pollutants (inorganic
and organic) [10].
Although the characteristics of aquaculture effluents
are highly variable following the cultivated species, the
type of production facility and the feed quality and man-
agement, some general features can be drawn (Table 1).
In a general way, the quality of aquaculture effluents is
rather comparable to raw surface water than to domestic
or secondary effluents, with low contents of total sus-
pended solids (TSS), organic matter, and total and ammo-
nium nitrogen. However, these low levels of pollution are
not conducive to easy treatment, at least concerning solids.
In fact, it has been reported that the efficiency of sedi-
mentation increases with higher concentrations of solids
[11].
Table 1. Comparison between aquaculture effluents and
other types of water.
Parameter Domestic
effluent
Domestic
secondary
effluent
Aquaculture
effluent
Raw
surface
water*
TSS [mg/L]400 - 500 30 5 - 50 50 - 400
TKN [mg/L]300 - 400 20 3 - 20 7
N-NH4+ mg/L]40 - 75 5 0.5 - 4.0 0.05 - 0.50
BOD5 [mg/L]300 20 0.2 - 0.5 2 - 4
*That requires treatment. Sources: [3,13].

J. RAMÍREZ-GODÍNEZ ET AL. 7
3.1. Conventional Pollutants
Conventional pollutants (TSS, BOD5 and nutrients) are
mainly derived from feed, excreta and fertilizers. The
main purpose of the addition of fertilizers is the stimu-
lation of both phytoplankton growth and fish produc-
tion. Inorganic compounds of N and P are among the
most usual fertilizers, but K, trace metals, and silicates
may also be presented [12]. Since fertilizers increase
the concentrations of nutrients in pond water, they may
cause eutrophication in receiving water bodies.
Even though the concentrations of conventional pol-
lutants are usually low, pond cleaning can increase them
considerably. In a study examining the quality of effluents
from a hatchery, TSS, BOD5 and total phosphorus in-
creased during cleaning from 1 to 88 mg/L, 3 to 32 mg/L
and from 0.22 to 4.00 mg P/L, respectively [14].
Due to their content of nutrients, aquaculture effluents
are well-suited for biological treatments (e.g., wetlands,
biofilters or algae-based systems) and agricultural reuse
(as in hydroponics and crop production). In the first case,
it must be noticed that aquaculture discharges have nitro-
gen levels disproportionately high regarding carbon con-
tents [4]. As balanced microbial growth requires a C:N
ratio of about 100:10, biological treatment of aquaculture
effluents is likely to involve the addition of exogenous
carbon substrates.
3.2. Pesticides, Heavy Metals and Emerging
Pollutants
Intensive aquaculture often relies on chemical additives
for health management, manipulation of reproduction or
growth promotion, among other purposes. Some pesti-
cides commonly used are rotenone, simazine, 2,4-D, di-
quat and diuron [3,10], essentially for weed control. Or-
ganophosphate compounds (as malathion and dichlorvos),
carbamates and pyrethroids are also employed as para-
siticides. A concern arises from their non-selective action
and their long-term effects on pond productivity [3].
However, the concentration of pesticides in aquaculture
effluents is scarcely reported, and there is a lack of in-
formation about their effects on non-target organisms.
Heavy metals can also be found in pond effluents be-
cause they are common constituents of proteinates and
vitamin/mineral premixes (e.g., Cu and Zn [15]). But
mainly, they can be added to ponds as oxidizing agents
for controlling phytoplankton and pathogenic organisms
(e.g., KMnO4 [12]) or as algicides (e.g., CuSO4 [12]).
Although these metals tend to precipitate as bottom sedi-
ments, the applied doses should be surveyed to avoid any
toxic effect on fish. For instance, CuSO4 is frequently
used for eradicating submerged weeds, but the safe Cu
levels have not been fully established for chronic expo-
sure [15]. In fact, sublethal effects of Cu such as reduced
swimming speed, reduced feeding and growth inhibition
have been widely reported in salmonids [15].
Nowadays, one of the main environmental concerns
about aquaculture is the release of bactericides (glutaral-
dehyde, formalin), therapeutants (as malachite green and
dipterex) and antibiotics (mainly tetracyclines, quinolones
and β-lactams) to the aquatic media. Some of these com-
pounds are added in appreciable amounts; for instance,
glutaraldehyde and formalin are regularly added at con-
centrations of 1 - 10 mg/L to avoid the proliferation of
pathogens [12]. In a survey of fish farms in England,
contents as high as 15.20 and 0.61 mg/L of formalin and
malachite green, respectively, were found [3]. It is worth
noting that malachite green is environmentally persistent,
mutagenic in rats and mice, cytotoxic to mammalian cells
and carcinogenic to experimental animals [16]. Even
though malachite green has been banned in several coun-
tries, it is still used in others due to its efficiency and low
cost [16]. Antibiotics are found in the water of intensive
farms rather than in extensive ones [17], most likely be-
cause in an intensive hatchery fish are subject to more
stressors that decrease the ability of their immune system
to deal with infections [18]. The concentrations measured
for antibiotics are usually low (e.g., from 0.17 to 10 μg/L
for oxytetracycline [19]), although they rise noticeably
through prophylactic treatments. Ormetoprim content has
been measured at 0.69 μg/L, but it can be found at levels
as high as 12 μg/L during fish treatment [20]. The main
consequence of the reliance of aquaculture on antibiotics
is probably the augmented antibiotic resistance in fish
pathogens, which raises the possibility for passage of their
antibiotic resistance determinants to bacteria of land ani-
mals and human beings via the food chain.
Intensive fish farming is also a source of steroid hor-
mones such as estrone, testosterone and androstenedione
[21]. In fact, estrone has been pointed out as the most
important natural endocrine disrupting compound found
in natural water due to its ubiquity and estrogenic potency
(higher than that of nonylphenol) [22]. Steroids are pre-
sented in the blood plasma of fish and can be excreted via
urine or bile, mainly during periods of reproduction [21].
The contents detected (of about 1 ng/L) of these emerging
pollutants in aquaculture effluents are similar to those
found in domestic secondary effluents and high enough to
lead to adverse reproductive effects on aquatic species as
trouts [21,22]. However, the removal and the effects of
hormones in the usual treatment systems of aquaculture
effluents have not been studied thoroughly yet.
4. Recirculating Aquaculture Systems (RAS)
The reduction of the wastewater volume is essential for
enhancing the sustainability of fish farming, and recircu-
lating aquaculture systems (RAS) have been proposed
with this purpose. In these systems, water is partially

J. RAMÍREZ-GODÍNEZ ET AL.
8
reused in the process after undergoing a proper treatment,
thereby reducing water usage and improving effluent
quality. By means of the life cycle analysis methodology,
RAS have been compared against a conventional flow-
through system [23]; it has been found that RAS reduce
water dependence by 93% in comparison to conventional
systems. Moreover, RAS eutrophication potential re-
sulted to be 26% - 38% lower than that of traditional
systems.
RAS technology relies considerably on biological fil-
tration as the mechanism for removing critical pollutants
[24]. In a study following the oxytetracycline content in
water of a sand biofilter-based RAS, peak concentrations
of 0.39 - 0.72 ng/L were detected in the water both enter-
ing and leaving the biofilter only during the 10-day treat-
ment of fish [25]. All through the therapeutic period, the
amount of oxytetracycline discharged by RAS was con-
siderably lower than that discharged by a conventional
flow-through system [25].
In addition, through nitrification, biofilters are able to
make recycled water suitable for fish production by oxi-
dizing TAN to nitrates. In first generation-RAS, the
maximum allowed concentration of nitrates steers the
external water exchange rate [23]. But recent technologi-
cal developments include a denitrification reactor for full
nitrogen removal. As a result, last generation-RAS reduce
water consumption, as well as the concentrations of ni-
trates and BOD5 in the final discharge [23]. Although
RAS are intended to reduce the water volume used, a
minimum water exchange ratio must be maintained. By
lowering the make-up water volume, an accumulation of
growth inhibiting factors (e.g. fish-produced cortisol,
bacterial metabolites and metals) is likely to occur. In a
low water exchange RAS, the accumulation of phosphate,
As and Cu led to higher mortality and reduced larvae
length and body weight in the culture of carp [26].
Activated carbon-based biofilters are well-suited for
RAS, because they offer the possibility of removing pol-
lutants either by adsorption or by biological mechanisms
such as biodegradation, nitrification or denitrification. It
has been demonstrated that biological activated carbon
filters (i.e., fixed beds of granular activated carbon sup-
porting bacterial growth) can effectively remove (>90%)
emerging pollutants such as pesticides, steroids, antibiot-
ics and other persistent chemicals from water [27] and
wastewater [28]. In this way, the accumulation of growth
inhibiting factors in RAS could be avoided.
5. Conclusion
Aquaculture effluents contain low concentrations of
conventional pollutants (TSS, organic matter and nutri-
ents), pesticides, heavy metals and emerging pollutants.
Biological treatments are environmentally-friendly alter-
natives for removing these pollutants in RAS and hence
for preventing the pollution originated by this industry.
To this end, activated carbon-based biofilters seem ap-
propriate for minimizing water exchange ratios in RAS
without compromising the quality of fish production by
the accumulation of growth inhibitors. However, the
typical unbalance between the contents of organic matter
and nitrogen could require the addition of easily assimi-
lable carbonaceous sources for achieving full nitrogen
removal.
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