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Estimates of nutrient discharge from striped catfish farming in the Mekong River, Vietnam, by using a 3D numerical model

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A simulation was carried out by using a 3D numerical model for estimate the nutrient level discharged from striped catfish, Pangasianodon hypophthalmus intensively farming in the Mekong River, Vietnam. The simulated period was dry season from April 24, 2007, to April 27, 2007. Both dissolved and particulate forms of nutrients were simulated. A real status of water environment and scenario of discharge after applying fishpond effluent for irrigation of rice field were estimated in My Hoa Hung fish farm, An Giang Province. Simulated results were verified by observed data. Our results showed that nutrient levels at farming area in dry season were temporarily high and local. Applying waste water from the fishpond for irrigation of rice field could greatly reduce nutrients level in the fish farming area, the nutrient levels were 77 % for total nitrogen and 73 % for phosphorus. Therefore, recycling nutrient from fishpond effluent for irrigation of rice field illustrated an effective technology for pollution reduction which is a crucial issue to enable sustainable development of intensively farmed striped catfish.
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Estimates of nutrient discharge from striped catfish
farming in the Mekong River, Vietnam, by using a 3D
numerical model
Tran Thi Ngoc Trieu Minjiao Lu
Received: 1 October 2012 / Accepted: 21 May 2013
Springer Science+Business Media Dordrecht 2013
Abstract A simulation was carried out by using a 3D numerical model for estimate the
nutrient level discharged from striped catfish, Pangasianodon hypophthalmus intensively
farming in the Mekong River, Vietnam. The simulated period was dry season from April
24, 2007, to April 27, 2007. Both dissolved and particulate forms of nutrients were sim-
ulated. A real status of water environment and scenario of discharge after applying fish-
pond effluent for irrigation of rice field were estimated in My Hoa Hung fish farm, An
Giang Province. Simulated results were verified by observed data. Our results showed that
nutrient levels at farming area in dry season were temporarily high and local. Applying
waste water from the fishpond for irrigation of rice field could greatly reduce nutrients
level in the fish farming area, the nutrient levels were 77 % for total nitrogen and 73 % for
phosphorus. Therefore, recycling nutrient from fishpond effluent for irrigation of rice field
illustrated an effective technology for pollution reduction which is a crucial issue to enable
sustainable development of intensively farmed striped catfish.
Keywords Numerical simulation Nutrient Pangasianodon hypophthalmus
Rice field Scenario
Introduction
Striped catfish production in the Mekong Delta, Vietnam, is one of the biggest freshwater
aquaculture industries globally (De Silva et al. 2010; De Silva and Phuong 2011). This
industry produced 1,128 thousand tons of fish in 2008 (Anh 2010) making Vietnam the
third-ranked nation in global aquaculture production (FAO 2010). Most fish farms are
T. T. N. Trieu (&)M. Lu
Department of Civil and Environmental Engineering, Nagaoka University of Technology, 1603-1,
Kamitomioka, Nagaoka, Niigata 940-2188, Japan
e-mail: ngoctrieu1232003@yahoo.com
123
Aquacult Int
DOI 10.1007/s10499-013-9656-3
small scale, and the land used for fishponds is very expensive. Fish are farmed mainly in
earthen ponds and produce a large amount of effluent. The treatment of effluent is required
by national legislation, but it is nearly impossible in practice. Less than 10 % of fish farms
have sedimentation ponds (Hoa 2008). Therefore, a large volume of fish effluent from
fishponds is discharged directly into rivers and canals. Besides this, fish farming in cages
located along two branches of the Mekong River, where waste cannot be treated, have been
considered as a source of pollution, causing degradation of water quality in the river.
Though nutrient emissions from this industry account for \1 % of the total nutrient in the
Mekong Delta (Anh 2010), environmental concerns of striped catfish farming are reported.
The most common environmental concern is the direct discharge of farm effluent to rivers
and canals (Phan et al. 2009; Anh 2010; De Silva et al. 2010; Khoi 2011), and environ-
mental impacts of feed used in catfish farming in the Mekong River, Vietnam, were also
mentioned by Da and Berg 2009; De Silva et al. 2010. Besides striped catfish farming, the
fish processing industries also contributed to water pollution in the Mekong Delta (Anh
2010).
Numerical models can be applied to make environmental impact predictions and test
various scenarios. The interest in monitoring and management of aquaculture wastes with
mathematical models has been quickly increasing (Kishi et al. 1994; Hevia et al. 1996;Wu
et al. 1999; Doglioli et al. 2004). Most models were applied for coastal water only. The
main environmental impact from fish farms has been identified as the direct discharge of
effluent to rivers and canals. Until now, details about nutrient loading on the river bed nor
the distribution of nutrient after discharging from fish farming into the Mekong River are
investigated. There is a need to assess whether the fish farming activities have an impact on
the surrounding environment.
In this study, a numerical model was developed and applied to two scenarios: (1) the
direct discharge of total dissolved and particulate nitrogen and phosphorus from ponds and
cages to river and (2) the effects of nutrient discharge on river water quality when passing
the fish pond effluent through rice fields as a means to reduce the need for applying
fertilizers to the fields.
Materials and methods
Study area
My Hoa Hung fish farming area in My Hoa Hung commune, An Giang province (Fig. 1)
was selected for this study because this is the largest concentration of floating cages and
ponds of striped catfish (about 300 cages and 466.16 ha pond culture—Hanh 2009). Two
typical grow-out farms were chosen. Hatchery and nursery farms were not included in this
study. Bathymetry was surveyed by The Southern Institute of Water Resources Research
(SIWRR) in 2004. The dimension of the study area was 10 km 95 km, and the river in
this area has depth from 0 to 25 m.
Model
This study used a numerical model to simulate flow velocities and spatial contribution of
waste concentration from catfish farming in the Mekong Delta. The model simulated
nitrogen and phosphorus which were discharged from multi-point sources (pond culture
Aquacult Int
123
and cage culture), continuously released (in cage culture) and periodically released (in
pond culture) to the Mekong river. The model was programmed by Fortran 77 code,
including a 2D flow model and a 3D pollutant transport model. Its flowchart is shown in
Fig. 2. In the flow model, the 2D continuum and momentum equations (Eqs. 13) were
solved by the finite volume method.
og
otþoqx
oxþoqy
oy¼0ð1Þ
oqx
otþoUqx
oxþoVqy
oyþgh og
oxfqyþfr qqx
h2swx
q¼1
q
o
oxhsxx
ðÞþ
o
oyhsxy


ð2Þ
oqy
otþoUqy
oxþoVqy
oyþgh og
oyþfqxþfr qqy
h2swy
q¼1
q
o
oxhsyx

þo
oyhsyy


ð3Þ
where U,V=two components of depth averaged velocity in x,ydirection; q
x
,q
y
=two
components of unit discharge (q
x
=Uh;q
y
=Vh and q¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffi
q2
xþq2
y
q); t=time;
g=gravity acceleration; g=water surface elevation; h=g-z
b
=total water depth;
z
b
=riverbed elevation; q=water density; f=Coriolis coefficient; f
r
=bed friction
coefficient; s
wx
,s
wy
=wind stress at free surface; s
ij
=turbulent stress in horizontal.
A 3D flow field was derived by logarithmically distributing 2D flow field.
uðx;y;zÞ¼ U
z0
h1þln h
z0
hi
ln z
z0ð4Þ
Fig. 1 My Hoa Hung fish farm, An Giang province, Vietnam
Aquacult Int
123
vðx;y;zÞ¼ V
z0
h1þln h
z0
hi
ln z
z0ð5Þ
w¼Zz
0
ou
oxþov
oy

dyð6Þ
where z
0
=height with zero velocity; u,vand w=velocity in the x,y,zdirection.
In the pollutant transport model, waste concentration (C) was calculated by solving its
full 3D transport equation (Eq. 7) by the finite volume method with ADI scheme of
Douglas and Gunn (1964) in the ‘‘sigma’’ transformed coordinate system (Fig. 3).
In this coordinate system, the calculation domain with free surface and curved bed
surface becomes a rectangular prism where the depth equals to one. The value of rat the
bottom and the free surface are constant in time and space; that is r=-1 and r=0,
respectively. The main advantages of the ‘‘sigma’’ transformed coordinate system are the
accurate assimilation of the bottom and free surface boundaries and the possibility of easily
incorporating boundary layers. Therefore, flow and pollutant transport near water surface
and bottom are described more exactly. The calculation domain was divided into
Initial Condition , Boundary condition
Input TN, TP Concentration
Water Level Calculation
2D Flow Velocities Calculation
Dissolved TN, TP Transport
or
Particle TN, TP Deposition
Calculation Finish
Start
End
Ye s
No
3D Pollutant Transport Model
-3D Transport Equation
2D Flow Model
3D Flow Velocities Calculation -2D Shallow Water Equations
MK4 software
-1D Saint-Vernant Equations
Fig. 2 Flowchart of the numerical model
z
D
η
σ
1
h
Fig. 3 Sigma transform coordinate system
Aquacult Int
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25 m 925 m cells in a horizontal plane and ten layers in the vertical direction. In each
vertical line, the grid cell next to the river bed was half height of the other grid cells
(Fig. 4).
The 3D transport equation:
oC
otþouC
oxþovC
oyþowxs
ðÞC
oz¼o
oxACoC
ox

þo
oyACoC
oy

þo
ozKCoC
oz

þSð7Þ
The vertical velocity xwas defined as:
x¼1
DwxsuroD
oxþog
ox

vroD
oyþog
oy

roD
otþog
ot

ð8Þ
The Eq. (7) was changed as follows:
oqC
otþo
oxuqCDACo
ox
qC
D


þo
oyvqCDACo
oy
qC
D


þo
orxxs
D

qCKC
D
o
or
qC
D


¼DSð9Þ
where qC¼DCpollutant discharge; D¼gþh;AC;KC=eddy diffusivity coefficients
in horizontal and vertical direction; x
s
=settling velocity; S=the source term. Con-
vection terms of pollutant transport equation were interpolated following a centered
scheme.
The time interval was Dt=1 s to ensure the model’s stability while solving water level
and velocities, and the time interval for solving transport equation was 5 s, to speed up
calculation. Waste concentrations were calculated in every layer.
Equation (9) was solved with following boundary conditions:
(a)
(b)
(c)
Fig. 4 The outline of calculated grid (a), position of variables in horizontal plan (b) and position of
variables in sigma coordinate (c)
Aquacult Int
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Free surface boundary: At z¼g, no pollutant flux went through the surface.
Bottom boundary: At interface of bed layer z¼zbþa, the pollutant flux going through
the interface was defined by Van Rijn (1987): DbEb¼xsðCbCbÞ
In which D
b
=fish waste accumulated in the bottom; E
b
=fish waste loaded from
bottom; Cb;Cb=current concentration and saturated concentration in the bed surface.
Van Rijn (1984) suggested as: Cb¼0:015 d50
a
T1:5
D0:3
.
In which d
50
=mean diameter of waste particle; D¼d50 s1ðÞg=m2
½
1=3;T¼
sbsb;cr

sb;cr with s=density of waste particle; sb;cr =the critical shear stress
Shields; sb=the bed shear stress.
Solid boundary: The pollutant flux was set to zero.
Open boundary: A value of pollutant concentration was defined at the inlet. At outlet
boundary: a zero-gradient condition of concentration was used.
In this paper, we made a simulation from April 24, 2007, to April 27, 2007, in dry
season when the fish were at growth phase. This period coincided with the lowest discharge
time in the Vietnamese Mekong River (SRHMC 2010) which resulted in a worse situation
of water quality in fish farming area. The flushing rate T
f
of the study area is defined as
Wang et al. (2004): Tf¼V:f=R¼0:35 ðd1Þwhich means flushing time is 2.86 days with
f=1; V=low tide volume of the segment, R=the river discharge. So the simulated
period is enough to simulate the pollutant transport problem.
There was lack of gauged hydrodynamic data for boundary conditions in the study area.
Hence, boundary conditions were generated by offline coupling with the river flow sim-
ulation software-MK4 which was developed by the Fluid Mechanic Department, Ho Chi
Minh City University of Technology, Vietnam. In this software, the Saint Venant equations
were solved by an implicit algorithm which included diffusive component at conflux nodes
to calculate flow field from observed hydrographs for the larger area of complex network
of rivers and canals. High accuracy and stable features were verified by practical calcu-
lations (Giang 1999; Hang and Anh 2006; Ngo 2007).
A discharge time series was used as upstream boundary. The discharge data which were
redistributed as: qni ¼QðtÞh5=3
i=ðDxRh5=3
iÞand oqs=on¼0 assigned to grid node i at
the upstream cross-section.
At downstream boundary, water level g¼gðtÞ,oqs=on¼0 and oqn=on¼0 were used.
In which, QðtÞ;gðtÞwere generated by MK4 and h
i
—water depth at node i,s;n
tangential and normal direction at boundaries.
Scenarios to be modeled
Scenario of discharging nutrients directly from fish ponds and cages
In pond culture, there are 466.16 ha where pellet feeds are used and the mean yield is
300 ton/crop ha (Phan et al. 2009). Most farmers exchange water everyday. The per-
centage of daily wastewater exchange is about 30–40 % of the total water volume of the
pond (with 4 m depth). The sludge from the bottom is pumped out every 2 months when
the sediment in the bottom of the ponds becomes 10–20 cm in thick. In the simulation, five
drain canals were taken into account where discharging wastewater from ponds into the
river (showed in Fig. 5).
Aquacult Int
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In cage culture, farm-made feeds are used. There are 300 cages, with volumes ranged
from 500 to 1,000 m
3
/cage and mean yield is 50–100 ton/crop cage (Hanh 2008). Each
cage was assigned as one source point, and waste was released continuously.
Scenario of discharging nutrients after passing fishpond effluent through rice fields
While the Vietnamese standard for industrial wastewater discharging into the receiving
water which is used for transport, aquaculture, irrigation (TCVN 5945-2005 Type B) is
30 mg/L for total nitrogen and 6 mg/L for total phosphorus, the nutrients level from
untreated fishpond wastewater discharged to river in observed data ranged from 23.46 to
31.8 mg/L for total nitrogen and 5.02 to 12.54 mg/L for total phosphorus that exceeded
Vietnamese standard. Beside this, the nutrients level in sludge was much higher that ranged
from 2,564 to 3,057 mg/kg for total nitrogen and 606.4–1,164 mg/kg for total phosphorus
(Hanh 2008). In the fish farms where there are no sediment ponds, it is recommended to
recycle nutrients by discharging wastewater through rice fields. For sludge, it is not fre-
quently discharged and has low volume. So it is suggested that sludge be dried out and used
as fertilizer or leveling embankment.
It is very appropriate to the Mekong Delta where fish ponds are located near rice field
that require frequent irrigation with a large volume of water. Although wastewater dis-
charged from fishpond is often viewed as waste, it still has value and its reuse could benefit
the rice crops. The wastewater is first passed through a main canal which receives dis-
charge from a large number of fish ponds. It is then directed through rice fields to another
main canal. Irrigation water depth is 50–100 mm. The effluent is kept in the field for
5–7 days, then drains away and dries out for 5–7 days. After that, irrigation is repeated.
The wastewater from fish ponds could be used to replace a significant proportion of the
fertilizer requirement for rice field (33 % nitrogen, 50 % phosphorus and 50 % potassium)
Fig. 5 Location of pond area, cage area, sampling stations and discharge canals in study site
Aquacult Int
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(Phung et al. 2010a). Moreover, there is an increase in yield by 1 t/ha in rice fields that
receive waste from catfish ponds. This method resulted in a removal rate for nitrogen and
phosphorus of 74.6 and 72.4 %, respectively, before discharging to the river (Phung et al.
2010a,b; Phung and Bell 2010). Therefore, nutrient levels in wastewater were reduced as
7.02 mg/L for total nitrogen and 2.42 mg/L for total phosphorus, which were applied for
input nutrients concentration in the simulation.
Waste input
There are two types of waste input: exchanged water and sludge. Data on sludge quality
from the catfish farming are scarce. The exchanged water everyday has high volume
(12,000–14,000 m
3
/ha day), and its nutrients level is not as serious as sludge. Nitrogen and
phosphorus discharged from fish farming exist in both soluble and particulate forms
through dissolved nutrients, feces and uneaten food. Experimental data of nitrogen and
phosphorus concentration from fishponds at the study site were obtained through Hanh
(2008). Wastewater and sludge in 2 typical ponds (0.7 ha) and 3 locations surrounding in
My Hoa Hung fish farming S1, S2 and S3 (Fig. 5) were sampled during the period of
effluent discharge from January to June, 2007, the dry season in this region. Wastewater
samples were analyzed for total nitrogen and total phosphorus in dissolved form according
to the Standard Methods for the Examination of Water and Wastewater (APHA 4500D and
APHA 4550 p-D American Public Health Association-APHA 1998). Sludge samples were
analyzed for total nitrogen and total phosphorus in particulate form by Vietnamese Stan-
dard TCVN 7209: 2000.
Concerning nutrients concentration in cage culture, De Silva et al. (2010) found that
nitrogen discharge levels for farm-made feeds have a median of 46.8 kg/ton fish, while
phosphorus discharge levels for farm-made feeds have a median of 18.4 kg/ton fish. The
ratio of dissolved nitrogen and phosphorus for catfish farming is unknown. Ackerfors and
Enell (1990) estimated that 78 % of nitrogen released is dissolved and 22 % is particulate,
21 % of phosphorus is dissolved while the rest is particulate in salmonid farm. So these
model input values were shown in Table 1.
About settling velocity of particulate waste, Elberizon and Kelly (1998) estimated
values of 2.9 cm/s for fecal sizes larger than 2 mm when they studied benthic impacts of
freshwater salmonid cage aquaculture. For catfish waste, there is no documentation
available. In this paper, the value 2.9 cm/s was used in the simulation of particulate
nitrogen and phosphorus.
Table 1 Pollutants loading from
cage culture
Source:
a
De Silva et al. (2010),
b
Ackerfors and Enell (1990)
Indicators Input values Unit
Total nutrients
a
Total nitrogen 46.8 kg/ton fish
Total phosphorus 18.4 kg/ton fish
Dissolved form
b
Total nitrogen 36.5 kg/ton fish
Total phosphorus 3.86 kg/ton fish
Particulate form
b
Total nitrogen 10.3 kg/ton fish
Total phosphorus 14.54 kg/ton fish
Aquacult Int
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Result and discussion
Flow field
Due to lack of field data, flow simulation results were compared with the result of MK4. In
Fig. 6, there was good agreement in comparisons of (Q-t) at downstream and water level at
upstream site for period of calculation between the model and MK4.
Figure 7showed a maximum magnitude of flow velocities in the study site simulated by
the model on ebb tide and flood tide, respectively. The higher value of velocity in the left
branch to the right one of the river resulted in an increase in diffusion and a lower
concentration of nutrients in station S3, although there were three hundred cages located in
the study area.
Waste transport
Scenario of discharge nutrients directly from fish ponds and cages
The results of simulation were compared with observed data in Table 2. Nitrogen and
phosphorus concentrations when waste discharged from ponds and cages were relatively
agreed with those of field data. Values of nitrogen and phosphorus concentration in
farming area depended on tidal process and schedule of waste released from ponds.
Wastewater from ponds was exchanged during high tide. The concentrations of nitrogen
and phosphorus at locations S1, S2 and S3 were increasing with rising tide. The transport
process as well as contribution of dissolved nutrients in fish farming area was shown in
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1 6 11 16 21 26 31 36 41 46 51 56 61 66 71
H (m)
t (hour)
Variation of H-t at upstream H-MK4
H-Model
-8000
-6000
-4000
-2000
0
2000
4000
6000
1 7 13 19 25 31 37 43 49 55 61 67 73
Q (m3/s)
t (hour)
Variation of Q-t at downstream Q- MK4
Q-Model
(a)
(b)
Fig. 6 Variation of water level at upstream (a) and discharge at downstream section (b) of the study site
simulated by the model and MK4 software
Aquacult Int
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Fig. 8a, b. The highest values only occurred at discharged points in the short time just
1–2 h coincident with discharge period. Then these concentrations began to decrease and
got minimum value on ebb tide because waste tided down and diffused away. This process
was repeated when it was flood tide again.
Fig. 7 Depth averaged velocity [arrows (m/s)] and elevation [contour lines (m)] fields in ebb tide (a) and in
flood tide (b) at the study site
Table 2 Distribution of nutrient
concentration in fish farming area
a
Source: Hanh (2008)
Indicators Station Calculated (min–max)
(mg/L)
a
Observed
(mg/L)
1. Dissolved
nitrogen
S1 5.6–39.0 39.5
S2 2.2–35.0 32.0
S3 4.4–31.0 36.0
2. Dissolved
phosphorus
S1 3.6–11.5 10.9
S2 1.0–12.5 11.2
S3 1.0–3.5 2.3
3. Particulate
nitrogen
S1 1,900–2,000 1,817.2
S2 800–1,000 1,044
S3 1,100–1,700 1,591
4. Particulate
phosphorus
S1 140–170 176.9
S2 270–320 329.2
S3 120–140 117.9
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When sludge was periodical pumped out, particulate wastes from sludge settled quickly
in the river bed. Accumulated particulate nutrients on the riverbed was shown in Fig. 8c, d.
Accumulation of particulate nutrients on the riverbed is within 200 m from the discharge
Fig. 8 Scenario of nitrogen and phosphorus distribution in fish farming when discharging fishpond effluent
directly to the river: afor dissolved nitrogen, bfor dissolved phosphorus, cfor particulate nitrogen and dfor
particulate phosphorus
Aquacult Int
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point. Accumulated area of particulate waste in this study is rather smaller than that of
study cases for coastal simulation in Cromey et al. (2002) and Doglioli et al. (2004). The
high flow velocity in the Mekong River is main reason because dispersion of particulate
wastes is more strongly influenced by dominating flow field. The concentration of nutrients
in discharged sludge was much higher than in daily exchanged water though it was not
frequent. Therefore, pumping out sludge to surrounding surface water could cause an
environmental problem.
On ebb tide, there was no discharge from ponds; the river water was only affected by
cage culture. Though 300 cages distributed along the riverbank (from 8,000 to 11,000 m in
the vertical axis), nutrient in cage culture was continuously released to the river with
concentration lower than that from ponds. So it was easily diluted by the river flow. The
highest concentration of nitrogen and phosphorus in this case was 2.3 and 0.75 mg/L,
respectively.
According to simulated results, pond farming contributed most pollutants to surface
water. However, the contribution of this type of farming to the water pollution depending
heavily on whether or not pond sludge was discharged into the river.
In simulation, the pollution situation was local. The nutrient discharge was high, but it
hardly modified river water quality. The concentration quickly decreased after stopping
discharge waste due to large river and high velocity in the Mekong River. Similar
assessments have been reported by Bosma et al. (2011), De Silva and Phuong (2011).
Although catfish farming does not cause a big problem to water environment in the
Mekong delta, it is necessary to mitigate the environmental impact from this industry
through effectively managing wastewater and sludge to ensure its sustainable development.
Scenario of discharge nutrients after passing fishpond effluent through rice fields
The solution passing wastewater through rice field was applied thoroughly in the study site.
As simulated by the model, the highest concentrations of nitrogen and phosphorus were 9.0
and 3.4 mg/L, respectively (Fig. 9). The nutrients in the wastewater from fish pond were
assimilated and utilized by rice field. Besides, after discharging, high velocity of flow field
continued to dilute nutrients concentration. As a result, the nutrient concentration in river
surrounding catfish farming area was reduced significantly by 77 % for total nitrogen and
73 % for total phosphorus.
To access the effluent effect from fish farming to downstream, hourly concentrations of
nutrients were estimated in downstream position (1 km away from station S1), in 24 h on
April 26, 2007. A large variation in concentrations depended on discharge schedule from
upstream and tidal process. With concentrations varied from 0.05 to 1.6 mg/L for total
nitrogen and 0.05–0.35 mg/L for total phosphorus, there was a clear impact on water
quality at downstream. When wastewater was passed through paddy field before dis-
charging, maximum concentrations reduced to 1.0 and 0.15 mg/L for total nitrogen and
total phosphorus, respectively (Fig. 10). The impact was reduced also.
Although the simulation was made for the worst case that both sludge and exchange
water from all fish farms were discharged without treatment to the river based on the
background of a minimum of river flow in the dry season. There is a potential eutrophi-
cation at the surrounding fish farming area due to high nutrients concentration. There is a
need to mitigate the environmental problem step by step. First, the levels of waste con-
centration should be limited under the river capacity by improving feed quality and
adopting of Best Management Practices (BMPs). Secondly, suitable management tech-
niques should be studied to treat wastewater in the Mekong Delta.
Aquacult Int
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Fig. 9 Scenario of nitrogen and phosphorus distribution in fish farming after passing fishpond effluent
through paddy field: afor dissolved nitrogen, bfor dissolved phosphorus
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
max average min
TN (mg/l)
Dissolved nitrogen at downstream
before treatment
after treatment
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
max avera
g
emin
TP (mg/l)
Dissolved phosphorus at downstream
before treatment
after treatment
(a)
(b)
Fig. 10 Comparison of current status and treatment case for nitrogen and phosphorus concentration at
downstream: afor dissolved nitrogen, bfor dissolved phosphorus
Aquacult Int
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Conclusion
In this study, a simulation of water environment in My Hoa Hung fish farming was
presented. The study focused on nutrient loading from catfish farming. All other factors
were neglected. The results showed that discharging fishpond effluent directly to river
resulted in a high level of nutrients in surrounding surface water. The main environmental
impact from fish farms is the direct discharge of effluent to surface water. The reduction in
environmental pollution is a crucial issue to enable sustainable development of catfish
industry in Mekong Delta. Recycling nutrients by passing wastewater through paddy fields
could be applied as an effective technology for improving quality of surface water in the
Mekong Delta. Monitoring water quality in fish farming is very necessary. The numerical
model that may be successfully used for planning and monitoring purposes illustrated an
economical solution in discharge management from fish farms.
Acknowledgments The authors gratefully acknowledge Professor Le Song Giang whose software MK4
has been used to verify the model.
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... Therefore, it was suggested using the sludge as a fertilizer, which consists of 33 % of nitrogen, 50 % of phosphorus and 50 % of potassium. And during high tide period, the concentration of both nitrogen and phosphorus will be raised (Trieu & Lu, 2014). ...
... The results suggest that antibiotic contamination in water was partly mitigated by draining wastewater to the rice fields (see Pangasius 1) prior to discharge to surrounding aquatic environments. The drainage removed excess nutrients from aquaculture effluents effectively, while serving as a fertilizer in rice production (e.g., [39,40]). ...
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