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Airlift fluidized bed filter

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Aquacultural Engineering 47 (2012) 16–26
Contents lists available at SciVerse ScienceDirect
Aquacultural Engineering
journal homepage: www.elsevier.com/locate/aqua-online
Hydrodynamic characterization and performance evaluation of an aerobic three
phase airlift fluidized bed reactor in a recirculation aquaculture system for Nile
Tilapia production
Iván Andrés Sánchez O. a,∗ , Tsunao Matsumoto b,1
a
b
Departamento de Recursos Hidrobiológicos, Universidad de Nariño, Ciudad Universitaria Torobajo, Carrera 22, No. 18-109 Pasto, Nariño, Colombia
Faculdade de Engenharia de Ilha Solteira, FEIS/UNESP, Departamento de Engenharia Civil, Alameda Bahia 550, Ilha Solteira, SP CEP 15385-000, Brazil
a r t i c l e
i n f o
Article history:
Received 9 August 2011
Accepted 6 December 2011
Keywords:
Three phase fluidized bed
Biofilters
Biological treatment
Hydrodynamic characterization
Recirculation
Aerobic reactor
a b s t r a c t
The hydrodynamic characterization and the performance evaluation of an aerobic three phase fluidized
bed reactor in wastewater fish culture treatment are presented in this report. The objective of this study
was to evaluate the organic matter, nitrogen and phosphorous removal efficiency in a physical and biological wastewater treatment system of an intensive Nile Tilapia laboratory production with recirculation.
The treatment system comprised of a conventional sedimentation basin operated at a hydraulic detention
time HDT of 2.94 h and an aerobic three phase airlift fluidized bed reactor AAFBR operated at an 11.9 min
HDT. Granular activated carbon was used as support media with density of 1.64 g/cm3 and effective size
of 0.34 mm in an 80 g/L constant concentration. Mean removal efficiencies of BOD, COD, phosphorous,
total ammonia nitrogen and total nitrogen were 47%, 77%, 38%, 27% and 24%, respectively. The evaluated
system proved an effective alternative for water reuse in the recirculation system capable of maintaining water quality characteristics within the recommended values for fish farming and met the Brazilian
standards for final effluent discharges with exception of phosphorous values.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Aquaculture has been growing more than other sectors of animal food production, with an average value of 8.8% per year since
1970. According to data provided by FAO (2006), the species with
the highest growing production tendency, between 1991 and 2000,
in Latin America and the Caribbean was Tilapia.
Wheaton (1993) states that the three basic kinds of aquaculture systems are open, semi-closed, and closed systems. In closed
systems water is reconditioned and recirculated to culture units. A
recirculating aquaculture system (RAS) can be defined as an aquaculture closed system that incorporates the treatment and reuses
water with less than 10% of the total water volume replaced per
day (Hutchinson et al., 2004); it represents a compact alternative
for intensive farming of different species, maximizes production
requiring a smaller proportion of water and land to generate the
same quantity of fishes than the required by other kinds of systems
(Timmons et al., 2002).
∗ Corresponding author. Tel.: +57 2 7311449x239; fax: +57 2 7314482.
E-mail addresses: ivansaor@hotmail.com, iaso@udenar.edu.co (I.A. Sánchez O.),
tsunao@dec.feis.unesp.br (T. Matsumoto).
1
Tel.: +55 018 3743 1125; fax: +55 018 3743 1125.
0144-8609/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.aquaeng.2011.12.006
Water quality is important in fishculture in order to optimize water conditions for the cultured species and to minimize
environmental effects on the receiving water bodies produced by
wastewaters. On RAS for tilapia culture, recommended values of
some water quality parameters are: temperature: 24–29 ◦ C; oxygen: 4–6 mg/L; total suspended solids: <15 mg/L; total ammonia
nitrogen (TAN): <3 mg/L; unionized ammonia NH3 –N: <0.6 mg/L
(Timmons et al., 2002).
The Nile tilapia (Oreochromis niloticus) was one of the first fish
species cultured. Illustrations from Egyptian tombs suggest that
Nile tilapia were cultured more than 3000 years ago (Popma and
Masser, 1999). Tilapia is one of the most widely farmed freshwater
fish in the world; on RAS with aeration it is common to produce
15–40 kg/m3 of fish (Kubitza, 2000).
According to Naylor et al. (2000), to produce 1 kg of fish meat,
2 kg of dry food (adopting a food conversion ratio of 2 for Tilapia)
are necessary; however, the values of feed conversion ratio FCR for
fishes depend upon the species and variables as the diet formulation and its composition. For example, Mbahinzireki et al. (2001)
evaluated the growth, feed utilization and body composition of
Tilapia fed with cottonseed meal-based diets in a RAS and reported
FCR values from 1.46 to 2.28; Azaza et al. (2009) evaluated the use
of waste date fruit as partial substitute for soybean meal in practical diets of juvenile tilapia and registered FCR values from 1.62 to
1.80.
I.A. Sánchez O., T. Matsumoto / Aquacultural Engineering 47 (2012) 16–26
Nomenclature
Cc
D10
D60
HDT
KL a
KL a20
UC
S
carrier concentration on liquid (g/L)
effective size, size of an opening which will pass only
the smallest 10% of the granular media (mm)
sieve size which will pass 60% of the granular media
(mm)
hydraulic detention time (h, min)
volumetric oxygen mass transfer coefficient (1/s)
volumetric oxygen mass transfer coefficient standardized to 20 ◦ C (1/s)
uniformity coefficient
density of a particle of media (g/cm3 )
List of abbreviations
AAFBR aerobic three phase airlift fluidized bed reactor
Biochemical oxygen demand (mg/L)
BOD
COD
Chemical oxygen demand (mg/L)
recirculating aquaculture systems
RAS
TAN
total ammonia nitrogen (mg/L)
dissolved oxygen (mg/L)
DO
UNESP Universidade Estadual Paulista
GAC
granular activate carbon
total solids (mg/L)
TS
TVS
total volatile solids (mg/L)
TFS
total fixed solids (mg/L)
TSS
total suspended solids (mg/L)
FSS
fixed suspended solids (mg/L)
volatile suspended solids (mg/L)
VSS
TN
total nitrogen (mg/L)
total phosphorous (mg/L)
TP
ORP
oxidation–reduction potential (mV)
concentrations measured at influent of sedimentaI.Sed
tion tank
E.Sed
concentrations measured at effluent of sedimentation tank
E.EBR
concentrations measured at AAFBR effluent
platinum cobalt units
PCU
CFB
cumulative feed burden, CFB = F/VFW (g feed/L of
freshwater/day)
daily mass of feed delivered (g)
F
VFW
equivalent volume of freshwater replaced per day
(m3 )
MBBR
moving bed bioreactor
fluidized bed reactors
FBR
From the food provided in a fish culture system about 36% is
excreted in the form of BOD, 75% of nitrogen as ammonia (Brune
et al., 2003) and almost 84.02% of P is released to the representative
culture system as metabolite wastes as reported by Rafieea and
Saad (2005).
High levels of nutrients and organic matter cause environmental degradation of receiving water bodies, the level of the impact
depends on the quantity or concentration evacuated and the assimilation capacity of the aquatic environment. The environmental
legislation is one of the options to establish sustainable limits to
the pollutant activities of the aquatic resources. In Brazil, the conditions and limits of effluent disposal are defined by the 357 CONAMA
Resolution of 2005, and in the São Paulo by the state Decree number
8468 of 1976.
Besides the environmental restrictions, other reasons as biosecurity in the collecting of natural waters; scarcity and high
costs of the liquid; quality control and transparency demands,
hard to achieve in semi intensive systems; and thermal control
17
(Avnimelech, 2006) justify the aquaculture effluents treatment,
especially for intensive aquaculture recirculating systems. The RAS
with biofilters treat the polluted waters internally with dissolved
organic matter and ammonia and can reduce the water used
quantity and throwing-away by aquaculture (Gutierrez-Wing and
Malone, 2006).
According to Crab et al. (2007), there is a wide variety of systems
to execute the biological treatment of the aquaculture waters that
could be grouped in two general types: the emerged fixed biofilm
(rotating biological contactors and down flow filters) and the fixed
submerged biofilm (as the fluidized sand biofilters and floating
bead filters). The RAS submerged fixed film biofilters that display
similar characteristics are clustered in three basic blocks: packed;
expandable; and expanded bed biofilters as downflow microbead,
moving bed bioreactor and fluidized sand biofilters (Malone and
Pfeiffer, 2006).
A microbead filter is a combination of trickling and granular type
of biological filters. These filters are operated in a downflow configuration where influent water is distributed over the top of the
media bed and the water then trickles down through the media and
flows by gravity out of the reactor vessel (Timmons and Ebeling,
2010). Experiences with these biofilters were reported by Greiner
and Timmons (1998) and Timmons et al. (2006).
The moving bed bioreactor (MBBR) is an attached growth
biological treatment process based on a continuously operating,
non-clogging biofilm reactor with low head loss, a high specific
biofilm surface area, and no requirement for backwashing. The bacterial biomass grows on the media carriers and moves freely in
the water volume of the reactor (Timmons and Ebeling, 2010). The
idea behind the development of the Kaldnes MBBR process was to
adopt the best features of the activated sludge process as well as
those of the biofilter processes, without including the worst (Rusten
et al., 2006). In recent literature, Pfeiffer and Wills (2011) described
the evaluation of different types of floating plastic media for total
ammonia removal in a RAS.
Fluidized bed reactors (FBR) are a kind of expanded bed biofilters in which the media is contained in a vertical vessel with a
cylindrical or square cross-section (Lawson, 1995). The fluidized
sand biofilters are FBR that use sand as a carrier and keep it in suspension or fluidized by the upward flow of water. Some of the main
advantages of the FBR described by Timmons and Ebeling (2010)
include the facts that fluidized bed biofilters are very economical
to build from commercially available materials, raw filter media
has very high specific surface area at low cost, and that they can
be field built using a variety of proven methods. Some of the main
disadvantages indicate that fluidized bed biofilters can have problems with media carryover on system start-up or with restarting if
not designed to account for bed re-fluidization and manifold/lateral
flushing distribution; media density changes over time with biofilm
accumulation in fine sand filters which necessitates a bed growth
management strategy.
Nam et al. (2000) hold that in fluidized bed reactors the
supporting media size, flow velocity and reactor hydrodynamic
properties influence the biofilm development characteristics and
state that the lesser the size of the grains the better the biofilm
structure.
In fish culture it is fundamental to maintain high dissolved oxygen (DO) levels to guarantee an optimal species growing. Fall of
the DO produced by the animal respiration, by the organic matter degradation process and the transformation or oxidation of
total ammonia nitrogen (TAN) is present in the RAS; Nicolella et al.
(2000) asserted that to overcome the high recirculation rate problems when oxygen demand is high, this could be directly aerated
becoming a three phase fluidized bed reactor by air injection.
A brief description of the three phase fluidized bed reactors for
use in aquaculture was presented by Lekang (2007).
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I.A. Sánchez O., T. Matsumoto / Aquacultural Engineering 47 (2012) 16–26
In aerobic biofilters, such as the fluidized bed reactors, oxygen is
consumed as penetrated in the biofilm, until values that determine
anoxic or anaerobic conditions are reached. Thus two biofilm layers
– one aerobic that could potentially develop nitrification, and other
anaerobic or anoxic that could develop denitrification processes
– could be developed depending on the OD concentration in the
external fluid and the biofilm thickness.
In fluidized bed reactors with circulation produced by airlift,
the three phases (liquid, solid and gas) circulation can be external or internal and is produced by hydrostatic pressure difference
between two sections: riser and downcomer (Merchuk and Berzin,
1995).
Owing to the fact that in airlift fluidized bed reactors part of
the oxygen dissolved is necessary to the organic matter oxidation,
the oxygen amount transferred during air injection is an important
parameter that can be quantified by the measurement of oxygen transfer, whose methodology is defined by ASCE (1990) that
reported the volumetric oxygen mass transfer coefficient (KL a).
In the airlift reactors, the liquid velocity circulation is an important parameter, because it determines the liquid mixing, bubble
recirculation and solids suspension. Van Benthum et al. (1999)
described three bubble circulation regimes: in regime I, at very low
input rates, no air bubbles are present in the downcomer, since
the liquid velocity in the downcomer is lower than the average slip
velocity of the air bubbles in the liquid; in regime II, the liquid velocity in the downcomer is equal to the slip velocity of the air bubbles,
the downcomer can either be partly or fully filled with bubbles;
finally, in regime III, the liquid velocity in the downcomer becomes
higher than the slip velocity, the air bubbles recirculate with the
liquid from the downcomer into the riser.
When airlift reactors work at low velocities, the carrier could
settle or undergo low shearing tension, increasing the biofilm thickness, causing anaerobic or anoxic layer conditions; when working
at high velocities, the biofilm thickness could be reduced. In both
cases the organic matter removal and nitrification processes are
affected (Martins, 2005).
The main objective of this research was to evaluate the
organic matter, nitrogen and phosphorous efficiency removal in
the wastewater treatment of a RAS by intensive Nile Tilapia production system. The treatment system was made up by a conventional
sedimentation tank and an aerobic three phase airlift fluidized bed
reactor with circulation.
2. Materials and methods
The experiment was done in the Hydrology—Hydrometry and
Sanitation Laboratories of the Engineering Faculty of the Universidade Estadual Paulista (UNESP), Ilha Solteira campus.
The hydrodynamic research was done in a fluidized by airlift bed
reactor with circulation in concentric tubes. The external diameter
was of 0.25 m and the internal diameter was 0.10 m.
The RAS evaluated during 56 days consists in three intensive
Tilapia farming tanks with a biomass density higher than 30 kg/m3
whose effluents were treated by a conventional settling tank; an
aerobic three phase aerobic fluidized by airlift bed reactor with circulation in concentric tubes, the reactor got a sedimentation unit
to retain particles of the granular activated carbon used as biofilter
media, and an outlet at the upper side of this unit; a CO2 removal
and DO transfer reactor. The final effluent of the treatment system
was recirculated by a centrifugal pump to the culture tanks. The
RAS configuration, volumes and hydraulic retention times of the
system elements is presented in Fig. 1.
2.1. Materials
The experimental RAS system evaluated in the research project
was composed of three plastic tanks containing 0.20 m3 of freshwater for Tilapia fish farming, with a bottom central drainage,
interconnected by a semi-circular gutter made in a 100 mm diameter PVC tube that collected the tanks effluents and helped to
maintain the water level on the tanks constant; a sedimentation
Fig. 1. Profile view of the intensive Tilapia production recirculating system evaluated.
I.A. Sánchez O., T. Matsumoto / Aquacultural Engineering 47 (2012) 16–26
tank constructed in sheets of steel 2.0 m long by 1.0 m wide in
the surface and 1.4 m3 of effective volume; an aerobic three phase
airlift fluidized bed reactor in PVC concentric tubes with external
diameter of 0.25 m and 2.60 m height; 0.10 m of internal diameter
and 2.17 m of internal height. The reactor got a sedimentation unit
and an outlet at the upper side of this unit, as showed in Fig. 1;
a reactor to OD transfer and CO2 removal to the AAFBR effluent,
made of a 0.40 m diameter and 1.60 m high PVC tube. Inside the
reactor, in order to increase the residence time of the injected bubbles at the bottom of the tube, a 0.30 m layer filled with 2 cm PVC
conduit flexible pieces of 3/8 in. diameter was arranged; a 7.5 HP
compressor with a pressure regulation valve; two flow meters,
one to control for the air injected in the central bottom part of
the AAFBR (600 L/h) by an injector made in a PVC 40 mm diameter tube that had six lines of 36 holes of 1.0 mm diameter; and
in the OD transfer and CO2 removal reactor (2000 L/h) by a micro
bubbles injection membrane located at the base of that reactor; a
0.25 m3 suction tank to pump the treated effluent; two centrifugal
pumps of 1/2 HP and 3/4 HP used to pump the treatment system effluent to a 0.050 m3 constant level distribution box which
divided the treated flow to the three Tilapia farming tanks; A 25 mm
diameter PVC tabulation system used to water distribution to the
fish farming tanks and the transportation of the effluents between
treatment units; a 1/2 HP blower with 25 mm diameter outflow
pipe used to inject air in the fish farming tanks by diffusion stones,
and dechlorination tanks of 1.0 and 0.5 m3 ; 66 Oreochromis niloticus
units divided in the three fish farming tanks maintaining approximately 6 kg of biomass in each culture unit (farming density of
30 kg/m3 ).
For the hydrodynamic experiments four probes to electric conductivity measurement, two located at internal and two located at
external tube; and piezometers placed at the internal and external
tubes were also used.
2.2. Methods
In the hydrodynamic experiments the different amounts of
injected air were regulated by two flow meters. The liquid velocity was measured in the reactor to 800; 1200; 1600; 2000;
3000; 4000; 5000; and 6000 L/h of air at a constant pressure of
2.5 MPa. The experiments in each reactor were done four times,
and NaCl at 15 g/L concentration was used as tracer. The probes
detected the tracer presence and transferred the conditioned signals, and then acquisition data software read the signals and
traced curves in which peaks showed the highest detected tracer
concentration moment. With the minimum tension peaks and
the distance between probes the liquid circulation velocity was
calculated.
The expansion and gas holdup of the two-phase (liquid and
air) condition values were registered at the reactor for different
air flow amounts that varied between 300 and 7000 L/h. For the
three-phase condition the gas holdup was measured at three different carrier concentrations 30; 70 and 100 g/L at an air flow of
2500 L/h.
The expansion was expressed by the volume difference between
aerated and without aeration condition, determined by piezometers. The three-phase gas holdup depended of the mentioned
volume differences, the carrier concentration, and the liquid and
carrier densities.
The KL a determination was done for two-phase and three-phase
conditions (carrier concentration of 100 g/L), at 1000, 1200, 1500,
1800 and 2100 L/h of air flow. The experimental methodology was
the one described by ASCE (1990).
During the RAS evaluation period, 600 L/h of air were used in
order to guarantee the right quantity to transfer the DO and suspension and recirculation of granular activate carbon (GAC). The
19
physic characterization of GAC and two additional possible carriers (anthracite carbon and filter sand) were done based on the
ABNT (1984a,b) methods for granulometric analysis and the specific mass of solids, by the NBR 7181 and NBR 6508 methods,
respectively.
At the inclusion of the GAC on the AAFBR an air flow of 2000 L/h
was used to maintain the carrier in suspension. The carbon was
added progressively at the top of the reactor by addition of mass
quantities equivalent to 10 g/L concentration of GAC until the concentration of evaluation of the system 80 g/L was reached, after that,
the air flow was reduced to 600 L/h.
In the first week of inoculation of the AAFBR and filling up of
the RAS system identical amounts of non-chlorine water and water
from the Ipê Lake, localized in the municipality of Ilha Solteira
SP were used. After GAC introduction into the reactor, during 1
week and a half three partial replacements of water to promote
the removal of a black color produced by the hydration of the carrier and the migration of a little amount of GAC grains that passed
through the bomb were performed. Finally, each day during 1 1/2
weeks, 200 L of siphoned water from ornamental fish farming tanks
were added to the RAS.
The RAS monitoring was done daily during 8 weeks. The samples
collection and the in situ parameters measuring in situ were done
at P1, P2, P3, P4 and P5 points arranged on the RAS as showed in
Fig. 1.
The measured parameters carried out twice a week were: total
solids (TS), total volatile solids (TVS), total fixed solids (TFS), total
suspended solids (TSS), fixed suspended solids (FSS), volatile suspended solids (VSS), by gravimetric method; turbidity with 2100NA
Hach Turbidimeter; apparent color, COD, total nitrogen (TN) and
total phosphorous (TP), nitrite and nitrate – only in the first four
weeks of the experiment – in a Hach DR2500 spectrophotometer. The daily controlled parameters by an YSI Inc. 6920 probe
were: dissolved oxygen (DO), pH, conductivity, temperature, oxidation reduction potential (ORP), ammonia NH3 , ammonium NH4 + ;
water flow at the tanks was measured by volumetric method; there
were also controlled and regulated water levels and air pressure.
The carrier concentration was controlled by volumetric method
weekly.
Physical chemical parameter measurements followed the recommendations of APHA, AWWA and WEF (1998), in Standard
Methods for Examination of Water and Wastewater and were done
at Sanitation and Hydrology and Hydrometrics Laboratories of the
Engineering Faculty on the UNESP Ilha Solteira’s campus.
Once a week the sedimentation tank was totally drained and
cleaned to remove the accumulated floating and settled solids; this
activity represented a weekly replacement of 55% of total water
volume of the RAS, equivalent to 7.8% of total volume per day; that
was between the values recommended by authors as Hutchinson
et al. (2004) and Timmons et al. (2002).
At the farming tanks between 2% and 3% of the live weight in
fish food with 30% of protein contents, divided in three meals were
provided daily, as recommended by Kubitza (2000).
In the fish tanks the oxygen concentration was monitored three
times a day, in case of a reduction of the DO concentration a
higher amount of air was provided by opening the respective
globe valve. The pH variations did not justify some actions in
order to control it because most of the time it was very near to
neutrality.
A treatment to reduce and eliminate the fungus presence on the
fish epithelium was done. The treatment consisted in a 50% tank
water volume reduction; the granulated and diluted marine salt
added to the farming units; the manual agitation to unify the saline
solution concentration; the action of the unified solution during
10 min; the siphoning on the bottom of the tanks, and refilled with
the previously removed water.
20
I.A. Sánchez O., T. Matsumoto / Aquacultural Engineering 47 (2012) 16–26
Fig. 2. Liquid velocity in the riser and downcomer zones of the reactor.
3. Results and discussions
3.2. Liquid velocity
3.1. Carrier physical characterization
Fig. 2 shows the average measured liquid velocities in the riser
and in the downcomer reactor zones.
The measured velocities at the riser zone varied between three
and eight times the velocities registered at the downcomer section,
its ratio increases with the air flow increasing.
The densities of the possible supporting media evaluated were:
S = 2.57, 1.74, and 1.64 g/cm3 , and the uniformity coefficients
(UC = D60 /D10 ) were 2.13, 2.17, and 1.68 to sand, anthracite carbon,
and GAC, respectively.
The GAC was selected as the carrier because it had predominance of certain material sizes that suggest their uniformity
(represented by the smallest value of UC); besides GAC was the
carrier with the smallest specific mass which made it of easier suspension requiring less air flow than the other materials. Those data
and the high specific surface of the GAC (Lawson, 1995) made of
this material with natural humidity of 13.9% and effective size D10 of
0.34 mm the best option for its use in the AAFBR and the subsequent
biofilm development conditions.
3.3. Gas holdup at two-phase and three-phase reactors
Fig. 3 shows the average measured gas holdup values of the
two-phase condition reporting high gas holdup difference values
between the riser and the downcomer zones. The values suggested
that the reactor presented the bubble recirculation regimes II and
III that produces the liquid circulation. The experiments registered
higher gas holdup values in the riser than the observed in the
Fig. 3. Gas holdup values in two-phase condition at the riser and downcomer zones.
I.A. Sánchez O., T. Matsumoto / Aquacultural Engineering 47 (2012) 16–26
21
Fig. 4. Gas holdup on the riser and downcomer at different carrier concentrations.
downcomer; this means that some portion of the bubbles injected
escaped to the atmosphere and the rest recirculated to the downcomer.
The gas holdup values in three-phase configuration of 2500 L/h
air flow and different carrier concentrations (Cc ) reported that at
higher concentrations the gas holdup inside the reactor is reduced.
According to Fig. 4, the carrier concentration increment causes
a reduction of the gas holdup values and could affect the gas transfer efficiency in the three-phase internal loop airlift reactors as
affirmed by Freitas and Teixeira (2001).
3.4. Oxygen transfer
The KL a measured was standardized at a 20 ◦ C temperature. The
KL a20 presented in Fig. 5, adapted from Araújo (2008), shows that
higher air injected flows increase the oxygen mass transfer. The
air mass transfer occurs mainly at the riser zone, and due to the
increases of liquid velocities at high air flows the liquid–gas contact
time will be reduced producing the stabilization of the KL a20 values.
In three-phase experiments, it was difficult to maintain the carrier (sand) in suspension, mainly to low air flow values.
It was noted that oxygen transfer in the three-phase condition was lower than the achieved at the two-phase experiments
as reported by Freitas and Teixeira (2001).
3.5. Characterization of the waters used in the inoculation of the
AAFBR
The total solids mean values measured in the ornamental fish
tank and the Ipê Lake waters were similar, however, the ornamental fish tank water presented higher dissolved solids concentration
in the organic matter form, produced by the food that was not consumed, feces and fish secretions. Lake Ipê water presented a higher
concentration of suspended solids and total volatile solids probably produced by the presence of some microalgae which is very
common in this kind of natural waters (Table 1).
The water of Lake Ipê had a higher DO concentration than the
ornamental fish tank because DO was consumed by the fish in the
tank. pH in the tank was lower because of the biological activity
of the fishes and their secretions. In Table 2, there are the values
of BOD, COD, total phosphorus, nitrite, TAN and TN, which were
higher in the tank because the biological waste produced by the
fishes metabolism and because of the food not consumed by the
animals.
3.6. Monitored parameters
Fig. 5. Comparison of the average values of KL a20 obtained in two-phase and threephase conditions.
At the beginning of the monitoring period (first 9 days), there
were important oscillations of the values of some parameters
due to the adaptation process of the AAFBR to carbonaceous
and nitrogenous organic matter removal. Parameters as the
oxidation–reduction potential (ORP) and pH depended on the
weekly exchanged water, and conductivity depended mainly on
the fish treatments practiced.
There was variability on some parameters in the culture units
mainly due to the non-uniform removal of the produced solids
at tanks 2 and 3. The small water outflow in the tanks did not
collect and transport feces and non-consumed food appropriately
because of the low velocity of the water; it causes the aggregation and accumulation of the solids in the tubes producing partial
blocking, reduction of the out flow, and rising of water level in the
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I.A. Sánchez O., T. Matsumoto / Aquacultural Engineering 47 (2012) 16–26
Table 1
Values of the solids measured in the AAFBR inoculation waters.
Water
Origin
TS
(mg/L)
FTS
(mg/L)
VTS
(mg/L)
SS
(mg/L)
FSS
(mg/L)
VSS
(mg/L)
DS
(mg/L)
DS
(%)
Lake Ipê
Tank
103
109
81
93
22
16
21
8
14
7
7
1
82
101
79.6
92.7
Table 2
Measured parameters values in the AAFBR inoculation waters.
Water
Origin
DO
(mg/L)
BOD
(mg/L)
COD
(mg/L)
TP
(mg/L)
Nitrite
(mg/L)
TAN
(mg/L)
TN
(mg/L)
pH
Turbidity
NTU
Lake Ipê
Tank
6.04
4.61
1.38
1.46
3.5
8
0.075
0.650
0.029
0.033
0.05
0.12
1.3
1.8
8.1
7.3
2.52
3.23
tanks. Such situation causes suddenly detachment of the material,
increase of the flow, and dragging of the solids to the sedimentation
tank changing the HRT in that unit and in the AAFBR temporarily.
Another reason of the variability of the measured parameters
could be the presence of a fine dust produced by the GAC hydration;
their friction inside the AAFBR and the passing through the pumps
rotors. This last condition was observed due to the loss of little
amounts of carrier because the effluent flow velocity exceeds the
sedimentation velocity of some particles at the decantation AAFBR
zone.
3.6.1. TAN removal monitoring
Table 3 illustrates the average and maximum values of ionized
ammonium (NH4 + ) and non ionized ammonia (NH3 ) forms and
their addition (TAN) registered during the project, including and
excluding the mean concentrations measured during the reactor
adaptation period. The division was made because the strong influence of the extreme values presented on the first day of operation
of the system, while the biofilm developed on the carrier surface
achieved the adaptation to the kind of wastewater of the RAS and
maintained an apparent equilibrium between ammonium excreted
by the fishes or produced by the bacterial decomposition of the
non-consumed food (Durborow et al., 1997) and the amount of
ammonium removed by the AAFBR. The high concentrations registered in the first 2 weeks of the experiment produced the 19 fishes
death, representing 90% of total mortality registered during the
project; the dead animals were replaced by new ones in order to
keep the biomass in the tanks.
Concentration variability of the different ammonium forms in
the culture tanks depended on biological wastes produced by the
fishes and food consumption. It tried to provide a uniform food
quantity (3% of tank biomass, 1% three times a day) from the beginning of the research; nevertheless, the food consumption was not
constant probably as a function of some water quality parameters
as temperature and DO concentration. Fig. 6 shows the TAN concentrations at influent and effluent of sedimentation tank (I.Sed and
E.Sed) and in the AAFBR effluent (E.FBR), the peak values were the
result of the permanent production of that substance by the fish
metabolism and their accumulation.
TAN capacity removal of the AAFBR and the partial water substitution of the RAS practiced once a week promotes the control of
the different ammonium forms, the reduction of the TAN levels can
be appreciated in Fig. 6.
After the AAFBR stabilization, TAN values observed in culture
tanks were within the range recommended by authors as Timmons
et al. (2002) and Colt (2006).
TAN removal efficiency was calculated based on mean concentrations measured in the three culture tanks mixed effluents at
sedimentation tank inlet; in its effluent and in the AAFBR effluent. The average efficiency measured after reactor stabilization was
of 27.06%, with a maximum value of 51.33%. The results obtained
with D10 = 0.34 mm GAC carrier were better than those reported
at two phase fluidized sand reactors by Timmons et al. (2002) and
Summerfelt (2006) with D10 between 0.45 and 0.80 mm; however,
the calculated efficiency at AAFBR was lower than the obtained by
using fine media as reported by Weaver (2006) and Davidson et al.
(2008).
3.6.2. TN removal
Most of the time, the AAFBR effluent (E.FBR) had lower TN values
than those registered on the sedimentation tank effluent (E.Sed); it
suggests nitrogen assimilation by the biofilm microorganisms and a
possible denitrification as a part of the ammonium process removal.
Fig. 7 represents the TN values measured.
According to Van Rijn et al. (2006), denitrification can occur
inside the RAS in anoxic environments and in presence of carbon
and nitrogen inorganic compounds, or also in the same biofilm
as observed by Dalsgaard and Revsbech (1992) in the form of a
“passive denitrification”.
TN of AAFBR influent and effluent mean values were 4.6 mg/L
and 4.1 mg/L, respectively. As observed in Fig. 7, there were
moments in which TN concentration at effluent was higher than
the measured at influent; this situation could be explained for the
thickness loss attached to carrier biofilm and its reactor output
produced by the effluent flow.
TN average removal efficiency during the project was of 24.32%,
value related with the 27.06% of TAN removal efficiency demonstrated by the AAFBR in the same period.
Table 3
Mean and maximum values of ammonium nitrogen forms registered in the AAFBR.
Sampling point
Tank 1
Tank 2
Tank 3
Sedimentation tank inlet
Sedimentation tank outlet
AAFBR outlet
Values including adaptation period (mg/L)
Values excluding adaptation period (mg/L)
NH4 +
NH3
TAN
TAN max
NH4 +
NH3
TAN
TAN max
0.133
0.200
0.189
0.145
0.139
0.101
0.004
0.004
0.005
0.004
0.003
0.003
0.136
0.205
0.193
0.149
0.143
0.104
0.338
0.866
0.612
0.352
0.325
0.194
0.331
0.413
0.415
0.309
0.315
0.291
0.009
0.009
0.010
0.009
0.009
0.012
0.340
0.422
0.424
0.318
0.325
0.303
1.820
2.307
2.108
1.750
1.787
1.802
I.A. Sánchez O., T. Matsumoto / Aquacultural Engineering 47 (2012) 16–26
23
Fig. 6. TAN concentrations in the influent and effluent of the sedimentation tank and the AAFBR effluent.
3.6.3. Nitrite and nitrate removal
The average nitrite values (as NO2 –N) measured on the AAFBR
influent and effluent were 0.82 and 0.88 mg/L, respectively, and the
average nitrate levels (as NO3 –N) calculated on the same sampling
points were 4.01 and 4.56 mg/L. Despite the significant scrubbing
and biomass attrition of the two-phases fluidized bed reactors
(Weaver, 2006; Sandu et al., 2002), increased in this case by the high
velocity circulation, the three-phase condition and the low density
of particles, a raise of the nitrite and nitrate levels was measured
on the AAFBR due to the nitrification processes developed inside
this reactor. Because the permanent circulation of the carrier on
the reactor, the zero OD conditions in its effluent or the behavior as
a nitrite generator were not registered, as commented by Weaver
(2006) for fine media two-phase fluidized bed reactors.
3.6.4. COD and BOD removal
There was a non-uniform behavior in the obtained data especially in the BOD, probably due to the situation presented in the
outlet of the culture tanks that produced sudden variations in the
flow and in the concentrations of the solids, carbonaceous matter and nutrients affecting the sedimentation tank and the AAFBR
hydraulic retention time.
The maximum COD value registered in the sedimentation tank
influent was of 113 mg/L and the average was 36 mg/L; and the
maximum and average values in the AAFBR effluent were 24 and
6.3 mg/L, respectively.
The maximum, average, and minimum BOD measured values in the sedimentation tank influent were 7.4 mg/L, 4.7 mg/L
and 1.7 mg/L, respectively, and in the AAFBR effluent they were
8.4 mg/L, 2.7 mg/L and 0.5 mg/L. The registered values suggest that
there was a trend to stabilization of the BOD amount produced in
the culture tanks and the organic matter removal by the treatment
system.
Based on the sedimentation tank influent and the AAFBR effluent
concentrations, COD and BOD removal efficiencies were calculated,
and are presented in Fig. 8. The average efficiency removal values
of BOD and COD were 47.44% and 77.33%, respectively.
Aquaculture wastewater treatment literature mainly focused
on the nitrogen compounds removal, but interesting articles as
the Sindilariu (2007) and Davidson et al. (2008) reported the performance of the wastewater treatment systems in terms of the
removal of the BOD. Davidson et al. (op. cit.) stated the high dilution
of BOD in the RAS waters and reported removal efficiencies above
the 60% in fluidized bed reactors with control of the biofilm thickness, using sand with D10 = 0.11 mm as carrier, a smaller effective
size than the presented by the GAC in this research.
The COD removal efficiency obtained in the AAFBR was much
higher than the reported by Ng et al. (1996) who used GAC as carrier
in a two-phase fluidized bed reactor. The three-phase condition and
the smaller effective size of the supporting media of the AAFBR that
increases the bed specific surface area contributed to improve the
absorption capacity of the carrier.
3.6.5. Total phosphorous removal
Total phosphorous values registered in the influent of the
sedimentation tank ranged between 0.57 and 3.84 mg/L and the
measured at AAFBR effluent between 0.48 and 1.53 mg/L; average
calculated concentrations in those points were 1.6 and 0.9 mg/L.
Fig. 7. TN concentrations in the AAFBR influent and effluent.
24
I.A. Sánchez O., T. Matsumoto / Aquacultural Engineering 47 (2012) 16–26
Fig. 8. BOD and COD removal efficiencies registered in the treatment system.
The calculated average total phosphorous removal efficiency was
of 38.49%, value similar to the reported by Davidson et al. (2008)
to fluidized bed biofilters in RAS, and Gebara (2006) to AAFBR in
domestic wastewater treatment.
The TP concentrations in the AAFBR effluent were higher than
the Brazilian legislation recommended values. Before dumping the
final effluents these could receive a post treatment in order to
remove organic matter, nitrogen, and especially phosphorous to
avoid problems related with stream damage, growth of algae and
the eutrophication of lakes. Phosphorous is a macronutrient that is
responsible for plants cell development and the promotion of good
root growth, it is because constructed wetlands could remove an
average of TP up to 35% (Von Sperling, 2005), and the three basic
types of land application: irrigation (slow rate); overland flow; and
infiltration-percolation (rapid rate) present expected performance
for phosphorous removal of 95%; 40–80%; and 60–95%, respectively
(Spellman, 2009). A source of vegetal phosphorous accumulation
are mature seeds or grains because they contain high amounts of
phosphorous in the form of phytic acid, typically between 50 and
85% of seed total phosphorus is found as phytic acid (Liu et al., 2005;
Frank et al., 2007; Dai et al., 2007).
3.6.6. Temperature and DO concentration monitoring
There was important water temperature variability during the
experiment. The minimum registered value was 15.7 ◦ C and the
maximum 25.1 ◦ C. Those variations produce thermal stress on
the fishes reducing food consumption and promoting the fungus
apparition on the epithelium.
The low initial values of DO associated with high ammonium
concentrations inside the tanks produced mortality in the first 2
weeks of the experiment. The lower OD concentration registered
in the farming tanks was 2.0 mg/L that coincided with the minimum
recommended value for culture units in RAS (Krause et al., 2006).
During the experiment, it was observed that temperature influences the DO concentration; low temperatures increased the
solubility and produced high concentrations (Metcalf and Eddy,
1996; Timmons et al., 2002). OD consumed by fish respiration –
that increases at high temperature and during digestion process –
affected the oxygen concentration too (Masser et al., 1999). Besides
the mentioned aspects, air flow provided in each culture tank
heterogeneity in replaced water flow, some outages, and two compressor changes influenced the DO behavior. According to Timmons
and Ebeling (2010), as operational and management experience is
gained, densities can be increased and systems added as needed
monitoring and control, so it is desirable to use automatic control
systems in future experiments.
The high DO values measured contributed to the good performance of the AAFBR; the average sedimentation tank effluent
OD concentration was 6.0 mg/L, recommended value by Krause
et al. (2006), and average DO concentration at AAFBR effluent was
8.1 mg/L.
3.6.7. pH, oxidation reduction potential, and conductivity
monitoring
In the experiment, pH stayed stable with lower value oscillations from 7.0 to 8.1 that were between the recommended values
for fish farming production by authors as Masser et al. (1999), and
Timmons et al. (2002). These values guaranteed a stable percentage of the toxic form of the ammonium: the NH3 , and helped the
growing and functions of nitrification bacteria, as stated by Metcalf
and Eddy (1996), and Krause et al. (2006).
Electric conductivity varied mainly due the four marine salt fish
treatments executed; progressive conductivity reduction on the
fish culture tanks and the other points of monitoring were produced by the weekly partial water substitution on RAS. Maximum
and minimum conductivity values registered in the tanks were
440 ␮S/cm and 174 ␮S/cm, respectively.
Only positive ORP values that vary between 265 and 510 mV
were registered in the system, the average measured ORP value
was 380 mV, most of the times the registered levels on RALFCC
effluent were near to that value. ORP measures the system oxidation or reduction capacity, oxidant agent presence, as oxygen
and chlorine represent positive values of ORP. Thus, ORP variability could be justified by DO variation concentrations and mainly
by little amounts of residual chlorine in water. The redox potential
measured levels were near the range defined by Lawson (1995) for
healthy aquaculture systems.
3.6.8. Solids removal
High variability of solids concentrations in the inflow and outflow of the sedimentation tank and the AAFBR was registered. In
sedimentation tank, variability was produced by the sudden nonuniformity of outflow at the farming tanks especially in tanks 2
and 3; in the AAFBR the variability could be generated by biofilm
detachment and little amounts of carrier loss.
Sedimentation tank influent registered TS concentrations
between 133 and 848.5 mg/L; at their effluent concentrations varied between 100 and 416 mg/L; AAFBR presented values between
I.A. Sánchez O., T. Matsumoto / Aquacultural Engineering 47 (2012) 16–26
135 and 490 mg/L. Those concentrations showed that the sedimentation tank removed TS with an average efficiency of 34.01%;
nevertheless, it could be necessary to improve the AAFBR sedimentation effluent system in order to reduce the carrier loss.
Sedimentation tank inflow presented SS concentrations
between 9 and 196 mg/L with an average of 79.2 mg/L. On AAFBR
effluent SS varied between 2 and 60 mg/L and the average value was
of 16.2 mg/L. Average SS removal efficiency in the sedimentation
tank–AAFBR treatment system was 64.45%.
Average VS concentrations at sedimentation tank influent and
effluent and AAFBR effluent were 208.5, 92.3, and 171.2 mg/L,
respectively; these data confirmed the biofilm (organic matter)
loss at AAFBR, probably produced by carrier grains friction due to
their permanent circulation into the reactor and the carrier loss
previously discussed.
An alternative to reduce that situation could be to increase the
sedimentation zone volume on AAFBR in order to diminish upward
flow and outflow liquid velocities, and to increase the outflow collecting diameter to reduce the effluent velocity.
3.6.9. Turbidity and apparent color removal
Turbidity registered values at sedimentation tank influent varied between 2.9 and 131 NTU; and at AAFBR effluent between 4.71
and 1.56 NTU. The average removal efficiency of this parameter
demonstrated by the sedimentation tank–AAFBR treatment system
was 65.51%.
The mixed three tanks effluents, that is, sedimentation tank
influent had the average apparent color value of 107.9 platinum
cobalt units (PCU), and the effluent average value was 35.2 PCU;
the AAFBR effluent reported an average value of 32.4 PCU. Based
on the 8-week values, the apparent color removal efficiency on the
RAS was calculated, whose average value was of 56.37%.
3.6.10. Cumulative feed burden
The cumulative feed burden (CFB, g feed/L of freshwater/day)
value was calculated as: CFB = F/VFW ; where F is the daily mass
of feed (g) delivered and VFW is the equivalent volume (m3 ) of
freshwater replaced per day (Colt et al., 2006; Malone and Beecher,
2000). The average feed ratio during the research was 2.5% of live
weight, and the maintained biomass IN the system was 30 kg/m3 ,
representing 450 g of feed/day; the equivalent volume of freshwater added on the RAS was 203 L/d. The calculated CFB was 2.28 g/L,
3.4 times lesser than the reported in a tilapia recirculating aquaculture system for Brazil (2006). The small value of CFB is due to the
low fish density maintained on the evaluated system.
The evaluated RAS showed that the investment of electricity on
water pumping and air injecting were higher than the commercial value of the fish kilograms produced, however, the average
stocking density maintained on the system was lesser than the
reported in other experiences with tilapia as Twarowska et al.’s
(1997) and Shnel et al.’s (2002) experiments in which the densities
were higher than 55 kg/m3 . The AAFBR could be evaluated at different HRT’s in order to optimize its treatment performance. High
stocking densities and productions are required in this recirculating
aquaculture system to be able to cover investment and operational
costs (Martins et al., 2010).
4. Conclusions
• The average TAN removal efficiency on the evaluated wastewater treatment system was of 27.1%, and average TN removal was
24.3%.
• The wastewater treatment system demonstrated a good capacity
for organic matter removal at the common RAS concentrations to
•
•
•
•
•
•
25
intensive Tilapia farming with an average BOD removal of 47.4%
and 77.3% to COD.
The evaluated wastewater treatment system performance suggested it as an alternative to water reuse on RAS, capable to
maintain the water quality characteristic at recommended values
for fish farming in closed systems.
The system maintained the water quality parameters in a range
of values below the maximum permitted values defined by the
Brazilian environmental legislation to final effluents disposal in
water corps, except for the phosphorous concentration that was
above the recommended limits.
The low value of the calculated cumulative feed burden was due
to the low fish density maintained on the evaluated system.
The operational costs were higher than the commercial value of
the fish produced, however, the AAFBR could be evaluated at different HRT’s and higher stocking densities in order to optimize
this treatment performance.
At three-phase and two-phase conditions, the gas holdup results
at the riser were always higher than the obtained at the downcomer.
In two-phase conditions the oxygen transfer was higher than registered for the three-phase conditions to the same air injected air
flow.
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